Enhanced Weathering in Agriculture

This Protocol provides the requirements and procedures for the calculation of net carbon dioxide equivalent (CO₂e) Removal from the atmosphere via Enhanced Weathering (EW) in agricultural settings. Agricultural EW is considered a subcategory of EW approaches.

Silicate weathering naturally sequesters approximately 0.1-0.3 Gt of CO₂ per year^{1 , 2 ,3} and is thus a critical feedback mechanism in global climate regulation on geologic timescales. In this process, atmospheric CO₂ dissolved in surface water reacts with silicate rocks and is converted to dissolved inorganic carbon or carbonate minerals, both of which constitute stable sinks for CO₂ on geologic timescales^{4, 5}. Natural weathering can be accelerated by applying crushed silicate rock to agricultural land, where the increased reactive surface area of the rock and damp conditions in the soil lead to elevated reaction rates. Alkaline species produced through weathering reactions allow for the increased storage of CO₂ in the aqueous phase, typically as dissolved bicarbonate. This dissolved inorganic carbon is eventually exported to the ocean where it can be stably stored for millennia⁶. This process is known as enhanced weathering and has been proposed as an effective natural Carbon Dioxide Removal (CDR) technique, in accordance with the Intergovernmental Panel on Climate Change (IPCC)⁷, to mitigate potential impacts of anthropogenic climate change⁸. Given that cropland covers approximately 38% of land surface on Earth, EW in agricultural settings has large scalability potential with relatively small changes in farm management. Scalability is aided by existing infrastructure of rock spreading as a soil amendment. Current estimates suggest that EW in cropland has the capacity to remove on the order of 1-2 Gt of CO₂ per year⁸. In addition to CO₂ removal, EW can potentially provide co-benefits to farmers in the form of increased crop yield and resiliency^{9, 10}, as well as replenishment of soil nutrients^{11, 12} and reduction of nitrogen loss^{9, 13}.

This Protocol accounts for the quantification of the gross amount of CO₂ removed via agricultural EW, as well as all cradle-to-grave, life-cycle Greenhouse Gas (GHG) emissions associated with the process.

The Protocol ensures:

• consistent and accurate procedures are used to measure and monitor all aspects of the EW process required to enable accurate accounting of net CO₂e removals
• consistent system boundaries and calculations are utilized to quantify net CO₂e removal for agricultural EW projects
• requirements are met to ensure the CO₂e removals are Additional
• evidence is provided and verified by independent third parties to support all net CO₂e removal claims

In addition to CO₂ removal, dissolution of silicate rocks in agricultural settings may provide co-benefits through the addition of nutrients, alkalinity and silica in soils. Potential co-benefits include:

• Combating soil acidification through addition of alkalinity⁹
• Increased soil nutrients, including K, P, Mg and Ca^{14, 15}
• Increased crop yields^{14, 15, 16}
• Enhanced crop resistance to common pests and disease^{17, 18, 19}
• Increased crop resilience to drought²⁰
• Decreased nitrogen loss, leading to reductions in fertilizer use, eutrophication and nitrous oxide emissions^{18, 13}

Increased soil nutrients and crop resilience by EW can lead to increased yields, which may help to alleviate global food insecurity if deployed at scale. EW also has the potential to further mitigate emissions via reduction of nitrous oxide fluxes in cropland, which is currently thought to represent 50-80% of global anthropogenic N₂O emissions¹³.

This protocol mainly utilizes and is intended to be compliant with the following Standards and protocols:

• Isometric Standard
• ISO, EN, 14064-2: 2019 - Greenhouse Gases - Part 2: Specification with guidance at the project level for quantification, monitoring, and reporting of greenhouse gas emission reductions or removal enhancements

Additional reference standards that inform the requirements and overall practices incorporated in this Protocol include:

• ISO 14064-3: 2019 - Greenhouse Gases - Part 3: Specification with Guidance for the verification and validation of greenhouse gas statements

• ISO 14040: 2006 - Environmental Management - Lifecycle Assessment - Principles & Framework

• ISO 14044: 2006 - Environmental Management - Lifecycle Assessment - Requirements & Guidelines

Additional standards, methodologies and protocols that were reviewed, referenced or for which attempts were made to align with or leverage during development of this Protocol include:

• Global Rock C-sink, Guidelines for the Certification of Carbon Sinks created by Enhanced Rock Weathering in Croplands, v0.9, Carbon Standards International, October 2022

• Enhanced Rock Weathering Methodology, PuroEarth, 2022

• Criteria for High-Quality Carbon Dioxide Removal, Carbon Direct, Microsoft, 2023

• BS EN 15978:2011 Sustainability of construction works - Assessment of environmental performance of buildings - Calculation method

• Foundations for Carbon Dioxide Quantification in Enhanced Rock Weathering Deployments, Cascade Climate, 2024

This Protocol was developed based on the current state of the art, publicly available science regarding EW in agriculture. Because EW is a novel CDR approach, with limited published literature, this Protocol incorporates requirements that may be more stringent than some current relevant regulations or other protocols related to EW for CDR.

Future versions of this Protocol may be altered, particularly regarding requirements for demonstrating Durability of EW, as the stability of CO₂ captured by EW from Feedstock dissolution in agricultural soils is better demonstrated and documented; Reversal risks are proven to be limited; and the overall body of knowledge and data regarding all processes, from feedstock supply to conversion and to permanent storage, is significantly increased.

This Protocol applies to projects or processes which:

• Utilize crushed rock or mineral feedstock applied to agricultural land to capture CO₂.
  • Rock or mineral feedstock is defined as silicate rock containing alkaline earth and alkali metals (i.e. Ca, Mg, K, and Na) which converts CO₂ to aqueous bicarbonate when applied in sufficient quantities to agricultural land.
  • Agricultural land includes all arable land and permanent cropland, as defined by the United Nations Food and Agriculture Organization (FAO), including row cropland and pastureland. Similarly-managed lands that do not meet this definition will be considered on a case-by-case basis.
• Export alkalinity generated through weathering reactions from soils to the ocean via riverine transport (Isometric will address alternative drainage pathways in future versions of this Protocol).

This Protocol applies to projects and associated operations that meet all of the following project conditions:

• the Project provides a net-negative CO₂e impact (net CO₂e removal) as calculated in the GHG Statement, in compliance with Section 7.

• the Project does not disproportionately harm underserved or marginalized communities, in compliance with Section 3.7 of the Isometric Standard and Section 5 of this Protocol.

• the Project is considered additional, in accordance with the requirements of Section 6.4.

• the Project provides long duration storage (>1000 year estimated) of CO₂ in seawater and/or soil.

Projects that are explicitly NOT eligible include the following:

• Projects that apply rock or mineral feedstock in non-agricultural settings

• Projects that utilize non-silicate feedstock

• Projects that lead to a sustained and substantial net decrease on crop yields

As previously stated, per the UN FAO definition of permanent and arable crop land, meadows and pastureland are eligible for EW projects. This Protocol will refer to monitoring requirements specific to row cropland, such as monitoring crop yield, health, and resiliency. Such measurements may not be applicable to projects taking place on pastureland and therefore may not be required in the Project Design Document (PDD). Justifications to omit measurements in such instances are allowable in the PDD.

The Project must consider environmental and social impacts at all Project locations, including the mine or quarry where feedstock is sourced, during feedstock transportation, and at the deployment site. Appropriate measures must be implemented to identify and eliminate potential risks to terrestrial and aquatic ecosystems and biodiversity. Where risks cannot be eliminated, the PDD must identify measures to monitor ecosystem health and mitigate adverse effects through a project-specific mitigation plan. Mitigation plans must be carried out by subject matter experts, in consultation with Isometric. Refer to Section 3.7 of the Isometric Standard for further guidelines on environmental and social impacts.

Credits issued under this Protocol are contingent on the implementation, transparent reporting and independent verification of comprehensive safeguards. These safeguards encompass a wide range of considerations, including environmental protection, social equity, community engagement and respect for cultural values. The process mandates that safeguard plans be incorporated into all major project phases, with detailed reports made accessible to stakeholders. Adherence to and verification of environmental and social safeguards, in accordance with Section 3.7 of the Isometric Standard, is a condition for all crediting projects.

Ongoing environmental assessment must be completed in accordance with the Isometric Standard to identify potential risks, followed by the development of tailored mitigation plans by subject matter experts where necessary. Project Proponents must first strive to avoid negative environmental impacts. In cases where adverse environmental effects take place, the Project Proponent must develop plans to minimize and rectify them. Effective implementation of these measures must also be accompanied by a robust monitoring plan to ensure efficacy. Project Proponents must demonstrate active stakeholder engagement throughout this process, in accordance with Section 3.5 of the Isometric Standard. All mitigation strategies must align with local and international environmental laws and contribute to sustainable project outcomes.

Enhanced Weathering of rock or mineral feedstock may be associated with the release of metals such as nickel (Ni) and chromium (Cr) or other harmful contaminants such as asbestiform minerals, which may pose an environmental risk. To prevent or mitigate such risks, the Project Proponent must take the following measures:

• Comprehensive analyses must be conducted in accordance with Isometric's Rock and Mineral Feedstock Characterization Module. Project Proponents should select rock or mineral feedstocks that minimize the risk of soil and groundwater contamination, e.g. by selecting feedstocks that do not contain harmful metals in concentrations that may lead to exceeding regulatory limits.
• A robust monitoring system must be established to regularly check for potentially harmful contaminants in soil and groundwater for projects with high contamination risk, as determined by the feedstock used. This will likely involve periodic sampling and analyses alongside field monitoring for removals and must be determined on a project basis. The concentration of contaminants in soil and water must not exceed the limits established by the local authority where the project is located. In the absence of local regulations, the Project Proponent must adhere to standards set by the European Union (EU), the World Health Organization (WHO) or the United States Environmental Protection Agency (US EPA). Justification behind the regulatory body selection must be provided in the PDD. An environmental monitoring plan is required for projects in which there is a significant risk of exceeding local regulatory limits of heavy metals (e.g. Ni, Co, Cr) or asbestiform minerals at the selected feedstock application rate. All Project Proponents must submit a description of contamination risks in the PDD and, where applicable, a description of the environmental monitoring plan.
• If pre-existing heavy metal concentrations exceed applicable regulatory limits or guidance (as identified in the baseline scenario), the Project may still be considered for crediting against this Protocol. This is contingent on the Project Proponent providing evidence of existing elevated metal concentrations. To qualify, the Project must undertake specific remediation strategies to mitigate the contamination. These strategies could include altering the variety or quantity of feedstock used, implementing soil amendments or introducing phytoremediation practices using plants adept at absorbing heavy metals. It is important to note, however, that any project with pre-existing elevated heavy metal concentrations which further aggravates soil contamination will not meet the criteria for this Protocol.

The Project Proponent must carefully consider and implement measures where necessary for the following potential impact areas before proceeding with an EW project. Appropriate measures must be implemented to identify and eliminate potential risks to human health and cultural rights at all project locations.

Maintaining agricultural productivity is critical to the environmental and social sustainability of EW projects. The Project Proponent must document how the project will monitor agricultural productivity and soil quality, including which productivity and soil characteristics will be tested and the frequency of testing. If justified, the Project Proponent may use proxy variables in lieu of direct testing or measurement. If productivity or soil quality are demonstrated or anticipated to be adversely affected, the Project Proponent must complete the following:

• The Project Proponent must collaborate with land managers or owners to implement soil management practices that maintain or enhance soil quality. For example, regenerative agriculture techniques such as diversifying crop rotation or utilizing cover crops.
• The Project Proponent must provide technical support, training and resources to help farmers adapt to any changes in soil conditions due to EW. This support could include advice on changes to soil amendments and sustainable farming practices.

All environmental and social safeguards will be verified to be implemented at all locations in the EW process, including at the feedstock source, transportation, and distribution sites. The Project Proponent must regularly assess the combined environmental impact of EW, which may include (but is not limited to) heavy metal concentrations in crops and groundwater. This may involve collecting data on soil and water quality, biodiversity indicators and agricultural productivity. The cadence of monitoring will vary based on the parameter. Accumulation of heavy metals in soils and crops must be assessed at a minimum of once per Reporting Period. Aqueous measurements will depend on the local water budget, but should be assessed quarterly at a minimum. All data indicators, data collection protocols and data interpretation must be developed or performed by subject matter experts. The Project Proponent must use the collected data to inform ongoing management of EW practices. This data must be shared with the public through Isometric's platform, in accordance with Section 6.6 of this Protocol. The Project Proponent must be prepared to adjust the EW strategy based on monitoring results and feedback from environmental studies and community engagement.

The following topics are covered briefly in this Protocol due to their inclusion in the Isometric Standard, which governs all Isometric Protocols. See in-text references to the Isometric Standard for further guidance.

For each specific Project to be evaluated under this Protocol, the Project Proponent must document Project characteristics in a Project Design Document (PDD) as outlined in Section 3.2 of the Isometric Standard. The PDD will form the basis for project verification and evaluation in accordance with this Protocol, and must include consideration of processes unique to each project, such as:

• detailed feedstock characterization (see Section 8)
• geographical designations of control plot(s) (see Section 9.3.1)
• description of measurement methods for all required analyses, cross-referenced with relevant standards where applicable
• description of any geochemical models used to quantify processes relevant to the calculation of CO₂ removal that are not directly measurable (see Section 7.4.1)
• a comprehensive sampling plan in accordance with Section 9, including climatic monitoring and field management plan

Projects must be validated and the Project GHG Statement (net CO₂e removal) verified by an independent third party consistent with the requirements described in this Protocol as well as in Section 4 of the Isometric Standard.

The Validation and Verification Body (VVB) must consider following requisite components:

1. Validate that feedstock adheres to the requirements listed in the Rock and Mineral Feedstock Characterization Module.
2. Verify that the quantification approach and monitoring plan adheres to requirements of Section 7, including demonstration of required records.
3. Verify that the Environmental & Social Safeguards outlined in Section 5 are met.
4. Verify that the project is compliant with requirements outlined in the Isometric Standard.

The threshold for Materiality, considering the totality of all omissions, errors and mis-statements, is 5%, in accordance with Section 4.3 of the Isometric Standard.

Verifiers should also verify the documentation of uncertainty of the GHG Statement as required by Section 2.5.7 of the Isometric Standard. Qualitative Materiality issues may also be identified and documented, such as²¹:

• control issues that erode the verifier's confidence in the reported data
• poorly managed documented information
• difficulty in locating requested information
• noncompliance with regulations indirectly related to GHG emissions, removals or storage

Project Validation and Verification must incorporate site visits to project facilities, namely agricultural fields being used as control, treatment and deployment plots, in accordance with the requirements of ISO 14064-3, 6.1.4.2. This is to include, at a minimum, site visits during validation and initial verification to the project site(s). Validators should, whenever possible, observe project operation to ensure full documentation of process inputs and outputs through visual observation.

A site visit must occur at least once during each project validation. Additional site visits may be required if there are substantial changes to field operations over the course of a project's validation period, or if deemed necessary by Isometric or the VVB.

Verifiers and validators must comply with the requirements defined in Section 4 of the Isometric Standard. In addition, teams shall maintain and demonstrate expertise associated with the specific technologies of interest, including soil sampling, analysis and data processing.

CDR via EW in agriculture is a result of a multi-step process (such as quarrying, transporting and spreading of rock), with activities in each step potentially managed and performed by a different operator, company or owner. When there are multiple parties involved in the process, and to avoid double counting of CO₂e removals, a single Project Proponent must be specified contractually as the sole owner of the Credits. Contracts must comply with all requirements defined in Section 3.1 of the Isometric Standard.

The Project Proponent must be able to demonstrate additionality through compliance with Section 2.5.3 of the Isometric Standard. The Baseline scenarios and Counterfactual utilized to assess additionality must be project-specific, and are described in Sections 7 and 9 of this Protocol.

Additionality determinations must be reviewed and completed at the time of initial Verification as well as following significant changes to project operating, including but not limited to:

• regulatory requirements or other legal obligations for project implementation change or new requirements are implemented
• project finance indicate Carbon Finance is no longer required, potentially due to:
  • increased tipping fees for waste feedstocks
  • sale of co-products that make the business viable without Carbon Finance
  • reduced rates for capital access

Any review and change in the determination of additionality shall not affect the availability of Carbon Finance and Carbon Credits for the current or past Crediting Periods, however, if the review indicates the Project has become non-additional, this shall make the Project ineligible for future Credits²².

The uncertainty in the overall estimate of the net CO₂e removal as a result of the Project must be accounted for. The total net CO₂e removed for a specific Reporting Period, $RP$, CO₂e_{Removal, RP}, must be conservatively determined in accordance with the requirements outlined in Section 2.5.7 of the Isometric Standard.

Projects must report a list of all input variables used in the net CO₂e removal calculation and their uncertainties, including:

• required measurements in Appendix 2 (e.g., soil and porewater measurements)
• data used to model and estimate riverine losses and marine losses
• emission factors utilized, as published in public and other databases used
• values of measured parameters from process instrumentation, such as truck weights from weigh scales, electricity usage from utility power meters and other similar equipment
• laboratory analyses, including analysis of rock or mineral feedstocks

More detailed uncertainty information should be provided if available, as outlined in Section 2.5.7 of the Isometric Standard. Uncertainty must be quantified at the project scale, which may include multiple fields within a region.

In addition, a sensitivity analysis that demonstrates the impact of each input parameter's uncertainty on the overall net CO₂e removal uncertainty must be provided. Details of the sensitivity analysis method must be provided so that the results are reproducible. Input variables may be omitted if they contribute to a < 1% change in the net CO₂e removal.

In accordance with the Isometric Standard, all evidence and data related to the underlying quantification of CO₂e removal and environmental and social safeguards monitoring will be available to the public through Isometric's platform. That includes:

• Project Design Document
• GHG Statement
• Measurements taken and supporting documentation, such as calibration certificates
• Emission factors used
• Scientific literature used
• Proof of approval for necessary permits

The Project Proponent can request certain information to be restricted (only available to authorized Buyers, the Registry and VVB) where it is subject to confidentiality. This includes emissions factors from licensed databases. However, all other numerical data produced or used as part of the quantification of net CO₂e removal will be made available.

The scope of this Protocol includes GHG sources, sinks and reservoirs (SSRs) associated with an EW CDR project. A cradle-to-grave GHG Statement must be prepared encompassing the GHG emissions relating to the activities outlined within the system boundary (Figure 1).

Emissions for processes within the system boundary must include all GHG SSRs from the construction or manufacturing of any project-specific physical site and associated equipment; closure and disposal of each site and associated equipment; and operation of each process (including feedstock production, transport, spreading and sampling for MRV).

Any emissions from sub-processes or process changes that would not have taken place without the CDR project must be fully considered in the system boundary. Paired with exclusion of waste input emissions when the criteria are met (see Section 7.1.1), this allows for accurate consideration of additional, incremental emissions induced by the CDR process.

The system boundary must include all SSRs controlled by and related to the project, including but not limited to the SSRs in Figure 1 and Table 1. If any GHG SSRs within Table 1 are deemed not appropriate to include in the system boundary, they may be excluded provided that robust justification and appropriate evidence is provided.

Figure 1. Process flow diagram showing system boundary for EW projects

Table 1. Systems boundary and scope of activities to be included for EW projects

The Project Proponent must consider all GHGs associated with SSRs, in alignment with the United States Environmental Protection Agency’s definition of GHGs, which includes: carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O) and fluorinated gasses such as hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluoride (SF₆) and nitrogen trifluoride (NF₃). For CO₂ stored, only CO₂ will be included as part of the quantification. For all other activities all GHGs must be considered. For example, the release of CO₂, CH₄, and N₂O is expected during diesel consumption.

All GHGs must be quantified and converted to CO₂e in the GHG Statement using the 100-yr Global Warming Potential (GWP) for the GHG of interest, based on the most recent volume of the IPCC Assessment Report (currently the Sixth Assessment Report).

Miscellaneous GHG emissions are those that cannot be categorized by the GHG SSR categories provided in Table 1. The Project Proponent is responsible for identifying all sources of emissions directly or indirectly related to project activities and must report any outside of the SSR categories identified as miscellaneous emissions.

Emissions associated with a project's impact on activities that fall outside of the system boundary of a project must also be considered. This is covered under Leakage in Section 7.4.3.4.

Ancillary activities (such as supplementary research and development activities and corporate administrative activities) that are associated with a project but are not directly or indirectly related to the issuance of Credits can be excluded from the system boundary.

EW may have additional impacts on GHG emissions beyond the scope of this Protocol. For example, there is some evidence that EW in agriculture may have secondary impacts on N₂O and CH₄ emissions associated with agricultural soils. These potential impacts are not currently included in the conservative GHG accounting framework and will be reviewed as scientific consensus evolves.

Embodied emissions associated with system inputs considered to be waste products can be excluded from the accounting of the GHG Statement system boundary provided the appropriate eligibility criteria are met.

If EC1 in Table 2 is satisfied, embodied emissions associated with the waste product input can be excluded from the system boundary. Market leakage emissions associated with waste inputs may also be excluded from the system boundary, as compliance with EC1 would result in no change to the waste producer behavior (i.e. no market leakage) and indicates there are no alternative users of the waste product (i.e. no replacement emissions).

Table 2. Waste input emissions exclusion criteria, EC1

If EC2 and EC3 in Table 3 are both satisfied, embodied emissions associated with the waste product input can be excluded from the system boundary. Market leakage emissions associated with waste inputs may also be excluded from the system boundary, as compliance with EC2 and EC3 would result in no significant change to the waste producer behavior (i.e. no market leakage) and there are no alternative use cases for the waste product (i.e. no replacement emissions).

Table 3. Waste input emissions exclusion criteria, EC2 and EC3

In some instances, the EW project activities may be integrated into existing activities, such as rock spreading whilst seeding. Activities that were already occurring and would continue to occur without the EW project may be omitted from the system boundary, if evidence that the activity was already occurring and would have continued to occur in the absence of the EW project can be provided.

The baseline scenario for EW projects assumes the activities associated with the EW project do not take place, no new infrastructure is built and business as usual agricultural practices occur.

The counterfactual is the CO₂ that would have been removed from the atmosphere and durably stored as a result of natural weathering or pre-existing land practices. This is determined through the use of a control plot, as described in Section 7.4.2, with detail on monitoring requirements described in Section 9.3.4.3.

EW in agriculture typically consists of an application of a characterized rock or mineral feedstock followed by discrete sampling periods to quantify CO₂ removals. Rock or mineral feedstock must be sourced, which may include quarrying, processing (e.g., crushing) and transportation, before it is spread over agricultural land. A combination of soil, aqueous and other geochemical measurements are then used to quantify the total CO₂ removals over a period of time.

The Reporting Period for EW represents an interval of time over which removals are calculated and reported for verification. Monitoring of CO₂ removals for EW may include a combination of discrete sampling (e.g., soil sampling) and continuous sampling. The equations used to calculate net CO₂e removals will pertain to all GHG emissions and CO₂ removals occurring over a Reporting Period. In most cases, this Reporting Period will be an interval of time bounded by sampling events.

GHG emission calculations must include all emissions related to the project activities that occur within the Reporting Period. This includes: a) any emissions associated with project establishment allocated to the Reporting Period, b) any emissions that occur within the Reporting Period, c) any anticipated emissions that would occur after the Reporting Period that have been allocated to the Reporting Period and d) leakage emissions that occur outside of the project boundary as a result of induced market changes that are associated with the Reporting Period. Requirements for allocated emissions to Reporting Periods are set out in Section 7.4.3. All allocated emissions must be accounted for by the Reporting Period in which 50% of total feedstock weathering potential has been realized.

Total net CO₂e removal is calculated for each Reporting Period, and is written hereafter as ${{CO}_{2}^{}e}_{Removal,\ RP}$.

Net CO₂e removal for EW in agriculture for each Reporting Period, $RP$, can be calculated as follows. The final net CO₂e removal quantification must be conservatively determined, giving high confidence that at a minimum, the estimated amount of CO₂e was removed (refer to Section 6.5 for details).

$${CO}_{2}^{}e_{Removal,\ RP}^{}\  = \ {CO}_{2}^{}e_{Stored,\ RP}^{}\ –\ {CO}_{2}^{}e_{Counterfactual,\ RP}^{}–\ {CO}_{2}^{}e_{ Emissions,\ RP}^{}$$

(Equation 1)

Where:

• ${CO}_{2}^{}e_{Removal,\ RP}^{}$ -- the total net CO₂e removal for the Reporting Period, RP, in tonnes of CO₂e
• ${CO}_{2}^{}e_{Stored,\ RP}^{}$ -- the total CO₂ removed from the atmosphere and stored as inorganic carbon in the solid or aqueous form in the treatment and deployment (in 3-plot approach) plots for the RP, in tonnes of CO₂e
• ${CO}_{2}^{}e_{Counterfactual,\ RP}^{}$ - the total counterfactual CO₂ removed from the atmosphere and stored as inorganic carbon in the solid or aqueous form for the RP, in tonnes of CO₂e
• ${CO}_{2}^{}e_{Emissions,\ RP}^{}$ -- the total GHG emissions for the RP, in tonnes of CO₂e

Note: Reversals occur after Credits have been issued so are not included in this equation. See Section 9.2 and Section 5.6 of the Isometric Standard for further information.

The total amount of CO₂ stored from an EW project must include the following terms:

$${CO}_{2}^{}e_{Stored,\ RP}^{}\  = \ \sum_{}^{}{CO}_{2}e_{Drawdown,\ RP} - \sum_{}^{}{CO}_{2}e_{Losses,\ RP}$$

(Equation 2)

Where:

• ${CO}_{2}^{}e_{Drawdown,\ RP}^{}$-- CO₂ removed from the release of base cations for Reporting Period, RP, in tonnes of CO₂e. These base cations may derive from newly weathered rock or mineral feedstock, net dissolution of carbonate minerals over the previous Reporting Period, or desorption of base cations that had previously come out of solution by surface sorption.
• ${CO}_{2}^{}e_{Losses,\ RP}^{}$ -- Inorganic carbon lost due to soil column (bio)geochemical processes for the Reporting Period, RP, as well as downstream riverine and marine losses, in tonnes of CO₂e. These losses include plant uptake of base cations, secondary silicate mineral formation, carbonate precipitation, non-carbonic acid neutralization, sorption of base cations to cation exchange sites, re-equilibration of the carbonate system in rivers and oceans and any other relevant processes.

This quantification framework considers the terms given in Equation 2 relative to two distinct zones, referred to as the Near-Field Zone (NFZ) and the Far-Field Zone (FFZ). The NFZ represents the portion of the upper soil column in which the weathering reaction and subsequent processes (e.g. soil and biomass uptake, secondary mineral formation) must be directly tracked and, for practical purposes, is defined as the depth of deepest soil sampling. The FFZ is defined as the transport path for the weathering products from the boundary of the NFZ to the final storage reservoir (typically the ocean). This includes the deep soil column, groundwater network, riverine transport and ocean discharge.

In this mass/charge balance approach, we explicitly account for both: 1) permanent losses of base cations (and corresponding CO₂ removals) that happen after base cations are released from a rock or mineral feedstock through weathering and 2) the possibility that base cations may be temporarily, but not permanently, rendered ineffective for removals.

Losses considered to be permanent include:

• Plant uptake of base cations by harvested biomass (annual crops) or new growth (perennial crops)
• The formation of secondary silicates (e.g., clays)
• Non-carbonic acid neutralization (e.g., neutralization of acid produced from nitrification of ammonia fertilizers)
• Degassing due to re-equilibration of the dissolved inorganic carbon system

Losses considered to be temporary include:

• The formation of carbonate minerals
• Base cation sorption to cation exchange sites

One particularly consequential aspect of these CO₂ removal loss terms is the potential time lag associated with cation sorption. A base cation may undergo many generations of sorption and desorption to cation exchange sites while migrating through the soil column, delaying the CO₂ removal effect until that cation is sufficiently deep in the soil column or more permanently transitions to the aqueous phase. There is currently no widely held scientific consensus on the best practices for modeling these cation sorption dynamics.

The sorption of base cations to cation exchange sites will cause re-equilibration of dissolved inorganic carbon species to maintain charge balance, which will result in degassing of CO₂ into soil pore space. If this degassing occurs sufficiently deep in the soil column, the resulting transient CO₂ may be sufficiently isolated from the atmosphere as to be considered a removal while the cation is still migrating through the soil column.

This Protocol defines removal relative to the primary quantification medium (e.g. solid or aqueous measurements). Where soil is used for CDR quantification, this Protocol considers removal to have occurred when base cations are exported from the NFZ. This Protocol currently recommends the depth of the NFZ to be the deeper of

1. 20 centimeters below the surface, or
2. the depth of tillage plus a buffer of 5 to 10 centimeters ^{23, 24}.

A shallower definition of the NFZ may be used in circumstances where meeting these criteria is not feasible (e.g., shallow water table which cannot be accessed through conventional sampling methods). Such deviations must be reported and justified in the PDD. Where aqueous phase measurements are used for quantification, base cations can be credited within the NFZ once they have entered the porewater.

All crediting projects must design a sampling plan that directly measures the initial weathering of rock or mineral feedstock. Once a statistically significant amount of feedstock weathering has occurred, the Project Proponent may be eligible for Credits. This Protocol currently allows for two primary quantification approaches for determining the the carbon stored from an EW project: 1) soil-based quantification and 2) porewater-based quantification. Additionally, this Protocol currently requires validation of the credit quantification through a secondary medium. For example, where soil-based measurements are used for quantification, aqueous phase measurements are used for validation. The determination used for crediting and for validation must be designated in the PDD. The details of quantification requirements are addressed in the following sections.

There are multiple approaches to quantifying enhanced weathering that have been employed in scientific studies and early enhanced weathering deployments. These methods currently fall into two major categories, solid phase measurements and aqueous phase measurements. Solid phase measurements quantify the loss of base cations from feedstock-amended soil over time, while aqueous phase measurements quantify the accumulation of base cations in soil pore water. In addition to these approaches, there are a number of other technologies at various stages of development that may prove useful for quantifying enhanced weathering in an agriculture setting (e.g., reactive transport models, ion exchange resin). The Isometric science team will regularly assess the state of scientific consensus around new and emerging technologies for application to measuring CDR via enhanced weathering.

This Protocol includes a list of quantification approaches that can be used to determine the amount of carbon stored. In addition to quantification approaches, this Protocol also includes a list of allowable validation approaches that may be used by crediting projects.

All projects seeking credits must quantify the amount of carbon stored using one of the listed quantification approaches in List 1. The quantification approach must be conducted within the framework of an appropriate sampling framework, as discussed in Section 9.3 (e.g., 2-plot, 3-plot). Additionally, the Project Proponent must select and implement at least one validation approach, listed in List 2. This validation is meant to be a localized, independent sense check on the quantification approach. The validation approach must be conducted within both a treatment and control area that represents at least 2.5% of the Project area. The same approach cannot be used for both quantification and validation.

The quantification and validation approaches currently accepted for crediting projects are:

List 1: Quantification Approaches

• Project scale soil-based quantification
• Project scale porewater-based quantification

List 2: Validation Approaches

• Local soil-based validation (so long as solid phase is not used for quantification)
• Local porewater-based validation (so long as aqueous phase is not used for quantification)
• Local ion exchange resin validation

The mean value from the validation method (List 2) must be within the 99.7% confidence interval of the quantification approach. If this is not the case, Projects must conduct an audit and work in consultation with the Isometric science team to surface an explanation for this discrepancy.

A previous version of this Protocol used the terms Determination 1 to describe soil-based quantification with porewater-based validation and Determination 2 to describe powerwater-based quantification with soil-based validation. These terms are no longer used in favor of the more flexible and forward-looking framework described here.

It is anticipated that new technologies and methods for quantifying and validating enhanced weathering in agriculture will emerge in the coming years. These new technologies will be regularly assessed by the Isometric science team for inclusion in this Protocol. New technologies and methods that have some demonstrated efficacy when compared to conventional soil and porewater approaches will be included in List 2 and may be used as a validation approach. New technologies and methods that have demonstrated efficacy and have gathered wide scientific consensus will be included in both Lists 1 and 2 (may be used for either quantification or validation).

Particularly, we anticipate that reactive transport models or other suitable models, which are currently insufficient for predicting enhanced weathering rates, will improve significantly in the coming years from large amounts of calibration data. Isometric will continue to evaluate reactive transport models for inclusion in Lists 1 and 2 in future iterations of this Protocol.

Formerly part of Determination 1

The total amount of carbon stored from an EW project can be determined from soil and porewater measurements according to:

$${CO}_{2}^{}e_{Stored,\ RP}^{} = {CO}_{2}^{}e_{Weathering, RP}^{}  - {CO}_{2}^{}e_{BiomassLoss,\ RP}^{}  - {CO}_{2}^{}e_{NetNewCarbonate,\ RP}^{} \\ - {CO}_{2}^{}e_{NetNewSilicate,\ RP}^{}- {CO}_{2}^{}e_{NetSorption,\ RP}^{} - {CO}_{2}^{}e_{NonCarbonicNeut, RP}^{} \\ - {CO}_{2}^{}e_{RiverineLoss,\ RP}^{} - {CO}_{2}^{}e_{MarineLoss,\ RP}^{}$$

(Equation 3)

Where:

• ${CO}_{2}^{}e_{Weathering,\ RP}^{}$ -- CO₂ removed from the release of base cations from feedstock for the Reporting Period, RP.
• ${CO}_{2}^{}e_{BiomassLoss,\ RP}^{}$ -- Amount of ${CO}_{2}^{}e_{Weathering,\ RP}^{}$ that is undone by the uptake of base cations by harvested plant biomass (annual crops) or new plant growth (perennial crops) for the $RP$.
• ${CO}_{2}^{}e_{NetNewCarbonate,\ RP}^{}$ -- Amount of ${CO}_{2}^{}e_{Weathering,\ RP}^{}$ that is undone from the net formation of new carbonate minerals in the soil column for the $RP$. This will typically lead to a ~50% decrease in the removal efficiency over aqueous phase export.
• ${CO}_{2}^{}e_{NetNewSilicate,\ RP}^{}$ -- Amount of ${CO}_{2}^{}e_{Weathering,\ RP}^{}$ that is undone from the formation of new silicate minerals in the soil column for the $RP$.
• ${CO}_{2}^{}e_{NetSorption,\ RP}^{}$ -- Amount of ${CO}_{2}^{}e_{Weathering,\ RP}^{}$ that is undone from the net sorption of base cations to cation exchange sites in the NFZ for Reporting Period $RP$. We note that, in some cases, this value may be negative, indicating a net release of cations that had accumulated in previous Reporting Periods.
• ${CO}_{2}^{}e_{NonCarbonicNeut,\ RP}^{}$ -- Amount of ${CO}_{2}^{}e_{Weathering,\ RP}^{}$ that is undone from neutralization of acids other than carbonic acid for the $RP$.
• ${CO}_{2}^{}e_{RiverineLoss,\ RP}^{}$ -- Amount of net in-field ${CO}_{2}^{}e_{Drawdown,\ RP}^{}$ that is expected to be released back to the atmosphere due to outgassing in river systems.
• ${CO}_{2}^{}e_{MarineLoss,\ RP}^{}$ -- Amount of net in-field ${CO}_{2}^{}e_{Drawdown,\ RP}^{}$ that is expected to be released back to the atmosphere due to outgassing in the ocean.

All terms have units of tonne CO₂e.

Below is an overview of how each term is determined, with more details provided in Section 9.

${CO}_{2}^{}e_{Weathering,\ RP}^{}$ must be determined from direct geochemical observation of the project area. Nearly all emerging soil-based methods for determining in-field ${CO}_{2}^{}e_{Weathering,\ RP}^{}$ rely on measuring the abundance of some insoluble tracer and its ratio to soluble base cations in the feedstock to determine weathering. This includes measuring the abundance of both insoluble tracer(s) and soluble base cations before and after feedstock application to determine the weathering potential, and measuring the evolution of these two groups of analytes at later time points to determine the gross CO₂ removal before accounting for the various losses. Project Proponents using soil-based measurements for quantification must demonstrate a statistically significant loss of cations between the start and the end of the Reporting Period to be eligible for crediting (see Section 9.3.4.2.1). The Project must clearly specify all of the following in the PDD:

• The depth of the NFZ for determination of weathering rate and a justification for depth (including tillage practices if applicable)

• The insoluble feedstock tracer(s)

• The soluble base cations being considered for CO₂ removal

• The method(s) used, including references to peer reviewed publications and/or standard methodologies

• Measures being taken to account for or limit interference from alternative sources of base cations (e.g., only using Ca and Mg to determine drawdown to avoid interference from monovalent cations in some fertilizers)

• The explicit form of the mass balance equation being used to determine the release of base cations from the feedstock over time

Once the statistical criteria for crediting have been met (see Section 9.3.4.2.1), ${CO}_{2}^{}e_{Weathering,\ RP}^{}$ should be determined by calculating the total CO₂ removal in the treatment and deployment areas separately before adding them together. In soils, this will take the following form:

$${CO}_{2}^{}e_{Weathering,\ RP,\ x}^{} = \  - \Delta{\lbrack ALK\rbrack}_{x} \times \rho_{x} \times d_{x} \times A_{x} \times \frac{CDR}{ALK}$$

(Equation 4)

Where:

• $\Delta{\lbrack ALK\rbrack}_{x}$ -- the change in alkalinity (base cations) in the NFZ of area $x$ (either treatment or deployment area) between the beginning and end of the Reporting Period, as calculated in Equation 16, in eq/kg
• $\rho_{x}$ -- the average bulk density of soil in the weathering horizon (typically 20 cm) in area $A_{x}$, in kg/m³
• $d_{x}$ -- the depth of the NFZ, in meters
• $A_{x}$ -- the area of the treatment or deployment (in 3-plot approach) plot, in m²
• $\frac{CDR}{ALK}$ is the pH dependent conversion of alkalinity to carbon stored in the aqueous phase (included for completeness, however, most projects will not use this conversation factor until considering equilibrium in rivers and oceans)

Additional details on soil based field monitoring are included in Section 9.3.4.

${CO}_{2}^{}e_{BiomassLoss,\ RP}^{}$ is determined from direct, representative sampling of plant tissues of harvested biomass (annual crops) or new plant growth (perennial crops) in an area in which feedstock is applied (treatment area if using 3-plot approach) and the control plot. This does not need to be directly measured in the deployment plot if using 3-plot approach. Sampling routines must be identical between areas with and without feedstock. Plant samples must be analyzed for C, N, Na, K, Mg and Ca concentrations (base cations not used for crediting may be omitted from this measurement). Project Proponents are required to cross reference the following standards for their measurement procedures:

• Total carbon content -- e.g., ISO 10694:1995

• Total nitrogen content -- e.g., ISO 13878:1998

• Cation concentrations -- e.g., ISO 17294-1:2024 for ICP-MS or ISO 11885:2007 for ICP-OES

For cation measurements, Project Proponents are required to outline and justify their method for digestion (dissolving sample into liquid phase) of plant material in the PDD.

Sampling plans targeting plant uptake must consider the total amount of biomass produced over the Reporting Period and the crop type (annual vs. perennial). Project Proponents are required to include all data and calculations in submitted reports, including information on standards and calibration curves. This data will likely include total shoot mass, total plant mass and cation concentrations.

The Project Proponent must describe in the PDD how measurements of base cations removed by plant uptake are conservatively extrapolated over the project area for the determination of ${CO}_{2}^{}e_{BiomassLoss,\ RP}^{}$.

${CO}_{2}^{}e_{NetNewCarbonate,\ RP}^{}$ represents the average net change in soil inorganic carbon (SIC) between the start and the end of the Reporting Period. This measurement pertains to all soils collected in the NFZ, typically 0 to 20 cm, but should also proportionally represent sample fractions if samples are partitioned for determination of initial weathering (e.g., total SIC is determined from a weighted average of 0-10 cm samples and 10-20 cm samples). This Protocol requires that SIC, when measured, is measured via either calcimetry or thermo-gravimetric analysis. This must be quantified using:

• Calcimetry -- e.g., ISO 23400:2021

• Thermo-gravimetric analysis -- e.g., ASTM D8474-22

We note that some methods for measuring enhanced weathering in soils (e.g., total acid digestion followed by bulk cation quantification) implicitly include SIC monitoring; this is generally acceptable under this Protocol.

Project Proponents utilizing other carbonate quantification methods, in consultation with Isometric, must outline and justify these alternative analyses in the PDD.

This value of ${CO}_{2}^{}e_{NetNewCarbonate,\ RP}^{}$ may be positive, zero or negative. A positive value corresponds to a net increase in soil inorganic carbon, which will result in less CO₂ being stored. A negative value corresponds to net dissolution of carbonate minerals, and may lead to net CO₂e removal if dissolution is the result of reaction with carbonic acid.

Soil inorganic carbon is typically determined in weight percent (e.g., kg calcium carbonate per kg soil). The corresponding amount of CO₂ stored in net new carbonate is determined as follows:

$${CO}_{2}^{}e_{CaCO_{3},\ t}^{}\  = (wt\%\ {CaCO}_{3}) \times (\frac{1}{100}) \times (0.44) \times \rho_{x} \times d_{x} \times A_{x} \times (\frac{1\ ton}{1000\ kg})$$

(Equation 5)

$${CO}_{2}^{}e_{NetNewCarbonate,\ RP}^{} = \ {CO}_{2}^{}e_{CaCO_{3},\ \ t = 2}^{} - {CO}_{2}^{}e_{CaCO_{3},\ \ t = 1}^{}$$

(Equation 6)

Where:

• ${CO}_{2}^{}e_{CaCO_{3},\ t}^{}$ -- the amount of CO₂ stored in CaCO₃ minerals at time point t, in tonnes.
• $wt\%\ {CaCO}_{3}$ -- the mass fraction of a soil sample that is CaCO₃, expressed as a percent
• $\frac{1}{100}$ -- the conversion factor from weight percent to decimal
• $0.44$ -- a unitless ratio of the molar mass of CO₂ to the molar mass of CaCO₃
• $\rho_{x}$ -- the average bulk density of soil in area $A_{x}$, in kg/m³
• $d_{x}$ is the depth of the NFZ, in m
• $A_{x}$ is the area of the control, treatment, or deployment plot, in m²
• $\frac{1\ ton}{1000\ kg}$ is the conversion factor from metric tonnes to kilograms

Some project areas may have soil conditions where carbonate precipitation is not likely and not observed above analytical detection limits (e.g., low pH soils). Project Proponents operating in such areas may omit large scale soil inorganic carbonate measurements from routine monitoring with adequate justification in the PDD. In such instances, Project Proponents must conduct smaller scale measurements of soil inorganic carbon with each major sampling event to demonstrate that these assumptions still hold. If local geochemical conditions change over the course of a project and lead to measurable soil carbonate mineral accumulation, soil inorganic carbonate measurements must be reinstated at the same density as other primary soil measurements.

${CO}_{2}^{}e_{NetNewSilicate,\ RP}^{}$ is the average net change in silicate mineral (e.g., clay mineral) content between the start and end of the Reporting Period. There is no widely accepted and operationally feasible quantitative method for determining modest changes in secondary silicate mineral content in soils; clay content of soils is typically expressed as a percentage of clay-sized particles, and mineralogy is determined by X-ray diffraction. Thus, detection of new secondary clays requires either significant enough clay formation to shift the percentage of clay in the soil column or quantitative formation of clay mineralogies that are distinct from the initial assemblage (unlikely for modest pH change in soils with similar moisture retention; see Wilson (1999)²⁵ for an overview of the parameters controlling clay formation in soil). This term is included for completeness; Isometric will not explicitly require measurement of this parameter at this time.

${CO}_{2}^{}e_{NetSorption,\ RP}^{}$ is determined by measuring any change in adsorbed base cations in the NFZ over the Reporting Period. Typically, this is determined using the difference in the product of cation exchange capacity and base cation saturation between the start and end of the Reporting Period. A positive value corresponds to a net increase in base cations sorbed to cation exchange sites, leading to outgassing of dissolved CO₂. Conversely, a negative value corresponds to a net decrease in base cations sorbed, leading to CO₂ removal. Cation exchange capacity measurements should comply with ISO 11260:2018, ISO 23470:2018 or the Chapman method. Quantification of base saturation should comply with ISO 11260:2018. Some agronomic soil testing facilities may use regionally specific methodologies that deviate from the standards listed above. Such methodologies are generally permissible, but require approval by Isometric and justification in the PDD.

We note that some methods for measuring enhanced weathering in soils (e.g., total acid digestion followed by bulk cation quantification) implicitly include net sorption monitoring; this is generally acceptable under this Protocol.

${CO}_{2}^{}e_{NonCarbonicNeut,\ RP}^{}$ may be determined using direct measurements of anions in porewaters or a conservative estimate based on documented fertilizer application.

If direct measurement of anions is used, samples must be collected at or below the depth of the NFZ (typically 20 cm). Major anions should include NO₃^-, PO₄^3-, Cl^-, SO₄^2- and any others that may be relevant to local land management practices and the feedstock being used. Measurement methods must comply with ISO 10304-1:2007. Unlike some soil based measurements in this Protocol, aqueous concentration of anions in porewaters will likely be dynamic and vary significantly over a Reporting Period. The Project Proponent must provide details of their sampling plan and describe how the chosen sampling frequency is appropriate for capturing any significant deviation in concentration at the project location.

The Project Proponent must describe in the PDD how site-based observations (e.g., precipitation, irrigation, other climatic variables, etc.) are used to determine the total volume of water infiltrated into the soil, and how discrete anion concentrations are combined with this data to determine the total amount of non-carbonic acid neutralization that occurred over the Reporting Period. Given a known volume of water infiltrated into the soil, ${CO}_{2}^{}e_{NonCarbonicNeut,\ RP}^{}$ can be calculated as:

$${CO}_{2}^{}e_{NonCarbonicNeut,\ RP}^{}\  = \ \sum_{i}^{}c_{i}n_{i}$$

(Equation 7)

Where:

• $c_i$ -- the charge of the major anion
• $n_i$ -- the concentration of that anion in solution in ppm

Where data is available, Project Proponents may choose to use fertilizer application rates as a proxy for non-carbonic acid weathering. Quantification of non-carbonic acid weathering from fertilizer records may come from:

• Documented fertilizer application rates, assuming that 100% of ammonium applied is nitrified
• Documented fertilizer application rates and measurement of nitrogen use efficiency (plant uptake of nitrogen)

Project Proponents must provide data on fertilizer application rates and calculations of the fraction of feedstock weathering resulting from non-carbonic acid weathering in the PDD. If non-carbonic acid neutralization is accounted for using fertilizer application rates, Project Proponents must provide evidence to justify the assumption that nitric acid is the only significant non-carbonic acid source. This includes measurements of total sulfur in the feedstock and in the soil.

${CO}_{2}^{}e_{RiverineLoss,\ RP}^{}$ includes all future losses that will occur in river systems downstream of in-field activities (the FFZ). In most cases, this will be a modeled result. Models used to estimate riverine losses must use historic river geochemical data to estimate relevant parameters. This may include publicly available datasets or scientific publications. The source of such data must be reported. Models must include explicit consideration of:

• Formation of new carbonate minerals

• Outgassing of CO₂ due to re-equilibration of DIC system

${CO}_{2}^{}e_{MarineLoss,\ RP}^{}$ includes all future losses that will occur after base cations are exported to the ocean. This must include explicit consideration of:

• Formation of new carbonate minerals

• Outgassing of CO₂ due to re-equilibration of DIC system

It may be appropriate in some cases to pool post-deployment samples for more resource-intensive analyses (e.g., analysis of trace metal abundance). Solid and liquid phase sample pooling is acceptable in crediting projects, with a maximum recommendation of 10 samples pooled per analysis. It is strongly recommended that Project Proponents do not pool samples during baseline sampling events to ensure baseline heterogeneity in soil properties and geochemistry are fully appreciated. Note that liquid samples may be pooled only for elemental analysis and not for analysis of carbonic acid system parameters. All sample pooling plans must be approved by Isometric and described in the PDD.

Project Proponents pursuing soil-based removal quantification must consider all terms listed in Equation 3, but alternative methods may be appropriate for rigorous quantification of CDR. For example, some Project Proponents may choose to monitor the loss of alkalinity in a project area by performing full acid digests on soil samples with limited pre-processing (e.g. removal of carbonates, extraction of the soil exchangeable fraction). In this instance, weathering, cation sorption, carbonate formation and clay formation will all be integrated into a single measurement. Such deviations may be appropriate and will be considered on a project by project basis. In such cases, the Project Proponent must provide a detailed description of the methods and describe how the chosen analyses map onto the terms in Equation 3 in the PDD.

Formerly part of Determination 2

For aqueous phase quantification, we make the simplifying assumption that all of the major soil column processes, including the release of base cations from weathering and soil losses, are accounted for in the aqueous geochemistry of water that has infiltrated to some depth. At this time, we are recommending this depth coincide with the depth of the NFZ, typically 20 cm. Therefore, the total amount of carbon stored from an EW project can be determined from porewater measurements in the top 20 cm of soil. Alternatives to 20 cm may be justified for similar reasons described in the previous sections. Well-defined catchment waters may be used in place of porewaters where the project area is contained entirely within such well-defined catchments. In the context of this Protocol, a catchment is considered well-defined if:

• Groundwater runoff from the Project area drains at known points, as determined by hydrologic maps, and
• Groundwater runoff from the Project area is isolated from other water sources, and
• The Project Proponent can demonstrate, given feedstock application rate, anticipated weathering rate, average annual water fluxes in the Project area and distance from the Project to the drainage point that the alkalinity flux from the Projectis is resolvable at a proposed measurement cadence.

In instances where catchment waters are used, the Project Proponent must provide supporting documents detailing site-specific hydrogeology. $CO_2e_{Stored, \ RP}$ can be calculated from the aqueous phase via the following equation:

$${CO}_{2}^{}e_{Stored,\ RP}^{} = {CO}_{2}^{}e_{Aqueous,\ RP}^{} \ – \ {CO}_{2}^{}e_{RiverineLoss,\ RP}^{} \ – \ {CO}_{2}^{}e_{MarineLoss,\ RP}^{}$$

(Equation 8)

Where:

• ${CO}_{2}^{}e_{Aqueous,\ RP}^{}$ -- CO₂ removed as determined from the infiltration of carbonate alkalinity to the depth of the NFZ, typically 20 cm, in the Reporting Period, $RP$
• ${CO}_{2}^{}e_{RiverineLoss,\ RP}^{}$ -- Amount of net in-field ${CO}_{2}^{}e_{Aqueous,\ RP}^{}$ that is expected to be released back to the atmosphere due to outgassing in river systems
• ${CO}_{2}^{}e_{MarineLoss,\ RP}^{}$ -- Amount of net in-field ${CO}_{2}^{}e_{Aqueous,\ RP}^{}$ that is expected to be released back to the atmosphere due to outgassing in the ocean

All terms have units of tonne CO₂e.

Below is an overview of how each term is determined, with more details provided in Section 9.3.5. It is important to note that, although ${CO}_{2}^{}e_{BiomassLoss,\ RP}^{}$ is not explicitly included in aqueous phase quantification, alkalinity may still be taken up by plants below the observation window. Project Proponents using aqueous phase quantification for Credits must still consider and quantify plant uptake if roots extend below the depth of porewater sampling.

${CO}_{2}^{}e_{Aqueous,\ RP}^{}$ is the integrated amount of CDR as determined from measurements of aqueous phase base cation abundance from water that has infiltrated to the depth of the NFZ, typically 20 cm. There are two generally accepted approaches for determining aqueous phase alkalinity export: 1) direct measurements of cation and anion concentrations (e.g., via ICP-MS or ICP-OES) or 2) measurement of at least two carbonic acid system variables in solution. In both cases, Project Proponents must also produce a water budget for the project area encompassing the Reporting Period. Carbonic acid system measurements may include carbonate alkalinity titration to the CO₂ equivalence point, pH, DIC and/or pCO₂, followed by calculating the concentration of bicarbonate using the 2-for-6 method. Further guidance is given in Section 9.3.5.1.2.

${CO}_{2}^{}e_{RiverineLoss,\ RP}^{}$: See Solid Phase Quantification

${CO}_{2}^{}e_{MarineLoss,\ RP}^{}$: See Solid Phase Quantification

Project Proponents pursuing aqueous phase quantification must consider all terms listed in Equation 8, but alternative methods may be appropriate for rigorous quantification of CDR. Such deviations may be appropriate and will be considered on a project by project basis. In such cases, the Project Proponent must provide a detailed description of the methods and describe how the chosen analyses map onto the terms in Equation 8 in the PDD.

Synthetic ion exchangers, more commonly referred to as ion exchange resins, have been employed in soil science for many years to constrain nutrient fluxes in the soil column (e.g. Lehman et al., 2001²⁶, Lang & Kaupenjohann, 2004²⁷, Johnson et al., 2005²⁸, Predotova et al., 2011²⁹, Grahmann et al., 2018³⁰). Typically, monitoring will consist of a vessel filled with a mixture of ion exchange resin beads and quartz sand that is buried in the soil column. Soil water then flows through the device and ions sorb to the resin beads, accumulating within the device. At the end of a sampling period, the accumulated ions are analyzed.

Ion exchange resins are a novel technique for EW measurements, with limited data on their use in EW deployments. For this reason, ion exchange resins may be used as a validation medium (List 2 in Section 7.4.1.1) under this Protocol. As more data becomes available, the use of ion exchange resins as a quantification medium will be revisited. Calculation of $CO_2e_{Stored, \ RP}$ can be calculated as a validation check using ion exchange resins following Equation 8. Project Proponents using ion exchange resins for validation must provide detailed information on:

• The resin(s) used, including documentation that demonstrates the resin(s) target the following:

  • Base cations, including Ca²⁺, Mg²⁺, K⁺ and Na⁺;
  • All non-carbonic acid sources, which may include NO₃^-, PO₄^3- and SO₄^2-;
  • Trace metal cations that pose environmental risk, including Ni, Co and Cr;
    • Project Proponents employing ion exchange resins for validation of alkalinity export may alternately choose to quantify trace metal cations that pose environmental risk using porewater samples.
  • Weak acid resins that target the carbonic acid system may also be used, but this is not a requirement. Weak acid resins, when used, must be coupled with resins that satisfy the above requirements. This will be revisited as scientific consensus evolves.

• Description of the installation procedure, particularly focusing on how the procedure mitigates against disturbance of pre-existing soil structure;

• Description of the extraction and measurement procedures used.

As ion exchange resins quantitatively remove dissolved ions from porewaters, the derived estimates of removals should utilize Equation 8, and should otherwise follow the recommendations and requirements pertaining to aqueous phase sampling wherever applicable.

Type: Counterfactual

${CO}_{2}^{}e_{Counterfactual,\ RP}^{}$ describes the CO₂ that would have been removed from the atmosphere and durably stored in the baseline scenario as a result of natural weathering or pre-existing land practices. This is calculated using the control plot, which must be maintained under business as usual farming practices. The control plot ensures that any removals associated with business as usual agricultural liming are not attributed to EW in agriculture projects. The control plot will serve as the baseline in modeling with respect to measured soil and porewater parameters. The approach used for this accounting, including the feedstock storage location, climatic conditions of the storage location, models used and/or analyses conducted, must be provided and justified in the PDD.

In addition to counterfactual CO₂ related to business as usual farming practices, in some instances it may be appropriate to consider weathering of feedstock that would have occurred without project intervention. For example, if the feedstock used constitutes a waste product that was not mined or quarried specifically for project activities and was stored in open-air conditions, some degree of surficial weathering could be expected over timescales relevant to a project lifetime. Project Proponents using these feedstocks must account for counterfactual weathering if the feedstock does not undergo additional processing prior to deployment. This Protocol defines the durability of an enhanced weathering credit as 1,000+ years; thus, this is the default assumption for the calculation timescale of counterfactual weathering if no additional information regarding the storage conditions and duration of the feedstock at the mine/quarry site can be provided. If additional information on the conditions and duration of feedstock storage at the feedstock supplier are available, Project Proponents may justify calculating the counterfactual across a time period relevant to the specific mine or quarry from which the feedstock is sourced in the PDD. For example, projects operating in conjunction with active mines may find it appropriate to use the time of mine closure and provide details of the closure plan in the PDD. Alternatively, if sufficient documentation exists suggesting that piles of waste materials generated by the feedstock will not be exposed to ambient environmental conditions for a period exceeding a set number of years, the counterfactual may be considered across that time span. It is important to note that studies have shown that the vast majority of weathering in tailings piles occurs in the surface layer that is exposed to the atmosphere, provided that there is no mechanical overturn. For this reason, counterfactual weathering needs to be accounted for in the top meter of the tailings pile.

Where relevant, counterfactual weathering must be calculated by a combination of direct measurements and modeling of the expected weathering rate of feedstock under storage conditions relevant to the source site for either 1,000 years or a time period justified in the PDD as described above. Models must be justified by empirical data from subsamples of the feedstock; guidelines for sampling procedures that adequately capture feedstock heterogeneity are described in the Rock and Mineral Feedstock Characterization Module. Models must take into account:

• Feedstock mineralogy (required; direct measurement)
• Feedstock surface area (required; direct measurement)
• Baseline carbonation of the tailings pile (required; direct measurement)
• CDR potential of the feedstock (required; calculated from direct measurements of the feedstock batch)
• Environmental conditions of the source site (required; direct measurement or publicly available data), including:
  • Temperature
  • Average yearly precipitation
  • Rainwater pH
  • Groundwater pH
  • Carbonate saturation
• Permeability (required; direct measurement or calculated from direct measurement)
• Water saturation (required; direct measurement or calculated from direct measurement)
• Microbial activity (recommended; direct measurement)

The measurements and model used to calculate counterfactual feedstock weathering must be provided to Isometric and the VVB. Where counterfactual weathering is considered, $CO_2e_{Counterfactual, \ RP}$ is calculated as the sum of counterfactual weathering from business as usual farming practice and counterfactual weathering of feedstock.

${CO}_{2}^{}e_{Counterfactual,\ RP}^{}$ from business as usual farming practice is determined using the same equations (Equations 7 and 8) as the previous section for ${CO}_{2}^{}e_{Stored,\ RP}^{}$ in which all the encompassed terms are determined using a control plot of land as described in Section 9.3.1. Because the chemical composition of agricultural lime does not typically have sufficient concentrations of elements commonly used as immobile tracers, such as Ti, $CO_2e_{Stored}$ in the control plot will be calculated as the change in concentration of Ca in the NFZ within a Reporting Period. This is a conservative assumption, as it assumes any change in Ca is attributable to weathering of agricultural lime.

For instances where the Project Proponent has discretion as to which methods can be used to determine a particular weathering or loss term, the methods used must be the same for both the area over which feedstock is applied and the control plot.

Type: Emissions

${CO}_{2}^{}e_{Emissions,\ RP}^{}$ is the total quantity of GHG emissions associated with a Reporting Period $RP$. This can be calculated as:

$${CO}_{2}^{}e_{Emissions,\ RP}^{}\ = \ {CO}_{2}^{}e_{Establishment,\ RP}^{}\ + \ {CO}_{2}^{}e_{Operations,\ RP}^{} + \ {CO}_{2}^{}e_{End-of-life,\ RP}^{}+ \ {CO}_{2}^{}e_{Leakage,\ RP}^{}$$

(Equation 9)

Where:

• ${CO}_{2}^{}e_{Emissions,\ RP}^{}$ -- the total GHG emissions in a Reporting Period, $RP$, in tonnes of CO₂e
• ${CO}_{2}^{}e_{Establishment,\ RP}^{}$ -- the total GHG emissions associated with project establishment in a $RP$, in tonnes of CO₂e
• ${CO}_{2}^{}e_{Operations,\ RP}^{}$ -- the total GHG emissions associated with operational processes in a $RP$, in tonnes of CO₂e
• ${CO}_{2}^{}e_{End-of-life,\ RP}^{}$ -- the total GHG emissions that occur after the RP and are allocated to the $RP$, in tonnes of CO₂e, see Section 7.4.3.3
• ${CO}_{2}^{}e_{Leakage,\ RP}^{}$ -- represents the GHG emissions associated with the project’s impact on activities that fall outside of the system boundary of a project, over a given Reporting Period, in tonnes of CO₂e, see Section 7.4.3.4

The following sections set out specific quantification requirements for each variable. It is anticipated that most emissions associated with EW projects will occur during the Calculation of ${CO}_{2}^{}e_{Establishment,\ RP}^{}$ phase.

GHG emissions associated with $CO_2e_{Establishment,\ RP}$ should include all historic emissions incurred as a result of project establishment, including but not limited to the SSRs set out in Table 1.

Project establishment emissions occur from the point of project inception through to after the spreading event has taken place. GHG emissions associated with project establishment may be allocated in one of the following ways, with the allocation method selected and justified by the Project Proponent in the PDD:

• As a one time deduction to the first Reporting Period(s)
• Allocated over the first half of the anticipated project lifetime (meaning the point at which 50% of the feedstock weathering potential is realized) as annual emissions
• Allocated per output of product (i.e., per tonne CO₂ removed) so long as establishment emissions are fully accounted by the time 50% of the weathering potential has been realized

The anticipated lifetime of the project should be based on reasonable justification and should be included in the Project Design Document (PDD) to be assessed as part of project validation.

Allocation of $CO_2e_{Establishment,\ RP}$ emissions to removals must be reviewed at each Crediting Period renewal and any necessary adjustments made. If the Project Proponent is not able to comply with the allocation schedule described in the PDD (e.g., due to changes in delivered volume or anticipated project lifetime), the Project Proponent should notify Isometric as early as possible in order to adjust the allocation schedule for future removals. If that is not possible, the Reversal process will be triggered in accordance with the Isometric Standard, to account for any remaining emissions.

GHG emissions associated with $CO_2e_{Operations\, RP}$ should include all emissions associated with operational activities including but not limited to the SSRs set out in Table 1. For EW projects, the Reporting Period begins after the spreading event on agricultural land has occurred and ends once the weathering potential has been realized and MRV activities have ceased.

$CO_2e_{Operations,\ RP}$ emissions occur over the Reporting Period for the deployment being credited and are applicable to the current deployment only. $CO_2e_{Operations,\ RP}$ emissions must be attributed to the Reporting Period in which they occur. Allocation may be permitted on a case by case basis in agreement with Isometric.

$CO_2e_{End-of-life,\ RP}$ includes all emissions associated with activities that are anticipated to occur after the Reporting Period, but are directly or indirectly related to the Reporting Period. For example this could include ongoing sampling activities for MRV for the specific deployment (directly related) if applicable, or end-of-life emissions for project facilities (indirectly related to all deployments).

GHG emissions associated with $CO_2e_{End-of-Life,\ RP}$ may occur from the end of the Reporting Period onwards, and typically through to completion of project site deconstruction and any other end-of-life activities.

GHG emissions associated with activities that are directly related to each deployment must be quantified as part of that Reporting Period. GHG emissions associated with activities that are indirectly related to all deployments may be allocated in the same ways as set out in $CO_2e_{Establishment,\ RP}$.

Given the uncertain nature of $CO_2e_{End-of-Life,\ RP}$ emissions, assumptions must be revisited at each Crediting Period and any necessary adjustments made. Furthermore, if there are unexpected $CO_2e_{End-of-Life,\ RP}$ emissions associated with a Reporting Period, or the project as a whole, that occur after the project has ended, then the Reversal process will be triggered to compensate for any emissions not accounted for.

$CO_2e_{Leakage,\ RP}$ includes emissions associated with a project's impact on activities that fall outside of the system boundary of a project.

It includes increases in GHG emissions as a result of the project displacing emissions or causing a knock on effect that increases emissions elsewhere. This includes emissions associated with activity-shifting, market leakage and ecological leakage.

It is the Project Proponent's responsibility to identify potential sources of leakage emissions. For an EW project, feedstock replacement must be considered as part of the leakage assessment, as a minimum.

$CO_2e_{Leakage,\ RP}$ emissions must be attributed to the Reporting Period in which they occur. Allocation may be permitted in certain instances, on a case by case basis in consultation with Isometric.

This section of the Protocol outlines requirements for EW emissions accounting relating to energy use, transportation, and embodied emissions associated with a CDR project.

This section sets out specific requirements relating to quantification of energy use as part of the GHG Statement. Emissions associated with energy usage result from the consumption of electricity or fuel.

Examples of electricity usage may include, but are not limited to:

• Electricity consumption for equipment for drying rock or mineral feedstock after milling

Examples of fuel consumption may include, but are not limited to:

• Handling equipment, such as fork trucks or loaders
• Fuel consumption of agricultural machinery for spreading, tilling, and sampling

The Energy Use Accounting Module v1.2 provides requirements on how energy-related emissions must be calculated in a CDR project so that they can be subtracted in the net CO₂e removal calculation. It sets out the calculation approach to be followed for intensive facilities and non-intensive facilities and acceptable emissions factors.

This section sets out specific requirements relating to quantification of emissions related to transportation.

Emissions associated with transportation include transportation of products and equipment as part of a Reporting Period’s process. Examples may include, but are not limited to:

• Transportation of feedstock to agricultural site
• Transportation of rock from quarry to crushing site
• Transportation and shipping related to collecting samples for environmental monitoring

The Transportation Emissions Accounting Module v1.1 provides requirements on how transportation-related emissions must be calculated in a CDR project so that they can be subtracted in the net CO₂e removal calculation. It sets out the calculation approach to be followed and acceptable emissions factors.

This section sets out specific requirements relating to quantification of embodied emissions as part of the GHG Statement. Embodied emissions are those related to the life cycle impact of equipment and consumables.

Examples of project-specific materials and equipment that must be considered as part of the embodied emission calculation include but are not limited to:

• Rock or mineral feedstock and associated production, processing, treatment and transportation equipment
• Sampling equipment and consumable materials such as augers and storage containers
• Raw materials and equipment used in the fabrication, assembly and construction of agricultural machinery for spreading, tilling, and sampling

The Embodied Emissions Accounting Module sets out the calculation approach to be followed including allocation of embodied emissions, life cycle stages to be considered, data sources and emission factors.

This Protocol requires feedstock characterization and reporting in accordance with Isometric's Rock and Mineral Feedstock Characterization Module.

Durability refers to the length of time for which CO₂ is removed from the Earth's atmosphere.

The long-term storage reservoir for the alkalinity generated through EW is the ocean, where the removed CO₂ is stored in the form of dissolved inorganic carbon (DIC). At typical sea surface conditions, about 90% of DIC is in the form of bicarbonate, while 9% is in the form of carbonate²⁰. The durability of the ocean DIC reservoir can be described by its residence time, which is the average amount of time a substance stays in a particular reservoir. Residence time is calculated by dividing the total inventory of a substance by the inflows or outflows, and assuming near-steady state conditions. It is scientifically well-established that the global ocean DIC residence time lies between 10,000 to 100,000 years^{6, 31}. This is based on decades of research that estimates the global ocean DIC inventory to be between 37,000 and 39,000 GtC,^{32, 33, 34} with a recent estimate being 37,200 ± 200 GtC³⁵. Additionally, the global riverine DIC inputs are well constrained to be between 0.3 to 0.4 GtC/yr^{4, 36, 37, 36}, which approximately balances the loss of DIC through carbonate precipitation and burial on the seafloor³¹. Thus, the durability of EW as generated alkalinity is at least 10,000 years.

In the near-term when CDR is operating on small scales (i.e. $<$Gt), it is unlikely that CDR activities will result in meaningful changes to the global ocean DIC inventory or its input and output fluxes. Longer-term, elevated alkalinity in the ocean may lead to increased carbonate mineral production, which would remove alkalinity and decrease the residence time and durability of the global DIC reservoir. More research is needed to better understand these potential effects⁶.

Based on the present understanding, projects applicable to this Protocol are generally categorized as having a Very Low Risk Level of Reversal according to the Isometric Standard Risk Assessment Questionnaire. This is because reversals in the global ocean DIC reservoir will not be directly observable with measurements and attributable to a particular project, and EW as a pathway does not yet have a documented history of reversals. Instead, larger uncertainty discounts must be used to ensure conservatism. Risk of reversal information is given in Appendix 4: Risk of Reversal Questionnaire, with further information provided within the relevant storage module storage module.

Reversals will be accounted for by projects and the Isometric Registry as detailed in Section 5.6 of the Isometric Standard.

This section outlines the monitoring approach that Project Proponents must take in crediting projects. Several of the monitoring requirements described below include measurements that will be used in the quantification of rock spread and determination of removals (see Section 7.4).

A Project is a field or group of fields that are to be managed together for the purposes of deployments and crediting. A single Project Proponent may choose to have multiple Projects, each with an individual PDD, or group operations under a single Project. All deployments and sampling events across a Project must occur on similar time frames, which must be stated in the PDD.

There is no minimum or maximum Project area, however, projects must designate at least one control, treatment and deployment plot (if using the 3-plot model) per 2,000 hectares. Project areas exceeding 2,000 hectares but less than 4,000 hectares will thus need to designate a minimum of 2 control, treatment and/or deployment plots; Projects exceeding 4,000 hectares but less than 6,000 hectares will need to designate a minimum of 3 control, treatment and/or deployment plots; and so on. This lower limit is set primarily to avoid heterogeneity in factors that will influence weathering rate (e.g., precipitation).

Project Proponents with project areas distributed over large distances must maintain a set of plots for each region even if the area criteria listed above do not require it. This may include additional sets of plots per climatic and/or geo-political region, in consultation with Isometric. Difference in field and climatic characteristics (e.g., soil type, crop type, precipitation, temperature range, etc.) may also necessitate the maintenance of additional treatment and control sites. In all projects, the total number of control and treatment plots (where applicable) should each total at least 2.5% of the project area. Control, treatment and/or deployment plots will, in many cases, be contiguous, but this is not a requirement. Furthermore, projects greater than 1,000 hectares must maintain a research plot. Research plot requirements are listed in Section 9.3.1.8. Projects may contain non-contiguous fields, managed by different farmers or landowners, so long as all plots are representative of each other, in accordance with Section 9.3.1.7.1.

Where applicable, analytical methods must be cross-referenced with appropriate standards (e.g., ISO, EN, BSI, ASTM, EPA) or standard operating procedures (SOP). Where a project utilizes a non-standardized methodology or SOP for the determination of a listed parameter, the Project Proponent is required to outline the relevant method within the PDD submitted to the Validation and Verification Bodies (VVB).

The Project Proponent has the choice between two monitoring frameworks in crediting projects: the 2-plot and the 3-plot approach.

Figure 2 Schematic of in-field monitoring approaches, illustrating the relative size of the control, treatment, and deployment plots in the 2- and 3-plot approaches, as well as CO₂ drawdown from the atmosphere.

Table 2. Summary of Project Area.

The 2-plot approach for quantifying removals from EW in agriculture calls for the designation of the Project area into one of two categories:

• Control - area representing CO₂ removal under business as usual practices

• Treatment (2-plot) - area representing CO₂ removal from application of rock or mineral feedstock

The Treatment area is used to directly observe the CO₂ removal resulting from EW in agriculture, while the control is used to directly observe counterfactual (or business as usual) CO₂ removal. The 2-plot approach should be generally thought of as a flat, per-area sampling scheme.

The 3-plot approach allows for intensive, high-resolution data collection and monitoring of EW projects and their counterfactuals at smaller scales, while monitoring the remainder of the project at lower resolution.

In this construction, the project area must be divided into three sections that will be designated as the control, treatment and deployment areas defined as follows:

• Control - densely monitored area representing CO₂ removal under business as usual practices

• Treatment (3-plot) - densely monitored area representing CO₂ removal from application of rock or mineral feedstock

• Deployment - less densely monitored area representing CO₂ removal from application of rock or mineral feedstock

Unless otherwise specified, the measurements collected in the control, treatment or deployment plots will be used to quantify removal terms contained within that particular area (i.e. deployment samples are used to quantify removals in the deployment plot). There may be some cases where measurements collected in the treatment area may be extrapolated to the deployment area (e.g., quantification of biomass uptake of base cations). Unless otherwise specified, the same number of samples must be taken from the control, treatment and deployment plots.

The purpose of a control plot is to quantify the removal that would have otherwise occurred without the application of rock or mineral feedstock. Most notably, this includes any CO₂ removal effect of agricultural liming that would have occurred in the absence of project activities. The control area must represent a minimum of 2.5% of the total project area. The control area must be maintained using a continuation of historical agricultural practices, to represent what would have occurred in the absence of the CO₂ removal project. This includes, for example, liming if EW is replacing current liming practices. If the Project Proponent is unable to maintain a control plot concurrent with the project deployment, the Project Proponent may opt to do the following:

• Provide a written explanation detailing why it was not possible to maintain a control plot.

• Provide liming records of the project area over the last 10 years.

• Assume as counterfactual that liming would have occurred at the average liming rate for the past 10 years and assume a 100% efficient CO₂ removal processes (i.e., all lime is exported in the aqueous phase as bicarbonate with no losses).

In instances where the last 10 years of liming records cannot be obtained, the Project Proponent must work in consultation with Isometric to identify the most plausible counterfactual land management scenario. Examples of possible sources of information include signed affidavits from knowledgeable local individuals or aggregated county level records. The Project Proponent must detail the sources of this information in the PDD.

The treatment (2-plot) encompasses the project area on which rock or mineral feedstock is applied. In most cases, this is 97.5% of the project area. The treatment area will be subject to monitoring as prescribed by the sampling plan. Application of feedstock to the treatment area must be uniform unless otherwise stated (see Section 9.3.1.6.1: Variable Application Rates for further details).

The treatment (3-plot) must also represent a minimum of 2.5% of the total project area. If a Project Proponent opts to maintain control and treatment areas that are larger than 2.5%, it is recommended that these two areas maintain parity in both size and sampling density. The treatment area will be subject to intensive monitoring as prescribed by the sampling plan. Application of feedstock to the treatment area must be at the same feedstock volume per unit area as the deployment area.

The deployment area shall encompass the remaining project area that is not within the control or treatment plots. In most cases, this will represent 95% of the project area.

In some cases, feedstock application rates may vary within a single deployment due to project-specific constraints, such as heterogeneity in soil characteristics or regional agricultural practice. These Projects may still be eligible for crediting under this Protocol.

As described in Section 9.3.4.4, this Protocol requires a soil-based determination of feedstock application to quantify the maximum Project CDR potential. Project Proponents utilizing variable application rates are required to submit application rate maps for the Project area. These maps can be generated from either recommended application rates supplied to the applicator (typically from farmer-supplied soil data) or directly from the applicator. This data must be used to design soil sampling plans such that the full range of application rate is captured. Projects seeking credits with variable application rates are required to designate control and treatment (if using a 3-plot model) areas that correlate with different application rates in the Project Area. For example, if application rate in a particular field varies from 1-4 tonnes/ha, the Project Proponent may subdivide this into two ranges of 1-2 tonnes/ha and 3-4 tonnes/ha, each with separate control plots. $CO_2e_{Stored,\ RP}$ will be determined for areas with similar application rates, which will then be used to calculate a weighted average of $CO_2e_{Stored,\ RP}$ for the full Project area.

The Project Proponent must designate one or more control areas that can be reasonably considered representative of the project area. Project Proponents should consider a range of climatic, soil, environmental, topographic, agronomic and hydrological properties contained within the project area when determining the representative plots. The following properties most relevant to weathering rate must be explicitly addressed in the PDD to demonstrate representativeness of the control: soil texture and composition, soil pH, local climate (temperature and precipitation), crop type and local hydrology. In irrigated fields, control plot designation must also consider spatial variation in irrigation rates. Project Proponents may use publicly available data sets for determining these site characteristics. All data and derived metrics used for designating field and control plots must be provided in the PDD. Where no data is available, the Project Proponent must undertake a soil survey to characterize relevant soil characteristics. Control plots based on the data provided in the PDD should capture both the central tendency and dynamic range of the listed control variables.

Some Project Proponents may choose to designate a contiguous area for the control area, however, this is not a requirement. The Project Proponent may designate two or more smaller areas that, in aggregate, represent 2.5% of the project area for either the control or treatment area. This might be done if field conditions do not readily allow for a single representative control and treatment areas.

In the 2-plot approach, the treatment area will consist of the remaining project area, which in most cases will be 97.5% of the total project area.

Project Proponents must report the boundaries of the 2-plot designations, including project area maps, soil pH maps and soil texture maps with clearly demarcated boundaries and the GPS coordinates for the boundaries. It is recommended that the Project Proponent designates "buffer zones" near the edges of the control plots wherein samples will not be collected due to an elevated risk of feedstock migration. Geographical details on such designations must be reported.

The criteria for designating the treatment and control in the the 3-plot approach are the same as those described for the control in the 2-plot approach. Project Proponents must report the boundaries of the 3-plot designations as described in the 2-plot approach above.

Projects exceeding a total project area of 1,000 hectares are required to maintain research plots for the purpose of furthering scientific understanding of outstanding research question in EW projects. Research plots are required to include a control and experimental (treatment or deployment) plot, and may be located within the control, treatment or deployment plots established for crediting. Project Proponents may choose the location and design of the research plot pursuant to specific scientific questions. It is recommended that research plots are designated such that maximum variability is captured. Research plot designations must be outlined and justified in the PDD for applicable projects.

Project Proponents meeting this criteria are required to trace the evolution of alkalinity down to at least 60 cm (but a recommended depth of 1 meter) using both soil and porewater measurements in the treatment and control areas. While only one analysis per plot is required per Reporting Period, the Project Proponent should use a similar depth partitioning and compositing framework applied in the rest of the project. Analyses of soil and fluid samples in the research plot must include all analyses required in Appendix 2.

Field management practices affect CO₂ removal both directly and indirectly ^{38, 39, 40}. For example, irrigation can significantly impact both moisture and pH, therefore acting as a strong control on weathering rate⁴⁰. Some irrigation sources might contain significant amounts of alkalinity, influencing some quantification approaches. Furthermore, soil tilling can drive increased carbon flux in the upper soil column^{38, 39, 41}, which can complicate the calculation of stored carbon. Thus, projects are required to provide detailed information on field management prior to feedstock deployment. Field management information includes:

• Crop type

  • Note that any changes to crop type (e.g. due to crop rotation cycles) during the Project Crediting Period must be reported to Isometric.

• Productivity levels

  • This may be either farmer-supplied or monitored as crop yield in control and treatment areas.

• Tillage practice

• Fertilizer use and composition, if being used to account for non-carbonic acid weathering

In the event that a field is irrigated with a non-rainwater source and maintaining control plots that account for spatial variation in irrigation is operationally infeasible, Project Proponents must provide information on irrigation schedule and the irrigation source, including the following geochemical parameters:

• pH

• Alkalinity

• DIC

• Major and trace elements that will be used for CDR quantification (see Analytical Methods)

• Anions (see Analytical Methods)

Project Proponents may use publicly available or farmer-supplied datasets where sufficient information on all of the above parameters is available.

If, at any point in the project duration, changes are implemented to any of the above field management strategies, the changes must be documented and reported. This includes a description of the change, the date the change was implemented and any required analyses associated with that field management strategy. For example, if the irrigation source changes, the fluid must be characterized according to the measurements listed above. Communication with agricultural partners should be established prior to deployment to facilitate data sharing. In extraordinary circumstances in which field management data cannot be provided, Project Proponents may still be eligible for Credits. In such cases, Project Proponents must justify why they are unable to provide this data in the PDD.

Climatic variables, such as temperature and precipitation, are strong controls of both feedstock reaction kinetics and the rate of alkalinity export ^{40, 42}. Thus, the project must provide climatic data from the project area. Climatic data includes:

• Rainfall

• Temperature

• Humidity

• Wind speed

• Solar radiation

The reported data must encompass the full time period for which crediting will take place. Acceptable data sources include sensors deployed directly in the project site or obtained from the geographically nearest weather station.

Inter-regional variability in rainfall can be significant even at the kilometer scale ^{43, 44}. Because of the direct impact of rainfall on CO₂ sequestration in agricultural EW, the project therefore must take microclimatic variation into account in collecting and reporting rainfall at the project site by deploying a rain gauge with data logging. This Protocol recommends that one rain gauge is installed per 2,000 hectares. Project Proponents may propose alternative methods to account for rainfall at the project site if installation of a rain gauge is infeasible; for example, data from a weather station within 10 km² distance from the project site.

This Protocol considers CO₂ removal in terms of the alkalinity flux beyond the depth of the NFZ. Given the complexity of determining the depth of the NFZ, as well as the operational difficulty of deep soil sampling, this Protocol sets a recommended sampling depth of 20 cm (or tillage depth with additional buffer), with the expectation that this recommendation will be updated as more information becomes available. CO₂ removal based on soil measurements will be calculated based on the alkalinity that reaches the depth of the NFZ. However, we recognize the importance of accounting for temporal lags in alkalinity export from the soil column associated with cation sorption/desorption and formation/dissolution of secondary carbonates. To this end, Project Proponents are required to provide some constraints on alkalinity export from the chosen sampling depth to the watershed (see Section 9.3.4.6.4)

To account for in-field heterogeneity, soil samples should consistent of multiple soil cores or subsamples. It is strongly recommended that soil samples be composed of 10-20 composited soil cores or subsamples randomly or arbitrarily distributed about a sample coordinate^{45, 46, 47}. While this is a recommendation, Project Proponents are encouraged to consider increasing the number of composited cores per sample, especially in fields that have been characterized with a high level of spatial heterogeneity. The overall compositing procedure must be reported and justified in the PDD.

Sampling technique, including sampling design and methodology, type of sampling materials or machinery, must be stated in the PDD. Significant changes to sampling procedures throughout the lifetime of the project must be reported to Isometric.

This Protocol has multiple statistical considerations related to soil sampling. The first is a requirement for crediting, and the second is a guidance on determining the number of control, treatment and deployment samples needed for statistical significance.

The Project Proponent must demonstrate a statistically significant weathering signal in the quantification medium. If a project is utilizing soil-based quantification methods, a statistically significant decrease in the concentration of base cations in the NFZ must be observed in the treatment plot between the beginning and end of a Reporting Period. We recommend using a one-tailed t-test to demonstrate this statistical significance⁴⁸. Alternative statistical tests may be appropriate and may be justified based on characteristics of the data distribution (e.g., use Mann-Whitney U-test if data is not normally distributed). The Project Proponent may alternatively use a two-dimensional interpolation framework to estimate the spatially integrated weathering over the project area (with uncertainty explicitly quantified). In such cases, the interpolation scheme, uncertainty quantification routine and an appropriate test for statistical significance must be described in detail in the PDD.

A significance level of 0.05 must be used for all statistical tests (with the null hypothesis that there is no change in base cation concentration at the end of the Reporting Period compared to the earliest time point). Although there are a range of analyses that may be conducted to measure the abundance of base cations (relative to some immobile tracer), all statistical tests must be performed on base cation concentrations (e.g., mol/kg or g/kg) or base cation concentrations converted to units of equivalents of charge per kilogram of soil (e.g., eq/kg). The statistical test used, the input data and the result of this statistical test must be reported.

In cases where Project Proponents opt not to measure the relative concentrations of cations in soil immediately after deployment, the mean and uncertainty distribution of cation abundance at the start of the corresponding Reporting Period may not be known. For the purposes of demonstrating statistical significance, the Project Proponent may estimate the uncertainty of cation abundance immediately after spreading to be the same as the uncertainty observed at the end of the Reporting Period. This estimation can only be done for the first Reporting Period after feedstock is applied.

Projects using the 3-plot approach have one additional statistical requirement. This second requirement for crediting is that the Project Proponent must demonstrate the treatment area is sufficiently representative of the deployment area. This can be demonstrated through one of the following: 1) at the end of the Reporting Period, the base cation concentration is not statistically different between the deployment area and the treatment area (if not significantly different at the start of the Reporting Period), or 2) the change in base cation concentration over the entire Reporting Period is not statistically different between the deployment area and treatment area. For this statistical test, a two-tailed t-test is appropriate if data is approximately normally distributed,⁴⁸ but an alternative test may be used for the same reasons described in the above paragraph. A significance level of 0.05 must be used. The statistical test must be performed on base cation concentrations (e.g., mol/kg or g/kg) or base cation concentrations converted to units of equivalents of charge per kilogram of soil (e.g., eq/kg). The statistical test used, the input data and the results of the statistical test must be reported. If the first statistical requirement is met, but it is found that the deployment is statistically different from the treatment area, an audit must be conducted to determine the likely source of the discrepancy. If no clear resolution is found, the deployment area may be credited at the lesser of the treatment and deployment plots (on a per area basis).

This Protocol recommends, but does not prescribe, a number of soil samples for quantifying removals. However, Credits will not be issued unless projects are able to achieve a statistically significant result within the Reporting Period. In some cases, this means that the Project Proponent may need to collect more than the recommended number of samples. The number of samples needed to demonstrate statistical significance will depend on several factors, including the underlying weathering rate and the in-field heterogeneity. Project Proponents may find it useful to have some guidance on how many samples may be needed to demonstrate statistical significance given an estimate of the weathering rate and the in-field heterogeneity determined from post-deployment soil sampling. The following provides simplified approach to estimating the minimum number of samples needed to achieve statistical significance, assuming normally distributed data^{44, 48, 49, 50}:

$$N\  \geq \ \frac{2\sigma^{2}{Z_{\alpha}}^{2}}{{S_{\min}}^{2}}$$

(Equation 10)

Where:

• $N$ -- number of samples needed for statistical significance
• $\sigma^{}$ -- the standard deviation of base cation concentration over an area of interest
• ${Z_{\alpha}}^{}$ -- the Z-score associated with the significant level of interest (1.96 for two-tailed, 1.645 for one-tailed)
• $S_{\min}$ -- the estimated change in base cation concentrations between the beginning and end of the Reporting Period

Using an estimate for weathering rates, the Project Proponent can derive the minimum number of samples needed to achieve statistical significance. We recommend that the Project Proponent uses an extremely conservative estimate of weathering rate or a safety factor above the calculated N when using this equation to determine the number of samples needed.

Establishing baseline (i.e., before feedstock application) soil conditions is critical to both verifying CO₂ removal through EW activities and to monitor potential environmental impacts. Project Proponents are therefore required to collect baseline soil samples prior to spreading rock or mineral feedstock. Baseline soil sampling should be conducted within the 2-plot or 3-plot framework described above. If pre-existing data on soil characteristics is unavailable, it may be appropriate to conduct additional baseline sampling prior to determining the boundaries of the control, treatment and deployment plots. Baseline samples are targeted to quantify heterogeneity in the soil characteristics most relevant to EW, including pH, soil texture, soil permeability, cation exchange capacity (CEC) and base saturation. Additionally, baseline sampling will allow the Project Proponent to determine the inherent heterogeneity of any tracer that may be used to determine rock or mineral feedstock addition. A full list of required analyses for baseline samples is given in Table 4. Baseline sampling must also quantify the abundances of all elements, isotopes and isotope ratios that will be used for quantification of feedstock weathering rate.

To minimize sampling bias, it is recommended that Project Proponents collect soil samples in the NFZ (typically 20 cm) at a high spatial sampling density²³ (e.g. 1 sample per hectare) in accordance with the quantification and validation approaches used for crediting, as outlined in Section 7.4.1. Sampling guidance for each quantification approach are given in Appendix 3. If a Project Proponent is utilizing a 3-plot approach, it is recommended that the total number of samples be evenly divided between the three areas (i.e., one-third of the total number of samples collected in each plot). While random sampling routines are generally preferred, the Project Proponent may use alternative sampling routines as long as they are documented and justified in the PDD. After sampling, the soil cores may be divided into separate soil increments or horizons and homogenized within each increment or horizon for analysis.

The Project Proponent is advised that the above baseline establishment sampling guidance represents the minimum sampling recommendation. We encourage the Project Proponent to consider the impact of increasing the number of samples on the statistical significance threshold required for crediting.

This section deals with the determination of the total amount of feedstock-based alkalinity applied to a field during a deployment. This is a crucial part of the overall removal quantification because it sets the maximum potential CO₂ removal. Given the importance of accurately determining the total amount of feedstock-based alkalinity added, this Protocol requires using average feedstock application rate data that is cross-validated against direct soil measurements.

While it is recommended that a sampling event occur immediately after application to directly measure feedstock application rate, this is not a requirement so long as redundant determinations of application rate are conducted as described below.

Determination of alkalinity added for a project is conducted by multiplying the average application rate (kg/m²) by the concentration of base cation alkalinity in the rock (eq/kg). Average application rate can be determined by either a) the average application rate of rock using records from the applicator or b) the total mass of rock added divided by the land area over which it was applied. This average application rate must then be multiplied by the moles of alkalinity per kilogram of rock, as outlined in the Rock and Mineral Feedstock Characterization Module.

The second method for determination of feedstock-based alkalinity added is meant to serve as an internal consistency check on the soils-based approach in the previous section. The total amount of feedstock-based alkalinity or base cations added to a site is determined by measuring the soil concentration of a particular immobile element, isotope or isotope ratio before and after feedstock application, and using the ratio between that analyte and feedstock alkalinity to determine the true feedstock application rate.

Prior to application, the project area will be separated into two or three plots as described in Section 9.3.1. Baseline soil samples will be collected prior to the spreading of feedstock. Soil samples shall be partitioned and analyzed as previously described. Following feedstock application, soil samples will be collected and analyzed in the same manner.

A mass balance approach is then used to determine the total mass of rock added in each field area (i.e., 2-plot or 3-plot) separately:

$${\lbrack ITE\rbrack}_{PA,\ x}{mass}_{soil - PA,\ x} = \ {\lbrack ITE\rbrack}_{BL,\ x}{mass}_{soil - BL,\ x} + {\lbrack ITE\rbrack}_{FS}{mass}_{FS,\ x}\$$

(Equation 11)

Where:

• ${\lbrack ITE\rbrack}_{PA,\ x}$ -- mean concentration of trace immobile analyte in plot x post feedstock application, weight/weight (e.g., ppm)
• ${\lbrack ITE\rbrack}_{BL,\ x}$ -- mean concentration of immobile analyte in plot x before feedstock application, weight/weight (e.g., ppm)
• ${\lbrack ITE\rbrack}_{FS}$ -- mean concentration of immobile analyte in feedstock, weight/weight (e.g., ppm)
• ${mass}_{FS,\ x}$ -- mass of rock added, kg, in plot x
• ${mass}_{soil - BL,\ x}$ -- mass of soil before application, kg, in plot x
• ${mass}_{soil - PA,\ x}$ -- mass of soil after application, kg, in plot x

The Project Proponent may, in consultation with Isometric, provide an alternate form of the mass balance equation that is more appropriate for the specific tracers being used, which must be included in the PDD.

The mass of soil can be estimated using the product of soil density, depth of the NFZ (typically 20 cm) and the area of interest. Using the assumption that the total soil mass in the top 20 cm is not significantly changed by rock application, Equation 11 can be rearranged to calculate the mass of rock added:

$${mass}_{FS,\ x} = \ \frac{(\lbrack{ITE\rbrack}_{PA,\ x} - \lbrack{ITE\rbrack}_{BL,\ x}){mass}_{soil,\ x}}{\lbrack{ITE\rbrack}_{FS,\ x}}$$

(Equation 12)

$${mass}_{soil,\ x} = \rho_{x} \times d_{x} \times A_{x}$$

(Equation 13)

Where:

• $\rho_{x}$ -- the average bulk density of soil in area $A_{x}$, in kg/m³
• $d_{x}$ -- the sampling depth, typically 0.2 m, in m
• $A_{x}$ -- the area of the control, treatment, or deployment (3-plot only) plot, in m²

The alkalinity application rate can then be calculated as follows:

$$Alkalinity\ application\ rate = \frac{{(mass}_{feedstock,\ Treatment} + {mass}_{feedstock,\ Deployment})\ \times \lbrack Alk\rbrack_{feedstock}}{(A_{Treatment} + A_{Deployment})\ }$$