Biochar Storage in Agricultural Soils

The Durability of a Carbon Dioxide Removal (CDR) process refers to the length of time for which CO₂ is removed from the Earth’s atmosphere and cannot contribute to further climate change. This Module details the durability, reversal risks and requirements for storage of carbon as biochar in soil in agricultural soils. This Module is intended for use in conjunction with other Isometric Protocols and Modules, and assumes the following:

• The full quantification of the net tonnes of CO₂e removal for Crediting occurs following an Isometric Protocol; and
• All environmental and social safeguards have been followed according to Section 5 in the Biochar Production and Storage Protocol.

The information and requirements outlined within this Module are based on the best available science at the time of writing. This Module will be reviewed at a minimum every 2 years and/or when there is an update to scientific published literature which would affect net CO₂e removal quantification or the monitoring guidelines outlined in this Module, and/or in line with changes in scientific consensus regarding the durability of biochar in agricultural soils.

This storage module applies to Projects or processes which apply biochar to agricultural land to capture CO₂. Biochar can be applied as pellets or fine-grained biochars. According to the definition by the United Nations Food and Agriculture Organization, agricultural land is defined in this Module as permanent and arable crop land, meadows and pastureland¹.

Projects that are explicitly not eligible include, but are not limited to, the following:

• Projects that apply or store biochar in non-agricultural settings; or
• Projects that lead to a sustained net decrease in crop yields.

Soils have the potential to act as a significant carbon Sink, as evidenced by the magnitude of soil organic carbon stocks - which exceed that of plant matter and atmospheric carbon combined². Utilizing this potential will be important for meeting ambitious climate goals, such as those put forth by the IPCC³. Soil organic carbon stocks can be increased by incorporating plant biomass into soil, a common agricultural practice⁴. This can be achieved by incorporating agricultural residues into topsoil, however, the labile nature of this source of organic matter results in CO₂ being released back into the atmosphere on timescales that are too short for meaningful climate change mitigation⁵. As an alternative, charred or pyrolyzed carbon has the potential to contribute similar soil health benefits, with a higher stable carbon fraction in soil over long timescales.^{6, 7}

Scientific consensus on stability of biochar in storage environments is growing ⁸, but there are still some key areas where further research is needed. While biochar is more stable than non pyrolyzed carbon, the mean residence time (MRT) of biochar will depend on its physical and chemical characteristics prior to application, as well environmental conditions at the storage location. Biochar can mechanically degrade over short timescales following application to soils. This degradation can cause significant changes in biochar physical properties. Degradation primarily affects biochar particle size, and to a lesser extent specific surface area and porosity^{9, 10}. However, these factors are not necessarily indications of biochar decay, and the carbon content of the biochar can remain intact and stably stored despite these variabilities.^{11, 12}

The MRT of biochar is highly variable, with available literature reporting values ranging over several orders of magnitude^{13, 14, 15}. As outlined in Section 3, only biochars which have been characterized with $H/C_{Organic}$ values <0.5 are considered, which provides high confidence that the biochar is stable on crediting time horizons of at least 200 years. Biochar that is durably stored is defined under this Module as biochar which is stable in soils after at least 200 years have elapsed.

This Module describes how the requirements in Section 3 should be used to quantify the number of credits that are issued for a Project applying biochar to soil. It also includes details of the environmental conditions that must be met and documented in the Project Design Document (PDD) to ensure that biochar-C is stably sequestered in surface land applications for at least 200 years.

Due to the rapid mechanical degradation of biochar in agricultural soils, efforts to directly measure and observe decay of biochar in soils, for example through changes in Soil Organic Carbon (SOC) stocks, are currently very challenging. Spectroscopic techniques such as near- and mid-infrared spectroscopy (NIRS/MIRS), coupled with comprehensive reference databases, show potential for distinguishing biochar-C from other SOC fractions¹⁶. However, these databases are currently limited in scope, require further verification, have a high cost, and are not well-suited for routine analysis. Therefore, quantification of the durability of biochar according to the current best available science must focus on the use of rigorous characterization of the biochar resulting from pyrolysis to calculate the fraction of biochar that is stable beyond the desired crediting time horizon. This should be coupled with conservative treatment of the Uncertainty associated with that calculation (see Section 6.5 of the Biochar Production and Storage Protocol).

This Module addresses storage site conditions and quantification of $CO_2e_{stored}$ for biochar storage in soils. For more information on pyrolysis conditions and biochar characterization, please refer to Sections 9 and 10 of the Biochar Production and Storage Protocol.

Maintaining agricultural productivity is critical to the environmental and social sustainability of CDR 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 measurements. 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. Examples of such practices include regenerative agriculture, such as diversifying crop rotation and 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 the CDR project. This support could include advice on changes to soil amendments and sustainable farming practices.

• When agricultural residues are used in projects, documentation of characterization of biomass must be provided in accordance with the Biomass Feedstock Accounting Module to demonstrate impact on nutrient (nitrogen, phosphorus, potassium) cycles. Pre-treatment of biochars is recommended prior to application where charging of nutrients may be necessary to ensure maintenance of crop yields.

As outlined in the Biochar Production and Storage Protocol, there are some potential risks to environmental and human health associated with biochar composition, for example:

• Heavy metals, including potassium and phosphorus that will impact soil health and soil amendment properties (reduced fertilizer needs); and
• Environmental impacts such as soil contamination for heavy metals and any POPs (Persistent Organic Pollutants).

Project Proponents should outline in the PDD what legal and regulatory requirements their Project adheres to in terms of environmental risks. In the absence of regulation, Projects should adhere to safe limits associated with the following upper bounds set for the following by the World Biochar Certificate (WBC)¹⁷ for storage of biochar in agricultural soils:

• Heavy metals, including Pb, Cd, Cu, Ni, Hg, Zn, Cr, As; and
• Organic contaminants, including PAHs, Benzo(a)pyrene, PCBs, PCDD/F.

Best practice for how to carry out measurements of these parameters is provided by the World Biochar Certificate¹⁷, and should be followed by Project Proponents and outlined in full in the PDD.

This Section provides the requirements for the characterization of biochar for durable storage. Durability refers to the length of time for which CO₂ is removed from the Earth's atmosphere. The durability of biochar will depend on its physical and chemical characteristics as well as the storage site conditions^{18, 19, 20}. This Section will provide requirements and guidance for characterizing the physical and chemical properties of biochar to determine if the material is eligible for crediting under this Protocol.

Biochar physical and chemical characteristics will be informed by the biomass feedstock type and pyrolysis conditions. This section will not set requirements or guidelines for biomass feedstock eligibility or pyrolysis conditions. Please refer to the Biomass Feedstock Accounting Module and Section 9 of this Protocol for guidance and discussion on these two topics.

Some of the required measurements in this Section are associated with minimum or maximum thresholds for eligibility for crediting by Isometric. Other measurements may be required but have no associated eligibility threshold. Some measurements may not be required but this Protocol strongly recommends that the Project Proponent measure and report these values to facilitate scientific advances in biochar durability in soil. Analytical methods provided are examples of eligible methodologies but are not meant to indicate the only acceptable methodologies. The methodology or analytical technique used for each measurement must be stated in the PDD.

Biochar must be characterized prior to application to ensure safety and suitability for CO₂ removal.

The analyses described in this section regarding the physical composition of biochar are recommended for the purposes of information gathering given the nascency of biochar durability quantification. Physical properties of biochar may affect the evolution of biochar in soil²¹, however there is not yet any evidence that the physical properties of biochar would materially affect its durability on the time horizon considered for crediting. Given the uncertainty surrounding biochar durability time horizons, coupled with the relation between physical characteristics of biochar and soil health, agricultural productivity, and albedo ^{22, 23}, the following analyses of biochar physical characteristics will be recommended, with no associated eligibility thresholds.

Table 1: Recommended Measurements of Biochar physical properties

The following analyses regarding the chemical composition of biochar will be used to assess the reactivity and durability of biochar-C in different storage environments. Some of these measurements will be used in the quantification of CO₂e_stored , as outlined in Section 8.3 and the relevant storage Module. The required and recommended measurements listed below investigate multiple mechanisms of reactivity (or prevention of), including aromaticity and aromatic condensation, functional groups, and volatility. The redundancy of characterizing reactivity potential via different mechanisms serves to reduce the uncertainty surrounding the durability of biochar, and provides multiple indicators of confidence that durability will exceed the crediting time horizon.

Table 2: Recommended and Required Measurements of Biochar chemical properties

For the required measurements hydrogen-to-carbon ratio ($H/C_{organic}$), oxygen-to-carbon ratio ($O/C_{organic}$), and random reflectance ($R_0$), samples should be taken using the same sampling regimes outlined in Section 8.3.1 for measuring carbon content.

A batch associated with any one project may have a unique history or set of characteristics that could require individual consideration for recommended measurements. A broad range of feedstock characteristics and pyrolysis conditions may influence biochar homogeneity, and so the sampling plan in place in Section 8.3.1 takes a conservative approach to sampling with enough frequency to minimize the impacts of any heterogeneity in biochars. These considerations include, but are not limited to, biomass feedstock type and particle size distribution, pyrolysis temperature, reactor type, etc. All relevant details of the sampling plan, number of analyses, and adequate justification for the choices made by the Project Proponent to adhere to sampling regimes outlined in Section 8.3.1 must be included in the PDD.

For frequency of measurement of Random Reflectance (R₀), please see Section 4.1, as this depends on whether credits are issued with 200 year or 1,000 year durability.

To ensure representative sampling, Project Proponents should follow the guidelines outlined in the European Biochar Certificate Guidelines for a Sustainable Production of Biochar, Version 9.3E. A composite sample can be sub-divided into a final (6) replicates for laboratory analytical classification per batch to allow for reliable estimation of mean values and Standard Deviation, and detection of potential outliers.

It is a requirement that all Projects demonstrate the degree of homogeneity within a single Storage or Production Batch. All projects are required to include in the PDD a detailed description of how the chosen sampling plan addresses any heterogeneity that might be present within the batch. This may include sampling across horizontal and vertical dimensions of a Production or Storage Batch to account for particle sorting that may occur during processing and transportation, as outlined in Section 8.3.1 “Minimum number of samples per Batch”. As a result of this, it is the responsibility of Project Proponents to undertake these routine batch characterizations of the biochar utilized within a Crediting Project and detail these in full in the PDD.

The Project Proponent is required to report the analytical laboratory/laboratories that have been utilized for the biochar characterization. It is the responsibility of the Project Proponent to ensure that the chosen analytical facilities are reputable and conduct characterization techniques to the required standards indicated within this Protocol. A qualified laboratory is evidenced by accreditation to ISO 17025 or equivalent standards for laboratory quality management for the specific test method.

It is recommended that Project Proponents should utilize accredited analytical services such as UKAS, MCERTS, DWTS, and ISO whenever feasible. Where a Project Proponent utilizes laboratory facilities within an academic institution, or a non-accredited commercial laboratory, periodic external validation must be undertaken with an accredited facility. The frequency of these external checks will vary by project and analytical procedure, and will be agreed with Isometric on a case-by-case basis.

Laboratories must complete standard quality assurance procedures on a schedule in accordance with their quality management plans and accreditation requirements to include:

• Analysis of blanks;
• analysis of duplicates; and
• Instrumentation calibrations and analysis of calibration standards.

Project Proponents are required to report calibration records from analytical facilities to the relevant VVB when submitting biochar characterization data. Projects are also required to outline specific analytical checks that have been carried out to maintain data quality, with specific reference to the relevant certified reference materials (CRM) used by the utilized laboratory facility.

Characterization data should be validated through set quality assurance and quality control (QA/QC) criteria within all crediting programs. All projects are required to report their QA/QC processes within the PDD, in accordance with the requirements of this Protocol. As part of QA/QC project proponents are required to clearly describe analytical checks (including duplicate, blanks and analytical standards checks) and calibration procedures.

Project Proponents are required and are responsible for the delivery of biochar characterization data to a project’s VVB. Although a Project Proponent is expected to carry out characterization data externally at an accredited facility, it is the responsibility of the Project Proponent to deliver data that is accurate and externally verifiable. Submitted data reports are required to include results of all standards to verify data quality. Project Proponents are required to maintain data records for a minimum of 5 years following the date of data collection.

Project Proponents are required to report data such that the data analysis methods used are easily identified, verified and replicated. This Protocol requires that any data reports include the raw data from which any data analysis/reduction was performed, including standards and replicate measurements. A summary containing information on analytical uncertainty, number of samples taken, standards used and number of standard runs, standard deviation and percentage error on the standards must also be included. This may, for example, take the form of a spreadsheet containing four sheets:

• Summary sheet detailing metadata:
  • Number of samples run;
  • Analytical uncertainty;
  • Standards used;
  • Number of standards run;
  • Standard deviation; and
  • Percentage error on standards.
• Reduced data sheet (data summary);
• Data reduction sheet (if applicable; e.g. processing of ICP-MS data); and
• Raw data.

There are two options for calculating the fraction of durable biochar ($F_{durable}$) in this Module. Option 1 results in credits issued at 200 year durability, and Option 2 results in credits issued at 1,000 year durability.

In either case, the formula to calculate CO₂e_stored is:

$$CO_2e_{stored} = C_{biochar} \times m_{biochar} \times F_{durable} \times \frac{44}{12}$$

Equation 1

Where

• $C_{biochar}$ is the carbon content of the biochar (empirical).
• $m_{biochar}$ is the dry mass of biochar applied.
• $F_{durable}$ is the fraction of durable biochar that remains in the soil for the full duration of the crediting timeline (i.e. either 200 or 1,000 years), and can be credited under this Module.
• $\frac{44}{12}$ is the mass fraction of carbon dioxide and elemental carbon.

Measurement of Carbon Content

See Section 8.3.1 of the Biochar Production and Storage Protocol for full guidelines on measurement of biochar carbon content, $C_{biochar}$.

Measurement of Mass of biochar applied

See Section 8.3.1.1 of the Biochar Production and Storage Protocol for full guidelines on measurement of mass of biochar applied, $m_{biochar}$.

Calculation of $F_{durable}$

Calculation of $F_{durable}$ is carried out differently in Option 1 (200 year durability) and Option 2 (1,000 year durability). Project Proponents must choose which quantification framework (Option 1, or Option 2, outlined in full below) they wish to use for crediting their Project. Option 1 and Option 2 cannot be used in conjunction with one another, or in combination. The choice of quantification framework and corresponding durability associated must be clearly outlined in the PDD.

Option 1: 200 year durability

The quantification framework for determining the CO₂e_stored for 200-year durability is based on Woolf et al. (2021)²⁶. The calculation of $F_{durable}$ requires two inputs: soil temperature ($T_{soil}$) and the $H/C_{organic}$ ratio. The formula to calculate $F_{durable}$ is:

$$F_{durable} =1 -[ c + (a+b \times ln(T_{soil})) \times H/C_{organic}]$$

Equation 2

Where

• $F_{durable}$ is the fraction of carbon remaining in durable storage.
• $c$, $a$, and $b$ are estimated parameters based on an analysis of the data of Woolf (2021), described in more detail below.
• $T_{soil}$ is the average annual temperature of the soil where the biochar is stored (°C).
• $H/C_{Organic}$ is the molar hydrogen-organic carbon ratio.

The parameters a, b, and c for the time horizon of 200 years are estimated using data available in the appendix of Woolf et al. (2021). Parameters theoretically could be recalculated for longer or shorter time horizons for crediting according to the following method. The coefficients a, b and c are calculated using 1 standard deviation below the mean which will result in a more conservative estimation of the decay function. The parameters are estimated using a two-stage regression analysis. First, a linear quantile regression of the non-durable portion ($1 - F_{durable}$) on $H/C_{Organic}$ is conducted for each unique soil temperature in the Woolf et al. (2021) data. Quantile regression allows for the estimation of the $1 - F_{durable}$ associated with a durability value that is in the 17th percentile of the durability distribution, conditional on the $H/C_{Organic}$ and $T_{soil}$ of the biochar²⁷. The estimated intercepts from this regression are regressed on a constant to obtain parameter c. The estimated $H/C_{Organic}$ coefficients from this regression are then regressed against the natural logarithm of soil temperature in a linear regression to obtain parameters a and parameter b, the effect of a change in $ln(T_{soil})$ on the relationship between $1 - F_{durable}$, and $H/C_{Organic}$, respectively. The following table (Table 1) provides these parameters for 200-year durability.

Table 3: Parameters calculated for Equation 2 for the time horizon of 200 years for crediting, using data available in the appendix of Woolf et al. (2021)²⁶.

If Project Proponents wish to make claims of durability on the time horizon of 200 years using Option 1, then the Random Reflectance (R₀) value reported during project verification for each batch should provide confidence that $H/C_{Organic}$ measurements are reasonable. In practice, this means that the durable fraction of biochar resulting from $H/C_{Organic}$ measurements and R₀ measurements are well correlated - examples can be found in Sanei et al. (2024). The number and frequency of R₀ values reported by the Project Proponent for crediting on a 200 year time horizon should be agreed with Isometric in advance, to ensure sampling is representative enough to compare with $H/C_{Organic}$ measurements. It is not necessary for the R₀ value to imply a higher degree of durability than implied by $H/C_{Organic}$ measurements. However, if there is a substantial divergence between the two methods, the Validation and Verification Body (VVB) may request additional sampling to ensure $H/C_{Organic}$ measurements are reliable.

Sampling guidelines for measurement of average annual soil temperature $(T_{soil})$:

• Project Proponents can choose to Baseline their own annual soil temperature measurements, in order to ensure that the data used in the calculation for crediting comes from direct, project-specific measurements. These should be carried out according to ISO 4974:2023²⁸, or equivalent, and justified in the PDD. A dataset of measurements for $T_{soil}$ taken for the year preceding crediting, and the average $T_{soil}$ calculated from that dataset, should be included in the PDD. Project Proponents should report the average of monthly soil temperature measurements from every application site. At least 10 measurements must be taken per site-month.
• If no baselining data for soil temperature is available, a justifiable $T_{soil}$ for calculation of the durable fraction of biochar should be obtained from a global database of soil temperatures such, as Lembrechts et al. (2022)^{20, 29}, or equivalent. Project Proponents should identify which region their Project best aligns with from the global dataset, and justify both the dataset used and the average annual soil temperature chosen in the PDD. Air temperatures should not be used as a proxy for average annual soil temperatures. While average air and soil temperatures are correlated, there is evidence that mean annual soil temperatures can be 2-4^oC warmer than mean annual air temperatures ^{30, 31}.

Option 2: 1,000 year durability

Project Proponents seeking to make claims of durability on the time horizon of 1,000 years should report a set of at least 500 measurements of R₀, calculated at the maceral-level, for their biochar (as outlined in Section 3 of this Module). Batches that adopt this measurement approach can be credited for the fraction of their biochar which passes the 2% R₀ benchmark outlined in Sanei et al. (2024) at this higher durability. The histogram of the R₀ values should submitted at the point of project verification for this crediting option. While biochar passing the benchmark of R₀ = 2% could be considered permanent⁹, peer-reviewed research further supporting the work of Sanei et al. (2024)³² is needed to validate the experimental work undertaken in that study to provide higher confidence in these claims. We take a conservative approach here to how biochar durability is credited. The fraction of biochar credited should be 1 standard deviation below the mean. Therefore, $F_{durable}$ must be calculated as:

$$F_{durable} = S- \sqrt{\frac{S*(1-S)}{N}}$$

Equation 3

Where

• $S$ is the share of samples passing the 2% R₀ benchmark
• $N$ is the number of samples taken per batch

Quantification of biochar storage for crediting in agricultural soils will be updated in future versions of this Module in line with the best available science as more research in peer-reviewed literature is published.

Field management practices affect CO₂ removal both directly and indirectly^{33, 34, 35, 22}. For example, irrigation could significantly impact both moisture and pH, and soil moisture has been shown to have an impact on biochar degradation rate¹³. Furthermore, soil tilling can drive increased carbon flux in the upper soil column²³, which can affect soil organic carbon stocks³⁶. Thus, Projects should provide information on field management prior to feedstock deployment in the PDD. Field management information includes:

• Irrigation schedule
• Irrigation source
• Tillage practice
• Fertilizer use
• Fertilizer composition
• Crop type and rotation
• Pre-deployment, deployment, and post-deployment monitoring

This section outlines the monitoring approach that Project Proponents should take in Crediting Projects. Projects should ensure that biochar application does not meaningfully change field management practices in a manner that results in additional CO₂ e emissions.

Where applicable, analytical methods should be cross-referenced with an appropriate standard (e.g., ISO, EN, BSI, ASTM, EPA) or standard operating procedure (SOP). Where a project utilizes a non-standardized methodology or SOP for the determination of a listed parameter, the Project Proponent should outline the relevant method within the PDD.

Establishing baseline (i.e., before biochar application) soil conditions is recommended to both (i) verify CO₂ sequestration through project activities, and (ii) facilitate monitoring of potential environmental impacts. Project Proponents are therefore recommended to collect baseline soil samples prior to spreading biochar. Baseline samples are targeted to quantify heterogeneity in the soil characteristics most relevant to biochar CDR, including pH, soil texture, soil moisture and soil organic carbon (SOC).

To minimize sampling bias, Project Proponents should collect soil samples to the maximum tillage depth or 30cm, whichever is deeper. While random sampling routines are generally preferred, the Project Proponent may use alternative sampling routines, provided these are documented and justified in the PDD. Where baseline measurements are taken, samples should be analyzed for the properties outlined in Table 3 below.

Table 4: Parameters for measurement to be used during Project baseline establishment.

The following aspects of biochar application must be included in the PDD (in addition to all others listed in this Module).

• Project boundaries:
  • Project Proponents must report the boundaries of the project area, including project area maps with clearly demarcated boundaries and the GPS coordinates for those boundaries.
• Application rate:
  • Application rate may be optimized for other soil health co-benefits, such as moisture retention, increased nutrient management/regulation and soil organic carbon stocks, but there is no evidence that biochar application rate acts as a lever on biochar-C stability. The application rate paired with the project boundaries will be used to confirm total mass of biochar applied.

As outlined in Section 2.5.9 of the Isometric Standard, the Buffer Pool is a mechanism used to insure against Reversal risks that may be observable and attributable to a particular project through monitoring. Based on present understanding, reversals in Biochar storage will not be directly observable with measurements and attributable to a particular project. Projects crediting against this protocol are credited conservatively to account for degradation of labile pools of biochar within the relevant crediting time horizon. Projects applicable to this Protocol are categorized as having a Very Low Risk Level of Reversal according to the Isometric Standard Risk Assessment Questionnaire. The Buffer Pool corresponding to this lowest risk score is 2% and is intended as an additional precaution against unknowns.

Following the Section 2.5.9 of the Isometric Standard, storage uncertainty for open systems is primarily accounted within the removal quantification framework. For more details on Reversals, refer to Section 2.5.9 and 5.6 of the Isometric Standard.

The following factors could contribute to a decrease in the expected durability of biochar storage in agricultural soils.

Beyond biomass feedstock type and physical and chemical characteristics of biochar, biochar durability is affected by environmental and anthropogenic factors. The following conditions may accelerate the degradation of biochar in soil over time. Project Proponents are advised to consider sites with minimal interference from the following factors.

Environmental factors

• Precipitation and weather events:
  • High soil moisture decreases biochar MRTs¹³
• Soil conditions:
  • Soil texture: fine and coarse grained biochars are more likely to have larger impacts on soil characteristics than medium grained³⁷.
  • Mean soil temperature: higher soil temperatures increase degaradation speed of biochar¹³.
  • pH: basic soil is more conducive to microbial growth, and therefore decreases biochar MRTs^{21, 24}.
  • Nutrient availability: higher nutrient availability in soil could potentially impact microbial activity, as outlined below.
• Extreme fluctuations in soil temperature, such as freeze-thaw events: effects as outlined above on soil temperature.
• Extreme fluctuations in soil moisture, such as wet and dry seasons: effects as outlined above on soil moisture.
• Root growth: Higher root growth could conceivably accelerate the physical breakdown of biochar particles in soil, as root growth is known to play a role in mechanical degradation of rocks and minerals during the weathering cycle.
• Microbial activity:
  • microbial activity spurs biochar degradation. Microbial activity can be increased by incorporation of labile organic matter, such as fresh agricultural residues²⁴.

Anthropogenic factors

All of the following activities taking place on agricultural land may impact the environmental factors above, and therefore impact biochar degradation.

• Irrigation source and schedule: as above, high soil moisture decreases biochar MRTs.
• Fertilizer use and composition: this could alter nutrient availability in soils, and by extension, microbial activity.
• Crop type and rotation: similar to the above discussion on root growth, different crops may interfere with mechanical breakdown processes, soil pH and/or soil moisture content.
• Land management practices such as tilling, plowing, seeding, and harvesting.

Project Proponents should outline in the PDD a full description of site conditions, including justification of how site suitability was chosen bearing in mind the factors listed above.

Isometric would like to thank following contributors to this Module:

• Meredith Barr, Ph.D. (London South Bank University)
• Segun Oladele, Ph.D. (University of Lincoln)

This document is a companion to the Isometric Biochar Storage in Agricultural Soils Module V1.0, providing supporting information regarding the rationale and factors considered when determining the requirements of the Module, specifically relating to the 200 year and 1,000 year durability crediting frameworks. This document should be read in conjunction with the Module and is provided as guidance. Should there be any discrepancy or inconsistency between this companion document and the Module itself, the requirements of the Module will prevail.

There are two options for calculating the fraction of durable biochar ($F_{durable}$) in the Isometric Biochar Storage in Agricultural Soils Module, which is then used to calculate the number of credits issued by Isometric. These are outlined in Section 4.1 of the Module.

The options for calculation of $F_{durable}$ are as follows:

• Option 1: 200 year durability: $F_{durable}$ calculated using $H/C_{Organic}$ and $T_{soil}$²⁶
• Option 2: 1,000 year durability: $F_{durable}$ calculated using a histogram of Random Reflectance (R₀) measurements

Option 1 estimates the amount of biochar that will remain stable beyond 200 years using regressions to estimate the decay rate, assuming all biochar will eventually degrade. This approach can potentially underestimate the fraction of biochar which remains durably stored over long timeframes.

Option 2 relies on new work published this year³², which pioneered the use of R₀ to measure the quantity of inertinite contained in the biochar. Inertinite is the stable fraction of biochar and has been shown to have a durability of >1,000 years, which indicates that there will be a certain portion of biochar (present as inertinite) that will not degrade within that time frame. Additional research on the use of random reflectance measurement is due to be published later this year. This will be reviewed and any necessary updates will be included in a future version of this Module.

The Module offers these two approaches to reflect the latest scientific advances and provide flexibility for suppliers to either credit at 1,000 year durability or continue crediting existing projects at a lower durability of 200 years.

If you are interested in diving into the detail of how these frameworks compare to one another quantitatively at 200 and 1,000 year durability, please refer to Table A1 and Table A2. These tables use example data from published literature to illustrate how the two options estimate the fraction of durable biochar ($F_{durable}$) for crediting. Please note: crediting using R₀ is not allowed under the module for 200 year durability, and crediting using $H/C_{Organic}$ is not allowed under the Module for 1,000 year durability - these values are included in the tables for reference only.

There are three key takeaways from this data shown in Table A1 and Table A2:

(1) Crediting using $H/C_{Organic}$ at 1,000 year durability would underestimate durable fraction compared to using R₀. This is because the R₀ measurement directly characterizes the biochar material, and can quantify the portion of inertinite (stable fraction), whereas the $H/C_{Organic}$ is a modeled approach which assumes a considerable volume of the biochar will eventually decay, therefore leaning towards underestimating the fraction of durable biochar.

(2) Using the Random Reflectance (R₀) for crediting at 200 years could lead to underestimation of credits, as there will be a percentage of labile carbon that might not degrade within the 200 year framework, and accounting only for the inertinite fraction discounts the percentage of semi-inertinite that can persist. The example data shows that the R₀ estimates are the same for 200 and 1,000 year durability.

For crediting at 200 year durability the Module relies on the $H/C_{Organic}$ quantification and requires a limited number of R₀ measurements for calibration. Whereas for 1,000 year durability the Module relies on a statistically significant number of R₀ measurements. For 200 year durability crediting, this is required because in theory, some samples could have a low $H/C_{Organic}$ ratio but only a small portion of inertinite. Mathematical relationships between $H/C_{Organic}$ and R₀ have not been published as of the time of writing, and so checking the agreement between these two metrics is important to avoid overcrediting or undercrediting until such time as that research is published. This Module will be updated and sampling requirements adjusted as appropriate once additional research in this area is available.

(3) We have included the mean R₀ value for reference in row 3 of the tables. This is to demonstrate the difference in estimations of $F_{durable}$ when using the mean % R₀ value compared to the inertinite which is below the 2% inertinite benchmark. This method is not included in this Module because the mean % R₀ value does not accurately represent the inertinite distribution within the biochar sample. This could lead to either underestimating or overestimating the durability, as shown in the table. Depending on the biochar composition, underestimating biochar’s durability could be possible where the mean % R₀ is used and is below the 2% R₀ benchmark. The latter would assume that the entire biochar is not inertinite, leading to a $F_{durable}$ of 0 (row 4) in comparison to Option 1.

Table A1: 200 year durability. Option 1 from Isometric’s Biochar Storage in Agricultural Soils Module for an average temperature of 14.9^oC, was used with biochar data from Sanei et al. (2024)³² to calculate the $F_{durable}$ for 200 years. Data for Option 2, in the form of a histogram of Random Reflectance (R₀) measurements³² were also used as comparison for the 200 year timeline. Reference values of feedstock, $H/C_{Organic}$ and reaction temperature have also been included in Table 1 below. Any changes in the durable fraction when using the two different methods are captured in the final column. The difference has been calculated using the % inertinite from the histogram value (column 6 - row 2) resulting in an $F_{durable}$ of 0.85 . Any changes in the durable fraction when using the two different methods are captured in the final column (difference has been calculated using the % inertinite from the histogram value - column 6). Two additional samples, from different feedstocks, have been included in rows 4 and 5 to demonstrate the overestimation and underestimation potential.

Table A2: 1,000 year durability. A modified version of Option 1 was applied, using relevant regression coefficients²⁶ for the 1000 year timeline and for an average temperature of 14.9^oC, calculated with biochar data from Sanei et al. (2024)³² to estimate $F_{durable}$ for 1,000 years, shown in Table 2 below. Data for Option 2 in the form of a histogram of Random Reflectance (R₀) measurements³² were also used as comparison for the 1000 year timeline. Any changes in the durable fraction when using the two different methods are captured in the final column (difference has been calculated using the % inertinite from the histogram value - column 6). Two additional samples, from different feedstocks, have been included in rows 4 and 5 to demonstrate the overestimation and underestimation potential.

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