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 characterization of the biochar physical and chemical properties, according to Section 10 of the Biochar Production and Storage Protocol.
• The full quantification of the net tonnes of CO₂e removal for Crediting occurs following an Isometric Protocol.
• 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. Per the United Nations Food and Agriculture Organization, agricultural land is defined as permanent and arable crop land, meadows and pastureland¹.

Projects that are explicitly NOT eligible include the following:

• Projects that lead to a sustained net decrease on crop yields
• Projects situated on wetlands, where “water covers the soil, or is present either at or near the surface of the soil all year or for varying periods of time during the year, including during the growing season”². Due to the high potential for flooding and variable water levels in wetlands, wetlands pose greater risks for durable biochar storage, and thus are not eligible storage sites at this time.

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 combined³. Utilizing this potential will be important for meeting ambitious climate goals, such as those put forth by the IPCC⁴. Soil organic carbon stocks can be increased by incorporating plant biomass into soil, a common agricultural practice⁵. 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 mitigation⁶. 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.^{7, 8}

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 of the storage location. Biochar can mechanically degrade over short timescales following application to soils. This degradation can cause significant changes in biochar physical properties, primarily 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 10.3 of the Biochar Production and Storage Protocol, this Module only applies to biochars which have been characterized with $H/C_{Organic}$ values <0.5, 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 10 of the Biochar Production and Storage Protocol 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.

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

Project Proponents should adhere to safe limits associated with the following upper bounds set by the European Biochar Certificate¹⁷ for storage of biochar in agricultural soils, for the relevant EBC-feed category their biochar should be categorized as:

• heavy metals; including Pb, Cd, Cu, Ni, Hg, Zn, Cr, As
• 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 European Biochar Certificate¹⁷, and should be followed by Project Proponents and outlined in full in the PDD.

Our quantification framework for determining the CO₂e_stored for 200-year durability is based on Woolf et al. (2021)¹⁸. The calculation requires two inputs: soil temperature (T_soil) and the H/C_organic ratio. The formula to calculate CO₂e_stored is:

$$CO_2e_{stored} = C_{biochar} \times m_{biochar} \times F_{durable} \times \frac{44}{12} \times (1- F_{dustloss})$$

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 after 200 years, and can be credited under this Module
• $\frac{44}{12}$ is the mass fraction of carbon dioxide and elemental carbon
• $F_{dustloss}$ is the percentage of biochar C lost during spreading of the biochar on agricultural land

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 $F_{durable}$

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 could be recalculated for longer or shorter time horizons for crediting according to the following method. 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 biochar¹⁹. 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 $H/C_{Organic}$. The following table (Table 1) provides these parameters for 200-year durability.

Table 1: 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).

If Project Proponents wish to make claims of durability on the time horizon of 200 years or beyond, then the average Random Reflectance (R₀) value reported should provide confidence that $H/C_{Organic}$ measurements are reasonable, e.g. $H/C_{Organic}$ measurements and R₀ values are well correlated (examples in Sanei et al. 2024). 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 Bodies (VVBs) may request additional sampling to ensure $H/C_{Organic}$ measurements are reliable.

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

Alternatively, if no baselining data for soil temperature is available, a justifiable $T_{soil}$ for calculation of the durable fraction of biochar should be either obtained from a global database of soil temperatures such as Lembrechts et al. (2022)^{20, 21} 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 ^{22, 23}.

Project Proponents can also 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 measurement. These should be carried out according to ISO 4974:2023²⁴ or equivalent, and justified in the PDD. The sampling frequency and dataset of measurements for $T_{soil}$ taken for the year preceding crediting should be included in the PDD and the average $T_{soil}$ calculated from that dataset. If Project Proponents elect to sample and calculate their $T_{soil}$ rather than rely on existing data, 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.

Use of Random Reflectance (R₀) for crediting

Project Proponents seeking to make claims of durability on the time horizon of 500 years should report a set of at least 500 measurements of R₀, calculated at the maceral-level, for their biochar (as outlined in Section 10.3 of the Biochar Production and Storage Protocol). 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 be reported in the PDD for the purposes of Validation and Verification. While biochar passing the benchmark of R₀ = 2% could be considered permanent⁹, peer-reviewed research further supporting the work of Sanei et al. (2024)²⁵ 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 using this novel method in two ways:

1. Biochar storage in agricultural soils using R₀ values is credited at 500 year durability, not 1,000 year durability
2. The fraction of biochar credited should be 1 standard deviation below the mean, that is, the $F_{durable}$ will be:

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

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 the Protocol in line with the best available science as more research in peer-reviewed literature is published.

Dust loss during biochar spreading $(F_{dustloss})$

The literature on dust loss (and associated emissions) is relatively limited²⁶. The only available estimates of fine-grained biochar dust loss during spreading on agricultural soils is 25-30%^{27, 28}. In order to be conservative, Project Proponents must include the appropriate dust loss factor, calculated using values from Table 2 below during calculation of $CO_2e_{stored}$ above. For projects burying biochar beneath the surface of agricultural soils, no discount for $F_{dustloss}$ is necessary. The categorization of biochars as fine-grained, medium-grained and coarse-grained is drawn from Sarfraz et al. (2020)²⁹.

Table 2: Factors for dust loss to be incorporated into Equation 4 below.

Calculation of applicable $F_{dustloss}$

Project Proponents must characterize the distribution of particle sizes using a sieving approach in accordance with ISO Standard 19.120. Project Proponents will determine the fraction of biochar, by mass, that falls into each of the particle size ranges above: ≤0.5 mm, 0.5–1.0 mm, > 1.0 mm. These fractions can be denoted as $X_{<0.5mm}$, $X_{0.5-1.0mm}$, and $X_{>1.0mm}$. These fractions should sum to the fraction of biomass that is not being buried beneath the soil surface, which will be $1$ for most cases. The relevant $F_{dustloss}$ for the application can be calculated as:

$$F_{dustloss}= (0.3X_{<0.5mm})+ (0.2X_{0.5-1.0mm}) + (0.1X_{>1.0mm})$$

Equation 4

Field management practices affect CO₂ removal both directly and indirectly^{30, 31, 32, 33}. 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 column³⁴, which can affect soil organic carbon stocks³⁵. Thus, Projects are required to provide detailed information on field management prior to feedstock deployment. 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 must take in crediting projects. Projects should ensure that biochar application does not meaningfully change field management practices in a manner that results in additional CO2e emissions.

Where applicable, analytical methods must 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 is required to outline the relevant method within the PDD submitted to the Validation and Verification Bodies (VVB).

Establishing baseline (i.e., before biochar application) soil conditions is critical to both verify CO₂ sequestration through project activities and to facilitate monitoring of potential environmental impacts. Project Proponents are therefore required 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 must 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 so long as they are documented and justified in the PDD. Samples must be analyzed for the properties outlined in Table 3 below.

Table 3: Parameters for measurement to be used during Project baseline establishment.

The following aspects of biochar application to surface land pertaining to pre-deployment 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 grained ³⁶
  • 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^{37, 38}
  • 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 residues³⁸.

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)
• Konstantina Stamouli, Ph.D.

1. Food and Agriculture Organization, Arable and Permanent Cropland Area (2007). https://www.un.org/esa/sustdev/natlinfo/indicators/methodology_sheets/land/arable_cropland_area.pdf. ↩

2. United States Environmental Protection Agency (2024). https://www.epa.gov/wetlands/what-wetland ↩

3. Lehmann, J., & Kleber, M. (2015). The contentious nature of soil organic matter. Nature, 528(7580), 60-68. ↩

4. Climate Change 2022: Mitigation of Climate Change. Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [P.R. Shukla, J. Skea, R. Slade, A. Al Khourdajie, R. van Diemen, D. McCollum, M. Pathak, S. Some, P. Vyas, R. Fradera, M. Belkacemi, A. Hasija, G. Lisboa, S. Luz, J. Malley, (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA. doi: 10.1017/9781009157926 ↩

5. Chenu, C., Angers, D. A., Barré, P., Derrien, D., Arrouays, D., & Balesdent, J. (2019). Increasing organic stocks in agricultural soils: Knowledge gaps and potential innovations. Soil and Tillage Research, 188, 41-52. ↩

6. EBC (2012-2023) 'European Biochar Certificate - Guidelines for a Sustainable Production of Biochar.' Carbon Standards International (CSI), Frick, Switzerland. (http://european-biochar.org). Version 10.3 from 5th Apr 2022 ↩

7. Woolf, D., Amonette, J., Street-Perrott, F. et al. Sustainable biochar to mitigate global climate change . Nat Commun 1, 56 (2010). https://doi.org/10.1038/ncomms1053 ↩

8. Smith, S. M., Geden, O., Gidden, M. J., Lamb, W. F., Nemet, G. F., Minx, J. C., Buck, H., Burke, J., Cox, E., Edwards, M. R., Fuss, S., Johnstone, I., Müller-Hansen, F., Pongratz, J., Probst, B. S., Roe, S., Schenuit, F., Schulte, I., Vaughan, N. E. (eds.) The State of Carbon Dioxide Removal 2024 - 2nd Edition. DOI 10.17605/OSF.IO/F85QJ (2024) ↩

9. Wang, H., Nan, Q., Waqas, M., & Wu, W. (2022). Stability of biochar in mineral soils: assessment methods, influencing factors and potential problems. Science of The Total Environment, 806, 150789. ↩ ↩²

10. Zhong, Y., Igalavithana, A. D., Zhang, M., Li, X., Rinklebe, J., Hou, D., ... & Ok, Y. S. (2020). Effects of aging and weathering on immobilization of trace metals/metalloids in soils amended with biochar. Environmental Science: Processes & Impacts, 22(9), 1790-1808. ↩

11. Das, O., Mensah, R. A., George, G., Jiang, L., Xu, Q., Neisiany, R. E., ... & Berto, F. (2021). Flammability and mechanical properties of biochars made in different pyrolysis reactors. Biomass and Bioenergy, 152, 106197. ↩

12. Shanmugam, V., Sreenivasan, S. N., Mensah, R. A., Försth, M., Sas, G., Hedenqvist, M. S., ... & Das, O. (2022). A review on combustion and mechanical behaviour of pyrolysis biochar. Materials Today Communications, 31, 103629. ↩

13. Singh, B. P., Cowie, A. L., & Smernik, R. J. (2012). Biochar carbon stability in a clayey soil as a function of feedstock and pyrolysis temperature. Environmental science & technology, 46(21), 11770-11778. ↩ ↩² ↩³ ↩⁴

14. Chiquier, S., Patrizio, P., Bui, M., Sunny, N., & Mac Dowell, N. (2022). A comparative analysis of the efficiency, timing, and permanence of CO 2 removal pathways. Energy & Environmental Science, 15(10), 4389-4403. ↩

15. Wang, J., Xiong, Z., & Kuzyakov, Y. (2016). Biochar stability in soil: meta‐analysis of decomposition and priming effects. Gcb Bioenergy, 8(3), 512-523. ↩

16. Rathnayake, Dilani, Hans-Peter Schmidt, Jens Leifeld, Diane Bürge, Thomas D. Bucheli, and Nikolas Hagemann. "Quantifying soil organic carbon after biochar application: how to avoid (the risk of) counting CDR twice?." Frontiers in Climate 6 (2024): 1343516. ↩

17. Premalatha R. P. , Poorna Bindu J. , Nivetha E. , Malarvizhi P. , Manorama K. , Parameswari E. , Davamani V. (2023) A review on biochar’s effect on soil properties and crop growth. Frontiers in Energy Research, 11, https://doi.org/10.3389/fenrg.2023.1092637 ↩ ↩²

18. Woolf D., Lehmann J., Ogle S., Kishimoto-Mo A. W. , McConkey B., and Baldock J. (2021) Greenhouse Gas Inventory Model for Biochar Additions to Soil. Environmental Science & Technology, 55 (21), 14795-14805 https://doi.org/10.1021/acs.est.1c02425 ↩

19. Hao, L., & Naiman, D. Q. (2007). Quantile regression (No. 149). Sage. ↩

20. Lembrechts, J. J., van den Hoogen, J., Aalto, J., Ashcroft, M. B., De Frenne, P., Kemppinen, J., ... & Hik, D. S. (2022). Global maps of soil temperature. Global change biology, 28(9), 3110-3144. ↩

21. Correction to Global maps of soil temperature. (2023). Global Change Biology, 29(22), 6423–6433. https://doi.org/10.1111/gcb.16910 ↩

22. S.W. Buol, in Reference Module in Earth Systems and Environmental Sciences, 2013 ↩

23. Sarfraz, R., Yang, W., Wang, S., Zhou, B., & Xing, S. (2020). Short term effects of biochar with different particle sizes on phosphorous availability and microbial communities. Chemosphere, 256, 126862. ↩

24. Buol, S.W. “Tropical soils | humid tropical.” Reference Module in Earth Systems and Environmental Sciences, 2013, https://doi.org/10.1016/b978-0-12-409548-9.05324-0. ↩

25. Sanei, H., Rudra, A., Przyswitt, Z. M. M., Kousted, S., Sindlev, M. B., Zheng, X., ... & Petersen, H. I. (2024). Assessing biochar's permanence: An inertinite benchmark. International Journal of Coal Geology, 281, 104409. ↩

26. Gelardi, D. L., Li, C., & Parikh, S. J. (2019). An emerging environmental concern: Biochar-induced dust emissions and their potentially toxic properties. Science of the Total Environment, 678, 813-820. ↩

27. Houses of Parliament Parliamentary Office of Science and Technology, Postnote, Number 358, July 2010. Available at: https://www.parliament.uk/globalassets/documents/post/postpn358-biochar.pdf ↩

28. Guidelines on Practical Aspects of Biochar Application to Field Soil in Various Soil Management Systems, International Biochar Initiative (2010). Available at: https://biochar-international.org/wp-content/uploads/2018/04/IBI_Biochar_Application.pdf ↩

29. Hou, R., Ouyang, Z., Maxim, D., Wilson, G., & Kuzyakov, Y. (2016). Lasting effect of soil warming on organic matter decomposition depends on tillage practices. Soil Biology and Biochemistry, 95, 243-249. ↩

30. Major, J. (2010). Guidelines on practical aspects of biochar application to field soil in various soil management systems. International Biochar Initiative, 8(1), 5-7. ↩

31. Sainju, U. M., Jabro, J. D., & Stevens, W. B. (2008). Soil carbon dioxide emission and carbon content as affected by irrigation, tillage, cropping system, and nitrogen fertilization. Journal of environmental quality, 37(1), 98-106. ↩

32. Dencső, M., Horel, Á., Bogunovic, I., & Tóth, E. (2020). Effects of environmental drivers and agricultural management on soil CO2 and N2O emissions. Agronomy, 11(1), 54. ↩

33. Cipolla, G., Calabrese, S., Porporato, A., & Noto, L. V. (2022). Effects of precipitation seasonality, irrigation, vegetation cycle and soil type on enhanced weathering–modeling of cropland case studies across four sites. Biogeosciences, 19(16), 3877-3896. ↩

34. National Research Council, Division on Earth, Life Studies, Ocean Studies Board, Board on Atmospheric Sciences, Committee on Geoengineering Climate, ... & Discussion of Impacts. (2015). Climate intervention: carbon dioxide removal and reliable sequestration. National Academies Press. ↩

35. Lal, R. (2007). Carbon management in agricultural soils. Mitigation and adaptation strategies for global change, 12, 303-322. ↩

36. Singh, H., Northup, B.K., Rice, C.W. et al. (2022) Biochar applications influence soil physical and chemical properties, microbial diversity, and crop productivity: a meta-analysis. Biochar 4, 8. https://doi.org/10.1007/s42773-022-00138-1 ↩

37. Campos, P., Knicker, H., Velasco-Molina, M., & De la Rosa, J. M. (2021). Assessment of the biochemical degradability of crop derived biochars in trace elements polluted soils. Journal of Analytical and Applied Pyrolysis, 157, 105186. ↩

38. de la Rosa, J. M., Rosado, M., Paneque, M., Miller, A. Z., & Knicker, H. (2018). Effects of aging under field conditions on biochar structure and composition: Implications for biochar stability in soils. Science of the Total Environment, 613, 969-976. ↩ ↩²