The mass of biomass feedstock can be directly measured and converted to CO₂e using a default factor. The feedstock quantity will also determine the feedstock-related emissions per ton of biochar produced (see Feedstock storage counterfactual, Feedstock replacement emissions, and Indirect land use change).

Biochar carbon content can be sampled to estimate the total mass of carbon in produced biochar and converted to CO₂e using a default factor. Mass of carbon in produced biochar can be compared to the mass of carbon in the biomass feedstock to estimate the CO₂ emitted during the biomass transformation process. This quantity can also be checked for consistency against operational data and expected conversion efficiency of the pyrolyzer. In addition to characterizing carbon content, a sampling protocol for produced biochar should characterize the physical and chemical characteristics that inform the expected durability of biochar (e.g. elemental ratios, volatile and ash content, porosity; see Biochar degradation) and the impact of biochar application (e.g. water retention, surface area, surface charge, liming capacity, heavy metal content, or polyaromatic hydrocarbon and polychlorinated biphenyls; see Soil carbon impacts, Change in agricultural lime use, Change in fertilizer use, Change in agricultural yields). The conversion efficiency, structure, and chemistry of produced biochar are contingent on the specific feedstock and operating conditions of the pyrolyzer, and a sampling regime should be designed accordingly.

While nearly all biomass carbon eventually makes its way back to the atmosphere, the effective drawdown from making biochar occurs when the carbon content of the feedstock would otherwise have been released in the form of atmospheric CO₂. If the counterfactual fate of the biomass feedstock would have resulted in little to no carbon storage — for example, burning or rapid decomposition — any durable storage achieved by biochar represents additional carbon removal. If, however, the counterfactual fate of the biomass feedstock would have resulted in medium-to-long term carbon storage — for example in soils or slow degrading environments — the mass of stored carbon in the counterfactual should not count as additional carbon removal until counterfactual emissions would have occurred.

If a biomass feedstock currently serves a function that will need to be replaced if the feedstock is turned into biochar, any emissions associated with the replacement must be considered. For example, if agricultural waste is currently used as animal feed or left on the fields to contribute to nitrogen, phosphorus, and potassium or to soil organic carbon, turning that agricultural waste into biochar could result in new demand for feed, fertilizer, or soil amendments, respectively. Current feedstock uses can be evaluated on a project-by-project basis, and the carbon impact of replacements can be estimated via lifecycle assessments. We recommend accounting for feedstock replacement emissions that involve existing feedstock uses rather than potential future uses. For example, agricultural feedstocks for biochar should be evaluated based on their current uses (e.g. for animal feed or soil nutrients) rather than potential future uses that are not practiced today (e.g. potential use for bioenergy). The counterfactual for existing utilization should be flexible, and re-evaluated in the future if prevailing practices change.

Any emissions associated with indirect land use change as a result of feedstock demand must be considered. For example, paying farmers for agricultural waste will likely increase the profitability of their operations, and could result in an increase in acres planted. Creating plantations could displace food production onto other lands or contribute to additional deforestation. Conversely, the creation of markets for forestry residues could help reduce the rate of conversion of forest systems to agricultural use. To be conservative, however, we recommend that this potential positive impact be excluded from the quantified carbon removal outcome. Estimating indirect land use change resulting from feedstock demand will likely be difficult in practice and require careful economic modeling.

In agricultural contexts, fine grained carbonate limestone (agricultural lime) is commonly applied to raise the pH of acidic soils. As the lime weathers, it may serve as a sink or source of carbon dioxide depending on the time frame considered, the extent of strong acid weathering, and the degree of secondary carbonate precipitation, among other factors (see the Enhanced Weathering pathway for more information). Biochar use could potentially reduce the need to apply lime – its liming capacity can be characterized as a chemical property of produced biochar (see Biomass transformation). Any change in lime use because of biochar application requires accounting for the corresponding change in both life cycle emissions and carbon removal. We consider the uncertainty due to changing lime use to be medium (5-20%) because lime weathering efficiency may be reasonably well-known at sites that have applied lime for years, but tracking the fate of lime-derived carbon remains a challenge. If no liming occurs prior to biochar application, the uncertainty of the liming counterfactual is not applicable.

Since biochar degrades over time, any claim about the volume of total carbon removal facilitated by biochar should be accompanied by a clear claim about the associated time horizon of storage. The degradation of biochar applied to soils is contingent on the physical and chemical characteristics of biochar (see Biomass transformation) and the environment in which it is applied. Simple models relating biochar elemental ratios or volatile matter content to its stability may provide some indication of expected longevity of carbon storage, and by extension, the mass of carbon remaining in biochar over a specified time horizon. However, these models do not extrapolate with confidence over long time horizons or capture variability critical environmental factors that drive degradation like climate, temperature, moisture, soil type and texture, and microbial and fungal communities. Estimates of carbon lost from biochar should also take into account the potential for biochar’s environment to change through time, for example by eroding off of agricultural fields or migrating through the soil column. Better characterizing biochar degradation is an active area of research. Note that although this component specifically focuses on biochar applied to soils, biochar could be stored (e.g. buried at depth) or productized (e.g. embedded in concrete) in a way that constrains degradation uncertainty.

Adding biochar to soils has the potential to induce changes to soil carbon (beyond the carbon embodied in the biochar), including by affecting pH or impacting plant or microbial activity. Theoretically, these impacts could increase or decrease soil organic carbon (SOC) storage (i.e. the priming effect), and are highly dependent on the physical and chemical properties of the applied biochar (see Biomass Transformation) and environment of application. There is relatively little research on these potential interactions to date, and they merit further exploration. If biochar application increases SOC storage beyond the carbon embodied in the biochar, we recommend that these changes not be used to counterbalance project-related fossil CO₂ emissions nor counted as additional durable carbon removal due to the impermanence of soil carbon storage.

Applied biochar could potentially change fertilizer use by shifting nutrient retention or availability. The interaction between biochar and fertilizer use is contingent on chemical and physical properties of biochar, like surface area and cation exchange capacity (See Biomass transformation) and the environment of application. Due to the potentially difficult nature of the counterfactual here — as well as the complexity of farmer decision-making — we recommend this factor not be included in the accounting of net carbon removal.

If applied to agricultural soils, biochar could impact agricultural yields, which is critical for understanding the broader impact of biochar use. While most literature suggests the potential for positive impacts on yields (dependent on biochar type and application strategy), there is some evidence documenting negative impacts on yields. Due to the potentially difficult nature of the counterfactual scenarios involved, we recommend this component not be included in the accounting of net carbon removal. If biochar application increases agricultural yields and by extension leads to avoided emissions, we conservatively recommend that these changes not be used to counterbalance project emissions nor counted as additional durable carbon removal.

The embodied emissions of any materials consumed during operation can be estimated based on a cradle-to-grave lifecycle assessment (LCA) of the material input. The embodied emissions of non-consumed project equipment and infrastructure must also be considered, amortized over the expected lifetime of operation. There are not yet consistent best practices around whether or how to account for the embodied emissions of equipment or infrastructure that is used but not owned by the project. Transparency around boundary assumptions, data sources, and uncertainties is critical for LCA consistency and comparability.

The emissions associated with energy use for the process. This component should be estimated based on an assessment of lifecycle emissions for the specific electricity or energy sources consumed by the project.

The durability of carbon storage in biochar applied to soils is contingent on the physical and chemical characteristics of biochar (see Biomass transformation and Biochar degradation) and the environment in which it is applied, and will change over time as a function of Biochar degradation. Better characterizing biochar storage and degradation is an active area of research. This uncertainty is not included in the calculation of this pathway's Verification Confidence Level (VCL), because it is also captured by the Biochar degradation component.