The mass of the biomass feedstock can be directly measured and converted to CO₂e using a default factor or direct measurements of biomass carbon and moisture content. The feedstock quantity can be checked for consistency against operational records, and will form the basis for estimating feedstock-related upstream emissions (see Carbon storage counterfactual, Feedstock use counterfactual, and Market effects) as well as understanding the removal efficiency of the process. In addition to characterizing carbon content, a feedstock sampling protocol should characterize embodied nutrients such as nitrogen, phosphorus, and potassium. This enables an assessment of potential nutrient depletion and may inform the estimation of upstream emissions (see Feedstock use counterfactual). Any emissions associated with the process of biomass cultivation or procurement must be included in the project’s lifecycle assessment (see Materials, Energy, and Non-CO₂ biomass emissions). The chemical and physical characteristics of the feedstock will also inform estimates of biochar degradation (see Biochar degradation).

Transforming biomass into biochar via pyrolysis results in the direct emission of some portion of the carbon originally contained in the biomass feedstock. Quantifying the emissions during pyrolysis supports the assessment of how efficiently a given process converts biomass into long-term carbon storage and/or useful co-products. Biomass carbon lost during biomass transformation can be estimated by comparing the mass of carbon in the produced biochar to the mass of carbon contained in the biomass feedstock (See Biomass procurement). The carbon content of the biochar can be directly sampled and converted to CO₂e using a default factor. 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 biochar sampling protocol should characterize the physical and chemical characteristics that inform the expected durability of biochar (e.g. total organic carbon and soluble organic carbon content, elemental ratios, pH, 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). If the transformation process produces other carbon-containing products, such as bio-oil or tar, we recommend the embodied carbon be conservatively classified as “emitted” unless the product has a clearly defined path to durable storage. 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.

The mass of biochar applied in a specific setting can be directly measured and converted to CO₂e according to the measured carbon content of the biochar (see Biomass transformation).

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 in 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.

While nearly all biomass carbon eventually makes its way back to the atmosphere, the effective drawdown from more durably storing biomass carbon occurs when the biomass feedstock would otherwise have released CO₂ into the atmosphere. 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 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, slow-degradation environments like some landfills, or living ecosystems — the mass of stored carbon in the counterfactual should not count as additional carbon removal unless and until counterfactual emissions would have occurred. This component may not apply in cases where carbon is purpose grown.

If a biomass feedstock currently serves a function that will need to be replaced if the feedstock is used for CDR, 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 in soils, using that agricultural waste for CDR could result in new demand for feed or fertilizer, respectively. Current feedstock uses can be evaluated on a project-by-project basis, and the carbon impact of replacements can be estimated via lifecycle assessment. We recommend accounting for feedstock replacement emissions that involve existing feedstock uses rather than potential future uses. For example, agricultural feedstocks used for CDR 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. The counterfactual for existing utilization should be flexible, and re-evaluated in the future if prevailing practices change. This component may not apply in cases where carbon is purpose grown.

Any emissions associated with market effects 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. Directly growing crops or creating plantations could displace food production onto other lands and contribute to deforestation. Estimating the system emissions resulting from new feedstock demand will likely be difficult in practice and require careful economic modeling that reflects the overall scale of new biomass demand.

Using a biomass feedstock for CDR could potentially avoid non-CO₂ emissions such as methane or nitrous oxide that would have occurred in the counterfactual, for example during biomass decay. We recommend that these avoided emissions be considered a co-benefit of the project, and not be included in the accounting of net carbon removal. GWP assumptions used to characterize any avoided non-CO₂ emissions should be transparently disclosed.

The embodied emissions of all non-consumable and consumable materials, including biomass feedstocks, should be estimated using a cradle-to-grave lifecycle assessment (LCA). If biomass is purpose-grown, emissions from biomass cultivation and direct land use change must be taken into account. Emissions associated with any built infrastructure, including monitoring and maintenance equipment, should include both construction emissions and material embodied emissions. 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 project activities should be estimated using an assessment of lifecycle emissions for the specific electricity or other energy sources required by the project. This should include the energy use associated with transporting biomass feedstock and biochar, combustion emissions associated with fuel use during the project, and energy use associated with monitoring and maintenance.

If a project’s management of the biomass feedstock before long-term storage leads to non-CO₂ emissions, these must be accounted for. Non-CO₂ emissions such as methane or nitrous oxide could be produced via processes like anaerobic decomposition during feedstock storage and transport, or during the management of biogenic waste products. Venting, leaking, or incompletely flaring methane produced during the biomass transformation process must also be considered.

A possible co-product of biochar production is energy (e.g. heat) that could potentially be used to displace fossil fuel emissions. Although the emissions impact of these energy products is a crucial co-benefit to evaluate alongside the carbon removal achieved, we recommend this component not be included in the accounting of net carbon removal.

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). If biochar application results in the reduction of liming in a context where liming would have led to carbon removal, this should be taken into account. 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 or if biochar does not displace lime use, the uncertainty of the liming counterfactual is not applicable.

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 scenarios involved, quantifying these impacts may be challenging. If they are quantified, we recommend any associated avoided emissions be understood as a co-benefit and excluded from the calculation 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 recommend that these changes be understood as a co-benefit and excluded from the calculation of net carbon removal.

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.

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.