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 feedstock decomposition (see Leakage).

Biomass storage prevents or slows the decay of biomass by putting it in dry, cold, or anoxic conditions. Here, we specifically consider storage via biomass burial. The mass of stored biomass carbon can be measured directly and converted to CO₂e using a default factor or direct measurements. This quantity can be checked for consistency against operational records around biomass sourcing and transportation.

If the construction of the biomass storage site disturbs the soil, for example by digging a pit for biomass burial, any impacts on soil carbon stocks must be accounted for. The amount of carbon lost from soils during a disturbance is dependent on carbon content of the soil, the amount of time soils are exposed at the surface, and the approach to reconstructing the soil profile. Soil carbon distribution varies by site and depth. Baseline soil carbon stocks should be established via soil sampling before any disturbance, and monitored after disturbance.

Biomass storage prevents or slows the decay of biomass by maintaining dry, cold, or anoxic conditions. In cases where decay is not fully prevented, leakage can be directly monitored with sensors in or around the storage site, but the potential for future leakage will likely be modeled. Modeling of future leakage must consider expected decomposition under optimal storage conditions, as well as suboptimal conditions (e.g. wetter, warmer, or more oxic than optimal), and should be informed by direct measurements. Long-tail risks like fire, pests, human disturbance, or physical changes to the storage site (e.g. erosion or settlement) must be estimated. GWP assumptions used to incorporate the leakage of non-CO₂ gasses into the calculation of net carbon removal should be transparently disclosed. Our uncertainty evaluation reflects the wide range of storage mechanisms being explored and the uncertainty associated with storing carbon for 1000+ years. Those interested in specific storage mechanisms, particular storage sites, or storage on longer timescales may find it appropriate to adjust these uncertainties.

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 that enable biomass storage — including biomass feedstocks and any materials or chemicals used to prevent biomass decomposition — 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, combustion emissions associated with fuel use during the project, as well as 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. These emissions are considered separately from any non-CO₂ gasses that are emitted after biomass storage (see Leakage). GWP assumptions used to incorporate these emissions into the calculation of net carbon removal should be transparently disclosed.

The durability of stored biomass carbon is a contingent on the characteristics of the biomass feedstock and the ongoing maintenance of conditions that prevent or slow biomass decomposition (see Leakage). An evaluation of durability claims must therefore consider the monitoring and maintenance plan, as well as any applicable regulatory structures that assign ongoing liability for maintaining the integrity of the storage mechanism. This uncertainty is not included in the calculation of this pathway's Verification Confidence Level (VCL), because it is also captured by the Leakage component.