Cation incorporation into biochar affects alkalinity-driven carbon dioxide removal accounting

Biochar production from biomass may be a scalable and cost-effective means of atmospheric carbon dioxide removal. Current accounting frameworks, however, focus primarily on persistent organic carbon and overlook how pyrolysis alters the return of biomass-derived ions to soils and waters. Here, we quantify a first-order alkalinity-related adjustment to biochar CDR by comparing the inorganic carbon cycling implications of biochar production with a biomass-decay counterfactual. During plant growth, net cation uptake generates soil acidity, which can consume bicarbonate alkalinity and shift the carbonate system toward dissolved CO2, causing CO2 emissions upon re-equilibration with the atmosphere. Biomass decay can eventually reverse this acid-base perturbation by returning cations to the biogeochemical cycle. Biochar formation delays or prevents part of this return flux by retaining cations and anions within the biochar matrix, making permanent or prolonging what would otherwise have been a temporary CO2 source. We estimate the resulting change in inorganic carbon cycling from the cation–anion charge equivalent imbalance associated with the retained elemental inventory relative to recalcitrant organic-carbon storage. Across observations reporting complete Ca, Mg, K, Na, N, and S data (n = 85), the mean alkalinity-related reduction in effective CDR is 3.2% when the retained elemental inventory is assumed to be released in proportion to biochar degradation. If cations and anions are instead retained preferentially as organic carbon is lost, the mean reduction increases to 3.9% and 5.1% when 82% and 63% of organic carbon persists, respectively. The fraction of observations showing a reduction in effective CDR of ≥5% similarly increases from 18.8% under proportional release to 21.2% and 22.4% under the two complete element retention scenarios. Individual biochars span a wide range of compositions and durabilities through time, including cases in which the inorganic carbon effect instead increases estimated net CDR.