Table 1_Cation incorporation into biochar affects alkalinity-driven carbon dioxide removal accounting.xlsx

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.</p>