Partitioning of microbially respired CO₂ between indigenous and exogenous carbon sources during biochar degradation using radiocarbon and stable carbon isotopes

Pyrolized carbon in biochar can sequester atmospheric CO ₂ into soil to reduce impacts of anthropogenic CO ₂ emissions. When estimating the stability of biochar, degradation of biochar carbon, mobility of degradation products, and ingress of carbon from other sources must all be considered. In a previous study we tracked degradation in biochars produced from radiocarbon-free wood and subjected to different physico-chemical treatments over three years in a rainforest soil. Following completion of the field trial, we report here a series of in-vitro incubations of the degraded biochars to determine CO ₂ efflux rates, ¹⁴ C concentration and δ ¹³ C values in CO ₂ to quantify the contributions of biochar carbon and other sources of carbon to the CO ₂ efflux. The ¹⁴ C concentration in CO ₂ showed that microbial degradation led to respiration of CO ₂ sourced from indigenous biochar carbon (≈0.5-1.4 μmoles CO ₂ /g biochar C/day) along with a component of carbon closely associated with the biochars but derived from the local environment. Correlations between ¹⁴ C concentration, δ ¹³ C values and Ca abundance indicated that Ca ²⁺ availability was an important determinant of the loss of biochar carbon.