Partitioning of Microbially Respired CO2 Between Indigenous and Exogenous Carbon Sources During Biochar Degradation Using Radiocarbon and Stable Carbon Isotopes

ABSTRACT 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.