Chemical Recalcitrance Rather Than Soil Microbial Community Determined Short-Term Biochar Stability in a Poplar Plantation Soil

The stability of biochar is fundamental to its soil carbon (C) sequestration potential. The relative importance of chemical recalcitrance and the soil microbial community on biochar stability is still unclear. To unveil the question, we conducted a 60-day incubation to explore the stability of two rice-straw-derived biochars pyrolyzed at 300 and 500 °C (denoted as BS300 and BS500), as well as the relative contribution of the soil microbial community and biochar chemical recalcitrance to biochar stability in a poplar plantation soil. Biochar-derived cumulative carbon dioxide (CO2) emission was estimated to be 41.3 and 6.80 mg C kg−1, accounting for 0.73 and 0.11% of the amended biochar-derived organic C (OC) in BS300 and BS500 treatments, respectively. The mean retention time (MRT) estimated by double-exponential model fitting was 49.4 years for BS300 and 231 years for BS500. Compared to control, BS300 and BS500 decreased β-D-glucosidase activity by 20.9 and 18.0%, while they decreased phenol oxidase activity by 31.8 and 18.9%, respectively. Furthermore, BS300 increased the soil microbial metabolic quotient (qCO2) by 155%, but BS500 decreased it by 13.4%. In addition, BS300 resulted in a 520% higher biochar-derived hot-water-extractable OC than BS500. Partial least-squares path modeling (PLSPM) showed that the path efficients of biochar’s chemical recalcitrance and microbial qCO2 were 0.52 and 0.25, respectively, and that of the soil microbial activity is neglected. We conclude from this short-term study that chemical recalcitrance imposed a greater effect than soil microbial community on biochar stability.