Effects of straw biochar on microbial-derived carbon: A global meta-analysis

Pyrolyzing biomass (e.g., crop straw) to produce biochar is a sustainable strategy in agricultural farmlands. Straw-derived biochar could increase soil organic carbon (SOC) and microbial-derived carbon (C) compared to no addition, while it is imperative to understand the effects of straw-derived biochar compared to its feedstock (e.g., straw). We retrieved 321 and 387 observations to investigate the effects of straw-derived biochar on microbial-derived C (e.g., microbial biomass C (MBC) and microbial necromass C (MNC)) taking no addition and straw as control, respectively. Notably, straw-derived biochar significantly increased dissolved organic C (DOC) by 24.9% and provided available substrates for microbial utilization, thus improving MBC by 16.7% and MNC by 19.7% compared to no addition. Nevertheless, compared to its feedstock (crop straw), straw-derived biochar significantly decreased MBC by 26.1% and MNC by 18.0% attributed to lower DOC, supported by a positive correlation between MBC and DOC (R² = 0.53). A negative correlation between changes in MBC and SOC indicated the adverse of microbial activity for C accrual under conversion from straw to biochar. Moreover, soil layer, experiment duration, and initial C/N ratio are the crucial factors affecting MBC under the conversion from straw to biochar. Specifically, with significant variations among subgroups, when compared to straw addition, straw-derived biochar had lower reduction in MBC observed on 0-5 cm layers, mean annual precipitation ≥550 mm, mean annual temperature ≥10 °C, clay loam soil, experiment duration≥1 yr, initial SOC≥14 g kg^-1, pH≥8, and bulk density ≥1.28 g cm^-3. Straw-derived biochar even increased MBC by 32.8% in an anaerobic environment, associated with biochar produced under limited oxygen and anaerobic microorganisms dominating the microbial community. This study concludes that the conversion from crop straw to biochar increases SOC but constrains microbial-derived C, which may disturb the microbial-mediated C-cycling process.