Variable effects of biochar on soil greenhouse gas emissions: A meta-analysis of climate, soil, and biochar property interactions.

Biochar application has been widely proposed as a climate-smart soil management strategy; however, related biochar studies result on soil greenhouse gas (GHG) emissions remain highly variable. Therefore, this study conducted a global meta-analysis to synthesize field-based evidence on the effects of biochar addition on soil carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) emissions, and to identify the key factors driving response heterogeneity under different environmental and management conditions. A total of 542 paired observations were compiled from published field experiments retrieved from Web of Science and the Chinese National Knowledge Infrastructure (CNKI). Standardized mean differences were calculated using mixed-effects models, and the relative importance of soil properties, climatic conditions, biochar characteristics, and management practices was further evaluated using multi-model inference and partial least squares path modeling. Overall, biochar application reduced soil N2O emissions by an average of 15.6%, whereas the responses of CO2 and CH4 were highly context-dependent. Soil properties exerted the strongest control on GHG responses, followed by management practices, climatic factors, and biochar characteristics. Under warmer climatic conditions (mean annual temperature >20 °C), biochar was associated with decreased N2O (-11.1%) and CH4 (-17.8%) emissions but increased CO2 emissions (+20.3%). PLS-PM further showed that soil exerted the strongest negative effect on overall GHG mitigation, whereas climate, biochar, and management had positive direct effects, although climate and management also generated negative indirect effects through soil. These results highlight that the mitigation potential of biochar is strongly regulated by site-specific conditions and management strategies. Although this synthesis is constrained by the predominance of short-term field studies and heterogeneous data reporting, it provides a quantitative basis for optimizing biochar application to mitigate soil GHG emissions across diverse agroecosystems.