Warming increases CO2 emissions in biochar-amended cropland soil

Biochar application is recognized as a promising strategy for enhancing soil carbon (C) sequestration, yet its influence on carbon dioxide (CO2) emissions under a warming climate remains inadequately understood. We conducted a global meta-analysis of 2079 paired observations from 32 peer-reviewed publications to quantify the responses of CO2 emissions in biochar-amended soils to warming and to identify their key drivers. Overall, warming increased CO2 emissions in biochar-amended soils by 77% on average, with a 117.5% increase in croplands and a 30.9% increase in forests. The effect was strongest for woody biochars, intermediate for crop-derived biochars, and weakest for grass-derived biochars. Increases in CO2 emissions were positively associated with warming magnitude, biochar characteristics, and soil properties. Warming magnitude was the dominant driver, followed by biochar application rate, soil C/N ratio, and biochar C/N ratio. The relative importance of these secondary drivers varies across cropland and forest ecosystems. These findings indicate that the climate mitigation potential of biochar may be overestimated if warming-induced C losses are ignored in biochar-amended cropland soil. To enhance the resilience of biochar applications, context-specific strategies should prioritize non-woody feedstocks, lower pyrolysis temperatures, and moderate application rates, particularly in vulnerable cropland systems. It is necessary to integrate the warming effects into biochar life-cycle assessments and soil C management frameworks to ensure realistic projections of its role in climate mitigation and carbon sequestration, particularly in cropland ecosystems under warming conditions.