Biochar’s effect on the soil carbon cycle: a rapid review and meta-analysis

Abstract Biochar offers opportunities for improving soil carbon (C) sequestration and reducing CO 2 emissions to the atmosphere. It has emerged as a strategy for mitigating climate change and improving the soil carbon cycle (SCC). While previous review studies have primarily investigated the effects of biochar on greenhouse gas (GHG) emissions, a considerable research gap remains regarding its impact on the SCC. The present study aims to bridge this gap by examining the main SCC components: total CO 2 flux, total microbial respiration, and C sequestration. We conducted a global meta-analysis which included 75 studies and 250 observations. The results show an average 11% increase in soil total CO 2 flux from biochar, but the confidence interval (CI) slightly touches the no-effect line (CI [0%, 23%]). Total microbial respiration remains unchanged after the application (10%, CI [− 2%, 23%]). In contrast, soil C sequestration benefits from biochar by 61% (CI [36%, 90%]). Our analysis identified key predictors affecting SCC components: experimental design, continent, biochar application rate, feedstock type, and pyrolysis temperature. Incubation experiments reveal benefits for all SCC components. The Middle East, Europe, and Asia exhibit potential for enhancing C sequestration with biochar. Higher application rates amplify C sequestration and total microbial respiration. Manure biochar enhances total microbial respiration, while woody biochar influences total CO 2 flux. Furthermore, lower pyrolysis temperatures show promise for improving C sequestration and total microbial respiration. In conclusion, while biochar holds promise for C sequestration, its impact on total microbial respiration and total CO 2 flux remains inconclusive.