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

Biochar offers opportunities for improving soil carbon (C) sequestration and reducing CO2 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 CO2 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 CO2 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 CO2 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 CO2 flux remains inconclusive. Meta-analyses revealed the impact of biochar on three key elements (C sequestration, total CO2 flux, and total microbial respiration) of the soil carbon cycle.Biochar increases soil carbon sequestration significantly.The effect of biochar wasn't significant for total microbial respiration and total CO2 flux responses.