Crop straw converted to biochar increases soil organic carbon but reduces available carbon

Biochar can potentially be used to sequester soil organic carbon (SOC). However, a comprehensive assessment of SOC and its fractions in response to biochar produced by crop straw is still lacking compared to straw return. Here, a global meta-analysis with 58 publications was conducted to quantify the impacts of straw biochar on SOC contents. The results showed that straw biochar (BC) addition increased SOC content by 49.4 % and 20.1 % compared to straw removal (S0) and straw return (ST), respectively. Random Forest model suggested that soil initial total N, mean annual precipitation (MAP), bulk density (BD), mean annual temperature (MAT), initial SOC, and biochar pyrolysis temperature were the critical factors affecting SOC contents under BC than that under S0 (P < 0.05). Compared to ST, experimental duration, soil initial total N, initial SOC, cropping system, soil pH, and land use were the main factors driving the response of SOC to BC (P < 0.05). Specifically, with significant variations among subgroups, the biochar-amended soil had higher relative changes in SOC content under experimental duration of 2–4 years (23.0 %), soil initial total N ≤ 0.9 g kg⁻¹ (28.0 %), initial SOC < 9 g kg⁻¹ (26.0 %), double cropping system (23.8 %), soil initial pH > 6.4 (22.6 %), paddy-upland (19.8 %) when compared to ST. Straw biochar had a higher microbial biomass carbon (MBC), humic acid carbon (HAC), and dissolved organic carbon (DOC) compared with S0. Whereas compared to ST, BC significantly decreased the concentrations of MBC, mineral-associated organic carbon (MAOC), fulvic acid carbon (FAC), and DOC, indicating that biochar produced by crop straw is not conductive to microbial utilization and growth. Overall, straw biochar application enhances SOC accumulation while it is difficult to be used by microorganisms. It is recommended that the co-application of crop straw and biochar from straw may benefit both SOC sequestration and the microbially mediated carbon cycle.