Applying biochar to soil has been recognized as a promising practice of climate-smart agriculture, with considerable potential in enhancing soil organic carbon (SOC) sequestration. Previous studies showed that biochar-induced increases in SOC stock varied substantially among experiments, while the explanatory factors responsible for such variability are still not well assessed. Here, we conducted an integrative meta-analysis of the magnitude and efficiency of biochar-induced change in SOC stock, using a database including 476 field measurements at 101 sites across the globe. Biochar amendment increased SOC stock by 6.13 +/- 1.62 (95% confidence interval, CI) and 7.01 +/- 1.11 (95% CI) Mg C ha(-1), respectively, compared to their unfertilized (R-0) and mineral nitrogen (N) fertilized (Rn) references. Of which approx. 52% (R-0) and 50% (R-n) were contributed directly by biochar-C input. Corresponding biochar carbon efficiencies in R0 and Rn datasets were estimated as 58.20 +/- 10.37% and 65.58 +/- 9.26% (95% CI), respectively. The change magnitude of SOC stock increased significantly (p < 0.01) with the increasing amount of biochar-C input, while carbon efficiency of biochar showed an opposite trend. Biochar amendment sequestered larger amounts of SOC with higher efficiency in acidic and loamy soils than in alkaline and sandy soils. Biochar amendments with higher C/N ratio caused higher SOC increase than those with lower C/N ratio. Random forest (RF) algorithm showed that accumulative biochar-C input, soil pH, and biochar C/N ratio were the three most-important factors regulating the SOC stock responses. Overall, these results suggest that applying high C/N ratio biochar in acidic soils is a recommendable agricultural practice from the perspective of enhancing organic carbon.
