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This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Biochar addition is a promising solution to improve soil nitrogen (N) availability and enhance crop productivity. However, a comprehensive understanding of the impact of soil N dynamics on crop productivity under biochar addition remains elusive. Here, we conducted a meta-analysis from 93 peer-reviewed field experiments to synthesize the linkage of crop productivity to soil N dynamics under biochar addition. The results show that biochar addition significantly enhanced crop productivity by 12.8% and plant N uptake (PNU) by 22.6%, and there was a strongly positive relationship between crop productivity and PNU. Biochar addition also significantly increased the contents of soil total N (TN), inorganic N (IN), microbial biomass N (MBN), and biological N2 fixation (BNF) by 3.34–18.7%, but reduced nitrous oxide (N2O) emission by 15.9%. Further analysis with the aggregated boosted tree model indicated that the TN and BNF played the most important roles in biochar-induced change in crop productivity. However, while crop productivity was positively correlated with TN under biochar addition, it showed a negative correlation with BNF. These findings suggest that biochar addition could enhance crop growth and productivity through increasing the soil N resource and N uptake, but this was not true for some N2-fixing crops, probably because they were usually constrained by nutrients other than N.

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