Integrating legumes into cropping systems increases soil carbon and nitrogen content while it reduces the soil carbon-to-nitrogen ratio. A meta-analysis

Abstract

Integrating legumes into cropping systems offers a promising strategy to mitigate the decline in soil fertility caused by continuous agricultural intensification. However, existing studies have mainly examined individual soil carbon or nitrogen indicators, or have been conducted within limited contexts, leaving a comprehensive understanding of soil carbon and nitrogen responses at the global scale lacking. We therefore conducted a global meta-analysis of 1199 observations from 173 field experiments to evaluate integrating legumes into cropping systems impacts on soil carbon and nitrogen under diverse management practices and environmental conditions. Our results showed that integrating legumes into cropping systems significantly increased the soil organic carbon and total nitrogen content by 4.60% and 7.90%, respectively, while reducing the soil carbon-to-nitrogen ratio by 2.37%. Changes in soil organic carbon and total nitrogen due to legume integration were positively correlated with mean annual precipitation and mean annual temperature, but negatively correlated with initial soil organic carbon and initial soil carbon-to-nitrogen ratio. Integrating legumes as cover crops led to greater increases in soil organic carbon and total nitrogen, whereas the reduction in soil carbon-to-nitrogen ratio was more pronounced when legumes were used as main crops. Moreover, soil organic carbon and total nitrogen increased when legumes replaced fallow or gramineous crops, whereas soil carbon-to-nitrogen ratio significantly decreased only when legumes replaced gramineous crops. Furthermore, the benefits of integrating legumes into cropping systems on soil organic carbon and soil carbon-to-nitrogen ratio were amplified by greater crop diversity. Overall, this study provides the first comprehensive global synthesis of the effects of integrating legumes into cropping systems on soil carbon and nitrogen status, quantifies how environmental conditions and management practices modulate these effects, and provides practical guidance for the site-specific implementation of legume-based cropping systems in sustainable agriculture.