Green manures and crop residues for soil fertility restoration in paddy-based cropping systems of monsoon Asia: balancing carbon sequestration and greenhouse gas emissions

Paddy-based cropping systems are the foundation of crop production in monsoon Asia, but their long-term sustainability is increasingly compromised by soil organic carbon depletion and declining soil nitrogen fertility, particularly where paddy fields are converted to upland use for wheat, soybean, and other crops. Green manures have long been used as on-farm sources of organic matter and biologically fixed nitrogen, but their role is being re-evaluated in the context of reduced fertilizer dependence, soil fertility restoration, and climate change mitigation. This review summarizes studies since the 1970s on the agronomic characteristics and utilization of green manure crops in paddy-based systems of monsoon Asia, with particular attention to Chinese milk vetch, hairy vetch, Sesbania, Crotalaria, green manure soybean, and Azolla. The integrated use of green manures with rice straw and other crop residues is examined as a strategy for maintaining soil organic carbon, enhancing nitrogen self-support capacity, and improving the resilience of rice-upland rotations. Finally, greenhouse gas trade-offs associated with organic matter incorporation, especially methane emissions under flooded conditions and nitrous oxide emissions under aerobic or intermittently aerobic conditions, are discussed. Although low-C/N leguminous green manures can partially replace synthetic nitrogen fertilizers and synchronize nitrogen release with crop demand, high-C/N residues such as rice straw are essential for long-term carbon maintenance. Future paddy-based cropping systems should therefore integrate green manures and crop residues in region-specific ways that balance soil fertility restoration, crop productivity, carbon sequestration, and greenhouse gas mitigation.