Long-term green manuring reduces net greenhouse gas emissions in upland cropping systems in China

Abstract

Green manuring enhances multiple agroecosystem services functions, yet its impact on net greenhouse gas mitigation remains controversial, primarily due to a limited number of long-term experiments. To address these challenges, this study investigated the long-term effects of green manure (GM) rotations on 100-cm-depth soil organic carbon (SOC) sequestration, N₂O emissions, and crop yields based on eight long-term experimental sites (7–16 years) in China's upland cropping systems combined with process-based modeling. The results demonstrated that green manuring significantly increased SOC concentration by 8.8 %–14.4 % (p < 0.001) across 0–100 cm soil profiles compared to fallow system, with annual SOC sequestration rates reaching 0.95–1.16 Mg C ha⁻¹ yr⁻¹ (p < 0.001). Notably, topsoil layer (0–40 cm) contributed 67.5 % of total profile SOC accumulation. Green manuring can replace approximately 40 % of synthetic fertilizers of N while maintaining long-term yield stability, though with potential trade-offs in elevated N₂O emissions. The optimal net global warming potential (NGWP) reached −16.47 Mg CO₂-eq ha⁻¹ yr⁻¹ under GM-based system with 30 % reduction in fertilizer N. Meanwhile, under the condition that GM substitution for fertilizer N achieved no yield reduction, the greenhouse gas intensity (GHGI) was optimized within the substitution rate range of 20 %–40 %. The results from process-based modeling demonstrate that substituting 30 % of N fertilizer with GM achieves optimal soil C sequestration while maintaining stable crop yields. These findings provide direct evidence that GM rotation increases C sequestration, addressing previous knowledge gaps in understanding the C sequestration and emission reduction effects of GM-based rotation.