Replacing urea-N with Chinese milk vetch (Astragalus sinicus L.) mitigates CH4 and N2O emissions in rice paddy

Chinese milk vetch (Astragalus sinicus L) is a widely used leguminous green manure in paddy rice culture and has potential to replace or partly replace inorganic nitrogen (N) fertilizer. However, there is little quantitative information regarding replacing inorganic N with milk vetch on CH4 and N2O emissions. This study investigated the relationship between greenhouse gas production and carbon (C) and N release under 0%, 25%, 50%, 75% and 100% substitution of urea-N with milk vetch aboveground litter in a paddy soil. CH4 and N2O emission fluxes were measured using a static chamber method and milk vetch decomposition rates were tested by mesh bag method. After incorporation, C and N release from milk vetch followed a single exponential decay model, with 82.3–89.1% and 98.8–98.9% of the original C and N released during 125 days. CH4 flux was positively correlated with milk vetch substitution ratio, while it was negative correlated with N2O flux. Compared with 0% substitution, 25% substitution decreased global warming potential (GWP) and greenhouse gas intensity (GWP per unit of grain yield) by 12.1% and 10.3%, respectively, while 100% substitution increased GWP and greenhouse gas intensity by 38.4% and 65.2%, respectively. The results showed that soil pH, NH4+-N and redox state were significantly correlated with CH4 emission, while C and N remaining in the incorporated milk vetch residue and NH4+-N were key factors regulating N2O emission. It could be concluded that replacing 25% of urea-N with milk vetch was most effective in greenhouse gas mitigation while maintaining rice yield.