Effect Of Replacing Nitrogen Fertilizer With Chinese Milk Vetch On Greenhouse Gas Emissions And Nitrogen Use Efficiency In Rice Fields

Chinese milk vetch（Astragalus sinicus L,CMV）is a widely used leguminous green manure in rice cultivation and has the potential to replace or partly replace inorganic nitrogen（N）fertilizer.However,the impact on greenhouse gas emissions and nitrogen use efficiency of replacing inorganic nitrogen fertilizers with leguminous green manure is less well quantified.This study investigated the changes in N utilization efficiency and greenhouse gas emissions under 0%,25%,50%,75%,100%substitution of urea-N with milk vetch aboveground litter and no fertilization（NF）in a paddy soil.The main research results are as follows:（1）All treatments with N fertilizer had higher rice yields than those without N fertilizer（NF）.The 25%CMV treatment had higher rice yield among all cotain CMV treatments,and there was no significant difference between 25%CMV and 0%CMV treatments（P>0.05）.The 25%CMV treatment had higher dry matter accumulation and N accumulation,and there was no significant difference in dry matter accumulation and N accumulation between the 25%CMV and 0%CMV treatments（P>0.05）.The N accumulation of 25%CMV treatment was 6.70%higher than that of 0%CMV treatment.Meanwhile,25%CMV treatment had relatively higher N fertilizer apparent utilization efficiency,N partial productivity and N harvest index,and there were no significant differences in N fertilizer apparent utilization efficiency,N partial productivity and N harvest index between 25%CMV treatment and 0%CMV treatment（P>0.05）.Therefore,25%CMV could both secure rice yield and effectively improve N use efficiency in paddy fields.（2）CH₄ and N₂O fluxes were measured using a static chamber method and CMV decomposition rates were tested by mesh bag method.After incorporation,C and N release from CMV followed a single exponential decay model,with 92.79%-95.97%and 99.09%-99.32%of the original C and N released during 192 days.CH₄ flux was positively correlated with CMV 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 10.24%and 9.62%,respectively,while 100%substitution increased GWP and greenhouse gas intensity by 41.23%and 69.23%,respectively.At the same time,the equation between GWP and Chinese milk vetch replacement ratio showed that 28%of replacement ratio was the most effective in reducing greenhouse gas emissions from rice fields.The soil p H,NH₄⁺-N and redox state（Eh）were significantly correlated with CH₄ emission,while C and N remaining in the incorporated CMV residue and NH₄⁺-N were key factors regulating N₂O emission.It could be concluded that replacing 25%of urea-N with CMV was most effective in greenhouse gas mitigation while maintaining rice yield.In conclusion,25%of urea-N with CMV can maintain the rice yield,facilitate the accumulation of dry matter and nitrogen in rice plants,improve the N use efficiency,and mitigate GHG in rice fields,and the best effect of 28%replacement ratio to reduce GHG from rice fields.