Nitrogen release from incorporated ¹⁵N-labelled Chinese milk vetch (Astragalus sinicus L.) residue and its dynamics in a double rice cropping system

Background and aims Chinese milk vetch (Astragalus sinicus L. CMV), a leguminous cover crop, has been shown to provide N benefits to rice crops, but little is known about the pathway of incorporated CMV and its N dynamics. In this study, effects of CMV under different application treatments (incorporated alone, applied in conjunction with urea fertilizer and applied with ryegrass (Lolium multiflorum Lam.)) on N dynamics, rice yields and N uptake were investigated and compared with those of chemical fertilizer (CF) and no fertilizer (NF) in a double rice cropping system. Methods Nitrogen release from incorporated CMV residue was quantified by using a bag method. Nitrogen dynamics of CMV were evaluated by using ¹⁵N-labelled fresh CMV tops and compared with those of CF (¹⁵N-labelled urea). Results CMV residue decomposition pattern and its N release pattern followed a single exponential decay model, with 87.8-89.5 % of the applied CMV decomposed and 95.1-96.1 % of the original N released in the double rice season (177 days after fertilizer application). CMV treatments had higher rice N uptake efficiency than CF (39.2-51.3 % vs. 29.9 %) at the sum of early and late rice seasons. Rice yield, N accumulation and mineral fertilizer efficiency in CMV treated treatments were higher than those in CF. After two consecutive rice seasons the amounts of residual N remained in the soil were higher in the CMV treated fields than in CF (29.4-33.2 % vs. 14.1 %). Conclusions CMV can be considered an efficient N source alternative to chemical fertilizer in double rice cropping systems.