Objectives Utilizing green manure is an important method for reducing chemical fertilizer application. In green manure-rice cropping system, the characteristics of soil nitrogen (N) supply and rice crop N uptake are still unclear.

Methods The field experiment was conducted in Jiangxi double-cropping rice area. Under the condition that the amount of N, P and K fertilizers for early rice was reduced by 20%, we set up winter planting with fresh milk vetch incorporation at 15000 (G₁F₈₀), 22500 (G_1.5F₈₀), 30000 (G₂F₈₀), and 37500 (G_2.5F₈₀) kg/hm², as well as winter fallow and rice with no chemical fertilization as control (CK), winter milk vetch and rice with no chemical fertilization (GM), and winter fallow with conventional fertilization (F₁₀₀). Rice yield, plant N uptake, N fertilizer use efficiency, and soil inorganic N content during rice growing stages were assessed, while the relationships between soil properties, rice yield, and plant N uptake were analyzed.

Results Compared with F₁₀₀, G₁F₈₀ treatment significantly increased the yield of early rice by 11.64%; G₂F₈₀ treatment significantly increased the yield of late rice by 7.81%; G₁F₈₀, G_1.5F₈₀ and G₂F₈₀ treatments significantly increased the total yield of double rice by 5.79%, 5.38% and 7.17%. Compared with F₁₀₀, other treatments with different incorporation amounts of milk vetch did not reduce the rice yield. Utilization of milk vetch combined with 80% chemical fertilizer significantly increased the N uptake of early rice, the N fertilizer use efficiency of early rice and the partial factor productivity of early rice, and increased soil total N and organic matter at the harvest stages of early rice and late rice. In the booting stage of early rice, harvest stage of early rice and harvest stage of late rice, the soil ammonium N content of G_2.5F₈₀ treatment was significantly higher than that of F₁₀₀, and was the highest among all treatments. From the booting to harvest stage of early rice, the N accumulation rate of each green manuring treatments was positive, indicating that the N uptake of rice plants increased. However, the N accumulation rate of F₁₀₀ was negative, indicating that the N uptake of rice decreased. During the growth period of rice, when the incorporation amount of milk vetch was less than 22500 kg/hm², the N uptake of rice plants increased with the increase of incorporation amount, and the amount of N uptake was significantly reduced when the incorporation amount was greater than 22500 kg/hm². Soil available potassium content had the largest contribution rate to rice yield and N uptake. The contribution rate of soil available K to early rice yield and N uptake by early rice was 35.17% and 40.16%, and to late rice yield and N uptake was 21.22% and 25.22%, and to double-cropping rice yield and N uptake was 34.83% and 27.86%, respectively.

Conclusions Under the condition of 20% reduction of fertilizer, planting and incorporating appropriate amount of milk vetch can improve the N uptake of early rice and increase rice yield. Incorporating a high amount of milk vetch is beneficial to soil carbon and N pools and improve soil N supply capacity. Based on various analyses, incorporation of milk vetch at 30000 kg/hm² in the double rice area of Jiangxi Province was the best.