Objectives Milk vetch is an important winter green manure crop in the southern rice region of China, which can fix carbon, improve soil physical properties and increase soil nutrient contents. It could also provide a suitable environment for the succeeding rice crop. Characterization of nitrogen (N) uptake and use in rice could provide the basis for optimal N fertilizer application and for significant improvement in N nutrition for high-yield cultivation.

Methods Completely field experiments of 4 × 4 two-factor were performed continuously for four years in four different sites under milk vetch-double rice cropping system. The returning proportion of the whole harvested milk vetch was set at 0, 30%, 60% and 100%; in each returning level, 0, 30%, 60% and 100% of the conventional N fertilizer application rate (N 150 kg/hm²)in this region were set up to form 16 treatments in total and triplicate with each treatment. The N uptake in the tillering, heading, filling and maturity periods of rice was analyzed and N fertilizer use efficiency was calculated.

Results In the tillering, filling and maturity periods, the highest dry matter accumulation was all from the treatment of M_60% + N_60%, and the lowest from M₀N₀, except that in the heading period the highest one was from M_30% + N₀. With the growth elongation, the difference among the treatments in dry matter accumulation became large. At maturity, the highest population dry matter, weighed 13.14 t/hm², was in the 60%M + 60%N, which was 53.15% higher than that in control. Also during these periods, the difference in dry matter accumulation of rice under different nitrogen application levels and winter green manure system was the greatest. Dry matter accumulation of M_30% + N₀ was 79.02% and 45.04% higher than that of M₀N₀ and the average of all the other treatments, respectively. The ratio of dry matter accumulation to total dry matter in the heading to filling period ranged from 23.19%–44.23%. N content in rice plants was maximized in the booting, heading and maturity periods in treatment of M_60% + N_60%, which was 20.71%, 14.84% and 15.44% higher than the average of other treatments, respectively. Correspondingly, the highest N uptake was in treatment of M_60% + N_60% at tillering, booting, heading and maturity, and the lowest in control, N application and green manure showed significant synergy in N contents. Periodic N accumulation and its ratio in total N accumulation in treatment M_60% + N_60% were significantly higher than all other treatments in the before tillering, booting-heading and heading-maturity periods. In the tillering-booting period, the periodic N accumulation was 51.87% higher in the treatment M_100% + N_100% than the average of other treatments.

Conclusions Overall, the 60% of vetch plowed plus 60% of normal N application is effective in increasing N use efficiency and N recycling in the "milk vetch-double rice" system. Therefore, the combination should be recommended for achieving effective nitrogen uptake and utilization.