Objectives The decompostion and nutrient release of Chinese milk vetch (CMV) and rice straw are complementary when jointly incorporated into soil. Thus, the effect of nitrogen application rates on rice yield, nitrogen-use efficiency, and soil organic nitrogen under incorporation of CMV and rice straw was studied.

Methods A two-factor complete split field trial was conducted for two years in Jianghan plain. The two factors were: 1) combined incorporation of CMV and rice straw (SM) or incorporation of rice straw alone (S); 2) N application rate 50% (N50), 100% (N100), and 150% (N150) of the conventional N input (165 kg/hm²), and no N input (N0). The grain yield and biomass were weighed, grain N concentration, plant N concentration, and inorganic and organic N concentrations in surface soil were analyzed

Results The rice yield of SMN50 treatment was 21%‒23% higher than that of SN50 treatment, but similar to those of SMN100 and SMN150. The grain N uptake in SMN0, SMN50, and SMN100 treatments were significantly higher than those recorded in SN0, SN50, and SN100 treatments, with increase rate of 65%, 27%, and 22%, respectively. The soil total N and non-acid-hydrolysable N concentrations after rice harvest were similar for all treatments, but the acid-hydrolysable N contents varied significantly. Among the four forms of the acid-hydrolysable N, the unidentified N increased significantly in SN150 treatment than that in other treatments.

Conclusions Compared with rice straw returning alone, the co-incorporation of rice straw and CMV increased rice yield and grain N uptake under 50% reduction of conventional N application rate, while conventional rate or 50% increase in N input did not. The co-incorporation of CMV and rice straw could improve the availability of soil N and significantly decrease the residual acid-hydrolysable unidentified N content under high N input.