Green manure mixed sowing combined with reduced nitrogen application can improve yield formation and soil nutrient characteristics in double-cropping rice systems

To explore a green and efficient double-cropping rice cultivation model that ensures stable yield and enhanced soil fertility, a mixed sowing of Chinese milk vetch and rapeseed was used as the basis, with five nitrogen application levels established: no nitrogen application (CK, N0MR), conventional nitrogen application (150 kg·hm^-2, N1MR), 20% nitrogen reduction (120 kg·hm^-2, N2MR), 40% nitrogen reduction (90 kg·hm^-2, N3MR), and 60% nitrogen reduction (60 kg·hm^-2, N4MR). A study was conducted under green manure mixed sowing conditions to investigate the effects of different nitrogen reduction levels on the yield, dry matter accumulation and distribution, and soil nutrient status of double-cropping rice. The results showed that moderate nitrogen reduction, especially the 20% reduction treatment (N2MR), resulted in higher yields and better yield components during both early and late rice growing seasons. The total grain yield under N2MR reached 14.51 t·hm^-2, which was 22.5% higher than the control. Analysis of dry matter accumulation indicated that N2MR significantly increased accumulation from tillering to panicle initiation and from heading to maturity stages, and notably enhanced dry matter translocation to the panicles. At maturity, the proportion of dry matter allocated to the panicles reached 56.14% for early rice and 64.63% for late rice, suggesting that moderate nitrogen reduction promotes the allocation of assimilates to reproductive organs, thereby increasing yield. Soil nutrient monitoring revealed that N2MR and N3MR treatments had better performance in terms of organic matter, available nitrogen, and ammonium nitrogen accumulation. Correlation analysis showed that grain yield was significantly or highly significantly positively correlated with soil organic matter, total nitrogen, available phosphorus, effective panicle number, and total biomass, while it was significantly negatively correlated with soil pH. In conclusion, under the basis of green manure mixed sowing, a 20% nitrogen reduction (N2MR) can simultaneously enhance yield and improve soil fertility, making it a sustainable and high-efficiency nitrogen management strategy for double-cropping rice systems.