Effects of reduced nitrogen application combined with organic fertilizer on yield formation and nitrogen utilization in mechanically transplanted rice

To investigate the effects of reduced nitrogen application combined with organic fertilizer on yield formation and related traits of machine-transplanted hybrid rice, this study used the indica hybrid rice F You 498 as the experimental material and employed a two-factor split-plot design. The main plots included two fertilization methods: no organic fertilizer (Mo) and organic fertilizer application (Mc, 1500 kg hm-2). The subplots consisted of nitrogen reduction treatments. Based on a conventional chemical nitrogen application rate of 180 kg hm-2 (N3, used as the control), four nitrogen levels were established: 25% reduction (N2), 50% reduction (N1), and no nitrogen application (N0). Yield, photosynthetic characteristics, dry matter accumulation, nitrogen transport, and nitrogen use efficiency were measured to clarify the synergistic mechanisms of reduced nitrogen application combined with organic fertilizer. The results showed that compared with Mo, the Mc treatment increased the photosynthetic leaf area index and photosynthetic potential, slowed chlorophyll degradation, effectively delayed leaf senescence, and enhanced photosynthetic performance. A moderate reduction in nitrogen combined with organic fertilizer optimized population structure, significantly increasing the number of effective panicles, grains per panicle, and 1000-grain weight. The treatment with 25% nitrogen reduction under organic fertilizer application (McN2) achieved the highest yield, with a two-year average increase of 6.04% compared with the treatment using chemical fertilizer alone (MoN3). Under the combined treatment, dry matter and nitrogen accumulation at maturity followed the order: N2 > N3 > N1 > N0. Additionally, compared with MoN3, McN2 significantly improved nitrogen dry matter production efficiency, apparent nitrogen recovery efficiency, physiological nitrogen use efficiency, agronomic nitrogen use efficiency, and partial factor productivity of nitrogen. These results suggest that applying organic fertilizer in combination with a 25% nitrogen reduction can enhance the targeted allocation of photosynthates to grains by synergistically optimizing the accumulation and distribution of assimilates and improving nitrogen transport efficiency, thereby establishing a population characterized by a “large source and sufficient sink”. This strategy achieves a balance between high yield and high nitrogen use efficiency in machine-transplanted rice and provides theoretical support for green, high-yield cultivation practices.