Mechanical transplanting has become an important part of modern Chinese rice production, and an inadequate sowing rate severely inhibits rice seedling growth and development. Precision drill sowing is an effective method for obtaining higher quality seedlings during machine transplanting. There is a lack of systematic research on the precision drilling of rice. Therefore, we carried out research on the quality of machine-transplanted seedlings and precision drill sowing transplantation. A greenhouse experiment (Liaoning Rice Research Institute) and field experiment (Sujiatun District, Shenyang City, Liaoning Province, China) were conducted between 2020 and 2021 to analyze the influence of precision drill sowing on rice growth and yield. Precision drill sowing was conducted at four sowing rates (3400, 3600, 3800, and 4000 seeds/tray), and traditional broadcasting was also conducted at a sowing rate of 4000 seeds/tray. We evaluated the seedling rice quality, physiological and biochemical characteristics and transplanting quantity. The results indicated that precision drill sowing at a sowing rate of 3400 seeds/tray resulted in the highest plumpness value (0.18) and seedling strength index (0.42) of individual plants. However, the empty hill rate was as high as 3.05%, which did not satisfy the field seedling number requirement. Precision drill sowing at a sowing rate of 4000 seeds/tray resulted in the lowest physiological (the average levels of SOD, POD and soluble protein were 311.78 µg/g, 8.25 µg/g and 1.28 µg/g) and biochemical indices of individual plants. The damaged seedling rate increased by 2.07%, and the dead seedling rate increased by 0.25%, resulting in poor seedling and transplanting quality. In this study, 3800 seeds/tray was the best option and had the highest yields of 10,776.60 kg/ha and 10,730.85 kg/ha over the two years. This sowing approach performs well in terms of field transplanting, provides a balance point between seedling number and quality and is conducive to rice yield production. The results of this study are important for improving rice seedling quality, enhancing field transplanting quantity and increasing rice yield and food security.

Introduction: The rice–crab coculture system is ecologically sustainable with efficient resource utilization, but the soil nitrogen cycling mechanisms underlying yield limitations in different coculture models remain unclear. Here, we aimed to identify yield-limiting factors by comparing rice productivity between the conventional rice–crab coculture model (CK) and an optimized model (12 rows cultivated-1 row empty, ERC-12). We hypothesized that ERC-12 enhances crab activity in empty rows, thereby stimulating nifH-mediated soil nitrogen fixation to offset yield losses caused by reduced planting density.Methods: Field experiments were conducted in Panjin, Liaoning Province, during 2023–2024 using two japonica cultivars, Yanjing 939 (YJ939) and Yanfeng 47 (YF47). Plots were arranged in CK and ERC-12 patterns; the latter was spatially divided into boundary (PB), intermediate (PM), and central (PC) zones. Yield components, aboveground dry matter (ADM), and nitrogen (N) accumulation were measured. Soil NH₄⁺-N, NO₃⁻-N, and other nutrients were analyzed at tillering and heading stages. Expression of nitrogen-cycling genes (nifH, nirK, nirS, etc.) was quantified by qPCR.Results: Our results showed that although ERC-12 increased per-plant yield via marginal effects in the boundary zone (PB), total yield decreased by 4.06%-5.20% compared to CK, primarily due to yield losses in the intermediate zone (PM) and empty rows. Correlation analysis revealed that the PB zone in ERC-12 had significantly higher soil ammonium nitrogen (NH+ 4-N) content and elevated expression of the nitrogen-fixing gene nifH (p < 0.01), which promoted aboveground dry matter accumulation and yield—consistent with enhanced biological nitrogen fixation under crab activity. In contrast, the PM zone suffered from nutrient competition and reduced activity expression of key nitrogen-cycle genes such as nifH, nirK, and nirS, becoming a key yield-limiting factor.Disscussion: ERC-12 partially compensates for yield losses through elevating soil nifH expression, which enhances NH4 +-N supply in the PB zone. To further improve ERC-12 yield, targeted strategies should be applied to optimize rice population structure in the boundary zone, the intermediate zone, and the central zone (PC), alleviating nutrient limitations in the PM zone while maintaining the boundary yield advantage.