Improved rice yield and macronutrient uptake through the optimization of organic fertilizer rates in the rice-crayfish co-culture system.

The rice-crayfish coculture system (RC) is an integrated planting-breeding model that balances ecological and economic benefits. The combined application of organic and chemical fertilizers is effective for crop production, but the optimal organic fertilizer rate in RC remains unclear. In 2024 and 2025, five organic fertilizer rates (0, 1250, 2500, 3750, and 5000 kg ha-1, coded as M0-M4) were tested, with chemical fertilizer inputs maintained at 130 kg N, 90 kg P2O5, and 120 kg K2O ha-1. Rice yield, soil available N (nitrogen), P (phosphorus) and K (potassium), plant NPK accumulation, and root traits were measured at key growth stages. Results showed that all organic fertilizer treatments increased yield and NPK uptake. M2 (2500 kg ha-1) performed best, achieving a two-year average grain yield of 8009 kg ha-1 (56.5% higher than M0), with agronomic N efficiency (AEN) of 22.25 kg kg-1 and partial factor productivity of N (NPFP) of 47.82 kg kg-1. At tillering, M2 increased soil available NPK by 60-70% and significantly enhanced root length, surface area, and vitality. In contrast, M4 (5000 kg ha-1) reduced NPFP and AEN and decreased root diameter. Structural equation modeling showed that aboveground N uptake was significantly associated with yield components. The integrated framework combined yield, nutrient-use efficiency, soil nutrient supply, root traits, plant NPK uptake, and nutrient balance to identify the agronomically optimal rate and clarify their relationships with yield formation. Therefore, 2500 kg ha-1 organic fertilizer is recommended as an agronomically optimal rate for high rice yield and nutrient-use efficiency while avoiding excessive nutrient accumulation in RC. In the Jianghan Plain, 2500 kg ha-1 organic fertilizer is recommended as basal fertilizer, combined with 130 kg N, 90 kg P2O5, and 120 kg K2O ha-1, with 70% of N applied basally and 30% at tillering.