Coupling of straw returning and nitrogen-water integration on rice nitrogen recovery and soil carbon pool

The traditional rice fertilization method of spreading increases yield at the cost of reducing nitrogen uptake efficiency and accelerating soil organic carbon loss. Given the rapid changes in rice production within Southwest China's crop rotation system, integrating short-term straw return with optimized water-nitrogen management could improve resource utilization and promote green agriculture. We examined the interactions between straw return and water-nitrogen coupling by analyzing soil carbon pools and yield formation. The results revealed that straw return and integrated water-nitrogen management regulated the soil-crop system synergistically. Straw return improved total organic carbon, dissolved organic carbon, and carbon pool indices, fostering a soil environment that supports the dynamic carbon balance. The optimized carbon pool structure established the foundation for integrated water-nitrogen management. With straw return, water-fertilizer coupling with 20% nitrogen reduction (1) promoted the targeted transformation of active organic carbon components and increased the carbon pool activity index; (2) enhanced the accumulation of post-flowering assimilates and promoted seed filling; (3) constructed a new nitrogen utilization pathway and increased the agronomic and physiological nitrogen use efficiency by 22.22%-22.44% and 3.11%-4.63%, respectively. Finally, the yield under this treatment reached the same as that under conventional nitrogen application. • Straw-returning (SR) enhances carbon pool indexes, improving the dynamic characteristics of soil carbon. • Integrated WNM achieves optimized yields via refined water-fertilizer management with 20% less N. • The coupling of SR and IWNM promotes grain filling through the later nutrient release. • The combination boosts post-flowering assimilates, enhancing yield components.