Abstract:To provide a theoretical and practical basis for establishing a water-nitrogen coupling model suitable for rice growth under straw return in the Chengdu Plain, the relationship between water management, straw return, N application, and rice root morphogenesis was studied through field experiments. The effects of water-nitrogen coupling on the rhizosphere environment and root development of hybrid indica rice ( Oryza sativa) ‘F You 498’ under wheat straw return conditions were investigated with the application of two water treatments, three N treatments, and two straw return treatments. The two water treatments were alternate dry-wet irrigation (W1) and submerged irrigation (W2); the straw return treatments were full-burying and return of wheat straw (S1) and no straw return (S0); and the N treatments involved the application of 150 kg∙hm ⁻²of N applied at ratios of basal∶tillering∶panicle fertilizers of 3∶3∶4 (N1), 7∶3∶0 (N2), and 0 (N0). The results showed that compared with W2, W1 promoted straw decomposition before the jointing stage, increased the total organic acid content of rhizosphere soil after the jointing period (1.38%–8.49%) and microbial biomass C content of rhizosphere soil before maturity (0.25%–12.93%), increased root activity during the entire growth period, and increased root morphological indices of the rice population, except at 10 days after transplanting and at the mature stage (18.53%–75.83% of root length, 10.57%–101.33% of root number, 2.49%–88.24% of root volume, and 8.91%−68.08% of root surface area). Compared with W1, W2 increase the total organic acids (3.34%) and microbial biomass C content (2.69%–6.23%) at the mature stage, and the decomposition of straw and the root morphological indices of single stems (12.03%–27.21% of root length and 9.05%–51.44% of root number). Compared with S0, S1 treatment inhibited root morphology development and reduced root activity (2.47%–45.83%), but increased the total organic acid content (8.02%–22.74%) and microbial biomass C and N levels of rhizosphere soil (1.58–31.22%), and the effect of improvement was highly noticeable under W1. The application of N fertilizer promoted the decomposition of straw, increased the total organic acids and microbial biomass C and N contents of rhizosphere soil, promoted root development, and increased root activity. Compared with N0, the optimal N application mode (N1) promoted the root development of single stems (8.27%–38.09% of root length and 2.96%–36.66% of root number) and promoted and maintained the root activity (2.26%–156.35%) in the middle and late growth stages; whereas the conventional N application mode (N2) increased the population root index (12.68%–44.32% of root volume and 4.91%–55.82% of root surface area) and root activity only in the early growth stage (22.01%–29.31%). W1 irrigation coupled with optimized N application promoted straw decomposition, significantly accelerated root growth and development, increased total organic acid content, microbial biomass C and N contents, and root activity in rhizosphere soil, and delayed root senescence. Therefore, under the condition of straw return, the optimized N application mode with W1 is the most suitable agriculture model.