Straw Return and Controlled-Release Fertilizers Improve Rice Yield by Alleviating Soil Salinity and Optimizing Nitrogen Uptake in Brackish Water-Irrigated Coastal Saline Soils

Brackish water irrigation induces secondary soil salinization and severe nitrogen (N) leaching, restricting crop N uptake and grain yield. To address these constraints in coastal saline farmlands, a 140-day field soil column experiment was conducted using coastal saline soil with a salinity of 2.60 g kg−1. We evaluated the individual and interactive effects of three straw return methods (straw removal (S1), straw incorporation (S2), and straw burial (S3)) and three N fertilizer managements (sole conventional urea (N1), 1:1 mixture of polyurethane-coated urea (PCU) and urea (N2), and sole PCU (N3)) on salt dynamics, rice agronomic traits, root morphological characteristics, N use efficiency (NUE), and yield components. The results indicated no significant interactive effects between straw return methods and N fertilizer managements on the measured variables (p > 0.05). S2 significantly decreased soil salinity and exerted the highest efficiency with regard to salt leaching, thereby promoting rice growth (p < 0.05). PCU markedly optimized root development, as evidenced by increased root tip number, branch number, and root crossing density (p < 0.05), which strengthened water and nutrient uptake, ultimately mitigating detrimental impacts of brackish water irrigation on grain yield and NUE. Notably, PCU application ratios showed no significant differences in crop yield (p > 0.05). Overall, straw incorporation combined with a 1:1 ratio of PCU and urea is verified as the optimal strategy for rice cultivation in brackish water-irrigated coastal saline regions. This practice effectively alleviates brackish water-induced salt stress and elevates crop yield and NUE. The outcomes provide solid scientific references and practical guidance for coordinated water–salt–nutrient management and sustainable utilization of fragile coastal saline soils.