Controlled-release fertilizer substitution boosts maize yield and water-nitrogen use efficiency via optimizing source-sink characteristics in saline-alkali soil

High-yield, resource-efficient maize production in saline-alkali soils is crucial for regional food security. However, how controlled-release fertilizer (CRF) substitution for conventional nitrogen fertilizer promotes maize yield and water–nitrogen use efficiency through source–sink regulation remains unclear, and the optimal CRF substitution ratio is yet to be determined. To determine the optimal nitrogen fertilization strategy for maize production in saline-alkali soils, a two-year field trial (2024–2025) was carried out in a saline-alkali maize field in the Ningxia Yellow River Irrigation Zone, China, with six treatments: no-N control (CK), conventional urea only (T0), and 25%, 50%, 75%, and 100% CRF substitution for urea N (T25, T50, T75, T100). Compared with T0, T50 significantly increased leaf SPAD values (11.65%), net photosynthetic rate (18.43%), and key nitrogen metabolism enzyme activities. Nitrate reductase (NR), glutamate dehydrogenase (GDH), glutamine synthetase (GS), and glutamate synthase (GOGAT) activities increased by 64.95%, 36.63%, 22.07%, and 60.41%, respectively. T50 also increased dry matter translocation rate (76.06%) and grain-filling rate, while advancing the time to maximum grain-filling rate by 9 days. T50 significantly increased grain yield (18.06%), agronomic efficiency of nitrogen (AEN, 72.79%), nitrogen recovery efficiency (NRE, 68.39%), nitrogen use efficiency (REN, 40.40%), and irrigation water productivity (IWP, 18.06%). Structural equation modeling showed that replacing conventional nitrogen fertilizer with controlled-release fertilizer positively regulated source–flow–sink coordination, while enhanced sink activity and assimilate translocation promoted grain yield and water–nitrogen use efficiency. Random forest analysis identified dry matter translocation rate, stomatal conductance, and glutamate synthase activity as key factors associated with maize yield in saline-alkali soils. Overall, 50% CRF substitution optimizes source–flow–sink coordination to achieve high yield and efficient water–N utilization in saline-alkali maize. These findings advance source–flow–sink regulation theory under optimized N management and support efficient large-scale maize production in saline-alkali regions.