Soil water regimes and nitrogen fertilization regulate maize leaf physiology, instantaneous water use efficiency, and yield formation in Northeast China

Soil water regimes (SWR) deficit remains a primary constraint to maize ( Zea mays L.) productivity in cold semi-arid regions, whereas excessive nitrogen fertilizer (N) inputs pose significant environmental risks. Addressing this, we hypothesized that optimizing the soil water–nitrogen nexus would enhance photosynthetic carbon gain and metabolic efficiency. Here, we present a three-year (2020–2022) field trial conducted in Heilongjiang Province, China, testing three soil water potentials (40%, 60%, and 80% of field capacity) across a gradient of N fertilization (0, 100, 200, 300 kg N ha –1 ). Key parameters measured included photosynthetic gas exchange parameters (Pn, Ci, Gs, Tr), chlorophyll fluorescence indices (Fv/Fm, ΦPSII, NPQ, qP), leaf area index (LAI), activities of phosphoenolpyruvate carboxylase (PEPC) and sucrose phosphate synthase (SPS), Na⁺-K⁺-ATPase and Ca 2+ -Mg 2+ -ATPase, soil total nitrogen (STN), dry matter translocation (DMT), and harvest index (HI). We demonstrate that maintaining SWR at 80% under moderate N application (200 kg N ha –1 ; W3N200) maximized the whole-plant carbon budget. W3N200 enhanced Pn by 226.97%, DMT by 61.31%, and intrinsic water use efficiency by 57.24% relative to unfertilized controls, coincident with a 24.44% increase in harvest index (HI). Mechanistically, W3N200 sustained peak activity of carbon-metabolizing enzymes (PEPC and SPS) and preserved cellular bioenergetics via the upregulation of Na + -K + -ATPase and Ca 2+ -Mg 2+ -ATPase. This enzymatic coordination facilitated superior partitioning of photo assimilates and dry matter remobilization. Crucially, structural equation modeling revealed that SWR was the primary determinant of carbon assimilation and HI, exerting a stronger direct effect than N status, while ATPase-mediated ion homeostasis acted as a pivotal intermediary linking the edaphic environment to yield formation. We conclude that SWR defines the upper limit for photosynthetic capacity, with N application serving to amplify these processes. The W3N200 regime offers a robust, evidence-based strategy for sustainable maize intensification in water-limited, cold-climate agroecosystems.