Canopy nitrogen addition reshapes leaf physiological-metabolic processes in Populus euphratica seedlings with a water dependency

Compared to soil nitrogen (N) addition, canopy N addition offers a more ecologically relevant simulation of natural N deposition, and plays a pivotal role in regulating plant physiological metabolism and adaptive strategies. However, it is uncertain whether this critical process is water-dependent. This study systematically revealed the synergistic response mechanism of Populus euphratica seedlings to water treatment (drought stress vs. well-watered) and N addition method (canopy N addition vs. soil N addition). The results showed that: (1) The interactive effects significantly regulated soil nutrient characteristics, with N addition method exerting a pronounced positive effects on ammonium nitrogen (AN) and soil organic carbon (SOC) (P < 0.05); (2) At the leaf physiological level, canopy N addition under drought stress significantly decreased the contents of Proline (LPRO) and Starch (LST), along with declining trends in Malondialdehyde (MDA) levels, Peroxidase (POD) and Ascorbate peroxidase (APX) activities, indicating its capacity to alleviate drought stress; (3) Metabolomic analysis revealed that drought stress induced changes in various secondary metabolites, including Carbohydrates and carbohydrate conjugates (7.55 %), Amino acids, peptides, and analogues (5.03 %), Flavonoid glycosides (4.40 %), enriched in pathways such as Arachidonic acid metabolism and Flavonoid biosynthesis. Canopy N addition markedly expanded drought-responsive metabolites, predominantly influencing primary metabolism (amino acid metabolism); (4) Structural equation modeling identified that water treatment exerted stronger direct effects on differential metabolites (DMs) variations than N addition method, while the latter indirectly drove metabolic reprogramming through modulation of leaf nutrients and soil N availability. These findings unveil desert plants’ balanced strategies of physiological defense and resource acquisition through water-nitrogen coupling, establishing nutrient stewardship frameworks for maintaining arid ecosystem equilibrium under global climatic perturbations scenarios.