Suaeda salsa strategically utilizes nitrate and ammonium nitrogen for salt tolerance

Soil salinization is a major environmental constraint on plant growth. Although halophytes can grow well in saline soils, their nitrogen-use strategies under saline conditions remain poorly understood. In this study, the typical euhalophyte Suaeda salsa was used as the study species to investigate how different salinity levels and nitrogen forms (nitrate-N, ammonium-N, and a nitrogen-free control) regulate its growth, physiological traits, nitrogen metabolism, osmotic adjustment, oxidative stress, and metabolomic responses. The results showed that S. salsa adopted distinct strategies for utilizing nitrate and ammonium under salt stress. Nitrate nutrition more strongly promoted biomass accumulation, shoot height, and root elongation, while also supporting nitrate accumulation as an inorganic osmoticum. Under high salinity, NO₃^--N accounted for 62.11% of total nitrogen, and the osmotic contribution of nitrate reached -0.16 MPa. This nitrate-retention strategy was associated with enhanced antioxidant capacity, reduced oxidative damage, and the accumulation of metabolites related to jasmonate signaling, glutathione turnover, and phenylpropanoid metabolism. In contrast, ammonium nutrition favored tissue hydration, canopy width, and root thickening. More than 84% of total nitrogen was maintained in organic forms under NH₄⁺-N supply, indicating efficient ammonium assimilation and detoxification. Metabolomic profiling further showed that ammonium treatment promoted the accumulation of nitrogen-rich osmotic and buffering metabolites, including arginine, histidine, and ornithine, particularly under high salinity. Overall, S. salsa exhibits dual nitrogen-use strategies, and this physiological and metabolic plasticity may contribute to its ecological success in saline environments.