Abstract: Nitrogen (N) is one of the primary limiting nutrients in wetland ecosystems. Increasing atmospheric N deposition and intensified wetland reclamation have led to substantial exogenous N inputs, altering wetland biogeochemical cycling. Here, we conducted a 13-year in situ N-addition experiment in the Sanjiang Plain with four treatments: CK (0 g N·m^-2·a^-1), N6 (6 g N·m^-2·a^-1), N12 (12 g N·m^-2·a^-1), and N24 (24 g N·m^-2·a^-1). We investigated the long-term effects of N enrichment on plant traits and soil nutrient stoichiometry by measuring leaf chlorophyll content, plant height, and aboveground biomass of the dominant plant species Glyceria spiculosa, as well as C∶N∶P stoichiometry in plant organs (leaves, stems) and soils (0-15 and 15-30 cm). Results showed that N enrichment significantly increased leaf chlorophyll content, plant height, and aboveground biomass. Leaf and stem N concentrations increased markedly under enrichment, reaching 3.93 times higher in N24 treatment than that in the control, accompanied by a pronounced decline in plant C∶N ratios. However, phosphorus (P) content remained largely unchanged. In the topsoil (0-15 cm), total N content increased and C∶N ratios declined significantly under N24 treatment, whereas soil carbon, nitrogen content, and stoichiometric ratios in subsoil (15-30 cm) showed no significant response. There were significant positive correlations between plant and soil N and P concentrations. These findings indicate that long-term N enrichment alleviates plant N limitation, thereby enhancing photosynthetic carbon assimilation and aboveground productivity, ultimately shifting nutrient balance by reducing C∶N ratios in both plants and soils. This study provides empirical evidence on how N enrichment alters nutrient balance between plants and soils in wetlands, offering valuable insights for wetland conservation and nutrient management under ongoing anthropogenic N enrichment.

Key words: marsh wetland, nitrogen enrichment, ecological stoichiometric ratio, Glyceria spiculose