Nitrogen deposition alters plant community and physiological traits via soil acidification and phosphorus limitation in an alpine meadow of the Qinghai-Tibet Plateau

Globally increasing nitrogen (N) deposition has emerged as a critical factor influencing alpine grassland ecosystems on the Tibetan Plateau. However, existing research on plant community structure and physiological responses under N deposition remains controversial, particularly regarding physiological adaptations to shifting soil conditions. To address this, we conducted a six-year in-situ controlled experiment with four N addition gradients (0, 10, 15, and 20 kg N ha^-1 yr^-1) on the alpine grassland of Sejila Mountain, measuring aboveground plant communities, foliar gas exchange parameters, plant biometric traits, and soil physicochemical properties. Our results indicate that moderate N inputs (10–15 kg N ha^-1 yr^-1) alleviated N limitation, increasing total aboveground biomass (AGB) by 24.21% under the N15 treatment. However, high N inputs (20 kg N ha^-1 yr^-1) induced soil acidification, reduced available phosphorus (P), and caused an N–P stoichiometric imbalance, leading to significant declines in species richness and the Shannon diversity index (p < 0.05). Conversely, N inputs significantly increased belowground biomass (BGB) and root-to-shoot ratios, which peaked under N20 (20 kg N ha^-1 yr^-1) with remarkable increases of 41.93% and 27.34% relative to the control, respectively. The net photosynthetic rate (P_n) and transpiration rate (T_r) of Elymus dahuricus increased slightly with N addition, whereas those of Kobresia royleana declined significantly under high N. Aboveground N and P contents exhibited nonlinear responses, with total community P increasing by 24.20% under N15. Moreover, N:P ratios in graminoids and the total community decreased by 12.02% and 14.49% under N20, respectively. Overall, excessive N deposition (≥ 15 kg N ha^-1 yr^-1) reduces soil pH and available P, shifting the ecosystem toward P limitation. This nutritional constraint, combined with intensified light competition from dominant species, drives competitive exclusion, while dominant groups further alter soil carbon and N pools through litter feedback. Future research should prioritize P cycling and plant responses under combined N and P enrichment to better inform nutrient management and conservation strategies in alpine ecosystems.