Abstract: Aims The study of plant functional traits in response to nitrogen deposition and warming is crucial for understanding plant growth and community assembly under environmental change. However, the differential effects of nitrogen deposition and warming on community traits, as well as the underlying mechanisms, remain unclear. Methods Therefore, this study was conducted in the Bayinbuluke Grassland, located in the southern Tianshan Mountains. We set up an experimental design with warming (open-top chambers, OTC) and nitrogen addition (10 g/m2), and measured the effects of nitrogen addition and warming on community species composition and five functional traits (plant height, leaf area, leaf carbon content, leaf nitrogen content, and leaf phosphorus content). The aim was to explore the impact and driving mechanisms of nitrogen addition and warming on community-level functional traits. Important findings The results indicated that: 1) Compared to the control, nitrogen addition significantly increased community-level plant height and leaf carbon content by 52.89% and 28.65%, respectively, while leaf phosphorus content decreased significantly by 32.23%. There were no significant changes in leaf area or leaf nitrogen content. For plant height, the relative contribution of species turnover to the community change was greater than that of intraspecific trait variability (ITV), whereas for leaf chemical traits (leaf carbon content, leaf nitrogen content, and leaf phosphorus content), ITV played a significantly larger role than species turnover. 2) Under warming, community-level leaf area, leaf nitrogen content, and leaf phosphorus content decreased significantly by 44.53%, 25.07%, and 42.06%, respectively, compared to the control, while plant height and leaf carbon content showed no significant changes. Changes in plant height were mainly driven by the covariation between species turnover and ITV, whereas for leaf morphology (leaf area) and leaf chemical traits (leaf carbon, nitrogen, and phosphorus contents), species turnover had a much greater impact than ITV. 3) In the warming + nitrogen addition treatment, community-level leaf phosphorus content decreased significantly by 52.19%, with no significant changes in plant height, leaf area, leaf carbon content, or leaf nitrogen content. The changes in plant height and leaf carbon, nitrogen, and phosphorus contents were primarily driven by species turnover, while leaf area was mainly influenced by ITV. This study suggests that nitrogen deposition and warming can alter the functional composition of alpine meadow communities through distinct mechanisms. Following nitrogen deposition, ITV increases functional variability among individuals, enhancing the community's responsiveness and stability to environmental changes. Under warming and warming + nitrogen addition conditions, species turnover influences the community by altering species composition, directly affecting species diversity and structure, thereby significantly impacting the community function in the alpine meadows of the Tianshan Mountains.

Key words: alpine grassland, functional traits, intraspecific trait variation, species turnover