Biotic nitrogen (N) retention is an important ecosystem function in the context of ongoing land-use intensification, N deposition and global warming. However, a paucity of experimental evidence limits understanding of how different plant community components influence N retention in terrestrial ecosystems. In this investigation, we conducted a ¹⁵ N labelling experiment to test how plant community properties, including plant species richness/diversity, dominance and functional traits, influence plant N uptake and retention under different nutrient availabilities. A 3-year experiment examined the effects of adding N (10 g N m ⁻²  year ⁻¹ ) and phosphorus (P) (5 g P m ⁻²  year ⁻¹ ) to an alpine meadow on the Qinghai-Tibetan Plateau. Results show that ¹⁵ N retention increased with the addition of N and P; the addition of P produced the largest increase of ¹⁵ N retention in plant and soil N pools. Changes in soil nutrient conditions also facilitated different plant community controls on ecosystem N retention. Ecosystem ¹⁵ N retention was influenced by species richness and root biomass in the control plots; whereas the N addition treatment showed an important effect of community-weighted means (CWM) of specific leaf area, and plots with additional P recorded lower CWM of root nitrogen content (root N) and larger CWM root:shoot ratios as important determinants. Synthesis. Ecosystem N retention was influenced by conservative and exploitative plant species and/or their traits under N deficient and abundant conditions, respectively, whereas species richness and community plant biomass were most influential under middle condition. The discovery of an interaction between plant community traits and nutrient biogeochemistry as a mechanism for ecosystem N retention offers a means to predict how vegetation in alpine meadow ecosystems will respond to expected global change.