Intraspecific genetic diversity is increasingly recognised as critical for ecosystem functioning, especially for dominant grassland species facing global change drivers like nitrogen deposition and determining community productivity. However, the mechanisms by which nitrogen enrichment regulates the biodiversity–ecosystem function relationship at the genotypic level remain poorly understood. Here, we conducted a two-year factorial experiment genotypic diversity (1-, 3-, and 6- genotypes) × nitrogen addition (0, 2.5, 5, and 10 g N \(\cdot\) m⁻² \(\cdot\) yr⁻¹) to examine how soil nitrogen addition influences the effects of genotypic diversity on population biomass in Leymus chinensis, a dominant perennial grass in the typical steppe of northern China. (1) Both genotypic diversity and nitrogen addition significantly affected total population biomass across the two years, whereas their interaction was not significant. (2) In the second year, nitrogen addition exerted a significant negative main effect on the net diversity effect (P < 0.05), a pattern driven by negative contributions from the dominance effect and the trait-dependent complementarity effect, despite a positive contribution from the trait-independent complementarity effect. (3) The observed values of most plant functional traits and functional diversity indices exceeded their expected values under null models, with the notable exceptions of leaf nitrogen content and root tissue density. These findings demonstrate that nitrogen addition can modulate the expression of genotypic diversity effects over time, highlight the importance of conserving or actively restoring high genotypic diversity within L. chinensis populations to maintain grassland productivity under increasing nitrogen deposition.
