Nitrogen enrichment threatens non‐N mineral nutrition and nutritional stability of forage due to biodiversity loss

Abstract Nitrogen (N) enrichment induced by anthropogenic activities increases forage production but reduces plant diversity in global grasslands. It remains unclear whether biodiversity losses following N enrichment affect non‐N mineral nutrition of forage and nutritional stability, key components of forage quality. We assessed the responses of forage non‐N nutrition (P, K, Ca, Mg, Fe, Mn, Cu, Zn) and nutritional stability to multi‐levels N addition (0, 2, 5, 10, 20 and 50 g N m −2 year −1 ) in a temperate steppe over 9 years from 2015 to 2023. Nitrogen addition decreased standardized mineral concentration ( MC ) and expected mineral stability ( MS ) of forage, with the effects being strengthened over time. There were positive but saturation relationships between plant species richness and forage MC and MS across the N addition gradient. Nitrogen addition significantly reduced the capacity of grasslands to supply non‐N minerals by decreasing plant species richness, and led to a trade‐off between forage yield and quality. In addition, the N‐induced decreases in MC and MS were more severe in dry years due to a stronger decline of species richness. Synthesis . Nitrogen enrichment reduced non‐N mineral nutrition and nutritional stability of forage, largely due to species richness losses. While the quantitative role of biodiversity in sustaining production has been widely reported, our results emphasize its qualitative role under scenarios of increasing N deposition and climate change. The trade‐off between forage yield and quality highlights the existence of trade‐offs not only among different ecosystem services but also between different facets of a particular service.