Fluctuations in the realized biogeochemical niche explain eutrophication effects across organizational levels in grasslands.

While eutrophication is known to alter community structure and functioning through ecological niche modification in grasslands, quantifying the contribution of realized niches to dynamics of plant diversity and ecosystem productivity remains challenging. Based on the biogeochemical niche (BN) hypothesis and the Price equation, we linked realized BN, estimated by leaf elemental composition, to diversity loss and productivity increases after fertilization in a long-term alpine nutrient addition experiment and across 18 Nutrient Network grasslands. We found that fertilization compressed community-wide realized BN hypervolume and reduced elemental diversity, yet expanded species realized BN hypervolume. These changes together decreased plant diversity in alpine grassland and Nutrient Network grasslands. Mechanistically, the alpine grassland experiment showed that the reduction in plant diversity after fertilization resulted from two simultaneous processes: first, the loss of species whose realized BNs closely resembled those of their close relatives; and second, the suppression of coexisting highly phylogenetically plastic species that continued to persist in the communities. Additionally, persistent species' BN expansion enhanced population productivity, scaling to ecosystem productivity increase after fertilization. Our findings formalize a predictive framework linking grassland biodiversity and ecosystem functions to global change through quantifiable ecological niche and species compositional dynamics.