Nutrient effects on plant diversity loss arise from nutrient identity and decreasing niche dimension

Abstract Two hypotheses have been used to explain the loss of plant diversity with nutrient addition. The nutrient identity hypothesis posits that biodiversity loss is due to a specific limiting nutrient, such as nitrogen (N) or phosphorus (P), while the niche dimension hypothesis posits that adding a larger number of limiting nutrients, regardless of their identity, results in biodiversity loss. These two hypotheses have not previously been tested together simultaneously. Here, we conduct that analysis to enable their relative effect sizes to be compared. We manipulated the supply of eight nutrients in the same experimental meadow grassland site to isolate the effects of the identity of added nutrients versus the number of added nutrients on biodiversity loss. We found support for both hypotheses, with the largest negative effects on biodiversity measures being due to N, or N and P treatment, with additional more minor effects of the number of added nutrients. Structural equation models (SEMs) suggested both identity and number of added nutrients had direct negative effects on biodiversity, likely caused by species' innate ability to competitively respond to nutrients, especially in response to disease, herbivory, and stress. SEMs also suggested indirect effects arising from nutrient‐driven increases in aboveground biomass, which resulted in intensified competition for light and the competitive exclusion of short‐statured species. These effects were exacerbated by the nutrients N and P which caused a shift in biomass accumulation from belowground to aboveground. The results highlight that a multi‐nutrient perspective will improve our ability to effectively manage, monitor, and restore ecosystems.