Nutrient enrichment intensifies plant competitive effects, favouring monocultures: a global meta-analysis

Excess nutrient deposition is a major driver of plant diversity loss, but the mechanisms of this loss remain unclear. Diversity loss has been interpreted through the niche dimension hypothesis, yet the mechanistic link via competitive effects remains unresolved. Establishing this link demands a global synthesis of how nutrients alter competitive effects, defined as a species ability to perform better or worse with heterospecific competitors than with conspecifics. Strong competitive effects from heterospecifics relative to conspecifics can lead to decline or exclusion of the focal species, whereas weaker competitive effects indicate niche differentiation that promotes coexistence. We conducted a global meta-analysis of 71 plant competition experiments to quantify how nutrient addition alters competitive effects, using biomass as the performance metric. Nutrient addition intensified competitive effects with heterospecifics by 1.5-fold, as focal species biomass was higher with conspecifics. Responses depended on the initial competitive effect under low nutrients: when effects were weak, nutrient addition increased biomass with conspecifics; when effects were strong, nutrient addition increased biomass with heterospecifics, alleviating competition and potentially countering species loss. Competitive effects were amplified most when higher temperatures occurred in dry quarters, suggesting nutrient additions may exacerbate competition under extreme climate conditions. Understanding these interactive effects of nutrient addition, changing climate, and competition is critical for predicting plant responses to global change in both managed and natural ecosystems. Significance StatementNutrient enrichment from agricultural runoff is a driver of plant diversity loss. While patterns of diversity loss from nutrient additions are clear, the processes driving competitive effects that lead to diversity loss remain uncertain. Our global meta-analysis reveals that multiple nutrient addition intensifies competitive effects favouring monocultures i.e, species accumulating more biomass with conspecifics than heterospecifics. Depending on the initial strength of competitive effects, nutrient addition amplified or alleviated them. Temperature and water availability interact with nutrient addition to amplify competitive asymmetries, likely further shaping species dominance under changing climate. These insights provide the first global-scale evidence of nutrient enrichment effects on plant communities and highlight the need to mitigate compounding impacts of fertilization and climate change on diversity.