Effects of plant diversity on grassland productivity, or overyielding, are found to be robust to nutrient enrichment. However, the impact of cumulative nitrogen (N) addition (total N added over time) on overyielding and its drivers are underexplored. Synthesizing data from 15 multi-year grassland biodiversity experiments with N addition, we found that N addition decreases complementarity effects and increases selection effects proportionately, resulting in no overall change in overyielding regardless of N addition rate. However, we observed a convex relationship between overyielding and cumulative N addition, driven by a shift from complementarity to selection effects. This shift suggests diminishing positive interactions and an increasing contribution of a few dominant species with increasing N accumulation. Recognizing the importance of cumulative N addition is vital for understanding its impacts on grassland overyielding, contributing essential insights for biodiversity conservation and ecosystem resilience in the face of increasing N deposition.

Cedar Creek Ecosystem Science Reserve University of Minnesota 2660 Fawn Lake Dr NE East Bethel MN 55005 USA

Centre for Development and Environment CDE University of Bern Mittelstrasse 43 3012 Bern Switzerland

Data Observatory Foundation ANID Technology Center No DO210001 Eliodoro Yáñez 2990 7510277 Providencia Santiago Chile

Department of Botany Faculty of Science University of South Bohemia Na Zlaté stoce 1 370 05 České Budějovice Czech Republic

Department of Crop production Ecology Swedish University of Agricultural Sciences 901 83 Umeå Sweden

Department of Ecology Evolution and Behavior University of Minnesota 1479 Gortner Ave St Paul MN 55108 USA

Department of Forest Resources University of Minnesota 1479 Gortner Ave St Paul MN 55108 USA

Department of Functional Ecology Institute of Botany of the Czech Academy of Sciences Zámek 1 252 43 Průhonice Czech Republic

Ecology and Biodiversity group Department of Biology Utrecht University Padualaan 8 3584 CH Utrecht The Netherlands

GEMA Center for Genomics Ecology and Environment Universidad Mayor Camino La Pirámide 5750 Huechuraba Santiago Chile

German Centre for Integrative Biodiversity Research Puschstrasse 4 04103 Leipzig Germany

Institute for Global Change Biology and School for the Environment and Sustainability University of Michigan 440 Church Street Ann Arbor MI 48109 USA

Institute of Plant Sciences University of Bern Altenbergrain 21 3013 Bern Switzerland

Landcare Research Private Bag 3127 Hamilton 3240 New Zealand

Rural Development Institute of Paraná IAPAR EMATER Av Euzébio de Queirós s n° CP 129 CEP 84001 970 Ponta Grossa PR Brazil

Swiss Federal Research Institute WSL Zürcherstrasse 111 CH 8903 Birmensdorf Switzerland

UFZ Helmholtz Centre for Environmental Research Physiological Diversity Permoserstrasse 15 04318 Leipzig Germany

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