Negative effects of nitrogen override positive effects of phosphorus on grassland legumes worldwide

Tognetti, P.M. and Prober, S.M. and Báez, S. and Chaneton, E.J. and Firn, J. and Risch, A.C. and Schuetz, M. and Simonsen, A.K. and Yahdjian, L. and Borer, E.T. and Seabloom, E.W. and Arnillas, C.A. and Bakker, J.D. and Brown, C.S. and Cadotte, M.W. and Caldeira, M.C. and Daleo, P. and Dwyer, J.M. and Fay, P.A. and Gherardi, L.A. and Hagenah, N. and Hautier, Y. and Komatsu, K.J. and McCulley, R.L. and Price, J.N. and Standish, R.J. and Stevens, C.J. and Wragg, P.D. and Sankaran, M. (2021) Negative effects of nitrogen override positive effects of phosphorus on grassland legumes worldwide. Proceedings of the National Academy of Sciences of the United States of America, 118 (28): e202371811. ISSN 0027-8424

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Abstract

Anthropogenic nutrient enrichment is driving global biodiversity decline and modifying ecosystem functions. Theory suggests that plant functional types that fix atmospheric nitrogen have a competitive advantage in nitrogen-poor soils, but lose this advantage with increasing nitrogen supply. By contrast, the addition of phosphorus, potassium, and other nutrients may benefit such species in lownutrient environments by enhancing their nitrogen-fixing capacity. We present a global-scale experiment confirming these predictions for nitrogen-fixing legumes (Fabaceae) across 45 grasslands on six continents. Nitrogen addition reduced legume cover, richness, and biomass, particularly in nitrogen-poor soils, while cover of non-nitrogenfixing plants increased. The addition of phosphorous, potassium, and other nutrients enhanced legume abundance, but did not mitigate the negative effects of nitrogen addition. Increasing nitrogen supply thus has the potential to decrease the diversity and abundance of grassland legumes worldwide regardless of the availability of other nutrients, with consequences for biodiversity, food webs, ecosystem resilience, and genetic improvement of protein-rich agricultural plant species.