Nitrogen but not phosphorus addition affects symbiotic N 2 fixation by legumes in natural and semi-natural grasslands located on four continents

Vazquez, E. and Schleuss, P-M. and Borer, E.T. and Bugalho, M.N. and Caldeira, M.C. and Eisenhauer, Nico and Eskelinen, Anu and Fay, P.A. and Haider, S. and Jentsch, A. and Kirkman, Kevin P. and McCulley, Rebecca L. and Peri, Pablo L. and Price, J and Richards, A.E. and Risch, A. and Roscher, Christiane and Schütz, Martin and Seabloom, Eric W. and Standish, Rachel J. and Stevens, Carly and Tedder, m.j. and Virtanen, R and Spohn, M.J. (2022) Nitrogen but not phosphorus addition affects symbiotic N 2 fixation by legumes in natural and semi-natural grasslands located on four continents. Plant and Soil, 478 (1-2). pp. 689-707. ISSN 0032-079X

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Abstract

Abstract : Background and aims: The amount of nitrogen (N) derived from symbiotic N2 fixation by legumes in grasslands might be affected by anthropogenic N and phosphorus (P) inputs, but the underlying mechanisms are not known. Methods: We evaluated symbiotic N2 fixation in 17 natural and semi-natural grasslands on four continents that are subjected to the same full-factorial N and P addition experiment, using the 15N natural abundance method. Results: N as well as combined N and P (NP) addition reduced aboveground legume biomass by 65% and 45%, respectively, compared to the control, whereas P addition had no significant impact. Addition of N and/or P had no significant effect on the symbiotic N2 fixation per unit legume biomass. In consequence, the amount of N fixed annually per grassland area was less than half in the N addition treatments compared to control and P addition, irrespective of whether the dominant legumes were annuals or perennials. Conclusion: Our results reveal that N addition mainly impacts symbiotic N2 fixation via reduced biomass of legumes rather than changes in N2 fixation per unit legume biomass. The results show that soil N enrichment by anthropogenic activities significantly reduces N2 fixation in grasslands, and these effects cannot be reversed by additional P amendment.