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

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 N 2 fixation in grasslands, and these effects cannot be reversed by additional P amendment. EEA Santa Cruz Fil: Vázquez, Eduardo. University of Bayreuth. Department of Soil Ecology. Bayreuth Center of Ecology and Environmental Research (BayCEER); Alemania Fil: Vázquez, Eduardo. Swedish University of Agricultural Sciences. Department of Soil and Environment; Suecia Fil: Schleuss, Per‑Marten. University of Bayreuth. Department of Soil Ecology. Bayreuth Center of Ecology and Environmental Research (BayCEER); Alemania Fil: Borer, Elizabeth T. University of Minnesota. Department of Ecology, Evolution, and Behavior; Estados Unidos Fil: Bugalho, Miguel N. University of Lisbon. Centre for Applied Ecology “Prof. Baeta Neves” (CEABN-InBIO). School of Agriculture; Portugal. Fil: Caldeira, Maria. C. University of Lisbon. Forest Research Centre. School of Agriculture; Portugal. Fil: Eisenhauer, Nico. German Centre for Integrative Biodiversity Research; Alemania Fil: Eisenhauer, Nico. Leipzig University. Institute of Biology; Alemania Fil: Eskelinen, Anu. German Centre for Integrative Biodiversity Research; Alemania Fil: Eskelinen, Anu. Physiological Diversity, Helmholtz Centrefor Environmental Research; Alemania Fil: Eskelinen, Anu. University of Oulu. Ecology & Genetics; Finlandia Fil: Fay, Philip A. Grassland Soil and Water Research Laboratory (USDA-ARS); Estados Unidos Fil: Haider, Sylvia. German Centre for Integrative Biodiversity Research; Alemania Fil: Haider, Sylvia. Martin Luther University. Institute of Biology. Geobotany and Botanical Garden; Alemania Fil: Jentsch, Anke. University of Bayreuth. Department of Soil Ecology. Bayreuth Center of Ecology and Environmental Research (BayCEER); Alemania Fil: Kirkman, Kevin P. University of KwaZulu-Natal. School of Life Sciences; Sudáfrica Fil: McCulley, Rebecca L. University of Kentucky. Department of Plant and Soil Sciences; Estados Unidos Fil: Peri, Pablo Luis. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Santa Cruz; Argentina. Fil: Peri, Pablo Luis. Universidad Nacional de la Patagonia Austral; Argentina. Fil: Peri, Pablo Luis. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Fil: Price, Jodi. Charles Sturt University. Institute for Land, Water and Society; Australia. Fil: Richards, Anna E. CSIRO Land and Water. Northern Territory; Australia. Fil: Risch, Anita C. Swiss Federal Institute for Forest, Snow and Landscape Research WSL; Suiza Fil: Roscher, Christiane. German Centre for Integrative Biodiversity Research; Alemania Fil: Roscher, Christiane. Physiological Diversity, Helmholtz Centre for Environmental Research; Alemania Fil: Schütz, Martin. Swiss Federal Institute for Forest, Snow and Landscape Research WSL; Suiza Fil: Seabloom, Eric William. University of Minnesota. Dept. of Ecology, Evolution, and Behavior; Estados Unidos Fil: Standish, Rachel J. Murdoch University. Harry Butler Institute; Australia. Fil: Stevens, Carly J. Lancaster University. Lancaster Environment Centre; Reino Unido Fil: Tedder, Michelle J. University of KwaZulu-Natal. School of Life Sciences; Sudáfrica Fil: Virtanen, Risto. University of Oulu. Ecology & Genetics; Finlandia. Fil: Spohn, Marie. University of Bayreuth. Department of Soil Ecology. Bayreuth Center of Ecology and Environmental Research (BayCEER); Alemania Fil: Spohn, Marie. Swedish University of Agricultural Sciences. Department of Soil and Environment; Suecia