Nitrogen But Not Phosphorus Addition Affects Symbiotic N2 Fixation in Grasslands Located on Four Continents

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 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 legume biomass by 65% and 45%, respectively, compared to the control, whereas P addition had no significant impact. Element addition 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 the world’s grasslands, and these effects cannot be reversed by additional P amendment.