Terrestrial ecosystem productivity is widely accepted to be nutrient limited1. Although nitrogen (N) is deemed a key determinant of aboveground net primary production (ANPP)2,3, the prevalence of co-limitation by N and phosphorus (P) is increasingly recognized4-8. However, the extent to which terrestrial productivity is co-limited by nutrients other than N and P has remained unclear. Here, we report results from a standardized factorial nutrient addition experiment, in which we added N, P and potassium (K) combined with a selection of micronutrients (K+μ), alone or in concert, to 42 grassland sites spanning five continents, and monitored ANPP. Nutrient availability limited productivity at 31 of the 42 grassland sites. And pairwise combinations of N, P, and K+μ co-limited ANPP at 29 of the sites. Nitrogen limitation peaked in cool, high latitude sites. Our findings highlight the importance of less studied nutrients, such as K and micronutrients, for grassland productivity, and point to significant variations in the type and degree of nutrient limitation. We suggest that multiple-nutrient constraints must be considered when assessing the ecosystem-scale consequences of nutrient enrichment.
Bibliographical noteFunding Information:
We thank the Minnesota Supercomputer Institute for hosting project data, the University of Minnesota Institute on the Environment for hosting Nutrient Network meetings, and each site investigator for funding their site-level operations. Network coordination and data management were supported by funds from the National Science Foundation Research Coordination Network (NSF-DEB-1042132) to E.T.B. and E.W.S., from the Long Term Ecological Research program (NSF-DEB-1234162) to the Cedar Creek LTER, and from the Institute on the Environment (DG-0001-13). P.A.F. acknowledges USDA-NIFA (2010- 65615-20632). USDA is an equal opportunity employer and provider.