The capacity of species to persist at low levels of a limiting resource is a key mechanism underpinning competitive success. Thus, anthropogenic nutrient deposition may differentially affect certain plant functional groups. N-fixing legumes have specialist nutrient-acquisition strategies and are expected to be especially vulnerable to increased global nutrient deposition, particularly in regions with nutrient poor soils. The diverse legume family is crucial for nutrient cycling, provisioning of protein-rich crop and pasture production, and as a source of genetic resources for crop and fodder plant breeding. Here, we evaluated the additive and synergistic effect of nutrient addition on legume cover, richness and biomass in grasslands, worldwide. We measured the response of native and non-native N-fixing legumes to 3-6 years of standardized experimental additions of N, P, and K (10:10:10 g m-2 yr-1) plus other nutrients (i.e. Sulphur, Magnesium and trace elements, hereafter K+) in a factorial combination replicated in 45 grasslands spanning 6 continents. Using linear mixed effects models, and the log-ratio (LR=log (legumeslast/legumesinitial )] as a relative effect, we analysed soil nutrients, local plant community composition, photosynthetically active radiationand climatic conditions, as contingences for the nutrient effects on legumes, globally.
Results/Conclusions
Chronic N addition reduced legume cover by 32%, richness by 12% and biomass by 43%, consistently among native and non-native legumes. In contrast, P addition significantly increased legume cover (34%), whereas legumes did not respond to K+. Despite the benefits of P addition, its combination with N resulted in a net reduction of legume cover (22%), indicating that N combined with P resulted in negatively sub-additive cover of N-fixers. In contrast, non-legume forbs and grasses increased with N addition, concomitant with the reduction in legume cover. Moreover, N and P addition led to lower PARnon-legume . Finally, N addition promoted greater legume decline at sites with lower initial soil N and less legume decline in soils with higher NPratio, highlighting the detrimental impact of N addition on legumes in N-poor soils. Our global assessment is consistent with broader conclusions regarding impacts of N and P enrichment on grasslands and highlights the potential scale of impact of anthropogenic N-fixation on the abundance and diversity of plants with specialized N-acquisition strategies as legumes, regardless of the supply of other nutrients.