Contrasting grass nitrogen strategies reflect interspecific trade-offs between nitrogen acquisition and use in a semi-arid temperate grassland

Background and aims Nitrogen (N) niche differentiations play an important role in community structure and biogeochemical cycling in terrestrial ecosystems. However, very few studies have examined how plant N strategies specialize via trade-offs between N acquisition and use abilities under natural field conditions. Methods A field experiment was conducted to investigate N strategies by coexisting grass species using the in-situ stable isotope labeling technique. We injected 15 N–labeled nitrate, 15 N–labeled ammonium, and 13 C- 15 N-labeled glycine solutions in early and late vegetative growing seasons. Shoot δ 15 N and N concentration were measured to determine the N uptake and use abilities of 4 common species. Results All plant species preferred to take up nitrate (the dominant N form) over ammonium and glycine, and N-acquisition capacity varied with temporal variation of soil N. However, the dominant species was more N-conservative than less-dominant species and had lower overall N uptake rates (shoot 15 N excess 48 hours after injection) and higher N use efficiencies (aboveground biomass : N ratio). Conclusions The different N strategies may reflect mixed effects of environmental filtering and interspecific competition and have significant implications for species coexistence as well as for ecosystem functions such as nutrient cycling.