Background and aimsThere is ample experimental evidence for shifts in plant community composition under climate warming. To date, however, the underlying mechanisms driving these compositional shifts remain poorly understood.

Methods The amount and form of nitrogen (N) available to plants are among the primary factors limiting productivity and plant coexistence in terrestrial ecosystems. We conducted a short-term N-15 tracer experiment in a ten-year warming and grazing experiment in an alpine grassland to investigate the effects of warming and grazing on plant uptake of NO3--N, NH4+-N, and glycine-N. Four dominant plant species (Kobresia humilis, Potentilla anseria, Elymus nutans, Poa annua) were selected.

Results We found that 10-years of warming decreased plant uptake of inorganic N by up to 80% in all species. In contrast, warming increased the uptake of organic N in K. humilis, P. anseria, and E. nutans but not in P. annua. Results showed that plant relative biomass increased hyperbolically with the ratio of the plant species total uptake of available N and plant community uptake of available N. And a significant positive correlation between plant species uptake of soil glycine-N and the uptake of total available N.

Conclusions The stable relative biomass of plant species is largely dependent on organic N uptake by plants. We conclude that plant organic N uptake maintains species dominance under long-term warming.