PLANT COMMUNITY COMPOSITION ALTERED BY LONG-TERM NITROGEN ADDITION HAS MINOR CONTRIBUTION TO PLANT NUTRIENT STATUS AT THE COMMUNITY LEVEL

Short-term nitrogen (N) deposition has exhibited great impacts on plant internal nutrient cycling in forest, grassland and desert ecosystems. However, the responses of plant nutrient status to chronic N deposition are not well understood, especially in alpine grasslands limited by soil N availability. This study used an N addition experiment that had been implemented since 2009 in the Tianshan mountains, northwest China, to investigate the effects of N addition at five levels (0, 1, 3, 9 and 15 g N m(-2) year(-1)) on the leaf nutrient concentrations and stoichiometric ratios of dominant perennial grasses (Leymus tianschanicus, Festuca ovina, Agropyron cristatum and Koeleria cristata) and forbs (Potentilla anserina and Potentilla bifurca) at the species, functional group and community levels. The results showed that increasing N addition significantly enhanced soil available N concentrations and soil available N:P ratios but had no detectable impacts on soil available phosphorus (P) concentrations. Nitrogen addition significantly reduced forb relative biomass, ranging from 13% in control plots versus 2% in the highest N addition plots. Furthermore, N addition increased the foliar N concentrations and N:P ratios for all the species, decreased foliar P concentrations for grasses, and had no significant effects on P concentrations in forbs. Plant carbon (C) concentrations remained relatively stable, resulting in reduced C:N as N addition increased. In contrast, N addition consistently increased leaf N concentrations, C:P and N:P and decreased leaf P concentrations and C:N at the community level. Regression analysis showed that soil available N and N:P ratios, rather than soil available P, were key parameters controlling plant N and P concentrations and stoichiometric ratios. Foliar N and P concentrations showed divergent responses to long-term N addition at the species and community levels, which implied that plant N and P cycling decoupled under N deposition. Grasses were more sensitive to N addition than forbs, resulting in significantly altered plant community composition. These findings are important for understanding plant nutrient allocation strategies and how plants adapt to environmental changes, such as N deposition in alpine ecosystems.