Effects of grazing on the acquisition of nitrogen by plants and microorganisms in an alpine grassland on the Tibetan plateau

Nitrogen (N) limitation leads to intense competition between plants and soil microorganisms for available N. However, it is unclear how grazing affects the acquisition of N by plants and microorganisms. We conducted short-term N-15 tracer experiments during the growing season (June, early growing season; July, mid-growing season; and September, late growing season) in an alpine grassland on the Tibetan Plateau to investigate the effects of grazing on the acquisition of NO3 (-)-N, NH4 (+)-N, and glycine-N by plants and soil microorganisms. Three dominant plant species (one graminoid, Kobresia pygmaea, and two forbs, Potentilla bifurca and Potentilla multifida) were selected for the study. As these species represented > 90% of the vegetation, the plant recovery of N-15 reflected competition at the plant community. Grazing decreased the recovery of N-15 by soil microorganisms and plants by 46 and 69%, respectively, indicating that grazing strongly reduced the uptake of N-15 by plants and microorganisms and altered the partitioning of N-15 between them. Significant interactions were found between grazing, season and the different forms of N. In the absence of grazing, plants acquired relatively more N than soil microorganisms for the three forms of N in July and September, whereas the microorganisms obtained relatively more N-15 glycine in July and all three forms of N in September under grazing conditions. Under grazing, the plant root biomass played an important role in controlling plant-microbial N acquisition. Grazing alters the partitioning of inorganic and organic N between plants and soil microorganisms by reducing microbial N-15 recovery to a lesser extent than plant N-15 recovery. This indicates that heterotrophic microorganisms play an important part in N cycling in N-limited ecosystems.