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

Background and aims: 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. Methods: We conducted short-term ¹⁵N 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 ¹⁵N reflected competition at the plant community. Results: Grazing decreased the recovery of ¹⁵N by soil microorganisms and plants by 46 and 69%, respectively, indicating that grazing strongly reduced the uptake of ¹⁵N by plants and microorganisms and altered the partitioning of ¹⁵N 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 ¹⁵N 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. Conclusions: Grazing alters the partitioning of inorganic and organic N between plants and soil microorganisms by reducing microbial ¹⁵N recovery to a lesser extent than plant ¹⁵N recovery. This indicates that heterotrophic microorganisms play an important part in N cycling in N-limited ecosystems.