Soil microbes influence nitrogen limitation on plant biomass in alpine steppe in North Tibet.

Background and aims: An increase in nitrogen deposition is expected to increase the phosphorus-limited primary productivity of grassland ecosystems. However, it remains unclear as to whether the primary productivity of grasslands is co-limited by nitrogen and phosphorus, or only by one or the other. Methods: Nitrogen and phosphorus addition experiments were conducted at an alpine steppe site in North Tibet from 2013 to 2017. The biomass of plant community, the N and P content of plant leaf, soil chemical property, and changes in abundance and function and composition of bacterial and fungal community with N and P treatment were analyzed after five years. Results: Results from analyzing the response ratios of shoot biomass and leaf nitrogen and phosphorus concentrations to nitrogen and phosphorus addition suggested that nitrogen is the main limiting factor for the biomass of alpine steppe. Based on the ratio of nitrogen and phosphorus stoichiometry, plants must increase their phosphorus absorption following nitrogen addition. From the results of a structural equation model, plants and fungi both promoted the activity of phosphatase. The abundance of fungi and the saprotrophic group associated with decomposition increased following nitrogen addition, and more substrate is provided by the plants, which will accelerate the organic phosphorus cycle. These results suggest that the phosphorus deficit of plants owing to nitrogen addition will be alleviated. On the contrary, phosphorus did not limit the biomass, but its addition did increase the absorption of nitrogen by plants. Although the activity of urease was enhanced by soil microbes and plants, there was little substrate for microbes because the plant biomass was limited by nitrogen. Conclusions: The biomass is mainly limited by nitrogen and soil microbes will influence the effect of nitrogen on biomass in alpine steppe. [ABSTRACT FROM AUTHOR]