Response of microbial physiology and stoichiometry to N addition in a species removal experiment at a Tibetan grassland

Global Nitrogen deposition is rapidly increasing, and this tendency is predicted to be even stronger in the future. The Tibetan Plateau (TP), a region extremely sensitive to environmental disturbances, is no stranger to these changes. The TP hosts the largest alpine pastoral ecosystem in the world: Kobresia grasslands, dominated by the sedge species Kobresia pygmaea. These ecosystems hold most of the terrestrial C at the plateau, which is mainly stored below ground, in the rhizosphere. Grassland ecosystems are particularly sensitive to N input, long term sustained N deposition is likely to result in species richness decline. Besides climatic perturbations and the observed increasing N deposition at the TP can alter the soil environment stoichiometry, and hence soil microbial processes. Our study focuses on the response of soil microorganisms to sustained yearly N addition in a field species removal experiment. It is hypothesized that long-term fertilization influences microbial energy allocation, growth patterns, and SOM decomposition. We carried out our studies at the Species Removal Experimental Platform at TU-ITPCAS Naqchu Integrated Alpine Grassland Ecosystem and Environmental Observation Station. We used three species removal treatments: removal of K. pygmaea, removal of the 5 most dominant species, and a control where no species were removed. Samples exposed to long-term N addition (+7 years of 300 Kg of Urea/ha per year) and a set of control samples without previous N addition were amended with an equivalent N amount from (NH4)2SO4at the beginning of the incubation. We measured at three sampling points net ammonium, nitrate content, gross ammonification, available P, OC, TN, DOC,15N/14N and 13C/12C, microbial C, N, and P, and potential enzyme activities. With the aforementioned parameters, we aimed to monitor the effect of the N addition and species removal on the microbial biomass stoichiometry, and the physiological development of the microbial community. The result from the experiment will contribute to a deeper understanding of how an increased N deposition in soil at the TP, together with plant community shifts, could impact microbial mediated processes and potentially the fate of the C stored in this key ecosystem.