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

<p><span>Global Nitrogen deposition is rapidly increasing, and this tendency is predicted to be </span><span>even </span><span>stronger in the future. </span><span>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: </span><em><span>Kobresia </span></em><span>grasslands, dominated by the sedge species </span><em><span>Kobresia pygmaea</span></em><span>. Thes</span><span>e ecosystems</span> <span>hold </span><span>most of the</span><span> terrestrial C </span><span>at the plateau</span><span>, which is </span><span>mainly </span><span>stored below ground, </span><span>in the rh</span><span>i</span><span>zosphere. </span><span>Grassland ecosystems are particularly sensitive to N </span><span>input</span><span>, </span><span>long term sustained N deposition is likely to result in species richness decline.</span><span> Besides </span><span>c</span><span>limatic 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 </span><span><em>Species Removal Experimental Platform</em></span><span> at TU-ITPCAS Naqchu Integrated Alpine Grassland Ecosystem and Environmental Observation Station. We used three species removal treatments: removal of </span><em><span>K. pygmaea</span></em><span>, removal of the 5 most dominant species, and a control where no species were removed. Samples exposed to long-term </span><span>N addition</span><span> (+7 years of 300 Kg </span><span>of Urea</span><span>/ha </span><span>per year</span><span>) and a set of control samples without previous N addition were amended with </span><span>an equivalent N amount from </span><span>(NH</span>₄)₂SO₄<span>at the beginning of the incubation. </span><span>We </span><span>measured at three sampling points</span><span> net ammonium, nitrate content, gross </span><span>ammonification</span><span>, available P, OC, TN, DOC,</span>^{<span>15</span>}<span>N/</span>^{<span>14</span>}<span>N and </span>^{<span>13</span>}<span>C/</span>^{<span>12</span>}<span>C, microbial C, N, and P, and potential enzyme activities. </span><span>With the aforementioned parameters, we aimed to</span><span> monitor the effect of the N addition and species removal </span><span>on the microbial biomass stoichiometry, and the physiological development of the microbial community</span><span>. </span><span>The result from the experiment will contribute to a deeper understanding of </span><span>how an</span><span> increased N </span><span>deposition</span><span> in soil </span><span>at the TP</span><span>, </span><span>together with</span><span> plant community shifts, could </span><span>impact</span><span> microbial mediated processes </span><span>and potentially the fate of the C stored in this key ecosystem.</span></p>