Response of microbial physiology and stoichiometry to N addition in a species removal experiment at a Tibetan grassland
Authors: Urbina Malo Carolina , Ruenzi Viola , Klausmeier Julia , Winkler Vanessa , Quzong Ciren , Stange Florian , Wu Dongmei , Tangarife Andres , Sierra Carlos , Pester Michael , Dorji Tsechoe , Guggenberger Georg 
Published: March 2022

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