Plant-microbe relationships determine winners and losers in response to nitrogen pollution
S. HICKS (1), E. Farrer (2), D. L. Taylor (1), A. P. Alfaro (3), K. Suding (2), R. Sinsabaugh (1). (1) University of New Mexico, Albuquerque, NM, U.S.A.; (2) University of California-Berkeley, Berkeley, CA, U.S.A.; (3) Western Illinois University, Macomb, IL, U.S.A.
In response to Nitrogen (N) enrichment, <i>G. rossii</i>, a co-dominant plant in moist meadow alpine tundra at Niwot Ridge (CO), dies back while <i>D. cespitosa</i>, its principal competitor, expands. We ask whether contrasting host responses to N are mediated by shifts in plant-microbe symbioses. Using 454 pyrosequencing, we compared the effects of N on root-associated microbes (RAM) of <i>G. rossii</i> and <i>D. cespitosa</i> in experimental plots. We found root-fungal communities from a nitrophobic host were more sensitive to N than communities from a nitrophilic host. The <i>G. rossii</i> fungal taxa most (negatively) affected by N (Helotiales) are also the most abundant (>80% of sequences), and may play a role in accessing organically bound N. Conversely, <i>D. cespitosa</i> bacterial communities were more sensitive to N relative to <i>G. rossii</i>’s. The <i>D. cespitosa</i> bacteria species most (positively) affected by N has fungicidal properties, and forms symbioses with animals to improve resistance to fungal infection. Mutualistic fungi can become parasitic under nutrient enrichment, these bacteria may improve <i>D. cespitosa</i>’s immunity to fungal parasitism. Pathogens were unaffected by N, indicating loss/gain of mutualists may be underemphasized in diagnosing plant health. This evidences the potential importance of belowground microbial dynamics in determining plant response to N deposition.
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