Thompson, B. E. 2019. Microbial communities regulate nitrogen use efficiency by balancing tradeoff between resource acquisition and growth rate. Thesis, University of New Hampshire, Durham, NH.

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Microbial nitrogen use efficiency (NUE) is the portion of N uptake that microbes allocate to growth versus mineralize as ammonium and is thus a critical parameter governing the transformation of organic to inorganic nitrogen. Microbial NUE is sensitive to changes in the soil environment, but its microbial controls remain untested. I performed an incubation where identical mesocosms were inoculated with three distinct microbial communities derived from agricultural land management (conventional, organic, and deciduous forest). Through this incubation, I explored three scales at which communities may exercise control over NUE: in how microbes alter their soil environment through system-level processes; through their stoichiometric and community composition; and through community physiology. My results indicate that microbial activity and physiology are most strongly related to NUE in this controlled environment. Specifically, I show that NUE is positively related to microbial growth rate and not resource acquisition. I also empirically demonstrate for the first time that CUE and NUE are positively related. From these results I conclude that microbes may be important regulators of NUE and govern its variation in ways unpredictable by stoichiometric theory alone.

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