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.

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.

Associated Treatment Areas:

• T1 Conventional Management
• TDF Deciduous Forest
• T4 Biologically Based Management

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