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Abstract
Many processes related to carbon-cycling in terrestrial ecosystems are carried out by soil microbes. Our understanding of the relationship between carbon cycling and microbial community structure is, at best, rudimentary. The role of microbial community structure in carbon-cycling processes has, for the most part, been treated as either functionally redundant or driven by binary distinctions (e.g. fungal:bacterial ratios). It was the purpose of this work to test these generalizations and advance our understanding how soil microbes regulate ecosystem carbon dynamics. This purpose was accomplished using field-based observation and controlled lab-based experimentation. In the first of the field-based studies, I determine the mineralization dynamics of glucose and its relationship to land-use, microbial community and edaphic characteristics across a southeastern U.S. landscape. I find that the size, activity, and fungal:bacterial dominance of the microbial community is unrelated to glucose mineralization but that land-use and the underlying edaphic variable, soil phosphorus, explains spatial variation in this process. In the second field-based study, I examine how an invasive grass affects belowground carbon-cycling in a southeastern forest. Again, I find no role of fungal:bacterial dominance but observethat the presence of the invasive grass accelerates carbon-cycling and depletes soil carbon stocks, likely via priming of the microbial communitys activity. Using lab-based experimentation, I explore the role of microbial community structure separate to the influence of confounding variables. I find that litter mineralization rates are dependent on the microbial community and that these communities exhibit home-field advantage. These results refute theories related to functional redundancy. In a follow-up experiment, I find that the perception of litter quality, of two chemically-distinct litters, is dependent on the microbial community. Specifically, communities sourced from herbaceous habitats perceive more chemically-simple litter to be of higher quality than more chemically-complex litter, whereas communities from forest habitats mineralize both litters similarly. Together, my findings show that although microbial community structure may appear unrelated to carbon dynamics in situ, controlled-experimentation reveals that this structure may play an important role in determining rates of ecosystem processes. Future work needs to resolve why there is an apparent disconnect between results from observational and controlled-experimental studies.