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Investigating Feedbacks From Soil Trace Gas Fluxes of Carbon Dioxide and Nitrogen Oxides to Anthropogenic Nitrogen Deposition and Climate Change

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Human alteration of the carbon (C) and nitrogen (N) cycles is having profound detrimental impacts on natural systems. This dissertation research focused on investigating soil feedbacks to multiple anthropogenic drivers to explore unifying mechanisms applicable to global biogeochemical modeling. In Chapter 1 and 2, I investigate how changes in C, N, and temperature regulate soil CO2 production (Rsoil) through changes to the Michaelis-Menten parameters (i.e. Vmax and kM) using soils from three contrasting ecosystems in southern California in Chapter 1 and in subtropical evergreen forests of southern China in Chapter 2. Overall, the response of Rsoil to N addition was generally dependent on C:N stoichiometry, consistent with predictions from the dynamic microbial carbon-use efficiency hypothesis. Furthermore, I show the first empirical results from whole soil measurements demonstrating temperature sensitivity of both Vmax and kM, showing strong support for substrate regulation of Rsoil temperature sensitivity across diverse soils. Results from Chapters 1 and 2 demonstrate great potential for Michaelis-Menten kinetics in describing Rsoil responses to N and temperature, with implications for understanding microbial physiology and broad applicability to global biogeochemical modeling. Chapter 3 extends this research by exploring N trace gas emissions and evaluating variation in the microbial community composition along an N deposition gradient in the Colorado Desert. Results from Chapter 3 present soil N fluxes that were considerably higher than expected, demonstrating a need for greater appreciation of arid systems in global N budgets. While short term effects of experimental N addition resulted in inconsistent responses in the microbial community composition, long-term N deposition resulted in a distinct differentiation of the microbial community, with possible implications for N cycling. Microbial communities associated with nitrification, identified from 16S rRNA sequencing and qPCR of amoA, demonstrate a shift from archaeal dominance at the low deposition site to more bacterial dominance at the high deposition site. Overall, results from this dissertation contribute to mechanistic understanding of soil feedbacks to climate change and nitrogen deposition, which is necessary for predicting future changes and developing strategies for mitigation of anthropogenic global change drivers.

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