The cycling of nutrients, such as nitrogen (N), is arguably one of the most critical ecosystem services provided by soil. Nitrogen is the limiting nutrient for plant growth in many terrestrial ecosystems and can consequently regulate net primary production, plant diversity, and community composition. Transformations of available N, which are catalyzed by soil microorganisms, can also affect air and water quality, with possible implications for climate change and human health. In an era of global environmental change, it is paramount to gain a mechanistic understanding of how soil N is affected by anthropogenically derived perturbations such as exotic plant invasion and elevated nutrient deposition. Using a multifactor global change experiment, I assessed how three principal global change factors - exotic plant invasion, N deposition (simulated by N fertilization), and aboveground vegetation removal (to simulate cattle grazing or mowing) - affected soil N cycling, namely NH₄ ⁺ and NO₃^- availability and potential rates of nitrification and denitrification, in a California grassland. In order to increase understanding of how soil microbial communities regulate changes in N cycling, I concurrently measured broad-scale community structure of bacteria and archaea and the abundances of ammonia-oxidizing bacteria and archaea. I found that two invasive plants, Aegilops triuncialis and Elymus caput-medusae, reduced soil N availability and nitrification and denitrification potentials compared to perennial-dominated native communities but not naturalized exotic communities. Aboveground vegetation removal, which is often used as a tool to manage invasive plant populations (through cattle grazing or mowing), tended to exacerbate the effects of invasion by further reducing nitrification potential and soil NO₃^- availability. Fertilization with NH₄NO₃ consistently increased nitrification potential and soil NO₃^- availability, yet NH₄ ⁺ remained unaffected and denitrification potential was reduced. When combined, defoliation and N fertilization always produced additive effects. Finally, despite the sometimes dramatic shifts in N availability and potential rates that were observed, microbial community composition remained unaffected by changes in plant composition, N fertilization and defoliation. Overall, these findings provide evidence that N cycling is uniquely affected by each individual global change factor, and that the interactive effects of N fertilization and defoliation can be predicted based on combining single factor studies. These results also suggest that microbial communities composition is insensitive to global change in this system, and that microbial activity - as measured by rates of N cycling - is decoupled from community composition.

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