Effects of land management and climate change on soil microbial communities in Appalachian forest ecosystems

In terrestrial ecosystems, microorganisms are the dominant drivers of virtually all ecosystem processes, particularly cycling of carbon (C), nitrogen (N), and phosphorus (P). These microbial functions are critical for promoting ecosystem services that support human well-being, such as provisioning of clean drinking water, nitrogen retention, and carbon storage. In forests of the Appalachian region of the eastern US, these ecosystem services are threatened by multiple anthropogenic influences, including present and past land use activities (e.g., logging, conversion to agriculture) and climate change (e.g., intensifying droughts). However, despite the central importance of microbial communities in promoting ecosystem functions, impacts of land management and climate change on soil microorganisms remain poorly understood in the region. This dissertation seeks to address the following questions: 1) How does a new forest management practice, Rhododendron understory removal, influence the ecosystem functions of soil microbial communities? 2) Do historical land management activities have long-term legacy effects on the structure and ecosystem functions of soil microbial communities? And 3) Does historical land use influence responses of soil microbial communities to intensifying drought? In chapter 2, I show that experimental Rhododendron understory removal increased soil C and N availability, thereby promoting increased total microbial biomass. This increased microbial biomass resulted in elevated production of microbial extracellular enzymes, which increased rates of C and N cycling in soils following Rhododendron removal. In chapter 3, I examined soils across several historically disturbed and adjacent undisturbed reference forests and show that historical management activities, e.g., logging, conversion to agriculture, have long-term effects on soil microbial communities 4-8 decades after management activities occurred. These effects included increased bacterial diversity, increased relative abundance of r-selected bacterial taxa, and increased abundance of arbuscular mycorrhizal fungi. In chapter 4, I show that key soil biogeochemical processes, i.e., C mineralization, N mineralization, and nitrification, exhibit generally higher rates in historically disturbed forests relative to adjacent reference forests. Further, I attributed these changes in ecosystem process rates to changes in key aspects of microbial communities, including microbial biomass, extracellular enzyme activities, and bacterial r- vs K-selection. In chapter 5, I conducted a drought-rewetting experiment and show wide-ranging effects of experimental drought on soil microbial communities, including altered diversity, community composition, and shifts in the relative abundances of several specific taxa. Further, drought responses were particularly evident in soils from historically disturbed forests, indicating influences of land management on responses of soil communities to climate change. Finally, in chapter 6, I show that the experimental drought also influenced several ecosystem-scale properties of soils, including increased soil N pools and increased respiratory C loss. Overall, my dissertation reveals wide-ranging effects of anthropogenic activities on soil microorganisms and shows that microbial communities will influence forest responses to global change at the ecosystem scale.