Effects of land management and climate change on soil microbial communities in Appalachian forest ecosystems
Osburn, Ernest Daniel
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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.
General Audience Abstract
Forest ecosystems of the southern Appalachian region provide numerous services that support human well-being, including provisioning of clean drinking water and the retention of nutrients and carbon (C). These ecosystem services are dependent on several processes that occur in soil, including the C and nitrogen (N) cycles. These cycles, in turn, are carried out primarily by microorganisms (bacteria and fungi) that live in soil. The ecosystem services provided by these forests are being threatened in the Appalachian region by a variety of land use activities such as logging and conversion of forests to agriculture and are also being threatened by climate change. However, despite the critical role of microorganisms in supporting ecosystem services, effects of land use activities and climate change on soil microorganisms are largely unknown in the Appalachian region. The goal of this dissertation is to answer the following questions: 1) How does Rhododendron understory removal, a new land management practice, influence soil microbial communities? 2) What are the long-term effects of historical land use activities, such as logging and conversion to agriculture, on soil microbial communities? And 3) In what ways will intensifying droughts influence soil microorganisms in Appalachian forests? I used a variety of approaches to answer these questions, including experiments and observational approaches. In chapter 2, I show that experimental Rhododendron understory removal increased the overall size of microbial communities (i.e., more microbial biomass) due to greater availability of soil resources (i.e., C and N). These larger microbial communities produced more enzymes involved in C and N metabolism, thereby increasing rates of C and N cycling in soils. In chapter 3, I surveyed soils from several forests that were disturbed by humans ~40-80 years previously (i.e., logged, converted to agriculture) and showed that these historical human activities have many long-term effects on soil bacteria and fungi. Specifically, in historically disturbed soils, I observed higher diversity of bacteria, higher abundance of rapidly growing (i.e., r-selected) bacteria and higher abundance of some groups of fungi that associate with plant roots and aid in acquiring nutrients for plants (i.e., mycorrhizal fungi). In chapter 4, I show that these historically disturbed soils also had altered rates of C and N cycling and that these altered cycling rates were associated with changes in several properties of microbial communities, including microbial enzymes, microbial biomass, and growth rates of bacteria (i.e., r-selection). In chapter 5, I conducted an experimental drought in these soils and show that drought has wide-ranging effects on many aspects of microbial communities, including effects on diversity, species composition, and abundances of many specific microbial taxonomic groups. Further, responses of microbial communities were larger in soils from historically disturbed forests, showing that past management will influence microbial responses to future climate change. In chapter 6, I show that many aspects of the soil ecosystems as a whole were also impacted by the experimental drought. In particular, soils exposed to drought released more CO2 over the course of the experiment and had higher N concentrations than control soils. Overall, my dissertation identifies many influences of land management and climate change on soil microbial communities and shows that these microorganisms will influence forest responses to global change at the ecosystem scale.
- Doctoral Dissertations