Browsing by Author "Osburn, Ernest D."

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    Effects of land management and climate change on soil microbial communities in Appalachian forest ecosystems
    Osburn, Ernest D. (Virginia Tech, 2021-03-26)
    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.
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    Effects of Rhododendron removal on soil bacterial and fungal communities in southern Appalachian forests
    Osburn, Ernest D.; Miniat, Chelcy F.; Elliott, Katherine J.; Barrett, John E. (2021-09-15)
    Rhododendron maximum, a native ericaceous evergreen shrub, is expanding in forests of the southern Appalachian region following eastern hemlock (Tsuga canadensis) mortality due to hemlock woolly adelgid (Adelges tsugae) infestations. The goal of our study was to examine soil microbial community responses to experimental R. maximum removal treatments. The experiment was implemented as a 2 x 2 factorial design, including two R. maximum canopy removal levels (cut vs. not cut) combined with two forest floor removal levels (burned vs. not burned). These treatments were designed as potential management strategies to facilitate hardwood tree establishment in forests that have experienced T. canadensis declines. We sampled soils after removals and characterized bacterial and fungal communities using amplicon sequencing. Shrub removal did not affect bacterial or fungal alpha diversity but did affect both bacterial and fungal community composition. Relative abundances of bacterial phyla and fungal classes exhibited no differences among R. maximum removal treatments. However, specific bacterial and fungal taxa that were responsive to R. maximum removal (i.e., differentially abundant sequences) did exhibit clear patterns at high taxonomic levels. Specifically, taxa that responded negatively to R. maximum removal were found primarily in two bacterial phyla (Proteobacteria and Bacteroidetes) and one fungal class (Archaeorhizomycetes) while positive responders were clustered in several other bacterial phyla (e.g., Actinobacteria, Planctomycetes, Cyanobacteria). Fungal functional guilds also responded to R. maximum removal, including negative responses of ericoid mycorrhizae and positive responses of arbuscular mycorrhizae and wood saprotrophs. Effects of R. maximum removal on soil microbial communities were minor overall, but clear effects on some key functional groups were evident (i.e., mycorrhizal fungi), suggesting that microbial responses to R. maximum removal may influence recovery of forests in the southern Appalachian region.
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    Historical land use has long-term effects on microbial community assembly processes in forest soils
    Osburn, Ernest D.; Aylward, Frank O.; Barrett, J.E. (Springer Nature, 2021-09-10)
    Land use change has long-term effects on the structure of soil microbial communities, but the specific community assembly processes underlying these effects have not been identified. To investigate effects of historical land use on microbial community assembly, we sampled soils from several currently forested watersheds representing different historical land management regimes (e.g., undisturbed reference, logged, converted to agriculture). We characterized bacterial and fungal communities using amplicon sequencing and used a null model approach to quantify the relative importance of selection, dispersal, and drift processes on bacterial and fungal community assembly. We found that bacterial communities were structured by both selection and neutral (i.e., dispersal and drift) processes, while fungal communities were structured primarily by neutral processes. For both bacterial and fungal communities, selection was more important in historically disturbed soils compared with adjacent undisturbed sites, while dispersal processes were more important in undisturbed soils. Variation partitioning identified the drivers of selection to be changes in vegetation communities and soil properties (i.e., soil N availability) that occur following forest disturbance. Overall, this study casts new light on the effects of historical land use on soil microbial communities by identifying specific environmental factors that drive changes in community assembly.
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    Soil Bacterial and Fungal Communities Exhibit Distinct Long-Term Responses to Disturbance in Temperate Forests
    Osburn, Ernest D.; McBride, Steven Glynn II; Aylward, Frank O.; Badgley, Brian D.; Strahm, Brian D.; Knoepp, Jennifer D.; Barrett, John E. (2019-12-11)
    In Appalachian ecosystems, forest disturbance has long-term effects on microbially driven biogeochemical processes such as nitrogen (N) cycling. However, little is known regarding long-term responses of forest soil microbial communities to disturbance in the region. We used 16S and ITS sequencing to characterize soil bacterial (16S) and fungal (ITS) communities across forested watersheds with a range of past disturbance regimes and adjacent reference forests at the Coweeta Hydrologic Laboratory in the Appalachian mountains of North Carolina. Bacterial communities in previously disturbed forests exhibited consistent responses, including increased alpha diversity and increased abundance of copiotrophic (e.g., Proteobacteria) and N-cycling (e.g., Nitrospirae) bacterial phyla. Fungal community composition also showed disturbance effects, particularly in mycorrhizal taxa. However, disturbance did not affect fungal alpha diversity, and disturbance effects were not consistent at the fungal class level. Co-occurrence networks constructed for bacteria and fungi showed that disturbed communities were characterized by more connected and tightly clustered network topologies, indicating that disturbance alters not only community composition but also potential ecological interactions among taxa. Although bacteria and fungi displayed different long-term responses to forest disturbance, our results demonstrate clear responses of important bacterial and fungal functional groups (e.g., nitrifying bacteria and mycorrhizal fungi), and suggest that both microbial groups play key roles in the long-term alterations to biogeochemical processes observed following forest disturbance in the region.
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    Soil microbial response to Rhododendron understory removal in southern Appalachian forests: Effects on extracellular enzymes
    Osburn, Ernest D.; Elliott, Katherine J.; Knoepp, Jennifer D.; Miniat, Chelcy F.; Barrett, John E. (2018-12)
    Rhododendron maximum is a native evergreen shrub that has expanded in Appalachian forests following declines of american chestnut (Castanea dentata) and eastern hemlock (Tsuga canadensis). R. maximum is of concern to forest managers because it suppresses hardwood tree establishment by limiting light and soil nutrient availability. We are testing R. maximum removal as a management strategy to promote recovery of Appalachian forests. We hypothesized that R. maximum removal would increase soil nitrogen (N) availability, resulting in increased microbial C-demand (i.e. increased C-acquiring enzyme activity) and a shift towards bacterial-dominated microbial communities. R. maximum removal treatments were applied in a 2 x 2 factorial design, with two R. maximum canopy removal levels (removed vs not) combined with two O-horizon removal levels (burned vs unburned). Following removals, we sampled soils and found that dissolved organic carbon (DOC), N (TDN, NO3, NH4), and microbial biomass all increased with R. maximum canopy + O-horizon removal. Additionally, we observed increases in C-acquisition enzymes involved in degrading cellulose (beta-glucosidase) and hemicellulose (B-xylosidase) with canopy + O-horizon removal. We did not see treatment effects on bacterial dominance, though F:B ratios from all treatments increased from spring to summer. Our results show that R. maximum removal stimulates microbial activity by increasing soil C and N availability, which may influence recovery of forests in the Appalachian region.