Soil microbial response to land management practices and wildfire in Appalachian ecosystems

Abstract

Carbon (C) and nitrogen (N) cycling within and across ecosystems is critically controlled by the activity of soil microbial communities. Understanding how soil microbial communities change as a result of different land management practices or natural disturbances can provide insight into the effects of these perturbations on ecosystem health, such as water quality and organismal growth. The studies presented in this thesis address how relevant land management or natural disturbances in Appalachia alter soil biogeochemistry and soil microbial community diversity, structure, and function. The management practices and disturbances in these studies include using biochar agricultural amendments, wildfires, prescribed fires, and partial harvesting. Amending soil with biochar (i.e. pyrolyzed biomaterial) is a practice intended to enhance soil water holding capacity, build soil organic C stocks, and improve nutrient retention. In addition to direct addition of pyrolyzed material in agricultural systems, wildfire and prescribed burning may also alter organic matter inputs, along with many other ecosystem effects. In Southern Appalachian forests, over a century of fire-exclusion has caused changes in forest structure and vegetative growth under a characteristically wet climate. More intense drought events are projected to reduce vegetative moisture, creating fuel that increases the likelihood and severity of wildfires. Prescribed burning can mitigate wildfire risk and provide additional benefits such as increased nutrient availability and promotion of desired plant species. In Chapter 2, I examine the long-term effects of hay, softwood, or hardwood biochars on soil physicochemical properties, microbial community composition, and microbial diversity in a Virginia pasture ecosystem. All biochar amendments increased C:N ratios four and half years following amendments, however, softwood and hay biochar treatments had greater modulating effects on soil moisture. Biochar or biochar type did not have a significant effect on microbial community composition, and microbial diversity in this system was correlated with variation in soil electrical conductivity. In Chapter 3, I used a taxonomic and multifunctional approach to characterize four forested watersheds in Southern Appalachia: an undisturbed and fire-excluded watershed, a watershed repeatedly burned with prescribed fires, a recently partially harvested watershed, and a watershed impacted by a severe wildfire in 2016. Nitrogen availability and N-acquisition extracellular enzyme activity increased in the prescribed burn watershed relative to the fire-excluded watershed, although microbial community composition was not significantly different. In contrast, microbial community structure in the wildfire and partially harvested watersheds were influenced by differences in the physicochemical characteristics of the soil imparted by their respective disturbances. The partially harvested watershed exhibited C-limited conditions and a less efficient use of C, while the wildfire watershed did not exhibit microbial function that significantly differed from any other watershed. These two studies could help land managers and scientists understand how soil microbial communities and nutrient levels change in response to land management practices in Appalachian ecosystems.