Characterizing Biogeochemical Shifts in Two Salix Caroliniana Michx. Encroached Freshwater Subtropical Marshes

Shrub encroachment is a common disturbance in wetlands, but little is known about how shrub encroachment impacts functions such as carbon (C) storage and nitrogen (N) cycling. The objective of this thesis was to identify differences in physiochemical properties (within soil, water and leaf tissue) and biogeochemical processes (soil respiration, enzyme activity, litter decomposition, and N cycling) in two subtropical freshwater marshes encroached by coastal plain willow (Salix caroliniana Michx.). Two study regions (Moccasin Island and Lake Apopka) were selected because of their location in the St. John\u27s River watershed and their unique histories and hydroperiod, allowing for an investigation of how synonymous the effects of willow-encroachment are across sites. A stratified random sampling design was employed in each region, identifying three plot types: willow ( \u3e 80 % willow aboveground coverage), adjacent marsh ( \u3e 80% herbaceous aboveground coverage and \u3c 1 m from willows), and control marsh ( \u3e 80% herbaceous aboveground coverage and \u3e 10 m from willows) plots. Triplicate soil samples were collected in each plot in the wet and dry seasons of 2017 and analyzed for physiochemical properties (bulk density, moisture, nutrient content) and used in laboratory assays to measure soil respiration, enzyme activity, and potential N mineralization and denitrification rates. Leaf tissue was collected from the dominant vegetation in each plot and analyzed for nutrient content (total C, lignin-C, and total N). Short-term litter decay rate was determined using a litter-bag field experiment. Bioavailable N and dissolved organic C (DOC) concentrations were determined from surface and porewater collected from the center of each plot. In Moccasin Island, lower decomposition rates and greater denitrification, potential N mineralization, and soil C and N content were detected in willow and adjacent marsh plots, suggesting greater C storage and N cycling in willow-encroached marshes compared to non-encroached control marshes in Moccasin Island. Conversely, soil C and N content was lowest in willow plots in Lake Apopka. Decomposition and microbial activity (enzyme activity and respiration) were lowest in willow dominated areas and correlated to soil nutrient concentrations. In both regions, microbial compositional changes (gene copy number) were detected between plot types, mainly in bacteria (β-proteobacteria and Bacteroidetes) for Moccasin Island and archaea and fungi abundance in Lake Apopka. Ultimately, willow plots in both regions had greater lignin-C content and short-term litter C storage. Greater bioavailable N was also observed in adjacent and/or willow plots in both regions. However, soil C storage and N cycling differences were not synonymous between the two regions. Future studies of willow effects will need to look at multiple sites or risk making inaccurate generalizations. From the findings from this study, wetland processes can be altered in willow-encroached marshes and this data can help land managers decide where to allocate resources based on valued ecosystem services