Soil microbial community structure and function along environmental gradients: implications for wetland nitrogen cycling

Riparian wetlands are sites of intense biogeochemical activity and play an important role in improving water quality and recycling nutrients through microbially-mediated ecosystem functions. To enhance restoration of nutrient cycling functions, it is critical to know how abiotic factors influence microbial community structure and ecosystem function. Microbial systems are especially important for investigating structure-function relationships because microbial taxa can be linked to specific biogeochemical transformations.;The goal of this study was to document factors that significantly contribute to plant and microbial community variation in order to identify potential environmental filters responsible for structuring communities among and within wetlands. Vegetation surveys and microbial community composition (assessed by molecular analysis) along with landscape and local soil factors were assessed at 27 restored wetlands. Surrounding land use and soil fertility, specifically inorganic nitrogen concentration, were associated with plant community variation. In contrast, local soil factors, especially soil pH, were strongly linked to bacterial and denitrifier community variation. Identifying environmental drivers that affect plant and microbial communities can inform managers of possible ecosystem functions that can be supported at a restoration site.;To better understand structure-function relationships, I focused on microbial functional guilds and activities for the processes of denitrification (NO ₃− → N2) and nitrification (NH₄ + → NO₃−). Together, these nitrogen transformations influence nitrate removal capacity within wetlands. Denitrification is a facultatively anaerobic process, while microbes responsible for nitrification are obligate aerobes. Thus, these nitrogen transformations are particularly sensitive to oxygen concentration and soil moisture. Denitrifier and ammonia oxidizer composition were initially studied along an environmental gradient within a single wetland. I demonstrated that denitrifier taxa occupied a wider moisture range compared to ammonia oxidizer taxa. To further investigate how environmental gradients related to microbial structure-function relationships, I compared denitrifier and ammonia oxidizer community composition and activity along an environmental gradient within different wetland sites. Denitrifier and ammonia oxidizer community composition were distinct between upland and wetland plots at all sites. Microbial community structure was relatively constant, whereas potential microbial activity decreased over time at most sites. In addition, potential denitrification and nitrification rates were mainly influenced by environmental conditions compared to community structure (community composition or abundance).;To understand if hydrologic history constrains contemporary microbial function, I investigated the response of initial microbial communities shaped by historical hydrologic regime to a drying/flooding treatment in a mesocosm experiment. Initial hydrologic history strongly affected community structure and function. Potential denitrification rate significantly increased under wetter conditions, whereas potential nitrification rates remained unchanged in many cases. Results suggest that denitrification activity was more sensitive to drying/flooding, whereas nitrification was constrained by the resident community structure. Variation in microbial response can result in a shift in dominant nitrogen cycling transformations within wetlands.;Restoring nitrate removal function by encouraging denitrification is not as straightforward as flooding an area. Evidence from this study suggests that nitrification, resulting in nitrate production, can still occur under saturated conditions (presumably low [O₂]) if not limited by low pH or ammonium. As a consequence, while trying to restore one microbial function, restoration practitioners must consider how other, potentially opposing biogeochemical functions will respond.