Understanding the response of microbial communities to land use change is essential for restoration of wetland water quality functions. Denitrification and nitrification are microbial processes that are sensitive to oxygen levels and are influenced by moisture fluctuations. The objective of this research was to investigate the link between microbial community composition and nitrogen cycling activity in response to a moisture gradient. Sediment samples were collected from transects spanning a moisture gradient perpendicular to the shore of Thompson Lake at Emiquon Preserve (Lewistown, IL), a restored floodplain in the early stages of development. Microbial function was assessed using denitrification and nitrification assays, while microbial community structure was evaluated using DNA fingerprinting techniques based on rRNA genes (for total bacterial community), or on nosZ and amoA (for the specific microbial populations responsible for denitrification and nitrification, respectively). Functional gene abundance was also assessed.

Results/Conclusions

Denitrification and nitrification rates were highest in the most saturated sediments along the moisture gradient. Similar to the nitrification potential, the amoA gene abundance was highest in the most saturated plots. Specific denitrifier populations (ANOSIM R = 0.819, P = 0.001) and total bacterial assemblages (ANOSIM R = 0.987, P = 0.001) under completely saturated conditions were distinct from communities experiencing fluctuating moisture conditions in response to changes in lake water level. Denitrifier assemblages (ANOSIM R = 0.440, P = 0.001) and total bacterial communities (ANOSIM R = 0.393, P = 0.001) experiencing fluctuating moisture levels were also different from microbial communities in drier sites furthest from the lake. Moisture conditions influence bacterial community composition. Distinct community assemblages were observed across all replicates and time points in completely saturated sediments. More variable community assemblages were observed across all samples in the driest plots, and the greatest fluctuations in moisture resulted in a discrete group of microbial assemblages across all transects and time points. Further understanding of the relationship between microorganisms and their activity in response to changing water regimes may enhance floodplain management, thus, contributing to the goal of improving downstream aquatic ecosystems.