Investigating Hydrologic Connectivity as a Driver of Wetland Biogeochemical Response to Flood Disturbances

Authors

Corianne Tatariw¹* (ctatariw@ua.edu), Delaney M. Peterson¹, Lidia Molina Serpas¹, S. Elaine Rice¹, Gabrielle Allred¹, Taylor C. Ledford¹, Xingyuan Chen², Amy E. Goldman², C. Nathan Jones¹, James Stegen², Behzad Mortazavi¹

Institutions

¹The University of Alabama, Tuscaloosa, AL; ²Pacific Northwest National Laboratory, Richland, WA

Abstract

Wetlands are terrestrial-aquatic interfaces that serve as biogeochemical control points regulating the removal of anthropogenic nitrogen (N) from local to watershed scales. Flood disturbances influence wetland biogeochemical activity by delivering dissolved organic matter (DOM) and nutrients to wetland soils. This delivery is regulated by the mode of hydrologic connectivity (i.e., hillslope-connected vs. floodplain-connected flowpaths) to the stream. However, researchers lack an understanding of how differences in flood-driven water and material delivery affect post-flood biogeochemical processing within wetlands. The quantity and composition of OM can affect whether N is removed or recycled via competing microbial pathways, therefore, researchers hypothesized that flood disturbances will heighten the differences in nitrate reduction pathways by delivering soil-derived OM to hillslope wetlands and stream-derived OM to floodplain wetlands. Researchers conducted a study investigating how hydrologic connectivity mediates wetland biogeochemical response to floods at a forested, headwater coastal plain system where elevational differences have resulted in wetlands along a gradient of hydrologic connectivity, ranging from shallow subsurface flowpaths in the hillslope and surface inundation in the floodplain. Researchers designed the project to adhere to ICON science principles that is, to Integrate work across disciplines and scales; Coordinate use of consistent protocols; support Open exchange of ideas, and to use a Networked approach to data generation through stakeholder engagement. Researchers installed piezometers in nine wetlands along a gradient of hydrologic connectivity to characterize wetland hydrology during flood events. Researchers measured nitrogen removal (denitrification, anammox) and recycling (dissimilatory nitrate reduction to ammonium; DNRA) potentials in wetland soils along the hydrologic gradient using isotope pairing on soil slurries. Researchers also measured dissolved inorganic nitrate-nitrite in surface water and percent OM in soil (loss on ignition). Researchers found that during the dry season (baseflow), hillslope-connected wetlands had a lower OM: nitrate ratio that likely promoted denitrification over DNRA, as denitrification rates were nearly twice as high in hillslope-connected wetlands compared to floodplain-connected wetlands. In contrast, DNRA dominated in floodplain-connected wetlands and rates were nearly double that of hillslope-connected wetlands. Future event-based sampling efforts will relate changes in flood-derived organic matter composition to nitrate reduction rates following flood disturbances. In the second year of the study, these data will be paired with event-based wetland- and watershed-scale N measurements to develop a hydro-biogeochemical model to assess how post-flood N processing impacts watershed N impact.