OOS 27-2 - CANCELLED - Hydrologic connectivity drives nitrogen removal rates in forested headwater wetlands

Wetlands are biogeochemical control points, regulating the removal of anthropogenic nitrogen (N) from local to watershed scales. Floods influence wetland biogeochemical activity by delivering dissolved organic matter (DOM) and nutrients to wetland soils. This delivery is affected by the mode of hydrologic connectivity (i.e., hillslope-connected vs. floodplain-connected flowpaths) to the stream. While N removal by wetlands has been studied extensively, we lack an understanding of how differences in flood-driven water and material delivery affects post-flood biogeochemical processing within wetlands, and the subsequent implications for watershed nutrient export to larger downstream systems. The objective of this study is to investigate wetland hydro-biogeochemical responses (specifically, N processing) to floods. We conducted a study investigating how hydrologic connectivity mediates wetland biogeochemical response to flood events at a forested, headwater plain coastal system where elevational differences have resulted in wetlands along a gradient of hydrologic connectivity. Here, wetlands are defined by their hydrologic connectivity via shallow subsurface flowpaths in the hillslope and surface inundation in the floodplain, respectively. We used isotope pairing on soil slurries to measure nitrogen removal (denitrification, anammox) and recycling (dissimilatory nitrate reduction to ammonium; DNRA) potentials in wetland soils along the hydrologic gradient. We also measured dissolved inorganic nitrogen (DIN; nitrate-nitrite and ammonium) in surface water and percent organic matter in soil (loss on ignition). We found that dry season (summer) conditions in the wetlands varied between modes of hydrologic connectivity. Surface water DIN was highly variable, and ammonium was the predominant form. Ammonium concentrations were similar between wetland types (hillslope: 40±30 µM N; floodplain: 32±36 µM N), but nitrate-nitrite concentrations were roughly twice as high in hillslope wetlands (16±23 µM N) compared to floodplain wetlands (8±2 µM N). Soil organic content was similar between wetland types (hillslope: 9±6%; floodplain: 10±6%). Denitrification rates were nearly twice as high in hillslope-connected wetlands compared to floodplain-connected wetlands (46±28 µmol N kg dry soil-1 h-1 vs. 24±25 µmol N kg dry soil-1 h-1). In contrast, DNRA dominated in floodplain-connected wetlands (56±40 µmol N kg dry soil-1 h-1) and was nearly double that of hillslope-connected wetlands (31±22 µmol N kg dry soil-1 h-1). We hypothesize that a lower organic matter: nitrate favored denitrification in hillslope-connected wetlands. Future work will measure nitrate reduction rates following flood events. We hypothesize that floods will heighten the differences in nitrate reduction pathways by delivering labile, soil-derived organic matter to hillslope wetlands.