Surface-subsurface hydrologic exchange and nutrient dynamics in the hyporheic zone of the Tanana River

Abstract

The aquatic-terrestrial interface is an active site of biogeochemical transformation, regulating the flux of nutrients between ecosystems. I addressed the hydrologic controls on nitrogen biogeochemistry in the hyporheic zone of a glacially fed river. I measured hyporheic concentrations of solutes and gases along subsurface flowpaths on two islands. Denitrification was quantified using an in situ [delta]⁻¹⁵NO₃⁻ push-pull technique. Nitrate concentration was consistently greater in river than in hyporheic water. Denitrification ranged from 1.9 - 29.4 mgN kg sediment⁻¹ day⁻¹. Hotspots of methane partial pressure, averaging 50,000 ppmv, were found in densely vegetated areas with low oxygen concentration (<0.5 mgO₂ L⁻¹). Hyporheic flow was an important source of nitrogen to microbes and vegetation, transporting on average 0.41 gNO₃⁻-N m⁻² day⁻¹ through surface sediments. Results suggest that denitrification is a major sink for river nitrate in boreal forest floodplain soils, particularly at the river-sediment interface. The stability of the river hydro graph is a key factor regulating anaerobic metabolism in the hyporheic zone.