Nitrogen transformations and NO₃^- removal at a soil-stream interface: A stable isotope approach

The natural removal of NO₃^- by denitrification within riparian zones of streams and rivers is an area of considerable interest owing to its potential to minimize the impacts of excess anthropogenic loadings. In this study we utilize natural variations in stable N isotopic compositions of NO₃^- and NH₄⁺ within a transect of shallow wells extending 4 m inland from Smith Creek, a southwestern Michigan stream, to provide insight into microbial processes and the extent of NO₃^- removal within a soil-stream interface. Within this region three water masses with unique biogeochemical characteristics intersect: a shallow flow rich in NH₄⁺ and dissolved organic carbon (DOC), a deep groundwater mass rich in NO₃^- but depleted in DOC, and stream water low in NO₃^-, NH₄⁺, and DOC. N isotope values for NO₃^- within the well transect were highly variable (-7.7-34.1‰) and reflected intense microbial activity within this narrow region. Isotopic variation was primarily controlled by upwelling of deep groundwater near the stream and partial loss of NO₃^- via denitrification that was dependent upon a supply of DOC from shallow groundwater. Quantitative estimates of the fraction of NO₃^- removed due to denitrification within the soil-stream interface were obtained from N isotope data using a modified Rayleigh equation. Conservative estimates of NO₃^- removal range from 0% to 86%. In conjunction with measurements of hydrological flows within the sampling wells we provide a novel estimate of NO₃^- removal based only on natural abundance isotope measurements. NO₃^- removal was found to vary from undetectable levels to 123 mmol·L^-1·m^-2·d^-1 for the narrow region of the soil-stream interface in which deep and shallow groundwater intersect.