Controls of nitrogen cycling under gaining and losing conditions in a first order agricultural stream

Low order streams drain a big proportion of river catchments. They are not only fed by groundwater, but may also lose water to their connected aquifers and, in turn, can contribute to a substantial fraction of groundwater recharge. In such cases, these streams are typically characterized by the coexistence of gaining and losing stream reaches. Along the bidirectional exchange flow paths large biogeochemical gradients can evolve so that the exchange zones can function as hotspots for biogeochemical processes (such as the important (de)nitrification processes in croplands), which can substantially change under these two conditions. An agricultural first order stream (Schönbrunnen) in south-western Germany was equipped with stream gauging stations and piezometers were installed in the adjacent shallow aquifer, in order to find out how these biogeochemical processes change under losing versus gaining conditions. Hydrological and hydrochemical variables within the immediate vicinity of the stream, as well as stable nitrogen isotopes have been monitored between August 2017 and May 2020 to spatially and temporally identify the controls of nitrogen cycling dynamics in the Schönbrunnen.

Gaining and losing conditions at the Schönbrunnen were determined by salt tracer experiments and the flow direction (upwelling groundwater or downwelling streamwater) of the exchanging fluxes was determined based on hydraulic head contour maps.

Hydrochemical data suggests that nitrate reduction occurs within the first 20 cm of the streambed in the losing reaches. In these reaches, isotopic data depicts that nitrate is reduced along the flow path between stream and groundwater. Ammonium and organic electron donors (DOC) were found at greater depths in these reaches. By contrast, increasing nitrite and nitrate concentrations were observed also along the last 20 cm of the upwelling flow paths (gaining reaches). In summary, assuming that the transition zone between groundwater and streams is only a hotspot for denitrification might not always be true, as our field data suggests that redox conditions in the streambed, and in turn, the resulting biogeochemical processes differed substantially between losing and gaining reaches.