Denitrification and nitrous oxide effluxes in boreal, eutrophic river sediments under increasing nitrate load: a laboratory microcosm study

Intact sediment cores from rivers of the Bothnian Bay (Baltic Sea) were studied for denitrification based on benthic fluxes of molecular nitrogen (N2) and nitrous oxide (N2O) in a temperature controlled continuous water flow laboratory microcosm under 10, 30, 100, and 300 μM of 15N enriched nitrate (NO3 −, ~98 at. %). Effluxes of both N2 and N2O from sediment to the overlying water increased with increasing NO3 − load. Although the ratio of N2O to N2 increased with increasing NO3 − load, it remained below 0.04, N2 always being the main product. At the NO3 − concentrations most frequently found in the studied river water (10–100 μM), up to 8% of the NO3 − was removed in denitrification, whereas with the highest concentration (300 μM), the removal by denitrification was less than 2%. However, overall up to 42% of the NO3 − was removed by mechanisms other than denitrification. As the microbial activity was simultaneously enhanced by the NO3 − load, shown as increased oxygen consumption and dissolved inorganic carbom efflux, it is likely that a majority of the NO3 − was assimilated by microbes during their growth. The 15N content in ammonium (NH4 +) in the efflux was low, suggesting that reduction of NO3 − to NH4 + was not the reason for the NO3 − removal. This study provides the first published information on denitrification and N2O fluxes and their regulation by NO3 − load in eutrophic high latitude rivers.