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 (N(2)) and nitrous oxide (N(2)O) in a temperature controlled continuous water flow laboratory microcosm under 10, 30, 100, and 300 mu M of (15)N enriched nitrate (NO(3) (-), similar to 98 at. %). Effluxes of both N(2) and N(2)O from sediment to the overlying water increased with increasing NO(3) (-) load. Although the ratio of N(2)O to N(2) increased with increasing NO(3) (-) load, it remained below 0.04, N(2) always being the main product. At the NO(3) (-) concentrations most frequently found in the studied river water (10-100 mu M), up to 8% of the NO(3) (-) was removed in denitrification, whereas with the highest concentration (300 mu M), the removal by denitrification was less than 2%. However, overall up to 42% of the NO(3) (-) was removed by mechanisms other than denitrification. As the microbial activity was simultaneously enhanced by the NO(3) (-) load, shown as increased oxygen consumption and dissolved inorganic carbom efflux, it is likely that a majority of the NO(3) (-) was assimilated by microbes during their growth. The (15)N content in ammonium (NH(4) (+)) in the efflux was low, suggesting that reduction of NO(3) (-) to NH(4) (+) was not the reason for the NO(3) (-) removal. This study provides the first published information on denitrification and N(2)O fluxes and their regulation by NO(3) (-) load in eutrophic high latitude rivers.