N2O production, nitrification and denitrification in an estuarine sediment

The mechanisms regulating N2O production in an estuarine sediment (Tama Estuary, Japan) were studied by comparing the change in N2O production with those in nitrification and denitrification using an experimental continuous-flow sediment-water system with N-15 tracer (N-15-NO3- addition). From February to May, both nitrification and denitrification in the sediment increased (246 to 716 mu mol N m(-2) h(-1) and 214 to 1260 mu mol N m(-2) h(-1), respectively), while benthic NO evolution decreased slightly (1560 to 1250 nmol N m(-2) h(-1)). Apparent diffusion coefficients of inorganic nitrogen compounds and O-2 at the sediment-water interface, calculated from the respective concentration gradients and benthic fluxes, were close to the molecular diffusion coefficients (0.68-2.0 times) in February. However, they increased to 8.8-52 times in May except for that of NO2-, suggesting that the enhanced NO3- and O-2 supply from the overlying water by benthic irrigation likely stimulated nitrification and denitrification. Since the progress of anoxic condition by the rise of temperature from February to May (9 to 16 degreesC) presumably accelerated N2O production through nitrification, the observed decrease in sedimentary N2O production seems to be attributed to the decrease in N2O production/occurrence of its consumption by denitrification. In addition to the activities of both nitrification and denitrification, the change in N2O metabolism during denitrification by the balance between total demand of the electron acceptor and supply of NO3- +NO2- can be an important factor regulating N2O production in nearshore sediments. (C) 2001 Academic Press.