Denitrification and N2O effluxes in the Bothnian Bay (northern Baltic Sea) river sediments as affected by temperature under different oxygen concentrations

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Denitrification rates and nitrous oxide (N2O) effluxes were measured at different temperatures and for different oxygen concentrations in the sediments of a eutrophied river entering the Bothnian Bay. The experiments were made in a laboratory microcosm with intact sediment samples. 15N-labelling was used to measure denitrification rates (Dw). The rates were measured at four temperatures (5, 10, 15 and 20°C) and with three oxygen inputs (<0.2, 5, and 10 mg O2 l−1). The temperature response was highly affected by oxygen concentration. At higher O2 concentrations (5 and 10 mg O2 l−1) a saturation over 10°C was observed, whereas the anoxic treatment (<0.2 mg O2 l−1) showed an exponential increase in the temperature interval with a Q10 value of 3.1. The result is described with a combined statistical model. In contrast with overall denitrification, the N2O effluxes from sediments decreased with increasing temperature. The N2O effluxes had a lower response to oxygen than denitrification rates. The N2O/N2 ratio was always below 0.02. Increased temperatures in the future could enhance denitrification rates in boreal river sediments but would not increase the amount of N2O produced.

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