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

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 Q 10 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.