Abstract :
A new laboratory device was used to study changes in N2O production and consumption within an 1 mm layer of settled seston material during different seasons. In separate water bodies above and below the seston layer, the conditions of open bottom water and sediment porewater were simulated, respectively, and the concentrations of nitrate and oxygen were controlled. The exchange of substances through the layer was limited to diffusion processes. The concentrations of dissolved oxygen, nitrate, ammonium, nitrite and nitrous oxide were monitored continuously at the outflows of the water bodies.
After sedimentation of seston material, denitrification occurred within the seston layer when oxygen consumption reduced O2concentrations to below 0•5 mg l−1. Twenty-four percent of aerobic denitrification was found as nitrous oxide that accumulated within the layer at concentrations of ≥0•5 μmol l−1. During the diffusion of N2O from the deeper parts of the seston layer to the overlying bottom water, it was consumed instantly at a rate between 0•14 and 0•25 μmol cm−3 day−1. Methane added as a substrate was mineralized both under aerobic and anaerobic conditions, and it increased aerobic N2O consumption by a factor of two.
When anaerobic conditions were initiated in the layer, nitrate consumption exceeded the net production of nitrate. Anaerobic N2O production accounted for between 8•9 and 15•3% of the total nitrate consumption, which varied between 8•2 and 10•1 μmol cm−3 day−1during the year. The results indicate that an aerobic seston surface layer might act as an effective sink for N2O derived from both the overlying bottom water and deeper parts of the sediment
