Denitrification in a coastal sediment measured in situ by the nitrogen isotope pairing technique applied to a benthic flux chamber

A free operating benthic flux chamber lander (ELINOR) was used to measure in situ denitrification rates in the Aarhus Eight, Denmark (16 m depth). After insertion of the flux chamber into the sediment (NO3-)-N-15 was automatically injected into the enclosed-water phase. After 3 to 4 h of incubation ELINOR was brought back to the surface with an intact water and sediment phase. Dinitrogen was extracted and later analyzed for N-15 enrichment by mass spectrometry. Parallel sediment cores were sampled for laboratory incubation under in situ conditions. In situ denitrification rates in June and November were 350 and 470 mu mol N m(-2) d(-1) respectively, and were not significantly different from the laboratory measured rates of 290 and 500 mu mol N m(-2) d(-1), respectively. Oxygen uptake was 40% lower in the laboratory incubated cores compared to in situ measurements, apparently due to lowered activity of the dominating species of infauna, the bivalve Abra alba. Nitrate concentrations were low in the bottom water (<8 mu M) and most of the denitrification activity (65 to 100%) was coupled to nitrification in the sediment. A laboratory test showed that the addition of (NO3-)-N-15 did not significantly impede the reduction rate of the natural unlabelled NO3-. The results support the validity of laboratory measurements for coastal waters and demonstrate the potential of combining benthic flux chamber landers and the isotope pairing technique for accurate measurements of denitrification in shell, deep sea, and freshwater environments.