Nitrate, nitrite, and nitrous oxide transformations in sediments along a salinity gradient in the Weser Estuary

Abstract

The dynamics of the nitrogen intermediates N 2 O and NO 2 - in estuarine sediments were studied along a salinity gradient in the eutrophied Weser Estuary (Germany) using microsensors for N 2 O, NO 2 - , NO x - , and O 2 . During dark incubations in the laboratory of sediment samples from a freshwater site, a brackish water site with fluctuating salinity, and a marine site, the effects of environmental changes in nitrogen availability and salinity on microscale sediment dynamics were examined. Generally, sediment levels of intermediates were low: 1 to 25 μM NO 2 - and 0 to 8 μM N 2 O. However, significant variation was found in accumulation patterns and in the potential of the residing microbial community to control sediment releases of the intermediates. At fresh- and brackish water sites, NO 2 - production was found in the anoxic denitrification zone, and release from the sediments was effectively prevented by activity of nitrite oxidisers in oxic surface layers. In contrast, high rates of NO 2 - release occurred in marine sediment, where NO 2 - production was predominantly associated with incomplete nitrification in oxic layers. Similarly, stimulated partial nitrification due to NH 4 + addition led to NO 2 - liberation from brackish water sediment. Production of N 2 O was never observed in sediment from the brackish water site, which is naturally exposed to a daily regime of water column variations, but transient N 2 O accumulation was observed in the other sediments. The production of N 2 O could be induced by an abrupt change in either NO 3 - or salinity, and was found in anoxic or micro-oxic sediment layers. Because oxic sediment layers showed little or no potential for N 2 O consumption (in contrast to NO 2 - ) the accumulation of N 2 O always resulted in release from the sediment surface. Results demonstrate that changes in environmental parameters such as salinity and NO 3 - can trigger sediment production and release of NO 2 - and N 2 O, but further suggest that microbial sediment communities are highly adaptive and can become resistant towards intermediate release when regularly exposed to fluctuating conditions.