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

摘要

ABSTRACT:  The dynamics of the nitrogen intermediates N₂O and NO₂– in estuarine sediments were studied along a salinity gradient in the eutrophied Weser Estuary (Germany) using microsensors for N₂O, NO₂–, NO_x–, and O₂. 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 NO₂– and 0 to 8 µM N₂O. 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, NO₂– 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 NO₂– release occurred in marine sediment, where NO₂– production was predominantly associated with incomplete nitrification in oxic layers. Similarly, stimulated partial nitrification due to NH₄+ addition led to NO₂– liberation from brackish water sediment. Production of N₂O was never observed in sediment from the brackish water site, which is naturally exposed to a daily regime of water column variations, but transient N₂O accumulation was observed in the other sediments. The production of N₂O could be induced by an abrupt change in either NO₃– or salinity, and was found in anoxic or micro-oxic sediment layers. Because oxic sediment layers showed little or no potential for N₂O consumption (in contrast to NO₂–) the accumulation of N₂O always resulted in release from the sediment surface. Results demonstrate that changes in environmental parameters such as salinity and NO₃– can trigger sediment production and release of NO₂– and N₂O, but further suggest that microbial sediment communities are highly adaptive and can become resistant towards intermediate release when regularly exposed to fluctuating conditions.