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

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, NOx, 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 freshand 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.