Effects of salinity on NH4+ adsorption capacity, nitrification, and denitrification in Danish estuarine sediments

The regulatory effect of salinity on nitrogen dynamics in estuarine sediments was investigated in the Randers Fjord estuary, Denmark, using sediment slurries and intact sediment cores and applying N-15-isotope techniques. Sediment was sampled at three representative stations varying in salinity, and ail experiments were run at 0 parts per thousand, 10 parts per thousand, 20 parts per thousand, and 30 parts per thousand. The sediment NH4+ adsorption capacity decreased markedly at all stations when salinity was increased from 0 parts per thousand to 10 parts per thousand; further increase showed little effect. In situ nitrification and denitrification also decreased with increasing salinities, with the most pronounced reduction of approximately 50% being observed when the salinity was raised from 0 parts per thousand to 10 parts per thousand. The salinity-induced reduction in NH4+ adsorption capacity and stimulation of NH4+ efflux has previously been argued to cause a reduction in nitrification activity since the nitrifying bacteria become limited by NH4+ availability at higher salinities. However, using a potential nitrification assay where NH4+ was added in excess, it was demonstrated that potential nitrification activity also decreased with increasing salinity, indicating that the inhibitory salinity effect may also be a physiological effect on the microorganisms. This hypothesis was supported by the finding that denitrification based on NO3- from the overlying water (D-w), which is independent of the nitrification process, and hence NH4+ availability, also decreased with increasing salinity. We conclude that changes in salinity have a significant effect on nitrogen dynamics in estuarine sediments, which must be considered when nitrogen transformations are measured and evaluated.