Although the majority of continental margins is covered by coarse-grained relict sediments (Emery, 1968; Johnson and Baldwin, 1986), most previous biogeochemical research has focused on muddy or fine-grained sediments. Recent research indicated the advective transport in permeable sediments can exceed the molecular diffusion in muddy sediments by several orders of magnitude (Huettel and Webster 2001), which cause the characteristic changes in the biogeochemical zonation and also affects biogeochemical processes.
In our work, based on the advective transport in Wadden Sea permeable sediments, we investigated denitrification rates under near in situ conditions by using multiple experimental approaches, including the nitrogen isotope pairing technique in modified whole core incubation, slurry incubations, a flow-through stirred retention reactor, and microsensor measurements. Results indicate that permeable Janssand sediments are characterized by some of the highest potential denitrification rates (≥ 0.19 mmol N m-2 h-1) in the marine environment. Moreover, and surprisingly, several lines of evidence showed that denitrification occured at O2 concentrations of 35 up to ~90 μM. It indicates that where O2 and NOx- co-occur, O2 may not act as the exclusive control of N2 production. Therefore we hypothesize the observed high aerobic denitrification rates may result from the adaptation of denitrifying bacteria to recurrent tidally-induced redox oscillations in permeable sediments at Janssand.
We also investigated the N-loss in those permeable sediments on the temporal and spatial scale. In contrast to conventional views, the further results indicate that the permeable sediments could play a significant role in the biogeochemical process of N-removal from the shelf environments, which may account for 50-70% of oceanic N-loss (Codispoti et al., 2001). As such their contribution to the global N-removal can be re-evaluated based on our measurements combined with modeled results.