Insight into the N-cycling in the North Sea

The sediments in the coastal zone are a key site for these processes of the N-cycling such as denitrification anammox and DNRA. It is important to understand what factors favour one process or another because, although denitrification and anammox have the same impact on the N-cycling by removing N from the marine environment, the impact on the C-cycling have different implication in the environment. Denitrification release greenhouse gases (N₂O and CO₂) to the atmosphere, while anammox removes CO₂ and does not release N₂O. DNRA, in turn may worsen eutrophic environment conditions by recycling N instead of removing it.

In order to better understand the effect of factors such as oxygen penetration depth, temperature and organic matter content in sediments on the processes of the N-cycling a study was carried out in the open North Sea and the adjacent Wash estuary. The sampling site in the Wash was visited four times in May, June, Sept and October. In the open North Sea five sampling sites were visited during August. All sites had relatively low sedimentary organic carbon content (0.5-0.48%). The results of this study showed a temporal variation at the Wash, and spatial variation at the North Sea. At both sites the main process contributing to total N₂ production was canonical denitrification (>95%) with on average >80% associated with coupled nitrification-denitrification. In general the average rates of denitrification were higher at the North Sea (7.62 μmol m^-2h^-1) than in the Wash (4.4 μmol m^-2h^-1). Anammox was not detected at the Wash site and the contribution of anammox to total N₂ production at the North Sea was <6.6%. DNRA was observed at The Wash, with the highest relative importance to nitrate reduction during June (~41%) and the lowest in October (~6.5%). DNRA was found in only one station at the North Sea where it was responsible of around 17% of nitrate reduction. A major factor identified as a control on the overall rates of denitrification was temperature.