Denitrification in intertidal black anoxic surface sediments forms rather N2O and Nitrite instead of molecular Nitrogen

During the 'Black Area Event' of June/July 1996 in the Ostfriesian Wadden Sea, many square kilometers of the intertidal zone in the Ostfriesian Wadden Sea (Southern North Sea, Germany) lost the oxic sediment cover and showed a black surface of sulfidic sediment. This and earlier work on smaller 'Black Spots' caused reason to examine the effects on crucial ecological functions such as denitrification in the sediments. This was done by examining the denitrification potential defined as a measured microbial denitrification activity which develops in a biologically active sediment sample after a non-limiting addition of nitrate and/or organic electron donors. Four types of sediment samples were examined: Black surface, black sediment from below a black surface, oxic surface sediment from a reference area and black sediment from below this oxic sediment. The oxic sample showed the highest activity. In the presence of added electron donors, added nitrate was completely converted to N2 within about 15 days. Transient N2O did not or only in traces appear which later were reduced to N2. In corresponding experiments with samples from the Black Spot (surface and depth) and black sediments from below the oxic cover nitrate was also converted to gaseous products. After about 7 days nearly 80% of the nitrate-N appeared as N2O and after 45 days the ratio between N2O-N and N2-N was still between 1:1 and 2:1, representing together 100% of the added N. While a change of the temperature from 20 to 30°C mainly resulted in a faster conversion rate and not so much in a different intermediate and product spectrum, a change to 7°C led to an accumulation of nitrite which only in the oxic sediment samples was converted to N2. The main result is that the transition of the surface sediment from oxic to sulfidic means a shift from the denitrification end product N2 to partially reduced ones such as nitrite and N2O.