Anaerobic nitrification-denitrification mediated by Mn-oxides in meso-tidal sediments: implications for N2 and N2O production

Field measurements in the Arcachon Bay (south-west France) indicated anaerobic production of NOx via nitrification which was coupled to the reduction of Mn-oxides. To provide evidence for the occurrence of this process, laboratory microcosm experiments were set up. A N tracer based approach was used to track if NOx produced through Mn-oxide mediated anaerobic nitrification would be reduced to N2 via denitrification or anammox. We also hypothesized generation of the potent greenhouse gas nitrous oxide (N2O) during nitrification-denitrification in the presence of Mn-oxides. The microcosms were prepared using sediment sectioned at varying depths (0-2.5, 2.54.5, 4.5-8.5, 8.5-12 and 12-17 cm) during two sampling campaigns in October (fall) and January (winter). Labelling with NO3 revealed low N2 production originating from NO3 in the water column (Pw) which did not increase significantly on amendment with Mn-oxides during both sampling periods. However, for both seasons, a significant increase of N2 produced via nitrification (Pn) was observed upon addition of Mn-oxides reaching 76 fold enhancement at ≤ 2.5 cm. To support these results, sediment slurries of October were subjected to amendment of NH4, NO3 with or without addition of Mn-oxides. A substantial production of P15 (N2 production from NH4) within 0-17 cm provided further evidence on nitrification-denitrification mediated by Mn-oxides probably with minimal intervention of anammox. In organically-rich sediments, anaerobic nitrificationdenitrification mediated by Mn-oxides could play an important role in lowering re-mineralized NH4 levels in the benthic system. As hypothesized, significant production of N2O through the pathway was observed revealing yet another mechanism for generation of the radiative gas.