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

Field measurements in the Arcachon Bay (southwest France) indicated anaerobic production of NOx via nitrification, which was coupled to the reduction of Mn-oxides. To prove the occurrence of this process, laboratory microcosm experiments were set up. A N-15 tracer-based approach was used to track if NOx produced through Mn-oxide-mediated anaerobic nitrification would be reduced to N-2 via denitrification or anammox. We also hypothesized the 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, 2.5-4.5, 4.5-8.5, 8.5-12 and 12-17 cm) during two sampling campaigns in October (fall) and January (winter). Labeling with (NO3-)-N-15 revealed low N-2 production originating from NO3- in the water column (P-w), which did not increase significantly on amendment with Mn-oxides during both sampling periods. However, for both seasons, a significant increase of N-2 produced via nitrification (P-n) 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+)-N-15, (NO3-)-N-14 with or without addition of Mn-oxides. A substantial production of P15 (N-2 production from (NH4+)-N-15) 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 nitrification-denitrification 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 newer mechanisms leading to the generation of the radiative gas. (C) 2014 Elsevier B.V. All rights reserved.