Anaerobic nitrification-denitrification mediated by Mn-oxides in meso-tidal sediments: Implications for N-2 and N2O production
Résumé
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 15N 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 the generation of the potent greenhouse gas nitrous oxide (N2O) during nitrification–denitrification
in the presence ofMn-oxides. The microcosmswere 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 15NO3− 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 15NH4+, 14NO3− with or without addition of Mn-oxides. A substantial production of P15 (N2
production from 15NH4+) 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+
evels 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.