In vitro simulation of oscillatory redox conditions in intertidal sediments: N, Mn, Fe, and P coupling. (1st April 2019) - Anschutz, Pierre
Abstract: In coastal environments, oscillating redox conditions represent a functional state affecting organic matter mineralization. Such transient diagenetic processes remain difficult to study in situ, and we therefore designed a specific reactor to provide experimental results that are environmentally relevant in this context. Here, we present the results of two independent experiments carried out with sediment having contrasting Fe, Mn contents, collected from a coastal tidal lagoon (the Arcachon bay) and a mesotidal estuary (Adour river). Sediment and overlying water were mixed to form slurries that were submitted to redox oscillations to assess the diagenetic mechanisms that affect N, P, Fe, Mn, and S. Changing from anoxic to oxic conditions, we observed a rapid oxidation of dissolved Fe(II) and dissolved inorganic phosphorus (DIP) was apparently trapped by the newly formed Fe-oxyhydroxides (Fe-ox). DIP was totally titrated in the coastal lagoon sediment, but not in estuarine sediment, where the initial amount of Fe available was lower. In both experiments, Mn(II) was only slowly oxidized during the oxidation events and a major part of Mn(II) was adsorbed on new Fe-ox. In coastal lagoon sediment, ammonium remained constant in oxic conditions while nitrate was produced from organic-N mineralization. On the contrary, in estuarine sediment, ammonium was quantitatively oxidized to nitrate. When the conditions became anoxic again, direct reduction of nitrate to ammonium occurred in coastal lagoon sediment. Anaerobic production of nitrate occurred in estuarine sediment, probably because Mn-oxides (Mn-ox), which had a high concentration, acted as an oxidant for ammonium. Consequently, nitrate production prevented Fe(II) accumulation. The Mn-N-Fe coupling outlined here is an apparent indirect oxidation of Fe(II) by Mn-ox through anaerobic nitrification (with Mn-ox) and denitrification (with Fe-ox). This coupling also implied P availability because of the strong control of P by Fe. These experimental results show that nutrient dynamics in oscillatory redox environments such as the estuarine turbidity zone, bioturbated sediment, or tidal permeable sediments highly depends on Mn-and Fe-ox availability. Highlights: We designed a reactor to simulate oscillating redox conditions. Two sediments with different Mn oxide content submitted to redox oscillations. Nutrient dynamics depends on reactive Fe and Mn content. We outline a Mn-N-Fe coupling, which also imply P availability. Anaerobic nitrification with Mn-ox and denitrification with Fe-ox.