The fate of ammonium in anoxic manganese oxide-rich marine sediment

Abstract

The possibility for anaerobic NH 4 + oxidation and N 2 formation was explored in a Mn oxide-rich continental basin sediment from Skagerrak. The surface sediment contained 2.9 weight-% Mn(IV), and reactive Mn oxide persisted to ≥10 cm depth. Microbial Mn reduction completely dominated anaerobic carbon oxidation, whereas neither Fe reduction nor sulfate reduction were significant. Accumulation rates of soluble NH 4 + during anoxic incubations scaled with Mn reduction rates and did not indicate any substantial oxidation of NH 4 +. No sustained production of 15N-labelled N 2 from added 15NH 4 + was detectable during the four-day incubations, which constrains the rate of NH 4 + conversion to N 2 to <2% of the NH 4 + production rate. Traces of 15N-labelled N 2 accumulated initially, and this transient N 2 production was possibly related to brief coupled nitrification/denitrification resulting from sediment handling. Oxidation of NH 4 + to NO 3 − was also insignificant as there was no accumulation of NO 3 − during the incubations and added 15NO 3 − was rapidly consumed with N 2 as a major product. Although the oxidation of NH 4 + with Mn oxide is thermodynamically favorable, our results demonstrate that such oxidation was insignificant and that NH 4 + can be considered the end product of nitrogen mineralization in this anoxic Mn oxide-rich sediment.