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

The possibility for anaerobic NH4+ 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 greater than or equal 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 NH4+ during anoxic incubations scaled with Mn reduction rates and did not indicate any substantial oxidation of NH4+. No sustained production of N-15-labelled N-2 from added (NH4+)-N-15 was detectable during the four-day incubations, which constrains the rate of NH4+ conversion to N-2 to <2% of the NH4+ production rate. Traces of N-15-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 NH4+ to NO3- was also insignificant as there was no accumulation of NO3- during the incubations and added (NO3-)-N-15 was rapidly consumed with N-2 as a major product. Although the oxidation of NH4+ with Mn oxide is thermodynamically favorable, our results demonstrate that such oxidation was insignificant and that NH4+ can be considered the end product of nitrogen mineralization in this anoxic Mn oxide-rich sediment. Copyright (C) 2000 Elsevier Science Ltd.