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 dom
inated anaerobic carbon oxidation, whereas neither Fe reduction nor sulfate
reduction were significant. Accumulation rates of soluble NH4+ during anox
ic incubations scaled with Mn reduction rates and did not indicate any subs
tantial oxidation of NH4+. No sustained production of N-15-labelled N-2 fro
m added (NH4+)-N-15 was detectable during the four-day incubations, which c
onstrains 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/denitri
fication resulting from sediment handling. Oxidation of NH4+ to NO3- was al
so insignificant as there was no accumulation of NO3- during the incubation
s and added (NO3-)-N-15 was rapidly consumed with N-2 as a major product. A
lthough 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 anox
ic Mn oxide-rich sediment. Copyright (C) 2000 Elsevier Science Ltd.