In two intertidal marshes, the vertical distribution in the sediment and inhibition by ammonium of methane
oxidation were investigated by slurry incubation experiments. The two sites differ in their dominant vegetation
type, i.e., reed and bulrush, and in their heights above sea level. The reed site was elevated with respect to the
bulrush site, resulting in a lower frequency and duration of flooding and, consequently, a higher potential for
methane oxidation. Methane oxidation decreased with depth in the bulrush and reed slurries, although
methane oxidation associated with root material from the bulrush plants increased with depth. Reed root
material had a limited capacity for methane oxidation and showed no significant increase with depth. Inhibition
of methane oxidation by ammonium was observed in all samples and depended on methane and
ammonium concentrations. Increasing ammonium concentrations resulted in greater inhibition, and increasing
methane concentrations resulted in less. Ammonium concentrations had to exceed methane concentrations
by at least 30-fold to become effective for inhibition. This ratio was found only in the surface layer of the
sediment. Hence, the ecological relevance for ammonium inhibition of methane oxidation in intertidal marshes
is rather limited and is restricted to the surface layer. Nitrate production was restricted to the 0- to 5-cm-depth
slurries
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