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