SPATIAL-DISTRIBUTION AND INHIBITION BY AMMONIUM OF METHANE OXIDATION IN INTERTIDAL FRESH-WATER MARSHES

In two intertidal marshes, the vertical distribution in the sediment a nd inhibition by ammonium of methane oxidation were investigated by sl urry incubation experiments. The two sites differ in their dominant ve getation type, i.e., reed and bulrush, and in their heights above sea level. The reed site was elevated with respect to the bulrush site, re sulting in a lower frequency and duration of flooding and, consequentl y, a higher potential for methane oxidation. Methane oxidation decreas ed with depth in the bulrush and reed slurries, although methane oxida tion associated with root material from the bulrush plants increased w ith depth. Reed root material had a limited capacity for methane oxida tion and showed no significant increase with depth. Inhibition of meth ane oxidation by ammonium was observed in all samples and depended on methane and ammonium concentrations. Increasing ammonium concentration s resulted in greater inhibition, and increasing methane concentration s resulted in less. Ammonium concentrations had to exceed methane conc entrations by at least 30-fold to become effective for inhibition. Thi s ratio was found only in the surface layer of the sediment. Hence, th e 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 slur ries.