Abstract
Abstract. Lake sediments in Amazonia are a significant source of CH4, a potential
greenhouse gas. Previous studies of sediments using 13C analysis found
that the contribution of hydrogenotrophic versus acetoclastic methanogenesis
to CH4 production was relatively high. Here, we determined the
methanogenic pathway in the same sediments (n=6) by applying
14Cbicarbonate or 2-14Cacetate and confirmed the high
relative contribution (50 %–80 %) of hydrogenotrophic methanogenesis. The
respiratory index (RI) of 2-14Cacetate, which is 14CO2
relative to 14CH4+14CO2, divided the sediments into
two categories, i.e., those with an RI < 0.2 consistent with
the operation of acetoclastic methanogenesis and those with an RI > 0.4 showing that a large percentage of the acetate-methyl was
oxidized to CO2 rather than reduced to CH4. Hence, part of the
acetate was probably converted to CO2 plus H2 via syntrophic
oxidation, thus enhancing hydrogenotrophic methanogenesis. This happened
despite the presence of potentially acetoclastic Methanosaetaceae in all the sediments.
Alternatively, acetate may have been oxidized with a constituent of the
sediment organic matter (humic acid) serving as oxidant. Indeed, apparent
acetate turnover rates were larger than CH4 production rates except in
those sediments with a R<0.2. Our study demonstrates that CH4
production in Amazonian lake sediments was not simply caused by a
combination of hydrogenotrophic and acetoclastic methanogenesis but
probably involved additional acetate turnover.
Subject
Earth-Surface Processes,Ecology, Evolution, Behavior and Systematics
Cited by
6 articles.
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