Oxygen effects on methane production and oxidation in humid tropical forest soils

Abstract

Abstract We investigated the effects of oxygen (O 2 ) concentration on methane (CH 4 ) production and oxidation in two humid tropical forests that differ in long‐term, time‐averaged soil O 2 concentrations. We identified sources and sinks of CH 4 through the analysis of soil gas concentrations, surface emissions, and carbon isotope measurements. Isotope mass balance models were used to calculate the fraction of CH 4 oxidized in situ . Complementary laboratory experiments were conducted to determine the effects of O 2 concentration on gross and net rates of methanogenesis. Field and laboratory experiments indicated that high levels of CH 4 production occurred in soils that contained between 9±1.1% and 19±0.2% O 2 . For example, we observed CH 4 concentrations in excess of 3% in soils with 9±1.1% O 2 . CH 4 emissions from the lower O 2 sites were high (22–101 nmol CH 4 m −2 s −1 ), and were equal in magnitude to CH 4 emissions from natural wetlands. During peak periods of CH 4 efflux, carbon dioxide (CO 2 ) emissions became enriched in 13 C because of high methanogenic activity. Gross CH 4 production was probably greater than flux measurements indicated, as isotope mass balance calculations suggested that 48–78% of the CH 4 produced was oxidized prior to atmospheric egress. O 2 availability influenced CH 4 oxidation more strongly than methanogenesis. Gross CH 4 production was relatively insensitive to O 2 concentrations in laboratory experiments. In contrast, methanotrophic bacteria oxidized a greater fraction of total CH 4 production with increasing O 2 concentration, shifting the δ 13 C composition of CH 4 to values that were more positive. Isotopic measurements suggested that CO 2 was an important source of carbon for methanogenesis in humid forests. The δ 13 C value of methanogenesis was between −84‰ and −98‰, which is well within the range of CH 4 produced from CO 2 reduction, and considerably more depleted in 13 C than CH 4 formed from acetate.