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

We investigated the effects of oxygen (O-2) concentration on methane (CH4) 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 CH4 through the analysis of soil gas concentrations, surface emissions, and carbon isotope measurements. Isotope mass balance models were used to calculate the fraction of CH4 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 CH4 production occurred in soils that contained between 9 +/- 1.1% and 19 +/- 0.2% O-2. For example, we observed CH4 concentrations in excess of 3% in soils with 9 +/- 1.1% O-2. CH4 emissions from the lower O-2 sites were high (22-101 nmol CH4 m(-2) s(-1)), and were equal in magnitude to CH4 emissions from natural wetlands. During peak periods of CH4 efflux, carbon dioxide (CO2) emissions became enriched in C-13 because of high methanogenic activity. Gross CH4 production was probably greater than flux measurements indicated, as isotope mass balance calculations suggested that 48-78% of the CH4 produced was oxidized prior to atmospheric egress. O-2 availability influenced CH4 oxidation more strongly than methanogenesis. Gross CH4 production was relatively insensitive to O-2 concentrations in laboratory experiments. In contrast, methanotrophic bacteria oxidized a greater fraction of total CH4 production with increasing O-2 concentration, shifting the delta C-13 composition of CH4 to values that were more positive. Isotopic measurements suggested that CO2 was an important source of carbon for methanogenesis in humid forests. The delta C-13 value of methanogenesis was between -84 parts per thousand and -98 parts per thousand, which is well within the range of CH4 produced from CO2 reduction, and considerably more depleted in C-13 than CH4 formed from acetate.