METHANE AND CARBON-DIOXIDE DYNAMICS IN A NORTHERN HARDWOOD ECOSYSTEM

There is evidence that oxidation of CH4 to CO2 by methanotrophic bacteria in forest soil is a major sink for atmospheric CH4, even though growth of the bacteria on such low CH4 concentrations (< 1.7 mu L L(-1)) is perplexing. Measurements of CH4 and CO2 in a northern hardwood ecosystem in the Adirondack Park in the state of New York indicated that: (i) soil CH4 concentrations were mostly higher than the concentration of CH4 in forest air, with concentrations as high as 500 mu L L(-1) at the 0.1-m depth in the early and late parts of the growing season; (ii) soil CO2 concentrations ranged from the atmospheric level to as high as 19 000 mu L L(-1) at the 0.2-m depth in midsummer; (iii) net consumption of atmospheric CH4 emission by soil in midsummer averaged only 0.25 mg m(-2) d(-1), which is low compared with findings for most forest ecosystems; and (iv) methanogenesis occurred in soil samples throughout the profile when incubated anaerobically as well as aerobically with CH3F to inhibit methanotrophs. The prominent activity of methanogenic bacteria producing CH4 in the spring and autumn could support the growth of methanotrophic bacteria that otherwise consume atmospheric CH4 when methanogens are not active. Therefore, CH4 dynamics in this Spodosol represent the balance between CH4 production and CH4 oxidation, which is similar to the CH4 dynamics in many wetland ecosystems.