Methane oxidation rates observed in a topsoil covering a retired landfill are the highest reported (45 g m(−2) day(−1)) for any environment. This microbial community had the capacity to rapidly oxidize CH(4) at concentrations ranging from <1 ppm (microliters per liter) (first-order rate constant [k] = −0.54 h(−1)) to >10(4) ppm (k = −2.37 h(−1)). The physiological characteristics of a methanotroph isolated from the soil (characteristics determined in aqueous medium) and the natural population, however, were similar to those of other natural populations and cultures: the Q(10) and optimum temperature were 1.9 and 31°C, respectively, the apparent half-saturation constant was 2.5 to 9.3 μM, and 19 to 69% of oxidized CH(4) was assimilated into biomass. The CH(4) oxidation rate of this soil under waterlogged (41% [wt/vol] H(2)O) conditions, 6.1 mg liter(−1) day(−1), was near rates reported for lake sediment and much lower than the rate of 116 mg liter(−1) day(−1) in the same soil under moist (11% H(2)O) conditions. Since there are no large physiological differences between this microbial community and other CH(4) oxidizers, we attribute the high CH(4) oxidation rate in moist soil to enhanced CH(4) transport to the microorganisms; gas-phase molecular diffusion is 10(4)-fold faster than aqueous diffusion. These high CH(4) oxidation rates in moist soil have implications that are important in global climate change. Soil CH(4) oxidation could become a negative feedback to atmospheric CH(4) increases (and warming) in areas that are presently waterlogged but are projected to undergo a reduction in summer soil moisture
