Rapid methane oxidation in a landfill cover soil

Abstract

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 CH4 at concentrations ranging from < 1 ppm (microliters per liter) (first-order rate constant [k] = -0.54 h-1) to > 104 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 Q10 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 CH4 was assimilated into biomass. The CH4 oxidation rate of this soil under waterlogged (41% [wt/vol] H2O) 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% H2O) conditions. Since there are no large physiological differences between this microbial community and other CH4 oxidizers, we attribute the high CH4 oxidation rate in moist soil to enhanced CH4 transport to the microorganisms; gas-phase molecular diffusion is 104-fold faster than aqueous diffusion. These high CH4 oxidation rates in moist soil have implications that are important in global climate change. Soil CH4 oxidation could become a negative feedback to atmospheric CH4 increases (and warming) in areas that are presently waterlogged but are projected to undergo a reduction in summer soil moisture.