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 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.