Seasonal variation in methane oxidation in a landfill cover soil as determined by an in situ stable isotope technique

Seasonal variations in the oxidation of methane during its transport across the soil cap of a landfill in Leon County, Florida, were determined in situ with a stable isotopic technique. The approach contrasted the δ 13 C values of emitted and anoxic zone CH 4 and utilized measurements of the isotopic fractionation factor α, which varied inversely with temperature from 1.025 to 1.049. Anoxic zone CH 4 did not vary seasonally and had a δ 13 C average value of −55.18 ± 0.15‰. Methane emitted from the landfill soil surface and captured in chambers ranged in δ 13 C from −54‰ in winter, when emission rates were high, to −40‰ in summer, when emission rates were lower. The antipathetic variation between the δ 13 C of emitted CH 4 and the rate of CH 4 emission is consistent with control of the emission rate by bacterial oxidation. Our interpretation of the isotope data indicates that methane oxidation consumed from 3 to 5% of the total flux in winter to a maximum of 43 ± 10% in summer. There was variation in the extent of methane oxidation in soil types, with mulch/topsoil averaging 55 ± 14% and clay averaging 33 ± 13% in summer. The seasonally integrated value for methane oxidation for areas of the landfill covered with mulch/topsoil was 26 ± 4% of the flux toward the soil surface, while for clay soil it was only 14 ± 2%. The overall annual average, which includes both types of soil, was 20 ± 3%. Covering landfills with additional mulch, which can be generated from yard waste, may attenuate methane emission by providing a loose noncompact substrate for bacterial attachment and an environment with moisture, methane, and oxygen. At specific sites within the landfill we studied, temperature was the main factor controlling methane oxidation.