CAPACITY FOR METHANE OXIDATION IN LANDFILL COVER SOILS MEASURED IN LABORATORY-SCALE SOIL MICROCOSMS

Laboratory-scale soil microcosms containing different soils were permeated with CH4 for up to 6 months to investigate their capacity to develop a methanotrophic community. Methane emissions were monitored continuously until steady states were established. The porous, coarse sand soil developed the greatest methanotrophic capacity (10.4 mol of CH4.m(-2).day(-1)), the greatest yet reported in the literature. Vertical profiles of O-2, CH4, and methanotrophic potential in the soils were determined at steady state. Methane oxidation potentials were greatest where the vertical profiles of O-2, and CH4 overlapped. A significant increase in the organic matter content of the soil, presumably derived from methanotroph biomass, occurred where CH4 oxidation was greatest. Methane oxidation kinetics showed that a soil community with a low methanotrophic capacity (V-max of 258 nmol.g of soil(-1).h(-1)) but relatively high affinity (k(app) of 1.6 mu M) remained in N-2-purged control microcosms, even after 6 months without CH4. We attribute this to a facultative, possibly mixotrophic, methanotrophic microbial community. When purged with CH4, a different methanotrophic community developed which had a lower affinity (k(app) of 31.7 mu M) for CH4 but a greater capacity (V-max of 998 nmol.g of soil(-1) h(-1)) for CH4 oxidation, reflecting the enrichment of an active high-capacity methanotrophic community. Compared with the unamended control soil, amendment of the coarse sand with sewage sludge enhanced CH4 oxidation capacity by 26%; K2HPO4 amendment had no significant elfect, while amendment with NH4NO3 reduced the CH4 oxidation capacity by 64%. In vitro experiments suggested that NH4NO3 additions (10 and 71 mu mol.g of soil(-1)) inhibited CH4 oxidation by a nonspecific ionic effect rather than by specific inhibition by NH4+.