The biogeochemical controls of N2O production and emission in landfill cover soils: the role of methanotrophs in the nitrogen cycle

Emissions of N2O from cover soils of both abandoned (> 30 years) and active landfills greatly exceed the maximum fluxes previously reported for tropic al soils, suggesting high microbial activities for N2O production. Low soil matrix potentials (< -0.7 MPa) indicate that nitrification was the most li kely mechanism of N2O formation during most of the time of sampling. Soil m oisture had a strong influence on N2O emissions. The production of N2O was stimulated by as much as 20 times during laboratory incubations, when moist ure was increased from -2.0 MPa to -0.6 MPa. Additional evidence from incub ation experiments and delta(13)C analyses of fatty acids (18:1) diagnostic of methanotrophs suggests that N2O is formed in these soils by nitrificatio n via methanotrophic bacteria. In a NH3(g)-amended landfill soil, the rate of N2O production was significantly increased when incubated with 100 ppmv methane compared with 1.8 ppmv (atmospheric) methane. Preincubation of a la ndfill soil with 1% CH4 for 2 weeks resulted in higher rates of N2O product ion when subsequently amended with NH3(g) relative to a control soil preinc ubated without CH4. At one location, at the soil depth (9-16 cm) of maximum methane consumption and N2O production, we observe elevated concentrations of organic carbon and nitrogen and distinct minima in delta(15)N (+1.0 par ts per thousand) and delta(13)C (-33.8 parts per thousand) values for organ ic nitrogen and organic carbon respectively. A delta(13)C value of -39.3 pa rts per thousand was measured for 18:1 carbon fatty acids in this soil, dia gnostic of type II methanotrophs. The low delta(15)N value for organic nitr ogen is consistent with N-2 fixation by type II methanotrophs. These observ ations all point to a methanotrophic origin for the organic matter at this depth. The results of this study corroborate previous reports of methanotro phic nitrification and N2O formation in aqueous and soil environments and s uggest a predominance of type II rather than type I or type X methanotrophs in this landfill soil.