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 tropical soils, suggesting high microbial activities for N₂O production. Low soil matrix potentials (< -0.7 MPa) indicate that nitrification was the most likely mechanism of N₂O formation during most of the time of sampling. Soil moisture had a strong influence on N₂0 emissions. The production of N₂0 was Stimulated by as much as 20 times during laboratory incubations, when moisture was increased from -2.0 MPa to -0.6 MPa. Additional evidence from incubation experiments and δ₁₃C analyses of fatty acids (18:1) diagnostic of methanotrophs suggests that N₂O is formed in these soils by nitrification via methanotrophic bacteria. In a NH_3(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 landfill soil with 1% CH₄ for 2 weeks resulted in higher rates of N₂O production when subsequently amended with NH_3(g) relative to a control soil preincubated 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 δ₁₅N (+1.0‰) and δ¹³C (-33.8‰) values for organic nitrogen and organic carbon respectively. A δ¹³C value of -39.3‰ was measured for 18:1 carbon fatty acids in this soil, diagnostic of type II methanotrophs. The low δ¹⁵N value for organic nitrogen is consistent with N₂ fixation by type II methanotrophs. These observations all point to a methanotrophic origin for the organic matter at this depth. The results of this study corroborate previous reports of methanotrophic nitrification and N₂O formation in aqueous and soil environments and suggest a predominance of type II rather than type I or type X methanotrophs in this landfill soil.