Mechanisms on nitrogen amendment stimulating methane oxidation in landfill cover soils

Landfill cover soil plays a pivotal role in mitigating CH₄ input to the atmosphere. However, the community and activity of methane-oxidizing bacteria (MOB), and their responses to nutrient amendment remain insufficiently understood in landfill cover soils. In this study, the influencing mechanisms of nitrogen amendment on MOB activity, and their functional microorganisms and genes were investigated in landfill cover soils. An exogenous ammonium and nitrate addition could enhance CH₄ oxidation activity of 13.9-34.1 times in the landfill cover soil. The NH₄⁺-N addition of 800 mg kg^-1 could cause a maximum nitrite accumulation of 41.2 mg kg^-1 in the landfill cover soils and inhibit CH₄ oxidation. Nitrogen was mainly deposited in the landfill cover soil in the form of organic nitrogen, with a slight loss of 1.47-3.21 % in the treatments amended with ammonium and nitrate at each stage. A high CH₄ oxidation could increase the secretion of CH₄-derived carbon and improve the organic matter of soil. Compared with ammonium, the nitrate addition had a greater stimulating effect on microbial and MOB growth. Type I MOB predominated in the soils amended with ammonium and nitrate, whereas type II MOB dominated in the nitrogen-deficient soils. Metagenomic analysis showed that the genes related to nitrogen fixation (nifDKH) were more abundant in the nitrogen-deficient soil than the others. These findings suggest that an appropriate ammonium and nitrate addition could induce carbon and nitrogen accumulation, and stimulate microbial metabolism such as CH₄ oxidation and element cycles in the landfill cover soils to mitigate CH₄ emission.