| CH4 is one of the most important greenhouse gases. The collection and utilization system of landfill gas is an effective method to reduce CH4 emission from municipal solid waste (MSW) landfill, however, the cost is quite high. Besides, the collection and utilization system of landfill gas begins to falter or fail as CH4 production diminishes after landfills are closed. Such low-level CH4 can escape to the air continuously for quite a long time. Consequently, large amount of CH4 inputs to the atmosphere and makes great contribution to greenhouse effect. What's more, shortage of landfill cover soil source and limitation of landfill capacity are emerging as a more and more serious problem. Therefore, it is of high urgency to find an effective and economical method to solve these problems. In this study, CH4 emission flux from closed MSW landfill was monitored by static chamber method. Meanwhile, characteristics of CH4 oxidation, such as the optimum determination method to methane monooxygenase (MMO) activity in soil and relationship between MMO activity and methane oxidation rate, were studied. Waste biocover soil as an alternative of normal landfill cover soil was optimized the compositions and properties, including particle size, moisture and pH value. Kinetics of CH4 oxidation in landfill gas and the effects of ambient conditions (temperature, oxygen concentration) on CH4 oxidation rate were studied by batch experiments. Simulated landfills and soil columns were established to characterize methane oxidation activity in waste biocover soil in laboratory. The components of landfill gas and its distribution in soil columns were determined periodically. CH4 emission flux of simulated soil columns was analyzed as well as the temporal and spatial variation of CH4 oxidation rate and MMO activity in simulated soil columns.These data demostrated the base mechanism of waste biocover soil for mitigating CH4 emission from landfills and provided technical basis for application of waste biocover soil. The main conclusions of this study were listed below.(1) Control for analysis of soil MMO activity was treated by 0.04 g kg-1 NaN3 (bioinhibitor) to eliminate propylene consumption of soil biological and adsorption. Propylene of 0.5% was a suitable substrate concentration. The incubation time didn't have an obvious effect on the measurement of soil MMO activity, but as far as measurement deviation and reliability were concerned,5 days was the optimum time. A strong linear relationship was observed between soil MMO activity and methane oxidation rate, and the correlation coefficient R2 was 0.9193.(2)The CH4 concentration in landfill cover soil was related to the depth of soil, and the lowest was observed in topsoil layer. CH4 emission flux in different landfill areas changed greatly in different seasons, in the range of 27.38～5611.67mmol·m-2·d-1 Landfill gas production decreased in accordance with the extension of landfill age, leading to the decrease of CH4 emission flux gradually.(3) CH4 oxidation rate and MMO activity of normal landfill cover soil were 0.36～1.81μmol·g-1·h-1 and 52.54～409.85nmol·g-1·h-1, relatively. CH4 oxidation rate and MMO activity in September were significantly higher than that in April. MMO activity in region A (landfill age,14～16 year) was higher than that in region B (landfill age, 9-11 year) and C (landfill age,6-8 year). There was a fairly good relationship between MMO activity and methane oxidation rate of landfill cover soil.(4) Waste biocover soil with original pH value (6.7-7.9),45% moisture and particle size of≤4mm has an excellent performance as a landfill cover material on CH4 oxidation. When CH4 concentration was lower than the saturated concentration (10% v/v), CH4 oxidation rate in waste biocover soil was enhanced with the increase of CH4 concentration. The methane oxidation process in waste biocover soil was described by Michaelis-Menten model with a maximum of CH4 oxidation rate of 9.03μmol·g-1·h-1. The kinetic parameters of CH4 oxidation showed that low affinity methanotrophs were abundant in waste biocover soil. CH4 consumption rate in waste biocover soil depended on temperature, oxygen concentration and nitrogen content. The content of NH4+-N or NO3---N in waste biocover soil which was below 600 mg·kg-1 might stimulate CH4 consumption. However, the content of NH4+-N or NO3--N of above 1200 mg-kg"1 would inhibit CH4 oxidation in waste biocover soil. Compared with NO3--N, NH4+-N had greater stimulating action as nutrient at low concentrations and inhibitory effect at higher concentrations on CH4 oxidation in waste biocover soil. Exposure to high CH4 concentrations might relieve nitrogen inhibitory effect of NH4+-N and NO3--N on CH4 oxidation. (5) Mean CH4 emission fluxes of the simulated soil columns of waste biocover soil were 0.12～0.20 mol·m-2·d-1, and they were almost zero on days 73,80 and 102 during the experiment. Meanwhile, mean CH4 emission flux of simulated soil columns of normal landfill cover soil were 3.15-2.81 mol·m-2·d-1. The CH4 and CO2 concentrations decreased progressively from bottom layer to the top layer in all simulated soil columns of waste biocover soil. The O2 concentration increased from bottom layer to the top layer in simulated soil columns of waste biocover soil, but showed little variations in landfill cover soil columns. CH4 oxidation rate of each simulated soil columns presented a similar trend:CH4 oxidation rate of bottom layer soil was the highest, followed by the middle layer and the top layer. MMO activity of waste biocover soil was twice higher than that of landfill cover soil. However, CH4 oxidation rate of waste biocover soil was nearly 10～20 times higher than that of landfill cover soil. A good relationship was observed between CH4 oxidation rate and MMO activity of both kinds of soil. Obviously, waste biocover soil has a great performance for CH4 biological oxidation in landfills, so it can be used as a good alternative landfill cover soil for the reduction of CH4 emission.
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