Effect of soil types and ammonia concentrations on the contribution of ammonia

Irrigation of stabilized landfill leachate to landfill cover soil is a cost-effective operation for leachate treatment. The contribution of ammonia-oxidizing bacteria (AOB) in the cover soil to CH4 oxidation, however, is unclear, because AOB and methane-oxidizing bacteria (MOB) can co-oxidize CH4 and NH4+-N. Thus, the contribution of AOB and the inhibitory effect of NH4+-N to CH4 oxidation were determined by using an acetylene pretreatment discrimination method. The results showed that the contributions of AOB to CH4 oxidation varied with the soil type and the concentration of NH4+-N addition. The relative contribution of AOB to CH4 oxidation for compost without NH4+-N addition was the highest (65.0%), and was 2.5 and 3.4 times higher than the corresponding values for aged refuse and landfill cover soil, respectively. The inhibitory effect of NH4+-N was enhanced by increasing the concentration of NH4+-N addition for all the soil samples. At equal NH4+-N addition concentrations, the inhibitory effect was always the lowest for the compost sample. The abundances of particulate methane monooxygenase (pmoA) and ammonia monooxygenase (amoA) genes were key factors influencing the CH4 oxidation rate and contribution of AOB to CH4 oxidation. The higher abundance of pmoA and lower abundance of amoA in landfill cover soil could explain the higher CH4 oxidation rate and lower contribution of AOB to CH4 oxidation in this soil type. Meanwhile, the higher contribution of AOB to CH4 oxidation for compost could be attributed to the higher abundance of the amoA gene and lower abundance of pmoA