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Nitrogen fertilizers are generally thought to have an important role in regulating methane oxidation.In this study, the effect of ammonium on methane oxidation activity was investigated in a paddy soil using urea at a concentration of 0, 50, 100, 200 and 400 μg N per gram dry weight soil (N/g.d.w.s)and ammonium sulfate at concentration of 0, 50 and 200 μg N/g.d.w.s.The results of this study demonstrate that urea concentrations of 200 μg N/g.d.w.s and above significantly inhibited methane oxidation activity, whereas no statistically significant difference was observed in methane oxidation activity among soil microcosms with urea concentration less than 200 μg N/g.dw.s after incubation for 27 days.Similar results were obtained in a sense that methane oxidation activity was inhibited only when ammonium sulfate concentration was 200 μg N/g.d.w.s in soil microcosms in this study.The inhibition of methane oxidation activity by urea was well explained by the changes in pmoA gene copy numbers.Phylogenetic analysis of pmoA genes show that nitrogen fertilization resulted in apparent changes in community composition of methane-oxidizing bacteria (MOB).The growth of type I MOB was stimulated by nitrogenous fertilizers, while type Ⅱ MOB dominated the native soil.Furthermore, methane oxidation activity was significantly negatively correlated with nitrification activity in the presence of urea or ammonium sulfate, although no statistically significant relationship was observed between pmoA gene and amoA gene abundances.Our results indicate that methane oxidation activity in paddy soils might be inhibited when the coneentration of ammonium fertilizers is high and the interactions between ammonia and methane oxidizers need to be further investigated.