Methane oxidation and response of Methylobacter/Methylosarcina methanotrophs in flooded rice soil amended with urea

Nitrogen, which is one of the most important factors controlling methane oxidation, affects rice production and methane emission from rice fields. Therefore, in this study, we aimed to investigate the effects of urea fertilization on methanotrophic communities and methane oxidation. Chinese rice soil was incubated with urea at concentrations of 0, 200, 400, and 800 mg N kg−1 dry soil, and methanotrophic communities were analyzed by terminal-restriction fragment length polymorphism (T-RFLP) of the particulate methane monooxygenase gene (pmoA). Metabolically active methanotrophs were identified by combining T-RFLP analysis of pmoA transcripts and rRNA stable isotope probing. The results showed that a higher concentration of urea delayed the initiation of methane oxidation but stimulated the final methane oxidation rate of the rice soil. As more urea fertilizers were applied, acceleration of the methane oxidation rate occurred more rapidly. These effects were cumulative and were not directly related to the ammonium concentration in rice soil. Moreover, the activity of the Methylobacter/Methylosarcina group was stimulated by urea fertilizer, and the composition of the dominant and active methanotrophic community changed only slightly after 90 h. Our results indicated that urea fertilization generally stimulated methane oxidation and methanotrophic activity, primarily by stimulating the relative abundance and activity of the Methylobacter/Methylosarcina group.