Methane (CH₄) is a potent greenhouse gas and its terrestrial sources account for more than 20% of the rising atmospheric CH₄ concentration. Wetland conversion to cropland turns CH₄ source to a sink; yet the underlying mechanisms remain elusive. Here, we investigated the seasonal variation of methanogenesis and methanotroph along 100 cm soil profiles for adjacent wetland and cropland in the Sanjiang Plain, China. The study confirmed that wetland reclamation converted a CH₄ source of 449.30 ± 101.65 kg·ka^-1·yr^-1 to a weak CH₄ sink of 0.56 ± 0.51 kg·ka^-1·yr^-1. The metagenomic data proved that the proportion of total CH₄ related genes, methanogenesis, as well as the CH₄ production marker genes – mcr were significantly decreased, and the proportion of methanotroph marker genes, pMMO and the sum of sMMO and pMMO were slightly increased. Conspicuously seasonal fluctuation of total CH₄ related genes, methanogenesis and methanotroph genes were observed in wetland, with highest in spring and lowest in autumn. We also observed significant variation across seasons of the proportion of marker genes including mcr, sMMO, pMMO and MMO in cropland, with highest in summer and lowest in winter. Moreover, all the proportion of CH₄ related genes were decreased along soil depth in both wetland and cropland, with two exceptions of genes that encoding pMMO and MMO in cropland, which were slightly increased in middle layers from 20 cm to 60 cm. Our observations suggest that the inhibition of functional genes involved in methanogenesis are responsible to methane emission reduction when wetland converted to cropland, following with slight stimulation of MMO especially pMMO genes. The finding that both methanogenesis suppression and methanotroph enhancement contributes to the shift of CH₄ source to CH₄ sink after wetland reclamation provides clear evidence for microbial mechanisms for CH₄ process, can serve as benchmark for CH₄ models.
 

methanogenesis; methanotroph; mcr genes; pMMO genes; wetland reclamation.