Microbial mechanisms for methane source-to-sink transition after wetland conversion to cropland

Wetland conversion to cropland substantially reduces methane (CH4) emission, turning a source into a sink on many occasions; how various microbial processes contribute to this source-to-sink transition remains elusive. We addressed this issue by examining the net CH4 flux, CH4 production potential, CH4 oxidation potential, and functional genes associated with methanogenesis and methanotrophy in a pristine wetland and a 23-year cultivated cropland in the Sanjiang Plain, China. The study confirmed that wetland conversion to cropland turned a CH4 source of 44.93 ± 10.17 g CH4·m−2·yr−1 to a small CH4 sink of −0.056 ± 0.051 g·CH4 m−2·yr−1. The proportion of total CH4-related genes, methanogenesis genes, as well as the CH4 production marker genes – mcr were significantly decreased by 24.14 %, 32.10 %, and 97.89 %, respectively in cropland. The proportions of methanotrophic marker genes, pMMO, and the sum of sMMO and pMMO were significantly increased by 48.74 % and 22.79 % after wetland cultivation. The 23-year cultivation yielded suppressing impacts on methanogenesis and mcr genes throughout the four seasons while stimulating effects on the functional genes of sMMO, pMMO, and MMO in spring and summer. The proportions of CH4-related genes decreased along soil depth in wetland and cropland, while pMMO and MMO slightly increased in the depth of 20–60 cm in cropland. A global synthesis supported this microbial mechanism for the CH4 source-to-sink transition, indicating the strong methanogenesis suppression and slight methanotrophy enhancement in explaining the source-to-sink transition after wetland conversion to cropland. This mechanism should be incorporated into CH4 models to predict CH4 dynamics under land-use change.