Metagenomic evidence of suppressed methanogenic pathways along soil profile after wetland conversion to cropland

Wetland conversion to cropland substantially suppresses methane (CH 4 ) emissions due to the strong suppression of methanogenesis, which consists of various pathways. In this study, we evaluated the cultivation impacts on four predominant CH 4 production pathways, including acetate, carbon dioxide (CO 2 ), methylamines, and methanol, in a wetland and cultivated cropland in northeastern China. The results showed significant suppression of CH 4 production potential and the abundance of genes for all four methanogenic pathways in cropland. The consistency between CH 4 production and methanogenesis genes indicates the robustness of genomic genes in analyzing methanogenesis. The suppression effects varied across seasons and along soil profiles, most evident in spring and 0 to 30 cm layers. The acetate pathway accounted for 55% in wetland vs. 70% in the cropland of all functional genes for CH 4 production; while the other three pathways were stronger in response to cultivation, which presented as stronger suppressions in both abundance of functional genes (declines are 52% of CO 2 pathway, 68% of methanol pathway, and 62% of methylamines pathway, vs. 19% of acetate pathway) and their percentages in four pathways (from 20 to 15% for CO 2 , 15 to 9% for methylamines, and 10 to 6% for methanol pathway vs. 55 to 70% for acetate pathway). The structural equation models showed that substrate availability was most correlated with CH 4 production potential in the wetland, while the positive correlations of acetate, CO 2 , and methylamine pathways with CH 4 production potential were significant in the cropland. The quantitative responses of four CH 4 production pathways to land conversion reported in this study provide benchmark information for validating the CH 4 model in simulating CH 4 cycling under land use and land cover change.