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.