Microtopography Matters: Belowground CH4Cycling Regulated by Differing Microbial Processes in Peatland Hummocks and Lawns

Water table depth and vegetation are key controls of methane (CH4) emissions from peatlands. Microtopography integrates these factors into features called microforms. Microforms often differ in CH4emissions, but microform‐dependent patterns of belowground CH4cycling remain less clearly resolved. To investigate the impact of microtopography on belowground CH4cycling, we characterized depth profiles of the community composition and activity of CH4‐cycling microbes using 16S rRNA amplicon sequencing, incubations, and measurements of porewater CH4concentration and isotopic composition from hummocks and lawns at Sallie's Fen in NH, USA. Geochemical proxies of methanogenesis and methanotrophy indicated that microforms differ in dominant microbial CH4cycling processes. Hummocks, where water table depth is lower, had higher porewater redox potential (Eh) and higher porewater δ13C‐CH4values in the upper 30 cm than lawns, where water table depth is closer to the peat surface. Porewater δ13C‐CH4and δD‐CH3D values were highest at the surface of hummocks where the ratio of methanotrophs to methanogens was also greatest. These results suggest that belowground CH4cycling in hummocks is more strongly regulated by methanotrophy, while in lawns methanogenesis is more dominant. We also investigated controls of porewater CH4chemistry. The ratio of the relative abundance of methanotrophs to methanogens was the strongest predictor of porewater CH4concentration and δ13C‐CH4, while vegetation composition had minimal influence. As microbial community composition was strongly influenced by redox conditions but not vegetation, we conclude that water table depth is a stronger control of belowground CH4cycling across microforms than vegetation. Northern peatlands are significant sources of the greenhouse gas methane (CH4) to the atmosphere. Patterns of CH4emissions across peatlands often mirror patterns in water table level, vegetation cover, and elevation over small spatial scales as these factors influence microbial CH4production and consumption. We investigated microbial CH4production and consumption across areas in a northern peatland with varying water table depth and vegetation cover using measurements of belowground CH4chemistry and microbial DNA sequencing. We observed consistent signals in the belowground concentration and stable isotope composition of CH4, which can indicate where different microbial processes are occurring, that suggest slightly elevated areas with lower water table depth are hotspots for CH4consumption while CH4production is more prominent in areas that are lower and wetter. Overall, both CH4chemistry and microbial communities were more strongly influenced by changes in moisture than vegetation. These insights are important for understanding how climate change may impact CH4cycling, as CH4producing and consuming microbes respond differently to changes in temperature and moisture. Better understanding of the distribution of CH4production and consumption across the landscape may also help scale predictions of CH4emissions across larger areas for global modeling efforts. Surface porewater δ13C‐CH4and δD‐CH3D values were more reflective of methanotrophy in hummocks and methanogenesis in lawnsCommunity composition of methanogens and methanotrophs was influenced by redox conditions and water table level across microtopographyVegetation cover had limited influence on microbial community composition and porewater CH4concentration and stable isotope composition Surface porewater δ13C‐CH4and δD‐CH3D values were more reflective of methanotrophy in hummocks and methanogenesis in lawns Community composition of methanogens and methanotrophs was influenced by redox conditions and water table level across microtopography Vegetation cover had limited influence on microbial community composition and porewater CH4concentration and stable isotope composition