Microtopography Matters: Belowground CH4 Cycling 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 CH4 emissions, but microform‐dependent patterns of belowground CH4 cycling remain less clearly resolved. To investigate the impact of microtopography on belowground CH4 cycling, we characterized depth profiles of the community composition and activity of CH4‐cycling microbes using 16S rRNA amplicon sequencing, incubations, and measurements of porewater CH4 concentration 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 CH4 cycling processes. Hummocks, where water table depth is lower, had higher porewater redox potential (Eh) and higher porewater δ13C‐CH4 values in the upper 30 cm than lawns, where water table depth is closer to the peat surface. Porewater δ13C‐CH4 and 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 CH4 cycling in hummocks is more strongly regulated by methanotrophy, while in lawns methanogenesis is more dominant. We also investigated controls of porewater CH4 chemistry. The ratio of the relative abundance of methanotrophs to methanogens was the strongest predictor of porewater CH4 concentration 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 CH4 cycling across microforms than vegetation. Northern peatlands are significant sources of the greenhouse gas methane (CH4) to the atmosphere. Patterns of CH4 emissions across peatlands often mirror patterns in water table level, vegetation cover, and elevation over small spatial scales as these factors influence microbial CH4 production and consumption. We investigated microbial CH4 production and consumption across areas in a northern peatland with varying water table depth and vegetation cover using measurements of belowground CH4 chemistry 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 CH4 consumption while CH4 production is more prominent in areas that are lower and wetter. Overall, both CH4 chemistry and microbial communities were more strongly influenced by changes in moisture than vegetation. These insights are important for understanding how climate change may impact CH4 cycling, as CH4 producing and consuming microbes respond differently to changes in temperature and moisture. Better understanding of the distribution of CH4 production and consumption across the landscape may also help scale predictions of CH4 emissions across larger areas for global modeling efforts. Surface porewater δ13C‐CH4 and 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 CH4 concentration and stable isotope composition Surface porewater δ13C‐CH4 and 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 CH4 concentration and stable isotope composition

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