Microtopography Matters: Belowground CH₄ Cycling Regulated by Differing Microbial Processes in Peatland Hummocks and Lawns

Abstract

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.

Citation

Perryman, Clarice R.; McCalley, Carmody K.; Ernakovich, Jessica G.; Lamit, Louis J.; Shorter, Joanne H.; Lilleskov, Erik; Varner, Ruth K. 2022. Microtopography Matters: Belowground CH₄ Cycling Regulated by Differing Microbial Processes in Peatland Hummocks and Lawns. Journal of Geophysical Research: Biogeosciences. 127(8): e2022JG006948. https://doi.org/10.1029/2022JG006948.