Waterlogging may reduce chemical soil C stabilization in forested peatlands

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Boreal forested peatlands are globally significant carbon (C) pools where soil water-table level largely controls decomposition rate. Here we aimed to investigate the contrarian hypothesis that waterlogging conditions do not lead to increased C stabilization in a forested peatland. We studied how waterlogging affects root and soil chemistry, fungal biomass and necromass, C pools, soil enzyme activities and fungal community composition in Pinus sylvestris rhizosphere. Samples were collected from 0–10 cm, 10–20 cm and 20–30 cm soil layers in a controlled-environment experiment with waterlogging and control treatments and in two forested peatland sites, wetter and drier. Tannin content, live fungal biomass, enzyme activities, and chemically stable SOC content and its proportion of total SOC content were generally lower in waterlogged than control treatment in the experiment, and lower in the wetter than the drier site. The proportion of ectomycorrhizal fungi was higher in the drier than the wetter site in the deepest soil layer studied, 20–30 cm. In the wetter site, chemically stable SOC content and proportion of ericoid mycorrhizal fungi were highest in topsoil that contained more roots than deeper soil layers. In conclusion, forested peatlands possess specific root-related chemical SOC stabilization mechanisms, and, consequently, waterlogging makes their SOC more vulnerable due to decreased chemical C stabilization via root-microbial interactions and lowering root production.