Differences in hydrophyte life forms induce spatial heterogeneity of CH_4 production and its carbon isotopic signature in a temperate bog peatland

Abstract

To clarify the effect of differences in hydrophyte life forms on methane (CH_4) production and its carbon stable isotopic signature (δ_13C-CH_4), we analyzed CH4 and carbon dioxide (CO_2) concentrations, their stable carbon isotope values, and chemical constituents dissolved in pore water in a small floating peat bog in Japan. Because eutrophication has modified the surrounding water quality, the bog vegetation on the mat has been, in part, replaced by fen-type vegetation. We hypothesized that differences in hydrophyte habitats affect redox conditions, including dissolved oxygen (DO) in water and therefore the amounts and carbon isotopic values of CH_4 and CO_2 dissolved in pore water. Between the habitats of two Sphagnum species, DO was considerably higher, and CH_4 concentrations were significantly lower in Sphagnum cuspidatum Ehrh. habitats in hollow (DO: 0.62 ± 0.20 mg/L (standard error (SE)) and CH_4: 0.18 ± 0.02 mmol/L) than in Sphagnum palustre L. habitats in hummock (DO: 0.29 ± 0.08 and CH4: 0.82 ± 0.06) in pore water (10 cm depth). Both DO and CH_4 concentrations in three vascular plant habitats (Rhynchospora fauriei Franch., Phragmites australis [reed], and Menyanthes trifoliata L.) in pore water (10 cm depth) were intermediate relative to the two Sphagnum species. However, CH_4 flux in M. trifoliata site was significantly higher than that at both Sphagnum sites, suggesting that the type of gas transport (diffusive or convective via root and stem) affected the depth profile of CH_4 concentrations and its flux. δ_13C-CH_4 values in pore water also varied among the vegetation types, even within Sphagnum species (e.g., at 10 cm depth, δ_13C-CH_4: R. fauriei, −55.3 ± 1.8‰ (SE); P. australis, −57.5 ± 1.6‰; M. trifoliata, −56.7 ± 1.5‰; S. cuspidatum, −71.2 ± 1.4‰; and S. palustre, −60.4 ± 0.6‰). Our results suggest that significant differences arise in CH_4 concentration and δ_13C-CH_4 values among the hydrophyte habitats even within a small peat bog and that change in vegetation relative to trophic conditions can affect CH_4 emissions and associated δ_13C-CH_4 values.