Differences in hydrophyte life forms induce spatial heterogeneity of CH₄ 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 13 C-CH 4 ), we analyzed CH 4 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 CH 4 : 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 13 C-CH 4 values in pore water also varied among the vegetation types, even within Sphagnum species (e.g., at 10 cm depth, δ 13 C-CH 4 : R. fauriei, À55.3 ± 1.8‰ (SE); P. australis, À57.5 ± 1.6‰; M. trifoliata, À56.7 ± 1.5‰; S. cuspidatum, À71.2 ± 1.4‰; and S. palustre, À60.4 ± 0.6‰). Our results suggest that significant differences arise in CH 4 concentration and δ 13 C-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 13 C-CH 4 values.

Multi-year effect of wetting on CH<sub>4</sub> flux at taiga–tundra boundary in northeastern Siberia deduced from stable isotope ratios of CH<sub>4</sub>

“…These study sites are underlain by continuous permafrost (Iwahana et al, 2014). Normally, snowmelt and the start of active layer thawing begin in the latter half of May through the first half of June, and the growing season occurs from the end of June through the beginning of August.…”

“…a Site K was previously named Kryvaya (Iwahana et al, 2014) or Kodak (Liang et al, 2014). b Observed for the surface soil layer down to 20 cm on 1 to 3 days in July 2011 at each observation point (see Table S2 for detailed observation dates).…”

“…In our study area, multi-year soil reduction may be important because soil temperature is generally lower than 11 • C (10 to 30 cm in depth; Fig. S1a and Iwahana et al, 2014) due to a shallow active layer underlain by permafrost. Therefore, decomposition of organic matter can be slow (Treat et al, 2015), which would slowly decrease soil redox potential, allowing it to remain relatively high in the first year of wetting.…”

Multi-year effect of wetting on CH<sub>4</sub> flux at taiga-tundra boundary in northeastern Siberia deduced from stable isotope ratios of CH<sub>4</sub>

“…Stable isotopes of CH 4 have been used to estimate production pathways of CH 4 (Sugimoto and Wada, 1993;Sugimoto and Wada, 1995;McCalley et al, 2014;Itoh et al, 2015), the fraction of oxidized CH 4 versus produced CH 4 (Marik et al, 2002;Preuss et al, 2013) and to study mechanisms of CH 4 transport by plants (Chanton, 2005). When CH 4 in soil is lost by oxidation or diffusion, both δ 13 C and δD of the remaining CH 4 increase.…”