Methane concentration and stable isotope distribution as evidence of rhizospheric processes: Comparison of a fen and bog in the Glacial Lake Agassiz Peatland complex
Ls. Chasar et al., Methane concentration and stable isotope distribution as evidence of rhizospheric processes: Comparison of a fen and bog in the Glacial Lake Agassiz Peatland complex, ANN BOTANY, 86(3), 2000, pp. 655-663
This study evaluates relationships between vegetation and stable isotope di
stribution within a large, northern peat-accumulating wetland. Concentratio
n and delta(13)C for both porewater and emitted methane were obtained from
June-September for two systems characterized by different plant assemblages
and hydrologic regimes: a Carer-dominated fen and a Sphagnum-dominated, fo
rested bog crest. Average methane emissions were higher in the fen than at
the bog crest across the entire growing season. Fen porewater methane conce
ntrations were maintained at consistently low levels in the upper one-third
of the peat column, and emitted methane was substantially C-13-depleted (b
y approx. 6 parts per thousand) relative to shallow porewater methane, tren
ds which are characteristic of passive plant-mediated transport of rhizosph
eric methane to the atmosphere. Fen porewater delta(13)C-CH4 values in shal
low pear (approx. -59 parts per thousand) suggest that microbial respiratio
n was primarily driven by acetate fermentation. CO2 reduction became more i
mportant deeper in the peat column with delta(13)C-CH4 values ranging from
approx. -65 to -69 parts per thousand between 1 and 2.8 m. In contrast to t
he fen, porewater methane concentrations in the bog were usually at near-ma
ximum levels just below the water table. delta(13)C values for emitted CH4
in the bog were enriched relative to those for shallow porewater CH4 by app
rox. 10 parts per thousand, indicating that methane was subject to oxidatio
n as it exited from the pear via passive diffusion. Methanogenesis in the p
eat at the bog crest appears to have been substrate-limited, with porewater
delta(13)C-CH4 (approx. -67 parts per thousand) suggestive of CO2 reductio
n at all depths. (C) 2000 Annals of Botany Company.