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Methane flux from Peltandra virginica: stable isotope tracing and chamber effects
Author(s) -
Chanton Jeffrey P.,
Whiting Gary J.,
Showers William J.,
Crill Patrick M.
Publication year - 1992
Publication title -
global biogeochemical cycles
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.512
H-Index - 187
eISSN - 1944-9224
pISSN - 0886-6236
DOI - 10.1029/91gb02969
Subject(s) - flux (metallurgy) , methane , environmental chemistry , environmental science , macrophyte , chemistry , geology , oceanography , organic chemistry
Methane flux from plots of Peltandra virginica in a bottomland hardwood swamp in southeast Virginia varied from 270 to 670 mg CH 4 m −2 d −1 from May to August and was 6 times greater than the flux from unvegetated adjacent open waters. Variations in chamber temperature, light, and CO 2 levels failed to produce systematic changes in CH 4 flux rate or its isotopic composition from vegetated plots, suggesting that CH 4 flux from Peltandra is independent of stomatal aperture or hourly variations in photosynthetic rate. Comparisons of different chamber techniques for the collection of CH 4 for isotopic analysis and for flux measurements from Peltandra suggest that the effect of using simple uncontrolled chambers is small relative to temporal and spatial variations in methane emission and its isotopic composition. These results may be extended to other aquatic macrophytes which also transport gas primarily by molecular diffusion such as Oryza, Carex, Pontederia, Sagittaria, and Cladium. The δ 13 C of CH 4 released from Peltandra varied between −61 and −71 °/oo. Isotopic variations in CH 4 associated with Peltandra appeared to be controlled by transport effects. Methane withdrawn from Peltandra stems was considerably 13 C enriched relative to sedimentary CH 4 and was not an indicator of the isotopic composition of CH 4 emitted by these plants. Hence emergent aquatic macrophytes rooted in organic‐rich sediments do not appear to be a source of 13 C enriched methane to the atmosphere. Emitted CH 4 was 13 C depleted relative both to the CH 4 within stems (−49 to −55 °/oo) and within sediments (‐56 to ‐58 °/oo), suggesting isotopic fractionation associated with the release of CH 4 by the plants. The preferential release of 12 CH 4 by the plants is partly compensated for by the enrichment of 13 C in CH 4 contained within the plant stem. Although the presence of methanotrophic bacteria within the rhizosphere of Peltandra was demonstrated, the stable isotopic composition of methane emitted from these plants or collected from sedimentary bubbles and plant stems presents no evidence for extensive oxidation of methane within Peltandra or its rhizosphere. Either stable isotopes are not a good indicator of rhizopheric methane oxidation or the microbes' in situ respiration is limited by competition for oxygen with the demands of root respiration, oxidation of complex organic matter, ferrous iron oxidation, and nitrification.