Quantification of methane oxidation in the rhizosphere of emergent aquatic macrophytes: defining upper limits

Abstract

Rates of rhizospheric methane oxidation were evaluated by aerobic incubations of subcores collected in flooded anoxic soils populated by emergent macrophytes, by greenhouse whole plant incubations, and by CH4 stable isotopic analysis. Subcore incubations defined upper limits for rhizospheric methane oxidation on an areal basis which were equal to or greater than emission rates. These rates are considered upper limits because O2 did not limit CH4 uptake as is likely to occur in situ. The ratio of maximum potential methane oxidation (MO) to methane emission (ME) ranged from 0.7 to 1.9 in Louisiana rice (Oryza sativa), from 1.0 to 4.0 in a N. Florida Sagittaria lancifolia marsh, and from 5.6 to 51 in Everglades Typha domingensis and Cladium jamaicense areas. Methane oxidation/methane emission ratios determined in whole plant incubations of Sagittaria lancifolia under oxic and anoxic conditions ranged from 0.5 to 1.6. Methane oxidation activity associated with emergent aquatic macrophytes was found primarily in fine root material. A weak correlation was observed between live root biomass and CH4 uptake in Typha. Rhizomes showed small or zero rates of methane uptake and no uptake was associated with plant stems. Methane stable isotope data from a S. lancifolia marsh were as follows: CH4 emitted from plants: −61.6 ± 0.3%; CH4 within stems: −42.0 ± 0.2%; CH4 within sedimentary bubbles: −51.7 ± 0.3%). The 13C enrichment observed relative to emitted CH4 could be due to preferential mobilization of CH4 containing the lighter isotope and/or the action of methanotrophic bacteria.