Locus of methane release and mass‐dependent fractionation from two wetland macrophytes

Abstract

Factors responsible for the alteration of CH 4 isotopic composition during gas transport by emergent macrophytes were investigated in Pontederia cordata (Pickerelweed) and Sagittaria lancifolia (Bull tongue). Measured rates of CH 4 emission from petioles and leaves indicated that the locus of gas release from these plants is the petiole and not the leaf. Methane concentration profiles of gases within petioles further indicated that most CH 4 is emitted from the lower portion of the petiole near the waterline. Ethane and propane tracer experiments confirmed mass‐dependent fractionation during gas transport through these plants. After injection with a gas mixture, both plant types emitted ethane 12% faster than propane. Twenty‐five minutes or more after injection, ethane was found to be depleted in gas within petioles relative to the injected gas. The degree of fractionation observed for the 13 CH 4 / 12 CH 4 and ethane/propane pairs was similar to values predicted by kinetic gas theory.