The effect of gas transport on the isotope signature of methane in wetlands

This paper reviews the C-13 isotopic fractionations of the three main modes of CH4 transport from wetlands: diffusion, ebullition and via emergent aquatic plants. Aquatic plants employ either diffusion or a combination of convective bulk flow in daylight and diffusion at night. Ebullition and diffusion across the air-water interface result in no to little isotopic fractionation. However, diffusion through air when conducted by plant lacunae or aerenchyma results in significantly greater rates of transport for (CH4)-C-12 relative to (CH4)-C-13. This difference causes the backup of C-13-enriched CH4 in plant lacunae, which partly but not totally compensates for the isotopic fractionation out of the plant. It is also suggested that the preferential plant transport of (CH4)-C-12 causes C-13 enrichment of methane in the rhizosphere but it is difficult to separate the effects of rhizospheric methane oxidation from this process. Convective bulk flow transport via plants results in little isotopic fractionation. However, the pattern of fractionation differs significantly in convective plants from day to night as diffusion is more important in darkness. (C) 2004 Elsevier Ltd. All rights reserved.