Methane transport mechanisms and isotopic fractionation in emergent macrophytes of an Alaskan tundra lake

The carbon isotopic composition of methane emitted by the Alaskan emergent aquatic plants Arctophila fulva , a tundra mid‐lake macrophyte, and Carex rostrata , a tundra lake margin macrophyte, was −58.6 ± 0.5 ( n =2) and −66.6±2.5 ( n = 6) ‰ respectively. The methane emitted by these species was found to be depleted in 13 C by 12‰ and 18‰, relative to methane withdrawn from plant stems 1 to 2 cm below the waterline. As the macrophyte‐mediated methane flux represented approximately 97% of the flux from these sites, these results suggest the more rapid transport of 12 CH 4 relative to 13 CH 4 through plants to the atmosphere. This preferential release of the light isotope of methane, possibly combined with CH 4 oxidation, caused the buildup of the heavy isotope within plant stems. Plant stem methane concentrations ranged from 0.2 to 4.0% ( x ¯ , 1.4; standard deviation (sd), 0.9; n =28) in Arctophila , with an isotopic composition of −46.1±4.3 ‰ ( n = 8). Carex stem methane concentrations were lower, ranging from 150 to 1200 ppm ( x ¯ , 500; standard deviation, 360; n = 8), with an isotopic composition of −48.3±1.4‰ ( n =3). Comparisons of the observed isotopic fractionations with those predicted from gas phase effusion and diffusion coefficients suggest a combination of one or both of these gas transport mechanisms with bulk (non‐fractionationating) flow.