Diel variation in lacunal CH4 and CO2 concentration and δ13C in Phragmites australis

We tested the hypothesis that the diurnal patterns of variationin lacunal gas concentrations and isotopic fractionationpreviously reported in a single plant genera (Typha)typified the patterns of all through-flow convective plantsby extending our observations to Phragmites australisCav. In daylight, Phragmites CH4 transport isdriven by internal pressurization which results in gas flowdown young green culms and its exit from one year old deadbrown culms. Flow rates of 10.4 ± 4.0 mL min−1 weremeasured in this study. At night, CH4 is transportedfrom the sediments to the atmosphere via the lacunal plantspaces by molecular diffusion. Within green culms, lacunalCH4 concentrations varied by a factor of 1000, from 3%(parts by volume) pre-dawn to lows of 25 ppmv during midday.Methane in brown culms varied by a factor of 10 diurnally,from 5% pre-dawn to 0.3% at midday. Lacunal CO2concentrations varied similarly.Concentrations of both gases varied inversely with lacunalpressure. In green culms, large isotopic fractionations wereobserved in CH4 and CO2 in the morning and eveningduring transitions in gas transport mode and were associatedwith slight downward flows counter to the upward diffusionof these gases. Methane δ13C as depletedas −100‰ was observed. In daylight, lacunal CH4 wassimilar to or 13C depleted relative to sedimentary andemitted CH4 isotopic values, but at night lacunalCH4 was 13C enriched relative to sedimentarymethane. Overall, the diurnal variations of CH4concentration and δ13C value inPhragmiteswere similar to those observed in Typha andindicate that these patterns should be consistent in otherconvective-flow plants. Furthermore, our results demonstratethat the large isotopic fractionations found in aquaticplants can result solely from isotopic fractionationassociated with gas transport.