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

We tested the hypothesis that the diurnal patterns of variation in lacunal gas concentrations and isotopic fractionation previously reported in a single plant genera (Typha) typified the patterns of all through-flow convective plants by extending our observations to Phragmites australis Cav. In daylight, Phragmites CH4 transport is driven by internal pressurization which results in gas flow down young green culms and its exit from one year old dead brown culms. Flow rates of 10.4 +/- 4.0 mL min(-1) were measured in this study. At night, CH4 is transported from the sediments to the atmosphere via the lacunal plant spaces by molecular diffusion. Within green culms, lacunal CH4 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 CO2 concentrations varied similarly. Concentrations of both gases varied inversely with lacunal pressure. In green culms, large isotopic fractionations were observed in CH4 and CO2 in the morning and evening during transitions in gas transport mode and were associated with slight downward flows counter to the upward diffusion of these gases. Methane delta(13)C as depleted as-100parts per thousand was observed. In daylight, lacunal CH4 was similar to or C-13 depleted relative to sedimentary and emitted CH4 isotopic values, but at night lacunal CH4 was C-13 enriched relative to sedimentary methane. Overall, the diurnal variations of CH4 concentration and delta(13)C value in Phragmites were similar to those observed in Typha and indicate that these patterns should be consistent in other convective-flow plants. Furthermore, our results demonstrate that the large isotopic fractionations found in aquatic plants can result solely from isotopic fractionation associated with gas transport.