Convective gas flow in Eleocharis sphacelata R. Br.: methane transport and release from wetlands

Murray-Darling Freshwater Research Centre

MDFRC item.

We investigated the importance of lacunar gas transport for the release of methane from beds of Eleocharis sphacelata R. Br. in a freshwater wetland in southeastern Australia. Gases were transported in the E. sphacelata lacunar system by pressurized convective flow, which was apparently driven by humidity induced pressurization. Internal culm temperatures were 1.9–4.2° C lower than at the external culm surface, suggesting that thermal transpiration was not responsible for lacunar pressurization. Some of the culms (influx culms) provide a net gas flux from the atmosphere into the plant, whereas others (efflux culms) act as conduits for flux from the rhizosphere to the atmosphere. The mean gas influx was 1.0±0.2 (SE) ml min−1 per culm, and the mean gas efflux was 0.2±0.0 (SE) ml min−1 per culm. The difference in influx and efflux flow rates is due to unequal numbers of the two culm types, and the total gas flux through three adjacent E. sphacelata beds was estimated from this flow ratio and the total culm density. It ranged from 1.1 to 2.5 1 m−2 h−1. The methane concentration in the efflux culms was 2–3%, resulting in a total methane efflux from E. sphacelata of 22–75 ml CH4 m−2 h−1. These rates represented 1–15 times the rate of methane release from the E. sphacelata beds by ebullition of methane in bubbles released from the sediment. Diffusive methane fluxes in the lacunar s system (< 300 μl m−2 h−1) were an insignificant mechanism of methane release compared with lacunar convective flow and ebullition fluxes from sediments. The interstitial methane concentration in sediments from E. sphacelata beds was approximately 0.6 times that of adjacent unvegetated sediment, highlighting the role of convective flow by E. sphacelata in accelerating methane release from sediments.