CONVECTIVE GAS-FLOW IN ELEOCHARIS-SPHACELATA R BR - METHANE TRANSPORTAND RELEASE FROM WETLANDS

We investigated the importance of lacunar gas transport for the releas e of methane from beds of Eleocharis sphacelata R. Br. in a freshwater wetland in southeastern Australia. Gases were transported in the E. s phacelata lacunar system by pressurized convective flow, which was app arently driven by humidity induced pressurization. Internal culm tempe ratures were 1.9-4.2-degrees-C lower than at the external culm surface , suggesting that thermal transpiration was not responsible for lacuna r pressurization. Some of the culms (influx culms) provide a net gas f lux from the atmosphere into the plant, whereas others (efflux culms) act as conduits for flux from the rhizosphere to the atmosphere. The m ean gas influx was 1.0 +/- 0.2 (SE) ml min-1 per culm, and the mean ga s efflux was 0.2 +/- 0.0 (SE) ml min-1 per culm. The difference in inf lux and efflux flow rates is due to unequal numbers of the two culm ty pes, and the total gas flux through three adjacent E. sphacelata beds was estimated from this flow ratio and the total culm density. It rang ed from 1.1 to 2.5 l 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 1-1 5 times the rate of methane release from the E. sphacelata beds by ebullition of metha ne in bubbles released from the sediment. Diffusive methane fluxes in the lacunar system (< 300 mul m-2 h-1) were an insignificant mechanism of methane release compared with lacunar convective flow and ebulliti on fluxes from sediments. The interstitial methane concentration in se diments from E. sphacelata beds was approximately 0.6 times that of ad jacent unvegetated sediment, highlighting the role of convective flow by E. sphacelata in accelerating methane release from sediments.