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Field measurements of internal pressurization in Phragmites australis (Poaceae) and implications for regulation of methane emissions in a midlatitude prairie wetland¹

²Department of Agronomy, University of Nebraska, Lincoln, Nebraska 68583-0817 USA; ³Department of Oceanography, Florida State University, Tallahassee, Florida 32306-3048 USA; ⁴School of Natural Resource Science, University of Nebraska, Lincoln, Nebraska 68583-0728 USA; and ⁵Department of Atmospheric Sciences, Yonsei University, Seoul 120-749, Korea

Emergent aquatic macrophytes in vegetated wetlands provide routes for methane (CH₄) transport from sites of production in oxygen-poor sediments, where CH₄ concentrations are relatively high, to the atmosphere, which typically has much lower CH₄ concentrations. Transport can occur through aerenchymatous tissue via simple diffusion. Recently, the importance of convective throughflow (i.e., mass transport of gases through plants driven by pressure gradients) in enhancing gas transport has been demonstrated in several genera (e.g., Nuphar, Nymphaea, Nelumbo, Typha, and Phragmites). This study was conducted to elucidate the governing plant-mediated gas transport mechanisms in a midlatitude prairie wetland and to determine both their diel and seasonal variations and the importance of environmental controlling factors. Pressures inside culms of the two dominant emergent aquatic macrophytes (Scirpus acutus and Phragmites australis) were measured directly throughout the growing season and on selected days in midseason. Supporting measurements included solar radiation, air temperature, relative humidity, and windspeed. Results indicated pressures inside green healthy culms of Phragmites were above atmospheric pressure by up to 1650 Pa during the day. At night culm pressures were at or slightly above atmospheric. No pressurization was detected in Scirpus. Highest pressures in Phragmites occurred during midseason when biomass and foliage area index were at their maxima (920 g/m² and 2.8, respectively). High internal pressures also coincided with periods of high solar radiation (>500 W/m²), high temperature (>20°C), and low relative humidity (<60%). Periods of high internal pressures also coincided with periods of high CH₄ efflux from the wetland as measured in concomitant studies. Convective throughflow driven by internal pressure gradients in Phragmites thus explains much of the diel variation in methane efflux previously reported from this wetland.

Key Words: convective throughflow • methane • Phragmites • Poaceae • prairie wetland