PHRAGMITES-AUSTRALIS - VENTURI-INDUCED AND HUMIDITY-INDUCED PRESSURE FLOWS ENHANCE RHIZOME AERATION AND RHIZOSPHERE OXIDATION

A Venturi-induced convection of gases through the Phragmites rhizome system is reported and the effects of this, and a humidity-induced convection previously described, are quantified. The Venturi-convection is driven by a pressure differential created by wind blowing across tall dead culms which sucks air into the underground system via culms snapped off close to ground level. The results demonstrate how wind, by increasing Venturi-induced convection, and how radiant energy, by increasing humidity-induced convection, raise the oxygen concentrations in the rhizome system, thereby causing much greater fluxes of oxygen into root and rhizosphere than are achieved if rhizome aeration is by diffusion alone. In laboratory experiments convective flow rates of as low as 0.7-1.4 x 10(-8) m3 s-1 (approx. 20-40 mm min-1 in the rhizome) were sufficient to raise rhizome oxygen concentrations to 90 % of atmospheric. The significance of this is discussed in connexion with pollution control by constructed wetlands, and photographic evidence is provided of the rhizosphere oxidizing activities in soil and anaerobic solution. A mathematical model is derived to help to evaluate the contributions of convection and diffusion in rhizome aeration, and to assess the likely effects of root and rhizosphere oxygen demands.