SOIL OXYGENATION IN CONSTRUCTED REED BEDS: THE ROLE OF MACROPHYTE AND SOIL-ATMOSPHERE INTERFACE OXYGEN TRANSPORT

Abstract

The in- and efflux of metabolic gases through the soil-atmosphere interface and through the hollow culms of reed (Phragmites australis) in a soil-based constructed reed bed with lateral sub-surface water flow was quantified. The total flux of gaseous oxygen into the bed substrate was 5.86 g m-2day-1 of which 2.08 g m-2 day-1 was through the hollow culms of standing-dead culms of P. australis. The respiratory oxygen consumption of roots and rhizomes almost perfectly balanced the oxygen influx through the culms leaving only 0.02 g O2 m-2day-1 to be released to the surrounding soil. The macrophyte-induced rhizosphere oxygenation was therefore of no quantitative importance for aerobic BOD degradation and microbial nitrification. The major drawbacks of the design are the lack of ability of the reeds to develop a sufficiently high hydraulic conductivity of the soil and to transfer oxygen into the substrate. Subapical regions of white young roots of P. australis, Glyceria maxima, Typha latifolia and Iris pseudacorus released oxygen, whereas no release was detected from old roots and rhizomes Studies on the potential diffusive oxygen transfer capacity of reeds showed that at 15° C the respiratory oxygen demand of the root-system would balance the diffusive transport capacity for root lengths of approx. 60 cm. The oxidation in constructed reed beds can be significantly improved by changes in the design and loading regime. In a vertical flow system consisting of several beds laid out in parallel with intermittent water loading, the oxygen transfer from the atmosphere to the bed substrate would be 30 to 150 g m-2day-1 depending on substrate texture.