Abilities of helophyte species to release oxygen into rhizospheres with varying redox conditions in laboratory-scale hydroponic systems

Plantlets of the wetland species cattail (Typha latifolia), reed (Phragmites australis), rush (Juncus effusus), and yellow flag (Iris pseudacorus) grown from seedlings or cuttings were investigated in laboratory-scale hydroponic systems in order to determine the intensity of oxygen release into the rhizosphere under various redox conditions. The initial redox conditions of the rhizosphere were modified by adding different amounts of titanium (111) citrate. All the plantlets investigated released oxygen into the rhizosphere (0.01 mg/h * plant to 1.41 mg/h * plant), depending on the actual conditions of each experiment. The intensity of release was found to be controlled by the external oxygen demand in the rhizosphere for the whole range of conditions from extremely reduced to oxidized. The capacities of the plantlets to release oxygen were found to be species-specific under reduced conditions in the rhizosphere (-400 mV to +200 mV). Under conditions of higher redox potential (> +200 mV) the oxygen release depends on the physiological status of the individual plant. Oxygen release rates are highest in the range -250 mV < Eh < -150 mV. For the species investigated, the highest rates were observed for Typha latifolia (1.41 mg/h * plant) followed by Phragmites australis (I mg/h * plant), Juncus effusus (0.69 mg/h * plant), and Iris pseudacorus (0.34 mg/h * plant). In general, the plantlets released oxygen with different rates into a nonbuffered rhizosphere until highly oxidized conditions in the root-near environment prevailed. The presented oxygen-releasing behavior is a process dominating natural conditions within the rhizosphere and is relevant to wetland systems and for conceptual approaches in phytoremediation.