Oxygen Release By Roots Into Rhizosphere Of Helophytes

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Oxygen reaching the root aerenchyma is respired by the root tissues and diffuses towards to the root apex and radially to the rhizosphere, to be consumed in the surrounding soil [54]. The oxygen flux from the root aerenchyma to the rhizosphere, known as "radial oxygen loss" (ROL), is determined by the concentration gradient, the consumption of oxygen by the cells along the radial path, and the physical resistance to oxygen diffusion [54, 56].

As a result, 30-40% of the oxygen supplied via the aerenchyma to the root apex is lost to the rhizosphere [54]. The radial oxygen loss causes oxygenation and therefore significant chemical and biological changes within the rhizosphere relevant to microbial populations [66], nutrient availability [67-70], and concentrations of potentially toxic substances [71-74]. However, the oxygen release from the root system to the rhizosphere mainly occurs at the apex, as well as at lateral parts which tend to be shorter, thinner and of lower porosity than the main axes [75, 76]. The lateral parts were described as the major source of oxygen loss to the rhizosphere in some species [77]. The oxygen released into the rhizosphere protects the young, sensitive parts of the root system against reduced soil compounds and helps the plants penetrate the reduced soil [19, 54, 78-81].

The oxygen released may result in a protective "layer" on the root surface [82]. This "layer" may have a thickness of between 1 and 4 mm depending on the reduced state of the root environment and it is characterized by a redox gradient ranging from about -250 mV as frequently measured in reduced rhizospheres to about +500 mV directly at the root surface [82]. Oxygen is more or less continuously released from the root system, counterbalancing chemical and biological oxygen consumption. This ability of helophytes to oxygenate their rhizosphere is of particular interest for biotechnological application [83, 84].

Different methods to estimate oxygen flow rates have been used, mainly in plant-physiological investigations [50]. Rates of 126 |mol O2/h g root dry mass for Juncus ingens (giant rush) and 120-200 |mol O2/h g root dry mass for Typha latifolia (cattail) determined by the titanium-citrate method are of technological relevance [85, 86]. Furthermore, model calculations for Phragmites australis (reed) resulting in oxygen input rates of 5-12 g O2/m2 patch area per day [86] and investigations with individual plantlets of different species in hydroponic cultures resulting in the highest oxygen release rates of 1.4 mg O2/h plantlet for T. latifolia [84] highlight from a more biotechnological view the considerable potential of helophytes to release oxygen. Some studies have revealed that the redox state of the rhizosphere has a significant effect on the intensity of oxygen release of various helophytes, with oxygen release rates increasing as the soil Eh becomes more reduced [85, 87, 88, 84]. In hydroponics model investigations, plantlets of various species showed the highest intensity of oxygen release at -250 mV < Eh < -150 mV and for extremely reduced conditions of Eh < -250 mV the release intensities were found to be lower [84].

Additional investigations emphasize the importance of the above and underground portions of the plants for the intensity of oxygen release [89]. The release rates by the roots of T. latifolia and J. effusus were found to be determined by the growth state of the aboveground part of the plants and relatively independent from the size of the root systems [89]. To clarify the influence of ambient conditions (air temperature and air humidity) determining the gas input at the leaves on the intensity of oxygen release into the rhizosphere, additional investigations should be performed.

The ventilation inside the helophytes causes air flow through the plants, i.e. the transport of mainly O2 and also N2. The behaviour of the atmospheric nitrogen (given radial losses from the roots) and its importance inside the rhizosphere has still not been investigated.

Gas exchange at the appropriate parts of the root system not only causes the oxygenation of the rhizosphere but also enables the flux of gases from the rhizosphere together with gases generated inside the plant tissues to the atmosphere by internal diffusion and/or ventilation. Although this exchange has been investigated for ethylene, carbon dioxide and methane, for example [90-92], information about nitrogen generated by denitrification inside the rhizosphere is still lacking. The processes involved depend on the species [83, 93-100]. In connection with the gas exchange, determining the phytohormone ethylene as a marker for plant-physiology studies is of particular interest [52, 92, 101-108]. Carbon dioxide may be generated by microbial mineralization in the soil or be derived from respiration in underground tissues of the plants [95, 109] and can also be fixed in photosynthesis by the plants [110]. Because of the role of methane as a "greenhouse gas" - after all, up to 90% of methane emissions from flooded soils may be transferred by the plants to the atmosphere [98] - interest in the process of methane release by plants has grown in recent years [111].

4. Release of organic compounds from plants

Knowledge about the input of carbon from plants into the rhizosphere (rhizodeposition) mainly comes from agricultural research. The quantity of C-compounds released by agricultural crops has been estimated at 10-40% of net photosynthetic production [112]. The composition of the exudates is highly diverse and species-specific. For example, sugars and vitamins such as thiamine, riboflavine and pyridoxine etc., organic acids such as malate, citrate, amino acids, benzoic acids, and phenolic compounds have all been identified [113]. Rhizodeposition may initiate the mobilization of nutrients [114], allelopathic effects [113], and the stimulation of microbial growth and activity inside the rhizosphere [112, 115-117].

Generally speaking, our understanding of the composition of root exudates of helophytes is very limited. Various substituted aromatic derivatives with hydroxyl, methoxyl, aldehyde and carboxyl groups were found in rhizome extracts of Scirpus lacustris (bulrush) [118]. These compounds can be used as a carbon source by microorganisms in the rhizosphere, causing oxygen depletion and decreasing the redox state. If not enough oxygen is available, other electron acceptor such as nitrate, nitrite, Fe3+, sulphate or carbonate (methanogenesis) are used. The resulting metabolic products, especially H2S, influence the redox conditions of the rhizosphere.

5. Dynamics of redox conditions and the removal of contaminants inside the rhizosphere of helophytes - results of model investigations

Because of the usually low hydrodynamics, rapid microbial consumption by more or less adsorbed micro-organisms and fast chemical reactions (fast oxidation in the strongly reduced surroundings) changes in redox reactions inside the rhizosphere due to gas exchange and the release and uptake of compounds mainly affect the area close to the root surface. Evaluating such micro-gradient processes near the root surface in constructed wetlands is very difficult due to the diurnal dynamics of different interacting redox reactions, such as nitrification, denitrification, the mineralisation of carbon substances, methanogenesis, sulphate reduction, sulphur/sulphide oxidation etc. [2, 7, 13].

Despite the overall reduced redox conditions detectable in the rhizosphere of constructed wetlands, highly effective oxidation processes can also be determined by evaluating the inflow and outflow concentrations of oxidizable contaminants. In addition to oxidation processes enhanced by the input of atmospheric oxygen at the surface of the wetland [119], diurnal micro-gradient oxidation processes near the rhizoplane clearly also play an important role. Furthermore, temporal gradients overlapping the diurnal gradients corresponding to changes in the oxygen release at the rhizoplane have to be considered, too, due to daily and annual variations of plant-physiological and/or ambient gas exchange conditions (temperature, humidity, illumination etc).

To shed more light on the dynamics of the redox conditions enhanced by the plants and the changes in micro-gradient redox reactions, investigations in a specially designed laboratory-scale reactor [120] were carried out [121]. This enabled the treatment of an artificial wastewater in a planted (J. effusus) gravel bed to be evaluated, and daily variations in the redox state depending on the intensity of illumination by daylight were observed, as shown in Figure 1.

Particularly in summer, these variations ranging from Eh ^ -150 to -230 mV at night to Eh ^ +50 to +350 mV at midday and in the early afternoon could only have been caused by changes in the light-enhanced ventilation inside the plants, resulting in changes in the oxygen release by the roots. This effect was interpreted as fundamental proof of the daily temporal variation of the redox state in the areas of the rhizosphere near the roots. Further investigations are needed to evaluate the correlations between these short-term redox changes and certain removal processes. The pH was also found to change daily to a small but still significant extent in response to the intensity of the daylight (Figure 2).

This pH variability has to be interpreted as a reaction to changes in the removal processes, including the consumption of nitrate, as well as changes to carbon dioxide transport out of the rhizosphere that are enhanced by the plants.

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