The rhizosphere

The rhizosphere, first described in 1904 by Lorentz Hiltner, Soil Bacteriologist at the Technical College of Munich, has been the focus of intensive research for many years because of its importance in plant nutrition and pathogenesis. More recently, the research on rhizosphere has been directed towards its influence on the transformation of pollutants in soils and now it is well established that microbially-mediated transformation processes in the rhizosphere play an important role in controlling the persistence, mobility and bioavailability of toxicants in soils.

The rhizosphere is a micro-zone at the root-soil interface that is under the influence of the plant root. A plethora of mutually interacting physical, chemical and biological processes operate within this zone. Although attempts have been made to unravel some of these processes, the understanding of the intricacies of this unique ‘twilight zone’ is still in its infancy. It is for this reason that noted microbiologists, G. D. Bowen and A. Rovira, correctly described the rhizosphere zone as - ‘the hidden half of the hidden half’.

Depending on plant species, the width of the rhizosphere zone has been shown to extend 2 - 80 mm from the root surface. The dimension of rhizosphere zone is affected by a number of factors that include soil characteristics, plant species, nutritional status of plants and climatic conditions. The rhizosphere zone is distinguished from the bulk soil zone, more commonly known as the ‘edaphosphere’, by enhanced microbial activity and increased concentration of root exudates. Nevertheless it is proving difficult to physically separate this zone from the root surface or ‘rhizoplane’. The rhizosphere effect is expressed quantitatively as the ratio of the number or activity of microorganisms or level of root exudates in rhizosphere soil (R) to that in the edaphosphere soil (E), the R/E ratio. The R/E ratios for microorganisms and root exudates are often found to range from 2 to 20 and from 5 to 100, indicating enhanced activity of microogranisms in the rhizosphere.

The emerging field of the use of green plants in the remediation of contaminated soils, called ‘Phytoremediation’ or ‘Green remediation’ is attracting research and commercial interests. Application of rhizosphere processes to phytoremediation of inorganic and organic contaminants in soil and aquatic environments requires greater understanding of these processes. This article provides some insight into the beneficial effects of rhizosphere on plant nutrition and contaminant attenuation.

The role of rhizosphere in bio(availability) and attenuation of nutrient ions and contaminants

Rhizosphere controls the transformation of nutrient ions and contaminants through changes in pH, redox potential, microbial population and mycorrhizal association. Changes in pH are brought about by the excretion of protons (H+), hydroxyl (OH-) or bicarbonate (HCO3-) ions due to cation/anion imbalance in the plant, the evolution of CO2 by respiration, and the excretion of low-molecular-weight organic acids. Plants taking excess cation over anion (cation charge surplus) tend to balance the charge by releasing H+, resulting in acidification of rhizosphere. Conversely, plants taking excess anion over cation (anion charge surplus) tend to balance the charge by releasing OH- or HCO3- ions, resulting in alkalinisation. The form of N supply has a major role in the cation/anion uptake ratio and its subsequent effect on rhizosphere pH. Plants take up N in three main forms – as an anion (nitrate, NO3-), as a cation (ammonium, NH4+) or as a neutral N2 molecule (from N2 fixation). Depending upon the form of N taken up and the mechanism of assimilation in the plant, excesses of cation or anion uptake may occur. To maintain charge balance during the uptake process, H+, OH- or HCO3- ions must pass out of the root into the surrounding soil. The H+ ions may be derived from the dissociation of organic acids within the cell, and OH- and HCO3- ions from the decarboxylation of organic acid anions. In general, while the uptake of NH4+ and N2 from fixation results in a net release of H+ ions, uptake of NO3- can result in a net release of OH- ions. Rhizosphere-enhanced acidification induces the solubilization of both nutrient ions, such as phosphate, copper and zinc, and toxic metal ions, such as cadmium and mercury. Thus, it is possible to manipulate the rhizosphere pH through appropriate use of N compounds, thereby controlling the transformation, mobility and bioavailability of nutrients and contaminants in soils.

Plant roots alter the redox potential in the rhizosphere soil directly by excreting CO2, and indirectly through the supply of readily available carbon for enhanced microbial respiration. In general, the redox potential is lower in the rhizosphere than in the bulk soil. The decrease in redox potential is likely to have important consequences on the redox reactions of pollutant metals such as arsenic, manganese and chromium. For example, enhanced reduction of Cr(VI) to Cr(III) reduces the toxicity and mobility of Cr in soils.

The enhanced activity of microorganisms, including mycorrhizal fungi, in rhizosphere is important in relation to the bioavailability and mobility of nutrients ions, inorganic metals and organic contaminants. For example, it has often been shown that the dissolution and release of water-insoluble fertilizer materials, such as elemental sulphur and apatite phosphate rocks is enhanced in the presence than in the absence of plants, which has been related to the increased activity of sulphur-oxidising microorganisms and the release of organic acids. The enhanced microbial activity is likely to reduce the mobility of non-reactive ions, such as nitrate and sulphate through microbial immobilisation. Plant roots have been shown to release a number of organic acids, such as citric, formic and oxalic which are believed to be involved in the solubilization of phosphate compounds and metal ions. It has been estimated that between 10 – 40% of the total net C assimilated by plants is released in the form of soluble root exudates, and insoluble materials such as cell wall and mucilage.

Microbial degradation is the major process by which organic contaminant residues are removed from the soil. The rate of degradation of organic pollutants has often been found to be faster in the presence of growing plants. The role of rhizosphere in the attenuation of contaminants is examined using a range of experimental techniques that include simple microcosm root cores under green house conditions, and mesocosm (Photo 1) and rhizotrons under field conditions. In a recent experiment, we examined the influence of rhizosphere on the degradation of 2,4-D herbicide using microcosm root cores. Two cores of soils, one containing the plant (clover plus ryegrass) and the other containing the herbicide were connected together by PVC rings. The soil core containing the herbicide was separated from plant roots by a 25 m nylon mesh. The nylon mesh allows only the root hairs to enter the soil containing the herbicide. The soil immediately close to the nylon mesh in the pesticide cores represents the transition zone between the soil in the root core (rhizosphere) and the bulk soil in the pesticide core (edaphosphere). After 4 weeks of plant growth the soil cores were separated. To examine the relative distribution of 2,4-D residues the soil in the pesticide core was cut into this sections.

The distribution of the water-soluble carbon, the microbial activity and the 2,4-D residues in soil samples were taken at different distances from rhizoplane in a microcosm root container experiment. There was an increase in the amount of soluble carbon in soil sections close to the root surface. The increase in soluble carbon is related to the rhizodeposition of root exudates that include low-molecular-weight organic acids, carbohydrates, nucleic acid derivatives and amino acids. There was a corresponding increase in microbial activity, as measured by the amount of oxygen consumed by the microorganisms, in the soil sections close to the root surface. The enhanced microbial activity in the rhizosphere is related to an increase in the supply of organic substrate as a source of carbon and energy for microbial growth. There was a decrease in the amount of 2,4-D remaining in the soil indicating that microbial degradation of pesticides occurred during the plant growth. The amount of residual 2,4-D remaining in the pesticide cores was less in the soil sections close to the root surface than in the other sections. Since grass roots are unlikely to absorb considerable amounts of 2,4-D residues, the low amount of 2,4-D residues close to the root core is attributed to the root-induced degradation of pesticides. The enhanced degradation of pesticides in soil sections close to the root surface is related to the rhizosphere-induced co-metabolism of pesticides. Co-metabolism requires the presence of a growth substrate other than the compound to be mineralised. Plant roots excrete a wide range of organic substances and the availability of these exudates is considered to be the major cause for the existence of an enriched microbial population in the rhizosphere, resulting in accelerated degradation of pesticides. The actively growing plant roots provide an excellent environment for intensive microbial activity, resulting in enhanced biodegradation of organic contaminants.

It is likely that an increase in the activity of pesticide-degrading microorganisms in the rhizosphere of certain plants is a mechanism by which the plants are protected from the toxic effects of the pesticides. Selective enrichment of microorganisms is likely to have a significant impact on the rhizo-remediation, rhizo-extraction or rhizo-filtration of recalcitrant organic contaminants in soils. It has become apparent that the role of rhizosphere in the natural attenuation of pollutants in soils is increasingly being recognised.

Summary

Historically, plant pathologists have been very active in research relating to rhizosphere influence on the infection of a number of root diseases. Soil scientists, on the other hand, seem to have neglected the importance of rhizosphere in the transformation of nutrients and contaminants in soil. A greater understanding of the biological and chemical changes in the rhizosphere will enable us to identify the processes involved in the mobilisation of nutrients and pollutants added to soils. Further in-depth study is required to describe the ecological and physiological characteristics of the microbial communities associated with plant roots and to identify the zone of influence of various plant species in relation to biodegradation of hazardous organic compounds. Basically two approaches have been used in the study of rhizosphere processes: experimental investigation and modelling. There have been increasing efforts on the experimental investigation of rhizosphere processes in relation to the attenuation of contaminants. However, mathematical modelling of the spatial extent of rhizosphere influence on chemical transformations in the soil is still in its infancy. Successful mathematical models have been developed to explain the mobilisation and transport of nutrients in the rhizosphere zone soil matrix. Transport of chemicals to the root from the bulk soil matrix is generally a function of a number of mechanisms which include; solution phase diffusion, surface phase diffusion, convection, mechanical dispersion, soil liquid exchange phenomenon and rhizosphere-induced solute changes. Taking into account the above mechanisms, mathematical models need to be developed, representing the chemical flux in the rhizosphere and edaphosphere. Kinetic and equilibrium boundary conditions should be applied at different interfaces that include rhizoplane, and rhizoplane-rhizosphere and rhizosphere-edaphosphere interfaces.
   

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