Worldwide, contamination of soil and ground water is a severe problem. The negative effects of pollutants on the environment and on human health are diverse and depend on the nature of the pollution. The search for alternative methods for excavation and incineration to clean polluted sites resulted in the application of bioremediation techniques. In this review, we describe some generally accepted bioremediation tools and subsequently focus on the combination of two approaches, phytoremediation and bioaugmentation, resulting in rhizoremediation. During rhizoremediation, exudates derived from the plant can help to stimulate the survival and action of bacteria, which subsequently results in a more efficient degradation of pollutants. The root system of plants can help to spread bacteria through soil and help to penetrate otherwise impermeable soil layers. The inoculation of pollutant-degrading bacteria on plant seed can be an important additive to improve the efficiency of phytoremediation or bioaugmentation.

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Rhizoremediation: A Beneficial Plant-Microbe Interaction

Significance of Soil Organic Matter to Soil Quality/Health - An Overview, R. Weil and F. Magdoff Organic Matter Management Strategies, F. Magdoff and R. Weil Soil Organic Matter Fractions and Their Relevance to Soil Quality/Health, M. Wander Stimulatory Effect of Humic Substances on Plant Growth, Y. Chen, M. De Nobili, and T. Aviad The Role of Soil Organic Matter and Soil Quality in Soil-Plant-Disease Interactions, A. Stone and H. Darby Contributions of Fungi to Soil Organic Matter in Agroecosystems, K.A. Nichols and S.F. Wright Connecting Belowground and Aboveground Food Webs: The Role of Organic Matter in Biological Buffering, P.L. Phelan Tillage and Residue Management Effects on Soil Organic Matter, A.J. Franzluebbers Strategies for Managing Soil Organic Matter to Supply Plant Nutrients, S. Seiter and W. Horvath Soil and Crop Management Effects on Soil Microbiology, A.C. Kennedy, T.L. Stubbs, and W.F. Schillinger Interactions between Organic Matter, Earthworms and Microorganisms in Promoting Plant Growth, C.A. Edwards and N.Q. Arancon

Soil Organic Matter in Sustainable Agriculture

This review concentrates on the description of various phytoremediation technologies, paying special attention to removal of organics and the application of in vitro systems for basic research in the role of plants for the remediation of contaminated sites or flows, and in the improvement of their effectiveness. Various aspects of xenobiotic metabolism in plant cells, the role of enzymes involved, and the cooperation with rhizospheric microorganisms accelerating remediation of organics are shown. Application of this approach as well as the possibility of introduction of foreign genes into plant genome that can enhance the rate of the bioremediation are discussed. © 2000 Elsevier Science Inc. All rights reserved.

Research review paper Exploitation of plants for the removal of organics in environmental remediation

Exploitation of plants for the removal of organics in environmental remediation.

Phytoremediation is a promising technology for the cleanup of polluted environments. The technology has so far been used mainly to remove toxic heavy metals from contaminated soil, but there is a growing interest in broadening its applications to remove/degrade organic pollutants in the environment. Both plants and soil microorganisms have certain limitations with respect to their individual abilities to remove/breakdown organic compounds. A synergistic action by both rhizosphere microorganisms that leads to increased availability of hydrophobic compounds, and plants that leads to their removal and/or degradation, may overcome many of the limitations, and thus provide a useful basis for enhancing remediation of contaminated environments. The review of literature presented in this article provides an insight to the nature of plant-microbial interactions in the rhizosphere, with a focus on those processes that are relevant to the breakdown and/or removal of organic pollutants. Due consideration has been given to identify opportunities for utilising the plant-microbial synergy in the rhizosphere to enhance remediation of contaminated environments. The literature review has highlighted the existence of a synergistic interaction between plants and microbial communities in the rhizosphere. This interaction benefits both microorganisms through provision of nutrients by root exudates, and plants through enhanced nutrient uptake and reduced toxicity of soil contaminants. The ability of the plant-microbial interaction to tackle some of the most recalcitrant organic chemicals is of particular interest with regard to enhancing and extending the scope of remediation technologies. Plant-microbial interactions in the rhizosphere offer very useful means for remediating environments contaminated with recalcitrant organic compounds. A better knowledge of plant-microbial interactions will provide a basis for improving the efficacy of biological remediations. Further research is, however, needed to investigate different feedback mechanisms that select and regulate microbial activity in the rhizosphere.

Utilising the Synergy between Plants and Rhizosphere Microorganisms to Enhance Breakdown of Organic Pollutants in the Environment (15 pp)

Rhizosphere microbial populations may increase bioremediation of soil contaminated with organic chemicals. A growth chamber study was conducted to evaluate rhizosphere microbial populations in contaminated and non-contaminated soil. Alfalfa (Medicago sativa L.) and alpine bluegrass (Poa alpina L.) were grown in soil containing a mixture of organic chemicals for 14 weeks. The equal millimolar mixture of hexadecane, (2,2-dimethyl-propyl)-benzene, cis-decahydronaphthalene (decalin), benzoic acid, phenanthrene, and pyrene was added at levels of 0 and 2000 mg/kg. Organic chemical degrader (OCD) populations were assessed by a Most-Probable-Number technique, and bacteria and fungi were enumerated by plate count methods. Different methods for expressing OCD rhizosphere populations were investigated to determine the effect it had on interpretation of the results. At 9 weeks, the OCD numbers were significantly higher in rhizosphere and contaminated soils than in bulk and non-contaminated soils, respectively. Alfalfa rhizosphere OCD levels were 4 × 107/g for contaminated and 6 × 106/g for non-contaminated soils. Bluegrass rhizosphere OCD levels were 1 × 107/g and 1 × 106/g in contaminated and non-contaminated soils, respectively. Selective enrichment of OCD populations was observed in contaminated rhizosphere soil. Higher numbers of OCD in contaminated rhizospheres suggest potential stimulation of bioremediation around plant roots.

Rhizosphere microbial populations in contaminated soils

The United States has more than 1000 individual areas of petroleum-contaminated soil at formerly used defense (FUD) sites located in cold regions This paper investigates biotreatment systems based on exploiting naturally occurring phenomena in the rhizosphere — the soil adjacent to and influenced by plant roots Rhizosphere-based remediation systems would be inexpensive to implement and maintain and would be applicable to remote or permafrost sites Herein, this paper provides the rationale for using rhizosphere-based biotreatment systems and some initial results In both laboratory and field studies, successful plant germination, plant growth, and root intrusion into and through contaminated soil are demonstrated Using a Captina silt loam in a 10-week laboratory study, the effects of vegetation and contamination on microbial numbers were compared The vegetation treatments included an unvegetated control and a vegetated treatment seeded with bahiagrass ( Paspalum notatum ) The contamination treatments included an uncontaminated control and a treatment with 2000 mg pyrene kg -1 soil added Microbial numbers at 10 weeks were not significantly influenced by the contaminant level of 2000 mg pyrene kg -1 soil compared to the control However, microbial numbers were greater in the rhizosphere of the bahiagrass-vegetated soil compared to the unvegetated soil In a 34-week field study, total petroleum hydrocarbon (TPH) concentrations of a diesel-contaminated soil decreased significantly more in the rhizosphere+nutrient treatment compared to the control that was not vegetated or fertilized Bacterial numbers in the field study were 287 times greater in the rhizosphere+nutrient treated soils than in the control treatments Measurable TPH compounds in the plant tissue were insignificant The data demonstrated that rhizosphereenhanced treatment of organic-contaminated soils can be effective in reducing soil petroleum concentrations and may be a cost-effective strategy particularly suited for treating cold-region sites where remediation options are limited by cost, remoteness of the site, and/or brevity of the treatment season

Plant enhancement of indigenous soil micro-organisms: a low-cost treatment of contaminated soils

A mixture of organic chemicals (MOC) containing equal molar amounts of benzoic acid, hexadecane, 2,2‐dimethyl 4,n‐propyl‐benzene, phenanthrene, pyrene, and either cycloheptane or cis‐decahydronaphthalene (cis‐decalin) was applied to soil at rates of 0 to 8000 mg/kg. In a plant‐screening experiment, growth responses of four legume and five nonlegume species were determined at 10 and 25°C. The MOC applied at 2000 mg/kg reduced the growth of several species without resulting in significant seedling death. At 10°C, the growth of alpine bluegrass (Poa alpina L.) in the 1000 and 2000 mg/kg treatments of soil increased by more than 185%. In a plant growth response experiment, alpine bluegrass and alfalfa (Medicago sativa L.) were grown in soil that had been contaminated at rates of 0 and 2000 mg/kg. At 14 weeks, the shoot and root dry weights of alfalfa were 97% lower in the contaminated soil, while the shoot dry weight, root dry weight, and root length of alpine bluegrass were 135,235 and 268% higher, ...

H. B. Rogers

Papers

Rhizosphere microbial populations in contaminated soils

Plant enhancement of indigenous soil micro-organisms: a low-cost treatment of contaminated soils

Selection of cold‐tolerant plants for growth in soils contaminated with organics