Phytostimulation, also referred to as enhanced rhizosphere biodegradation, rhizodegradation, or plant-assisted bioremediation/degradation, is the breakdown of organic contaminants in the soil via enhanced microbial activity in the plant root zone or rhizosphere. Microbial activity is stimulated in the rhizosphere in several ways: (1) compounds, such as sugars, carbohydrates, amino acids, acetates, and enzymes, exuded by the roots enrich indigenous microbe populations; (2) root systems bring oxygen to the rhizosphere, which ensures aerobic transformations; (3) fine-root biomass increases available organic carbon; (4) mycorrhizae fungi, which grow within the rhizosphere, can degrade organic contaminants that cannot be transformed solely by bacteria because of unique enzymatic pathways; and (5) the habitat for increased microbial populations and activity is enhanced by plants [1]

Five enzyme systems in soils and sediments have been investigated by the U.S. EPA Laboratory in Athens, GA: (1) dehalogenase - important in dechlorination reactions of chlorinated hydrocarbons; (2) nitroreductase - required in the first step of nitroaromatic degradation; (3) peroxidase - important in oxidation reactions; (4) laccase - breaks aromatic ring structures of organic compounds; and (5) nitrilase - important in oxidation reactions [2].

This method is useful in removing organic contaminants, such as pesticides, aromatics, and polynuclear aromatic hydrocarbons (PAHs), from soil and sediments [2]. Chlorinated solvents also have been targeted at demonstration sites.

Locations at which phytostimulation is to be implemented should have low levels of contamination in shallow areas. High levels of contaminants can be toxic to plants [3].

Tests performed at the Oak Ridge National Laboratory showed a disappearance of trichloroethylene (TCE) over time. Differences among five different plant species were observed [2]. Another study confirmed the direct relationship between microbial mineralization of atrazine and the fraction of organic carbon in the soil [4]. Demonstrations of phytostimulation have been performed to investigate remediation of chlorinated solvents from groundwater (Fort Worth, TX), petroleum hydrocarbons from soil and groundwater (Ogden, UT), petroleum from soil (Portsmith, VA), and PAHs from soil (Texas City, TX) [3].

Data Requirements
Degradation by microbes and dense root systems are needed for a successful design. Toxicity and fate of contaminants need to be evaluated and understood prior to implementing this technology. Vegetation may include trees, grasses, and legumes [2].

Phytostimulation is potentially more cost effective than many other technologies. A comparison performed in New Jersey using fine-rooted grasses showed that phytostimulation ranges from $10 to $35 per ton of soil. Other technologies, such as incineration, range from $200 to $1,000 per ton of soil [2].

Status of Technology
Phytostimulation is being tested at a Chevron site in Ogden, Utah using alfalfa to address fuel contamination and at the University of Iowa using poplar trees to address atrazine contamination [3]. Demonstrations have been conducted at Oak Ridge National Laboratory to investigate TCE contaminated soil [2].

1. Anderson, T.A., E.A. Guthrie, and B.T. Walton, 1993, Bioremediation in the Rhizosphere, Environmental Science and Technology 27 (13), pp. 2630-2636.

2. Miller, R., 1996, Phytoremediation, Technology Overview Report, Ground-Water Remediation Technologies Analysis Center, Series O, Vol. 3, October.

3. Schnoor, J.L., 1997, Phytoremediation, Technology Overview Report, Ground-Water Remediation Technologies Analysis Center, Series E, Vol. 1, October.

4. Nair, D.R. and J.L. Schnoor, 1993, Effect of Soil Conditions on Model Parameters and Atrazine Mineralization Rates, Water Research, 28 (5), pp. 1199-1205.

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