There is a great deal of concern about numerous organic and inorganic contaminants, and their fate in the environment. Some of these substances can have direct or indirect effects on human health, while others may impact the productivity of the soil. Plants can be used in a variety of ways to reduce the environmental impact or damage from these contaminants. Common effects of plants used to remediate soil and water contamination include accumulation, microbial enhancement, and retention of contaminants in the rhizosphere.
Phytoaccumulation refers to the uptake and sequestration of contaminants into the plant root and shoot biomass, where they are prevented from movement into groundwater or deeper soil layers. Phytoaccumulators then can be harvested and combusted to reclaim heavy metal contaminants or to destroy organic contaminants. Microbial enhancement refers to rhizosphere effects of plants on soil environmental conditions, which enhance the growth of microbes capable of decomposing contaminants. This is particularly true of organic contaminants, including petroleum hydrocarbons.
Tall fescue has been recognized as a phytoaccumulator of heavy metals, including Zn. Zinc can be a serious soil contaminant in mining regions, near smelting operations, or in association with wastewater from the metal-plating industry. Palazzo et al. (2003) found that tall fescue accumulated the most Zn (1553 mg kg-1) in its top growth of four species tested in a greenhouse study using contaminated soil obtained from a smelter site. Even at this high rate of Zn accumulation, the tall fescue plants showed no sign of phytotoxicity. This indicates that tall fescue could be used to extract excessive Zn from contaminated soils and the harvested biomass burned to reclaim the Zn.
The effectiveness of tall fescue for remediation of soils contaminated with organic compounds is related to the impact of the tall fescue root system on microbial populations in the rhizosphere. In studies designed to evaluate the effectiveness of grasses at accelerating the degradation of aged petroleum sludge, tall fescue reduced total petroleum hydrocarbons (TPH) in the soil column by 65% within 1 yr, while bermudagrass reduced TPH by 68%, compared to a 57% reduction of TPH in unvegetated soil in the first year (Hutchinson et al., 2001a). All vegetated treatments resulted in an increase in soil microbial activity. The rate of TPH reduction decreased with time in unvegetated treatments when compared with the vegetated treatments. Additional studies indicated that grass root growth improved soil structure, which in turn increased the transport of O2 through root channels to soil macropores and allowed better microbial growth and TPH degradation (Hutchinson et al., 2001b). Similarly, Siciliano et al. (2003) found that microbially produced genes for catabolic enzymes responsible for naphthalene degradation increased in the rhizosphere of grasses.
Grasses also are useful in reducing the environmental impact of agricultural herbicides. Atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) has been identified as a critical groundwater contaminant throughout the U.S. Corn Belt (Kolpin et al., 1996). Grass cover increased the degradation of atrazine by microbial dealkylation compared to bare ground in a field lysimeter study (Lin et al., 2003). In addition to increasing atrazine breakdown, the presence of grass also increased soil microbial biomass, indicating that the observed improved breakdown was due to enhanced microbial growth in the rhizosphere. It is generally believed that microbial growth is enhanced by the improvement of soil environmental conditions, such as aeration and drainage in the rhizosphere, and by the release of organic compounds, such as soluble carbohydrates by roots.
Grasses can reduce the environmental impact of contaminants by maintaining them in upper soil horizons instead of allowing them to leach into the groundwater. Even though Lin et al. (2003) did not see any increase in degradation of isoxaflutole in grass plots, presence of grass reduced its transport in leachate. Tall fescue has been shown to reduce leaching of the carcinogen nitrosodimethylamine (NDMA) by holding the contaminant in the rhizosphere (Yang et al., 2005). This contaminant results from the chlorination of wastewater, and it is very mobile in percolating waters. Microbial degradation of NDMA was also enhanced in tall fescue plots in this study.
In summary, tall fescue is a potentially important component of phytoremediation systems through a number of mechanisms. There is strong evidence for the enhancement of microbial degradation of organic contaminants in the rhizosphere of tall fescue, the retention of contaminants in surface soils through reduced leaching, and the accumulation of some heavy metals in plant biomass. These mechanisms support the use of tall fescue for phytoremediation of contaminated sites.
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