|Lerch, Robert - Bob|
Submitted to: Journal of Environmental Quality
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/22/2007
Publication Date: 1/11/2008
Citation: Lin, C.H., Lerch, R.N., Garrett, H.E., George, M.F. 2008. Bioremediation of Atrazine-Contaminated Soil by Forage Grasses: Transformation, Uptake, and Detoxification. Journal of Environmental Quality. 37:196-206. Interpretive Summary: Vegetative buffers have been shown to be effective at reducing the transport of nutrients and sediments from cropped fields, but little research has addressed their ability to effectively degrade herbicides (i.e. weed killers) that are commonly used in row crop production. The objective of this study was to evaluate the effectiveness of five grass species to enhance degradation of the commonly used corn herbicide, atrazine. The study was conducted using lysimeters under field conditions. Lysimeters are basically large cylinders filled with soil and connected to a drain pipe for sampling of shallow ground water. The grasses were planted in the lysimeters and a known amount of atrazine was added to each one. Treatments included switchgrass, orchardgrass, tall fescue, timothy, smooth bromegrass, and a bare ground control. Ground water samples were collected after each significant rainfall event, and soil and plant samples were collected after 25 days. Results showed that the majority of the applied atrazine remained in the soil (~88%) and only a relatively small fraction of herbicide leached to shallow groundwater (<9%) or was taken up by plants (<3%). All forage treatments significantly enhanced atrazine degradation compared to the control. Switchgrass had the greatest ability to facilitate atrazine degradation, with only 18% of the added atrazine present after 25 days, and the remainder converted to less toxic breakdown products. The grasses enhanced atrazine degradation in soil through their ability to increase microbial growth and activity in surface soil. Because of its ability to tolerate high levels of exposure to atrazine and the high degree of enhanced soil degradation, switchgrass is recommended for use in vegetative buffers designed to reduce atrazine transport to surface or ground waters. This research benefits conservation agencies, such as NRCS and state conservation departments, since it provides the needed science for improving vegetative buffer designs for improved protection of water resources impacted by row crop production.
Technical Abstract: A sound multi-species vegetation buffer design should incorporate the species that facilitate rapid degradation and sequestration of deposited herbicides in the buffer. A field lysimeter study with six different ground covers (bare ground, orchardgrass, tall fescue, timothy, smooth bromegrass, and switchgrass) was established to evaluate their phytoremediation capacity for transformation, uptake, and detoxification of atrazine (ATR). Results suggested that the majority of the applied ATR remained in the soil and only a relatively small fraction of herbicide leached to shallow groundwater (<15%) or was taken up by plants (<3%). Biological degradation or chemical hydroxylation of soil ATR was enhanced by 20% to 45% in forage treatment compared to the control. Of the ATR residues remaining in soil, switchgrass degraded more than 80% to less toxic metabolites, with 47% of these residues converted to the less mobile hydroxylated metabolites 25 days after application. The strong correlation between the degradation of N-dealkylated ATR metabolites and the increased microbial biomass carbon in forage treatments suggested that enhanced biological degradation in the rhizosphere was facilitated by the forages. Hydroxylated ATR degradation products were the predominant ATR metabolites in the tissues of switchgrass and tall fescue. In contrast, the N-dealkylated metabolites were the major degradation products found in the other cool season species. The difference in metabolite patterns between the warm and cool season species demonstrated their contrasting detoxification mechanisms, which also related to their tolerance to ATR exposure. Based on this study, switchgrass is recommended for use in riparian buffers designed to reduce ATR toxicity and mobility due to its high tolerance and strong degradation capacity.