HERBICIDE DEGRADATION BY RHIZOSPHERE MICROBIAL COMMUNITIES

T.A. Anderson, E.L. Kruger and J.R. Coats

Pesticide Toxicology Laboratory, Iowa State University, Ames, IA, 50011-3140, 515-294-8667


ABSTRACT

The use of vegetation at waste sites is a logical approach to improving microbial degradation of xenobiotics by overcoming some of the inherent limitations to biological cleanup approaches such as low microbial populations or inadequate microbial activity. Plants are known to influence soil microorganisms in positive ways by exuding organic substances into the root zone and providing a microhabitat conducive to proliferation. Previous research has demonstrated that rhizosphere soils or microorganisms isolated from rhizosphere soils often exhibit accelerated rates of xenobiotic metabolism, suggesting that plants might be valuable as a cost-effective approach for biological remediation of waste sites.

An area where cost-effective approaches to remediation are in particular need is retail agrochemical dealer sites. Many of these dealerships have experienced soil and water contamination problems from normal operating procedures and accidents during the last 40 years. In most instances, the costs associated with current cleanup technologies preclude their use at these facilities.

A potential limitation to using vegetation exists at these sites because of the presence of mixtures of herbicide contaminants at concentrations several-fold above the field application rate. Nonetheless, herbicide-tolerant and herbicide-resistant plants, including kochia (Kochia scoparia), barnyardgrass (Echinochloa crus-galli) and pigweed (Amaranthus retroflexus) routinely inhabit these environments. Previously, we demonstrated that the degradation of atrazine, metolachlor and trifluralin was significantly greater in rhizosphere soils from Kochia scoparia than in nonvegetated soils. In addition, mineralization of 14C-atrazine in a mixture of atrazine and metolachlor (50 g/g each) was significantly greater in Kochia scoparia rhizosphere soils than nonvegetated soils. In both studies, soils were collected from retail agrochemical dealer sites where mixtures of herbicides were present in the soil at concentrations several-fold above the field application rate. In addition, rhizosphere soils from other plant species were tested for their ability to mineralize atrazine or metolachlor at concentrations typical of point-source contamination (50 g/g). Several rhizosphere soils tested positive for 14C-atrazine mineralization (greater than or equal to 8.5%) including lambsquarters (Chenopodium berlandieri), foxtail barley (Hordeum jubatum), witchgrass (Panicum capillare), catnip (Nepeta cataria) and musk thistle (Carduus nutans). These results suggest that plants might be managed at pesticide-contaminated sites to help facilitate microbial degradation of wastes in soils.

KEY WORDS

rhizosphere, bioremediation, atrazine, vegetation

This paper is from the Proceedings of the 10th Annual Conference on Hazardous Waste Research 1995, published in hard copy and on the Web by the Great Plains/Rocky Mountain Hazardous Substance Research Center.