Graduate Thesis Or Dissertation

Microbial degradation of atrazine in soils

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  • Atrazine is an asymmetrical s-triazine herbicide used pre- and post-emergence for the control of weeds in many crops. Under conditions considered unfavorable for microbial activity, atrazine may persist in soils for extended periods of time. However, the significance of chemical versus microbial degradation is not known. This study was conducted to determine the significance of microbial degradation of atrazine by pure cultures and the native soil population in non-sterile soils. Isolated bacterial cultures were used to inoculate seeds in an attempt to provide protection against atrazine residues. Atrazine-treated soil was incubated at 30°C for varying periods and the subsequent loss of activity was correlated to evolution of ¹⁴CO₂ from labeled-atrazine in a radiorespirometric system. Microorganisms, mostly bacteria, were isolated from a soil solution; pour plates of atrazine-treated and non-treated soil; and the rhizosphere of corn, oats, tomatoes, and soybeans. Viable cell counts were used as an index to test for the utilization of atrazine as the sole source of carbon. Eight bacterial isolates did not show an appreciable difference in cell counts with or without atrazine as the sole source of carbon. Seed inoculation with a mixture of three bacterial isolates did not increase the growth of oats grown in atrazine-treated soil as an indication of crop protection. In synthetic media bacterial cultures evolved a small amount of ¹⁴CO₂ from chain-labeled atrazine during the first 24 hours and none thereafter. In sterile soil the same isolates evolved 0.4-0.7 percent of the input activity in two weeks. A mold respired 4.0 percent. No ring breakage was observed. In non-sterile soils, 1.4-1.6 percent of chain and 0.6-1.0 percent of ring-labeled atrazine was evolved in two weeks and 1.1-1.6 percent of ring-labeled hydroxyatrazine. The latter rate was 2-3 fold greater than from ring -labeled atrazine and indicated the formation of hydroxyatrazine as the rate limiting step in the dissipation of atrazine from soils. Data from the incubation experiment showed a 73 percent loss of the initial atrazine after 3-4 weeks. In a similar time period, only 2.2-2.6 percent of chain and 1.0-1.2 percent of ring-labeled atrazine was respired. Thus, the ¹⁴CO₂ data did not account for the loss of atrazine and further supports the formation of hydroxyatrazine as the rate limiting step. Extraction of the soils containing labeled-atrazine showed the presence of hydroxyatrazine in non-sterile and sterile soils after two weeks. The radiorespirometric system designed for these studies is proposed as a means to obtain a relative index of the residual life of herbicides or pesticides. The ¹⁴CO₂ data may be extrapolated to give an index based on microbial participation. Extraction of the soils would provide a test for possible non-toxic metabolites that may be formed via chemical reactions. Such data would be most beneficial in selecting and recommending new herbicides.
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