Graduate Thesis Or Dissertation
 

Interaction between dipyridylium herbicides and microbes in soil

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  • The herbicides Diquat (1, 1'-ethylene-2, 2'-dipyridylium dibromide), Paraquat (1, 1'-dimethyl-4, 4'-dipyridylium dichloride), and Gramoxone (1, 1'-dimethyl-4, 4'-dipyridylium dimethyl sulphate), and the spreader X-77, a mixture of alkylaryl-polyethylene glycol, free fatty acids and isopropanol, were tested by several soil microbial methods in an attempt to find out if concentrations of these compounds at field ratio, and at higher ratio that might result from over-application or reapplication, would either stimulate or inhibit the microorganisms and hence affect soil fertility. Three different soils were used: Chehalis silt loam, Woodburn silt loam, and Chehalis silty clay loam. Plate counts of bacteria and molds in the soils treated with 0.25, 0.5 and 2.5 ppm of the herbicides for periods ranging from 0 to 20 days averaged higher than counts from untreated control samples. The dipyridylium slightly depressed ammonification and sulfur oxidation in Chehalis silty clay loam soil. In the two silt loam soils minor increases resulted. Nitrification was stimulated by the lowest rate, 0.25 ppm, of the herbicides; the higher rates had little influence. A respiration study using carbon dioxide evolution as an index of the metabolic activity of microorganisms in the soil showed that addition of the herbicides resulted in some decrease in microbial activity. The spreader X-77, depressed CO₂ evolution only at the excessive rate of 100 ppm in one soil; otherwise it had essentially no effect. Four of six bacterial cultures were able to use Gramoxone as the sole source of carbon and nitrogen in synthetic media. Nitrogen in this herbicide promoted growth better than did the carbon. This should be true for Paraquat also, which differs only in the anion. Analytical recovery of Paraquate and Gramoxone from the different soils was correlated with soil type, more specifically with cation exchange capacity, amount and kind of clay, and content of organic matter. Recoveries ranged from 54 to 114 percent depending upon concentration and soil type. Recoveries generally were greater for lower concentrations. Higher concentrations of the herbicides were recovered less completely. Gramoxone appeared to persist in soil longer than Paraquat. In complete recoveries after incubation are attributed in part to some microbial degradation of the herbicides, and in part to certain soil properties responsible for losses in various stages of the assay. The most significant conclusion from these results is that the dipyridylium herbicides and X-77 spreader when used at recommended rates would produce no serious changes in microbial activities and their contributions to soil fertility.
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