Soil Photolysis of Herbicides in a Moisture- and Temperature-Controlled Environment

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Abstract
The problem of maintaining the moisture content of samples throughout the course of a soil photolysis study is addressed. The photolytic degradations of asulam, triclopyr, acifluorfen, and atrazine were independently compared in air-dried soils and in moist (75% field moisture capacity at 0.33 bar) soils maintained at initial conditions through the use of a specially designed soil photolysis apparatus. Each pesticide was applied at 5 μg/g. The exposure phase extended from 144 to 360 h, depending on the half-life of the compound. A dark control study, also using moist and air-dried soils, was performed concurrently at 25 °C. The results showed significant differences in half-life. The dissipations generally demonstrated a strong dependence on moisture. In most cases, photolytic degradation on air-dried soil was longer than in the moist dark control soils. Half-lives in dry soil were 2−7 times longer, and in the case of atrazine, the absence of moisture precluded significant degradation. Moist soil experiments also tended to correlate more strongly with linear first-order degradations. The dark control experiments also demonstrated shorter half-lives in moist soil. Moisture was also observed to affect the amount of degradate formed in the soils. Keywords: Soil photolysis; asulam; triclopyr; acifluorfen; atrazine; moisture; half-life