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Dissipation of triallate [S‐(2,3,3‐trichloroallyl) diisopropylthiocarbamate] and trifluralin (a,a,a‐trifluoro‐2,6‐dinitro‐N,N‐dipropyl‐p‐toluidine) in air and soil was measured following their application as a pre‐emergence treatment to a wheat (Triticum aestivum L.) field. Drift losses during application and incorporation were less than 1% of the amounts applied. Air samples, collected at six heights ranging from 30 to 200 cm above the soil surface initially and then above the crop canopy following emergence during the 67 d after application, showed distinct gradients of each herbicide in the air, with the highest concentrations in samples closest to the ground. The highest flux rates for triallate and trifluralin were 4 and 3 g ha−1 h−1 during the 4‐ to 6‐h period after application, when the concentrations at 30 cm were 2500 and 1700 ng m−3, respectively. Fluxes of both herbicides decreased with time, but were dependent mainly on soil moisture conditions. The total vapor losses for the 67‐d sampling period were 17.6 and 23.7% triallate and trifluralin, respectively. About half of these losses were in the first week. There were three distinct phases in the dissipation of both herbicides from the soil. The initial rapid phase, with vapor losses as the major route (Phase I), was followed by slow and continual dissipation over the entire growing season (Phase II), with volatilization and degradation as the potential pathways of dissipation. The third phase with little or no dissipation was reflective of the Canadian winter conditions. The gross dissipation of both herbicides during Phases I and II, however, followed the first‐order rate equation, with half‐concentration time of 88 ± 7 and 99 ± 9 d for triallate and trifluralin, respectively, with volatilization as the dominant process during Phase I.

Dissipation of iso‐octyl ester of 2,4‐dichlorophenoxyacetic acid (2,4‐D) and its acid metabolite in air, crop, and soil components were measured in a wheat (Triticum aestivum L.) field during and following application. Drift losses during application were only 0.2% of the amount applied. Air samples, collected at six heights ranging from 30 to 200 cm above the crop canopy during the first 7 d after application showed distinct ester gradients in the air, with concentration highest in the samples closest to the crop canopy. The highest concentration was measured during the afternoon of day 1 when 1604 ng m−3 were recorded 30 cm above the crop canopy. The vertical flux of the ester showed distinct diurnal variations with maxima reached in the early afternoon of day 1 and 2, followed by a rapid decline of the ester flux thereafter, corresponding with the depletion of the ester from the crop canopy. The total or cumulative vapor losses of the iso‐octyl ester over the 5‐d sampling period were estimated to be 93.5 g ha−1 or 20.8% of the amount applied. The crop canopy intercepted 52% of the applied ester and acted as the major source of vapor losses. The magnitude of vapor activity was controlled primarily by the atmospheric stability and air temperature following application. On entry into the crop, the ester was hydrolyzed to the acid metabolite, which reached its maximum level on day 3. There appeared to be a rapid initial metabolism of the acid followed by a slow decline. Ester losses from the soil surface occurred only when the soil surface was moist, i.e., after a rainfall event or in the early hours of the morning following the deposition of dew. In addition, both the hydrolysis of the ester and the subsequent degradation of its acid metabolite in the soil were dependent on the availability of surface soil moisture. No detectable 2,4‐D remained in the soil after 34 d.

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