John Wiley & Sons, Ltd.

Quantifying soil surface photolysis under conditions simulating water movement in the field: A new laboratory test design

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Soil surface photolysis can be a significant dissipation pathway for agrochemicals under field conditions, although it is assumed that such degradation ceases once the agrochemical is transported away from the surface following rainfall/irrigation and subsequent drainage of soil pore water. However, as both downward and upward water movement occurs under field conditions, relatively mobile compounds may return to the surface, prolonging exposure to UV light and increasing the potential for degradation by photolysis. To test this hypothesis, a novel experimental system was used to quantify the contribution of photolysis to the overall dissipation of a new herbicide, bicyclopyrone, under conditions that mimicked field studies more closely than the standard laboratory test guidance. Soil cores were taken from three US field study sites and the surface treated with [14C]‐bicyclopyrone. The radioactivity was redistributed throughout the cores using a simulated rainfall event following which the cores were incubated under a xenon‐arc lamp with continuous provision of moisture from below and a wind simulator to induce evaporation. After only two days, most of the test compound had returned to the soil surface. Significantly more degradation was observed in the irradiated samples than in a parallel dark control sample. The degradation rates were very similar to those observed in both the thin layer photolysis study and the field dissipation studies and were significantly faster than in the soil metabolism studies conducted in the dark. Thus for highly soluble, mobile agrochemicals such as bicyclopyrone, photolysis is not terminated permanently by rainfall or irrigation but can resume following transport to the surface in evaporating water. This article is protected by copyright. All rights reserved

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