An improved method for determination of fumigant degradation half-life in soil

Authors: ASHWORTH, DANIEL - University Of California; Yates, Scott

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/20/2015
Publication Date: 2/25/2016
Citation: Ashworth, D.J., Yates, S.R. 2016. An improved method for determination of fumigant degradation half-life in soil. Soil Science Society of America Journal. 80(1):64-68. doi: 10.2136/sssaj2015.07.0249.

Interpretive Summary: Fumigants are a class of pesticides used to kill pests in soil prior to planting crops. As gases, fumigants can leave the soil and contaminate air. However, they may also degrade, or decompose, within the soil via chemical and biological reactions. Studying how fast fumigants degrade is important in better understanding their overall fate within the environment, e.g., if a fumigant degrades very slowly, it remains in the soil longer and its emissions to air may be relatively high. We report a new method for determining the half-life of fumigants in soil in the laboratory. This is an improvement over the traditional method because the fumigant is in complete contact with the soil during the experiment, much like under field conditions. The new method gave shorter half-lives (by 22-64%) for two important soil fumigants, at two different temperatures, when compared to the traditional approach. This was due to the greater contact between the soil surfaces and the fumigant gas, which increased degradation. This suggests that half-lives for soil fumigants previously reported in the literature may have been overestimated. Where such half-lives have been used to predict soil-air emissions, the emissions may also have been overestimated. This has implications for fumigant regulations. The new method may also be applicable to other volatile chemicals. This research would be of interest to scientists, regulators, cooperative extension personnel and grower groups.

Technical Abstract: Using the current approach, measurement of fumigant degradation half-lives under realistic soil conditions is problematic due to the large headspace that is necessary above the soil during incubation. This results in a poor degree of contact between the fumigant and the soil’s degrading surfaces; differing markedly from field conditions. We report a simple, novel approach for determining fumigant degradation half-lives in which the fumigant is in complete contact with the soil during incubation (no headspace) and which uses gas-tight, non-sorbing apparatus commonly found in pesticide research laboratories. At both 23 and 50°C, the novel approach yielded much lower degradation half-lives for cis and trans 1,3-dichloropropene (1,3-D) isomers and chloropicrin (CP) than the traditional approach. Percentage reductions ranged from 22-64% and were greater at the higher temperature. The results suggest that literature reported half-lives of soil fumigants and, consequently, their modeled soil-air emission rates, may be overestimated. The novel approach could likely be applied to determining degradation kinetics of other classes of volatile chemicals in soil.