Incorporation of Fumigants into Soil Organic Matter

Abstract

Halogenated fumigants are some of the most heavily used pesticides worldwide. A number of studies have shown that fumigant transformation in soil is correlated with soil organic matter content. However, relatively little is known about the mechanisms of fumigant interaction with soil organic matter. In this study, we used 14C-labeled 1,3-dichloropropene (1,3-D) and methyl bromide (MeBr) to characterize their incorporation into soil organic matter and the association of bound radioactivity with the different organic matter components. The 14C activity of bound residues increased with time and reached 38−49% for 1,3-D and 37−42% for MeBr after 72 d of incubation at 25°C. More bound residues were produced for 1,3-D than for MeBr in the same soils. The distribution of 14C activity in soil humic substances followed the order of fulvic acids ≫ humin > humic acids. These observations suggest that incorporation into soil organic matter is the predominant pathway for transformation of halogenated fumigants in soil and that fulvic acids are likely the most significant sink of all soil organic matter fractions. It is further speculated that bound residues formed as a result of alkylation of organic matter by the fumigants through nucleophilic replacement.