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Title: 2,4-DICHLOROPHENOXYACETIC ACID (2,4-D) SORPTION AND DEGRADATION DYNAMICS IN THREE AGRICULTURAL SOILS

Author
item BOIVIN, ARNAUD - UC RIVERSIDE, CA
item AMELLAL, SAMIRA - UMR, VANDOEUVRE, FRANCE
item SCHIAVON, MICHEL - UMR, VANDOEUVRE, FRANCE
item Van Genuchten, Martinus

Submitted to: Environmental Pollution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/21/2005
Publication Date: 7/1/2005
Citation: Boivin, A., Amellal, S., Schiavon, M., Van Genuchten, M.T. 2005. 2,4-Dichlorophenoxyacetic acid (2,4-D) sorption and degradation dynamics in three agricultural soils. Environmental Pollution. 138:92-99.

Interpretive Summary: The extensive use of pesticides in agriculture is compromising soil and water quality. Sorption and degradation are key processes affecting the fate and transport of pesticides in the environment. Degradation is a fundamental attenuation process for pesticides in soil. This process, catalyzed by soil microbes, is governed by both abiotic and biotic factors. Degradation is affected by a variety of interactions among microorganisms, various soil constituents, and the specific pesticide involved. Sorption is similarly key to controlling the transport, transformations and bioaccumulation of pesticides in the subsurface. The aim of this study was to assess the dynamic interactions between 2,4-D sorption and degradation, and to identify the main factors influencing both processes. Soil incubation experiments were carried out to monitor both 2,4-D mineralization and time-dependent soil/liquid phase partitioning of 2,4-D and its metabolites in three cultivated soils (clay, loam, sand) from initial treatment to 60 days of incubation. Mineralization appeared to be the main process limiting the availability of this chemical. Hence, while this herbicide is one of the most mobile pesticides, its very rapid mineralization (50% of the applied dose in 10 days) lessens some of its potentially adverse effects on the environment (groundwater and surface water contamination). Bound residues were found to be formed quickly in the three soils, with the amounts slowly decreasing again near the end of the 60-day incubation experiments, possibly due to some overestimation of the amount of 2,4-D or metabolites being taken up by soil microorganisms (bioaccumulation). Results of this study are important to understanding pesticide fate and transport processes in the subsurface, and are helpful for designing farming practices that reduce pesticide loadings to streams and groundwater.

Technical Abstract: The fate and transport of 2,4-dichlorophenoxyacetic acid (2,4-D) in the subsurface is affected by a complex, time-dependent interplay between sorption and mineralization processes. 2,4-D is biodegradable in soils, while adsorption/desorption is influenced by both soil organic matter content and soil pH. In order to assess the dynamic interactions between sorption and mineralization, 2,4-D mineralization experiments were carried using three different soils (clay, loam and sand) assuming different contact times. Mineralization appeared to be the main process limiting 2,4-D availability, with each soil containing its own 2,4-D decomposers. For the clay and the loamy soils, 45 and 48% of the applied dose were mineralized after 10 days. By comparison, mineralization in the sandy soil proceeded initially much slower because of longer lag times. While 2,4-D residues immediately after application were readily available (more than 93% was extractable), the herbicide was present in a mostly unavailable state (less than 2% extractable) in all three soils after incubation for 60 days. We found that the total amount of bound residue decreased between 30 and 60 incubation days. Bioaccumulation may have led to reversible immobilization, with some residues later becoming more readily available again to extraction and/or mineralization.