2,4-Dichlorophenoxyacetic acid (2,4-D) sorption and degradation dynamics in three agricultural soils

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 (>93% was extractable), the herbicide was present in a mostly unavailable state (<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.

Introduction

The extensive use of pesticides in agriculture is compromising soil and water quality. One major concern is protecting contamination of water resources (Younes and Galal-Gorchev, 2000). Sorption and degradation are key processes affecting the fate and transport of pesticides in the environment (Linn et al., 1993, Rudel et al., 1993). Degradation is a fundamental attenuation process for pesticides in soil (Guo et al., 2000). 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 pesticide advective-dispersive transport, transformation and bioaccumulation processes (Calvet, 1989).

Several interactions are known to exist between sorption and degradation (Guo et al., 2000). It is commonly accepted that sorption limits pesticide degradation by reducing their partitioning into the soil liquid phase (Smith et al., 1992, Guerin and Boyd, 1997). Sorbed chemicals are generally assumed to be less accessible to microorganisms, which preferentially or exclusively utilize chemicals in solution. Nevertheless, an inverse relationship between sorption and degradation does not necessarily imply that degradation of sorbed chemicals is negligible. Reductions in the degradation rate are often not proportional to increases in sorption (e.g., Moyer et al., 1972). Sorption is a slow, time-dependent process that may proceed over a relatively long period of time (e.g., more than 1 day) to reach equilibrium (Pignatello, 1998). With longer contact times between the soil and the chemical, the fraction of extractable residues is expected to decrease, while conversely the amount of bound residues should increase (Boivin et al., 2004, Mordaunt et al., 2005). These findings suggest that aging of pesticides has an effect on sorption-desorption behavior and hence on the biological availability of pesticides in soil. Aging is known to be one of the more critical factors governing the fate and transport of pesticides in soils (Sharer et al., 2003, Walker et al., 2005). Aging enhances the retention of sorbed organic chemicals by facilitating the formation of bound residues via chemical and/or physical non-equilibrium sorption processes, thus causing the chemical to become less susceptible to desorption and degradation. Still, relatively few laboratory studies have been carried out to assess the dynamic interplay between sorption-desorption and degradation as a function of different residence times in soil.

2,4-Dichlorophenoxyacetic acid (2,4-D) belongs to a group of chemicals known as phenoxy compounds, which are potentially toxic to humans. The herbicide is widely used to control broad leaf weeds and grasses in crops, and has been frequently detected in groundwater supplies in Europe and North America (e.g., Gold et al., 1988, IFEN, 2004). We selected 2,4-D as a model pesticide for our study because of its unique behavior in soil. This chemical is both very degradable and vulnerable to leaching. For example, 2,4-D can be completely dissipated within 20 days in various land-use types (Voos and Groffman, 1997). Microflora able to mineralize this chemical have been found naturally in cultivated soils (Vieublé Gonod et al., 2003). At the same time several factors combine to make the chemical vulnerable to rapid advective-dispersive transport and leaching, such as having a relatively low molecular mass, a limited volatilization rate (i.e., a relatively low Henry's constant), a negative charge at low soil pH, and relatively high solubility. Several studies indicate that 2,4-D sorption in soil is a function of soil pH, and to some extent also on soil organic matter content (e.g., Dubus et al., 2001, Spark and Swift, 2002).

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. Measurements were made of time-dependent ¹⁴C and carbon dioxide release rates, the overall kinetic release process, extractable residues, and bound residues.