Dissipation of acetochlor and its distribution in surface and sub‐surface soil fractions during laboratory incubations

Abstract

Pesticides in soil are subject to a number of processes that result in transformation and biodegradation, sorption to and desorption from soil components, and diffusion and leaching. Pesticides leaching through a soil profile will be exposed to changing environmental conditions as different horizons with distinct physical, chemical and biological properties are encountered. The many ways in which soil properties influence pesticide retention and degradation need to be addressed to allow accurate predictions of environmental fate and the potential for groundwater pollution. Degradation and sorption processes were investigated in a long-term (100 days) study of the chloroacetanilide herbicide, acetochlor. Soil cores were collected from a clay soil profile and samples taken from 0-30 cm (surface), 1.0-1.3 m (mid) and 2.7-3.0 m (deep) and treated with acetochlor (2.5, 1.25, 0.67 microg acetochlor g(-1) dry wt soil, respectively). In sterile and non-sterile conditions, acetochlor concentration in the aqueous phase declined rapidly from the surface and subsoil layers, predominantly through nonextractable residue (NER) formation on soil surfaces, but also through biodegradation and biotic transformation. Abiotic transformation was also evident in the sterile soils. Several metabolites were produced, including acetochlor-ethane sulphonic acid and acetochlor-oxanilic acid. Transformation was principally microbial in origin, as shown by the differences between non-sterile and sterile soils. NER formation increased rapidly over the first 21 days in all soils and was mainly associated with the macroaggregate (>2000 microm diameter) size fractions. It is likely that acetochlor is incorporated into the macroaggregates through oxidative coupling, as humification of particulate organic matter progresses. The dissipation (ie total loss of acetochlor) half-life values were 9.3 (surface), 12.3 (mid) and 12.6 days (deep) in the non-sterile soils, compared with 20.9 [surface], 23.5 [mid], and 24 days [deep] in the sterile soils, demonstrating the importance of microbially driven processes in the rapid dissipation of acetochlor in soil.

Persistence of acetochlor, atrazine, and S‐metolachlor in surface and subsurface horizons of 2 typic argiudolls under no‐tillage

“…For example, Mueller et al reported a DT50 value of 6.3 d for the upper 8 cm of a soil, Ma et al reported a value of 9 d for the top 10 cm of 2 soils, and Newcombe et al reported 3 to 9 d for the top 15 cm of soil depth. Similarly, Taylor et al reported laboratory‐determined DT50 values of approximately 9, 12, and 13 d for depth ranges of between 0 and 30 cm, 100 and 130 cm, and 270 and 300 cm, which suggest no clear effect of depth on DT50, something that was also observed by Vaughan et al and Dictor et al . However, the results obtained in the present study are in agreement with those of Mills et al , who reported half‐life values ranging from 1 to 18 d for surface soils and from 2 to 88 d for subsurface soils, as well as those of Oliveira et al , who determined values from approximately 7 to 14 d at the surface (0–15 cm) and 20 to 27 d in subsurface soils (60–90 cm).…”

Enhanced Dissipation of Selected Herbicides in a Simulated Organic Matrix Biobed: A System to Control On‐Farm Point‐Source Pollution

“…Acetochlor has a high water solubility of 223 mg L −1 and a vapor pressure of 4.5 × 10 −3 mPa (Janniche et al, 2010). The reported half‐life of acetochlor ranges from 4.6 to 9.3 d in surface soils (Ye, 2003; Taylor et al, 2005; Oliveira et al, 2013). Atrazine (2‐chloro‐4‐ethylamino‐6‐isopropylamino‐s‐triazine) is a triazine herbicide that is widely used in the United States to limit broadleaf and grass weeds.…”

Performance of Slow‐Release Formulations of Alachlor

“…This fraction is included in the calculations, but is not susceptible to leaching and degradation processes; therefore, the calculations will yield an overestimation of the residual amounts in the soil, as noted in Table 3. The effect of sequestration of the active compound in the soil is manifested by the difficulty in extracting part of this recalcitrant fraction, which requires increasingly harsh solvents (Taylor et al, 2005).…”