Transformation of aldicarb sulfoxide and aldicarb sulfone in four water‐saturated sandy subsoils

Abstract

The transformation of aldicarb sulfoxide and aldicarb sulfone was studied in incubations with water‐saturated subsoils under simulated field conditions at 10°C. The subsoils were collected at four locations from beneath the water table at a depth of 2.5 to 3.5 m. In three of the subsoils, the half‐life of sulfoxide, incubated at concentrations of 0.14‐0.17 mg litre−1, ranged from 0.7 to 2.8 years. At higher concentrations (8‐13 mg litre−1), its half‐life ranged from 3.4 to 6.4 years. At the lower concentration, a large fraction of sulfoxide was transformed into sulfone. The rates of transformation of the sulfone at the lower concentration in the three subsoils corresponded to half‐lives of 3.3 to 8.1 years, but in only one subsoil was a significant transformation rate (half‐life 6.7 years) measured at the higher concentration during the 2.3‐year incubation period. The half‐lives at the lower concentrations were more like those in field studies, and perhaps would still underestimate transformation rates under field conditions. After a year, 2.5‐15% of the higher sulfoxide and sulfone doses had been trapped as [14C] carbon dioxide. In the fourth subsoil, with more anaerobic conditions, the half‐life of sulfoxide at both concentrations was less than 0.02 year and that of sulfone was about 0.04 year. Four or five radio‐labelled transformation products could be traced in this subsoil and about half of the dose of both compounds was trapped as [14C] carbon dioxide.

Accelerated Microbial Degradation of Nematicides in Vineyard and Orchard Soils

“…Pesticides volatise readily from coarsetextured soils; the texture of soil also affects the amount of pesticide that is lost due to run-off (Harrison, 1990). Smelt et al (1995) found the longest half-lives to occur in humic sandy soils, whereas the shortest were found in sandy loam soils.…”

“…Degradation rates in subsurface layers might differ significantly from those in the surface layer because of changes in soil conditions such as organic matter content, microbial activity, moisture content and temperature (Fomsgaard & Albaiges, 1995;Smelt et al, 1995). Decomposition rates in subsoils are generally much slower than they are in surface soils, because microbial activity is much less deep in the soil (Bromilow et al, 1986).…”

An appraisal of methods for measurement of pesticide transformation in the groundwater zone

“…An advanced method for collecting (anaerobic) soil material from depths of a few metres in the groundwater zone was described by Smelt et al 13 Plastic casing tubes fitted with a steel cutting ring were inserted into the soil and the soil material was removed from inside the tube by augering and bailing. When the boreholes were at the required depth, subsoil material from the permanently water‐saturated layer below was collected with a stainless steel sampling cylinder having a steel screw‐on cover incorporating a hole fitted with a connector from which a rigid plastic tube led to a switch.…”

“…The redox potential is an important characteristic of aquifers; it can be measured both in the field and in the laboratory by using a platinum electrode and a silver/silver chloride reference electrode, connected to a voltmeter. Long electrodes can be used in the field measurements: a piece of platinum wire extending from a rubber stopper at the bottom end of a long poly(methacrylate) tube can be connected via an internal copper wire to the voltmeter 13. The reference electrode is connected to the groundwater via a salt bridge consisting of a potassium chloride solution in a polythene tube provided with a tensiometer cup at the end.…”

“…The reaction mechanism for the fast anaerobic transformation, involving catalysis by iron(II) ions, was studied in detail 12. By contrast, the rates of transformation of aldicarb sulfoxide and aldicarb sulfone in aerobic water‐saturated subsoil with low pH were very low,13 with half‐lives up to several years. At comparatively low pH values, the rate of hydrolysis of the carbamoyl oximes is low,14, 15 but the rate increases with pH.…”