Aldicarb degradation in sandy soils from different depths

Aerobic and anaerobic degradation rates for aldicarb were measured in soil samples collected at different depths from a citrus grove near Davenport, Florida, together with the changes in concentration of its two toxic oxidation products, aldicarb sulfoxide and aldicarb sulfone. These data were used to estimate the first‐order rate‐constants for concurrent oxidation and hydrolysis of aldicarb, aldicarb sulfoxide, and aldicarb sulfone, and for the loss of total carbamate residues (TCR = aldicarb + aldicarb sulfoxide + aldicarb sulfone). Both oxidation and hydrolysis rate‐constants for aldicarb in surface samples (0‐30 cm) were larger than those in sub‐surface samples (30‐300 cm). Under aerobic conditions, microbial oxidation appeared to be the major route for aldicarb degradation in surface and shallow sub‐surface samples (30‐150 cm), whereas chemical hydrolysis appeared to be the dominant pathway in deep sub‐surface samples (150‐300 cm). Aldicarb in deep sub‐surface samples under anaerobic conditions was primarily degraded through hydrolysis and only trace amounts of aldicarb sulfoxide were detected. Time to 50% disappearance (DT50) values for TCR loss were generally invariant with depth. In sub‐surface samples, DT50 values under aerobic and anaerobic conditions were similar.