The Impact of Disturbed Soil Structure on the Degradation of 2 Fungicides Under Constant and Variable Moisture

Degradation of agrochemicals in soil is frequently faster under field conditions than in laboratory studies. Field studies are carried out on relatively undisturbed soil, whereas laboratory studies typically use sieved soil, which can have a significant impact on the physical and microbial nature of the soil and may contribute to differences in degradation between laboratory and field studies. A laboratory study was therefore conducted to determine the importance of soil structure and variable soil moisture on the degradation of 2 fungicides (azoxystrobin and paclobutrazol) that show significant differences between laboratory and field degradation rates in regulatory studies. Degradation rates were measured in undisturbed cores of a sandy clay loam soil (under constant or variable moisture contents) and in sieved soil. For azoxystrobin, degradation rates under all conditions were similar (median degradation time [DegT50] 34-37 d). However, for paclobutrazol, degradation was significantly faster in undisturbed cores (DegT50 255 d in sieved soil and 63 d in undisturbed cores). Varying the moisture content did not further enhance degradation of either fungicide. Further examination into the impact of soil structure on paclobutrazol degradation, comparing undisturbed and sieved/repacked cores, revealed that the impact of sieving could not be mitigated by repacking the soil to a realistic bulk density. Examination of fungal and bacterial community structure using automated ribosomal spacer analysis showed significant initial differences between sieved/repacked and intact soil cores, although such differences were reduced at the end of the study (70 d). The present study demonstrates that disruption of soil structure significantly impacts microbial community structure, and for some compounds this may explain the differences between laboratory and field degradation rates. Environ Toxicol Chem 2021;40:2715-2725. © 2021 SETAC.