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

Abstract

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.

Building a Conceptual Model for the Environmental Fate of the Fungicide Benzovindiflupyr

“…Many of these developments have utilized undisturbed field soil cores rather than the sieved soil that is mandated in the regulatory study design. Studies with soil cores have been used to investigate the impact of loss of soil structure through sieving on CPP degradation (Hand et al, 2021) as well as the impact of upward movement of compounds on the prolonged photodegradation on the soil surface under both laboratory (Hand et al, 2015) and outdoor (Hilton et al, 2019) conditions. Such intact core systems also provide an opportunity to study the contribution of phototrophic organisms on the soil surface to reducing persistence, as previously reported by Davies, Bramke, et al (2013), under laboratory conditions, by allowing a representative microbiotic soil crust, which is a feature of many agricultural fields (Davies, Schäfer, et al, 2013; Jeffrey et al, 2009; Knapen et al, 2007), to develop.…”