Ubiquity of microbial capacity to degrade metaldehyde in dissimilar agricultural, allotment and garden soils

Abstract

Metaldehyde is a molluscicide used to control slugs and snails. Despite its extensive use, very little is known about the capacity of soil microbial communities to degrade this chemical. This research provides a synopsis of the latent capacity of soil microbial communities, present in agricultural (n=14), allotment (n=4) and garden (n=10) soils, to degrade metaldehyde. Extents of 14C-metaldehyde mineralisation across all soils ranged from 17.7 to 60.0%. Pre-exposure (in situ, in the field) to metaldehyde was not observed to consistently increase extents of metaldehyde mineralisation. Where soils were augmented, (ex situ, in the laboratory) with metaldehyde (28 mg kg−1), the mineralisation capacity was increased in some, but not all, soils (uplift ranged from +0.10 to +16.9%). Results indicated that catabolic competence to degrade metaldehyde was evident in both surface (16.7–52.8%) and in sub-surface (30.0–66.4%) soil horizons. Collectively, the results suggest that catabolic competence to degrade metaldehyde was ubiquitous across a diverse range of soil environments; that varied in texture (from sand to silty clay loam), pH (6.15–8.20) and soil organic matter (SOM) content (1.2%–52.1%). Lighter texture soils, in general, were observed to have higher capacity to mineralise metaldehyde. Weak correlations between catabolic competence and soil pH and soil organic matter content were observed; it was noted that above a SOM threshold of 12% metaldehyde mineralisation was always >34%. It was concluded that the common occurrence of metaldehyde in EU waters is unlikely the consequence of low potential for this chemical to be degraded in soil. It is more likely that application regimes (quantities/timings) and meteorological drivers facilitate the transport of metaldehyde from point of application into water resources.