Antimicrobial resistance genes in biosolid-amended pasture soils: Insights from a pilot study on a sheep farm

Biosolids, a by-product of municipal wastewater treatment, are increasingly used as organic fertilisers to amend farm soils. Biosolids must meet regulatory criteria regarding the presence of pathogenic agents and heavy metals but may also be associated with the dissemination of antimicrobial resistance genes (ARGs), mobile genetic elements (MGEs) and other chemical contaminants like antimicrobial residues into soil. In this real-world pilot study, we evaluated the diversity and relative abundance of ARGs and MGEs in pasture soils and faecal samples from sheep grazing on biosolid-treated and control fields, using a high-throughput SmartChip™ qPCR platform. Thermally dried biosolids (96% dry solids) were applied twice annually, and soil and faecal samples were taken from both fields across multiple seasons. Analysis revealed that all biosolid samples had Escherichia coli levels significantly below the regulatory limit, indicating effective treatment during processing. Resistome analysis detected a range of ARGs and MGEs in all samples, but no clear evidence of differences was identified in gene diversity or abundance between biosolid-treated and control soils. Consistently, exploratory, principal component, and linear mixed models analyses showed no statistically detectable effect of biosolid application on gene abundance. While further field-level replication is recommended to distinguish site-specific characteristics from treatment effects, the present study provides no evidence of an effect within this particular Before-After-Control-Impact (BACI) comparison and under these management conditions. Given the single-farm design with one field per treatment, the work is framed as a pilot case study and conclusions are limited to this site and setting.