From biowastes to risks? Impact of biosolids treatment and dose on antibiotic resistance in agricultural soils – A mesocosm study

Biosolids application to farmlands enhances soil fertility and promotes sustainable agriculture; yet it is vital to co-evaluate the interconnected risk of spreading antibiotic resistance throughout the agri-food sector. The impacts of sewage sludge treatments (dewatered, air-dried, limed, and thermally dried) and application rates (1-3%) on the abundance of antibiotic-resistant bacteria (ARB) and antibiotic resistance indices (ARI) in biosolids and biosolid-amended agricultural soils were tested. Thermal drying markedly reduced total heterotrophic and antibiotic-resistant bacterial abundance by nearly 5 log₁₀ units, resulting in the lowest ARI across multiple antibiotic classes, including ciprofloxacin and neomycin. Liming also effectively lowered some resistance indices through pH elevation and ammonia volatilization. Soil mesocosm experiments with 1-3% (<i>w</i>/w) biosolid amendment doses revealed that increasing biosolid application rates resulted in a dose-dependent increase in bacterial counts and ARI, especially for air-dried biosolids. ARB and ARI were generally lower in soils amended with limed or thermally dried biosolids compared to untreated or air-dried sludge, underscoring the importance of treatment intensity. Notably, antibiotic resistance was detected even in unamended control soils, highlighting a baseline resistance background. While biosolid amendments enhance soil fertility, insufficiently treated sludge or high application rates (greater than 1%) can promote the proliferation and spread of clinically relevant antibiotic resistance in soils. These findings highlight the crucial role of advanced biosolid processing and prudent application management in mitigating antimicrobial resistance risks across the agri-food ecosystem, aligning with the One Health framework for safeguarding human, animal, and environmental health.