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

Biosolids are widely applied to agricultural lands as fertilizers due to their rich organic matter content; however, the risk of spreading antibiotic resistance remains high. This study assessed the impact of various sewage sludge treatment methods and biosolid application rates on the abundance of antibiotic-resistant bacteria (ARB) and antibiotic resistance indices (ARI) in biosolids and biosolid-amended agricultural soils. Dewatered, air-dried, limed, and thermally dried biosolids were compared in terms of their microbial loads and resistance profiles. 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% ( w /w) biosolid amendment doses revealed that increasing biosolid application rates caused a dose-dependent rise 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 emphasize the critical role of advanced biosolid processing and prudent application management to mitigate antimicrobial resistance risks within the agricultural ecosystem, aligning with the One Health framework for safeguarding human, animal, and environmental health. • Biosolid treatment significantly reduced antibiotic-resistant bacteria (ARB) abundance. • Higher dose-dependent biosolid application in soils elevated ARB and antibiotic resistance index (ARI). • Air-dried biosolids at high doses increased penicillin and neomycin resistance indices. • Limed and thermally dried biosolids limited ciprofloxacin and neomycin resistance in soils. • Thermally dried biosolids showed the lowest antibiotic resistance index (ARI) in soils.