Application of sewage sludge to agricultural soil increases the abundance of antibiotic resistance genes without altering the composition of prokaryotic communities Julen Urra a, ⁎, Itziar Alkorta b , Iker Mijangos a , Lur Epelde a , Carlos Garbisu a a NEIKER-Tecnalia, Department of Conservation of Natural Resources, Soil Microbial Ecology Group, c/Berreaga 1, E-48160 Derio, Spain b Instituto BIOFISIKA (CSIC, UPV/EHU), Department of Biochemistry and Molecular Biology, University of the Basque Country, P.O. Box 644, 48080 Bilbao, Spain HIGHLIGHTS • The long-term impact of sewage sludge application on agricultural soil was studied. • We observed Cu and Zn accumulation in soil, but without increased bioavailabil- ity. • We found correlation between soil Cu and Zn concentration and antibiotic re- sistance. • The composition of soil prokaryotic communities was not significantly al- tered. • The abundance of some antibiotic resis- tance genes increased in amended soils. GRAPHICAL ABSTRACT abstract article info Article history: Received 1 May 2018 Received in revised form 20 July 2018 Accepted 2 August 2018 Available online 08 August 2018 Editor: Avelino Nunez-Delgado The application of sewage sludge as soil amendment is a common agricultural practice. However, wastewater treatment plants, sewage sludge and sewage sludge-amended soils have been reported as hotspots for the ap- pearance and dissemination of antibiotic resistance, driven, among other factors, by selection pressure exerted by co-exposure to antibiotics and heavy metals. To address this threat to environmental and human health, soil samples from a long-term (24 years) field experiment, carried out to study the impact of thermally dried and anaerobically digested sewage sludge (at different doses and frequencies of application) on agricultural soil quality, were investigated for the presence of genes encoding antibiotic resistance (ARGs) and mobile genetic elements (MGEs). Sewage sludge-induced changes in specific soil physicochemical and microbial properties, as indicators of soil quality, were also investigated. The application of sewage sludge increased the total concentra- tion of copper and zinc in amended soils, but without affecting the bioavailability of these metals, possibly due to the high values of soil pH and organic matter content. Soil microbal quality, as reflected by the value of the Soil Quality Index, was higher in sewage sludge-amended soils. Similarly, the application of sewage sludge increased soil microbial activity and biomass, as well as the abundance of ARGs and MGE genes, posing a risk of dissemina- tion of antibiotic resistance. In contrast, the composition of soil prokaryotic communities was not significantly al- tered by the application of sewage sludge. We found correlation between soil Cu and Zn concentrations and the abundance of ARGs and MGE genes. It was concluded that sewage sludge-derived amendments must be properly treated and managed if they are to be applied to agricultural soil. © 2018 Elsevier B.V. All rights reserved. Keywords: Antibiotic resistance Heavy metals Horizontal gene transfer Mobile genetic elements Sewage sludge Soil quality Science of the Total Environment 647 (2019) 1410–1420 ⁎ Corresponding author at: NEIKER-Tecnalia, Department of Conservation of Natural Resources, c/Berreaga 1, E-48160 Derio, Spain. E-mail address: jurra@neiker.eus (J. Urra). https://doi.org/10.1016/j.scitotenv.2018.08.092 0048-9697/© 2018 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv