Sludge Conditioning Treatments Impact the Fate of Antibiotic Resistance Genes in Agricultural Soils Amended with Sludge Composts

Sewage sludge is one of the major environmental reservoirs of antibiotic resistance genes (ARGs), while its recycling releases abundant ARGs into the agricultural soils. Sludge conditioning treatment, as an indispensable step to improve sludge dewatering, can enhance the dewaterability of sewage sludge and meanwhile attenuate ARGs in sludge compost, but it remains unclear how sludge conditioning treatments impact the ARG profiles in different agricultural soils amended with sludge composts. In the present study, the fates of 18 ARGs and 2 mobile genetic elements (MGEs) in two types of agricultural soils (i.e., red soil and yellow–brown soil) were investigated for 170 days, after the land application of sludge composts derived from different conditioning treatments, including bioleaching and chemical conditioning using Fe[III]/CaO or polyacrylamide (PAM). The results showed that the absolute abundance of total ARGs and MGEs was the lowest in both the red soil and the yellow–brown soil amended with bioleached sludge compost (Day 170), which were only 26.4–76.8% of that in the corresponding soils amended with the compost products of raw, PAM-conditioned, or Fe[III]/CaO-conditioned sludge. Besides, in comparison with other conditioning treatments, the bioleaching conditioning treatment more greatly limited the enrichment of typical sludge-borne ARGs (sul2, aadA1, aadA2-01, aadA2-02, and aadA2-03) and their potential hosts. It was found that the much less enrichment of sludge-borne ARGs achieved by bioleaching conditioning most probably resulted from both the low abundance of ARGs in bioleached sludge compost and the limited growth of bacteria carrying ARGs in the amended agricultural soils. Therefore, bioleaching conditioning is superior to the chemical conditioning using Fe[III]/CaO or PAM in mitigating antibiotic resistance in different agricultural soils amended with sludge composts, which was contributed by the preremoval of ARGs in sludge compost and the potentially limited growth of bacteria carrying ARGs after the land application of sludge compost.