Freeze-thaw cycles and their interrelation with microplastic contamination affecting the spread and propagation of antibiotic resistant genes in black soil of Northeast China

Freeze-thaw cycles (FTCs) and microplastic (MP) contamination are common co-phenomena in the black soil of Northeast China. Climate warming increases the frequency of FTCs of soils in northern regions. Nowadays, the effects of repeated FTCs on the spread and proliferation of antibiotic resistance genes (ARGs) have received limited understanding. Whether the existence of MPs will complicate the freeze-thaw effect remains unclear. Therefore, microcosm experiments with two frequencies of FTCs (i.e. 14 and 28 cycles) and two types of MPs (polyethylene and polybutylene adipate terephthalate, added at 1% w/w) were conducted to unravel the effects of FTCs and their interrelation with MP contamination on the spread and proliferation of ARGs. Results showed that FTCs decreased the total abundance of target ARGs in soils. The existence of MPs further reduced their abundance, which was more significant at high FTC frequencies. Changes in the ARG abundance during the above processes were attributed to the decrease in the abundance of integrons intI1 mediating ARG horizontal gene transfer among bacteria and the potential host bacteria. Despite an increase in the abundance of freezing-tolerant bacterium Sphingobium carrying ARGs under FTCs, the total abundance of host bacteria carrying ARGs decreased. There were significant correlations between the available soil nitrogen (AN) and the abundances of ARGs. Moreover, the abundance of pathogenic bacteria carrying multiple ARGs increased after adding MPs under low-frequency FTCs, and their abundance decreased, especially under high-frequency FTCs. These findings expand our understanding of ARG spread and proliferation in the black soil of Northeast China and provide a theoretical basis for the prevention and control of ARGs in the soil.