Reducing water use by alternate-furrow irrigation with livestock wastewater reduces antibiotic resistance gene abundance in the rhizosphere but not in the non-rhizosphere

One effective approach to treating large amounts of wastewater produced during livestock production is to use it to irrigate crops. However, antibiotic compounds and antibiotic resistance genes (ARGs) associated with livestock wastewater may enter the soil and plants. ARGs are spread readily among microbial populations by mobile genetic elements, and may pose threats to human health. Compared with conventional furrow irrigation (CFI), alternate-furrow irrigation (AFI) can reduce water use and still achieve high yields. These different irrigation methods may influence the fate of ARGs in soil however, few reports have studied the combined effects of AFI and irrigation with livestock wastewater upon the distribution of ARGs in soil. Here, swine wastewater was used to irrigate cultivated peppers, and compared to relatively ARG-free groundwater. AFI was compared to CFI (100%) at three AFI irrigation rates (80%, 65% and 50% of CFI). The results showed that wastewater irrigation resulted in greater accumulation of antibiotic compounds and ARGs in soil than groundwater. The effect of wastewater was much more pronounced in the rhizosphere than non-rhizosphere soil. Compared with CFI, AFI using wastewater reduced the relative abundance of ARGs in the pepper rhizosphere, but the concentration of antibiotic compounds was largely unaffected; though antibiotic compound concentrations in roots were significantly lower, the abundance of ARGs in roots at 50% and 65% rates and in fruits at 50% rate were significantly increased when using wastewater. The soil bacterial communities did not change significantly between the different irrigation rates. Different behaviours were observed between ARGs and antibiotic compounds at different irrigation rates. Antibiotic compound availability plays an important role in the diffusion of ARGs. In conclusion, AFI of livestock wastewater can reduce the relative abundance of ARGs in rhizosphere soil, but low irrigation amount should be employed carefully for the safe agricultural production.