Phyllosphere of staple crops under pig manure fertilization, a reservoir of antibiotic resistance genes

In China, the common use of antibiotics in agriculture is recognized as a potential public health risk through the increasing use of livestock derived manure as a means of fertilization. By doing so this may increase the transfer of antibiotic resistance genes (ARGs) from animals, to soils and plants. In this study two staple crops (rice and wheat) were investigated for ARG enrichment under differing fertilization regimes. Here, we applied 4 treatments, no fertilizer, mineral fertilizer, clean (reduced antibiotic practice) and dirty (current antibiotic practice) pig manure, to soil microcosms planted with either rice or wheat, to investigate fertilization effects on the abundance of ARGs in the respective phyllospheres. For both rice and wheat, samples were collected after two separate fertilization periods. In total, 162 unique ARGs and 5 mobile genetic elements (MGEs) were detected from all rice and wheat samples. The addition of both clean and dirty manure, enhanced ARG abundance significantly when compared to no fertilizer treatments (P < 0.001), though clean manure enriched ARGs to a lesser extent than dirty manure, in all rice and wheat samples (P < 0.001). The classes of ARGs recorded were different between crops, with wheat samples having a higher ARG diversity than rice. These results revealed that staple crops in China such as rice and wheat may be a reservoir for ARGs when clean and dirty pig manure is used for fertilization.