Long-Term Effect of Different Fertilization and Cropping Systems on the Soil Antibiotic Resistome

Different fertilization and cropping systems may influence short- and long-term residues of antibiotic resistance genes (ARGs) and mobile genetic elements (MGEs) in soil. Soils from dryland (peanut) and paddy (rice) fields, which originated from the same nonagricultural land (forested), were treated with either chemical fertilizer, composted manure, or no fertilizer for 26 years before sampling, which occurred one year after the last applications. ARGs and MGEs were investigated using highly parallel qPCR and high-throughput sequencing. Six of the 11 antibiotics measured by LC-MS/MS were detected in the manure applied soil, but not in the nonmanured soils, indicating their source was from previous manure applications. Compared to the unfertilized control, manure application did not show a large accumulation of ARGs in either cropping system but there were some minor effects of soil management on indigenous ARGs. Paddy soil showed higher accumulation of these ARGs, which corresponded to higher microbial biomass than the dryland soil. Chemical fertilizer increased relative abundance of these ARGs in dryland soil but decreased their relative abundance in paddy soil. These results show how long-term common soil management practices affect the abundance and type of ARGs and MGEs in two very different soil environments, one aerobic and the other primarily anaerobic.