Effects of long-term manure applications on the occurrence of antibiotics and antibiotic resistance genes (ARGs) in paddy soils: Evidence from four field experiments in south of China

• The occurrence of antibiotics and ARGs in soil affected by manure applications at known rates in field sites was investigated.
• High concentrations of tetracyclines were detected in long-term manured soils.
• Long-term manure applications did not always increase the abundance of ARGs in soil.
• The abundance of ARGs correlated well with antibiotics and soil properties.
• The concentrations of tetracyclines and abundance of ARGs generally decreased with increasing soil depths.

Most studies of the effects of manure amendment on the occurrence of antibiotics and antibiotic resistance genes (ARGs) in soil employ the investigation of grab samples or short-term laboratory studies. However, the effects of long-term manure applications on antibiotics, ARGs and their vertical distribution in paddy soil in field experiments are lacking. We assessed the concentrations of antibiotics, ARGs and their vertical distribution in paddy soil receiving long-term manure applications in four field experiments. High concentrations of tetracyclines were detected in most manured soils, while sulfonamides were not detectable. Long-term manure amendments generally increased the antibiotic concentrations and ARGs abundances in the paddy soil over decades. However, in some sites such significant trends of ARGs could not be observed. The abundance of ARGs was statistically correlated with antibiotics and soil properties including pH and soil organic matter (SOM), indicating their importance in the selection of resistance genes. Tetracyclines could be detected in soil at different depths and the concentrations of tetracyclines and abundance of ARGs generally decreased with increasing soil depths.