Factors influencing the survival and leaching of tetracycline-resistant bacteria and Escherichia coli through structured agricultural fields

Intense use of antibiotics in agricultural production may lead to the contamination of surface and groundwater by antibiotic-resistant bacteria. In the present study, the survival and leaching of E. coli and tetracycline-resistant bacteria were monitored at two well-structured agricultural fields. Non-spiked pig slurry was injected in accordance with agricultural practice in the area. In both fields, the concentration of E. coli and tetracycline-resistant bacteria in the injected part of the plough layer decreased to the detection limit within 46–49 days. At Silstrup, the decay was initiated with a lag phase and a decimal reduction time of 16 days for E. coli and 18 days for tetracycline-resistant bacteria. At Estrup, the decay was immediate and the decimal reduction time was 22 days for E. coli and 26 days for tetracycline-resistant bacteria. Despite the two fields being prone to rapid preferential transport through macropores, it was found that faecal bacteria were only leached to drainage water at Estrup. Here E. coli and tetracycline-resistant bacteria were detected at concentrations up to 3 and 130 CFU mL−1 respectively. A PCA plot revealed that leaching of faecal bacteria was negatively correlated to soil water content and number of days after slurry application. At Silstrup, the drainage system remained hydraulically inactive until two months after the second slurry application and resulted in undetectable leaching to the aquatic environment. There were indications that factors such as slurry properties, agricultural practices, crop cover and hydrological setting influenced leaching. The degree by which these factors influence leaching from agricultural fields is currently not well understood and highlights the need for field-scale studies to increase understanding of faecal bacteria leaching.