Large-scale livestock farming is accelerating the spread of antibiotic resistance and heavy metal contamination in agricultural soils at a pace and scale that poses new risks to global food safety and public health, new research reveals. Scientists have uncovered how even "low-risk" organic fertilizers like dried poultry manure can inadvertently drive a dramatic surge in dangerous antibiotic resistance genes, once released into vegetable plots used for food crops.
The peer-reviewed study, published this week in Biocontaminant, focused on pig and chicken farms near Poyang Lake, China's largest freshwater lake. Researchers examined how heavy metals and antibiotic resistance genes (ARGs) interact and proliferate in soils, using advanced DNA sequencing and molecular tools to track their movement from livestock manure to farmland.
"Antibiotic resistance genes are recognized as a global threat, leading to 700,000 deaths each year," said lead author Dr. Wenbin Liu, of the School of Resources and Environment at Nanchang University. "Our findings show that intensive livestock farming doesn't just pollute the air and water. It seeds the soil with hidden genetic time bombs that can move rapidly between bacteria and spread into the wider environment."
Key Findings
Pig farms released far greater quantities of heavy metals such as zinc and copper into neighboring soils than chicken farms. Both metals are widely used as feed additives.
Antibiotic resistance genes were transferred to vegetable-growing soils mainly through manure applications, with the greatest surge in risk found in plots fertilized with chicken manure.
In one testing site, the health risk index for antibiotic resistance genes rose more than 16,000-fold compared to untreated soil after chicken manure was applied. Even manure classified as low-risk for resistance genes initially, once spread, created new opportunities for risk gene multiplication and exchange.
Soils treated with manure harbored a wider variety of high-risk resistance genes, including those conferring resistance to tetracycline, sulfonamides, and multiple other drug classes. Many of these genes were not present in the original manure but appeared post-application, indicating environmental selection and spread.
The team found that the presence of heavy metals plays a subtle but powerful role in this process. Rather than promoting resistance directly, these metals increase the abundance of mobile genetic elements (MGEs), tiny segments of DNA that transfer resistance genes between bacteria like pieces of genetic machinery. The bacterial group Firmicutes was identified as a particularly important host. This mobile DNA accelerates the spread of resistance genes from manure bacteria to soil microbes, making the soil a hotbed of hidden antibiotic resistance exchange.
Public Health and Food Safety Implications
The findings challenge common assumptions that dried or processed livestock manures are universally safe for use on food crops. "Simply drying manure may lower the overall number of resistance genes but does not eliminate those with the greatest risk," said co-author Dr. Huijun Ding. "Our data underlines the urgency of developing science-driven management strategies for manure treatment and application on farmland."
Solutions and Recommendations
The researchers point to advanced manure treatment options as vital to reducing soil contamination and protecting food safety. Processes such as hyperthermophilic composting, which uses high temperatures to disable both pathogens and resistance genes—can produce safer fertilizers. Similarly, hydrothermal carbonization, though technologically demanding, can remove resistant bacteria and genes entirely.
The team also advocates for prevention at the source, including smarter regulation of antibiotic and heavy metal use in animal feed, especially in large-scale farming. "Managing livestock waste is no longer just about odors or nutrients, but about safeguarding the genetic integrity of our food system," the study concludes.
About the Study
