Researchers from Massey University have found a link between elevated levels of heavy metals in Waikato pastoral soils and an increase in antibiotic-resistant soil bacteria.
In a paper recently published in Antibiotics magazine, the researchers have determined a link within bacteria that can develop a level of resistance to heavy metals including cadmium and zinc, and their ability to also increase resistance to antibiotics under lab conditions.
Lead researcher Dr Barry Palmer says the issue of New Zealand’s pastoral soils containing elevated levels of cadmium has long been understood as coming from natural contaminants of the phosphate rock used to make mineral fertilisers.
A legacy of cadmium remains from high-cadmium fertilisers used before the cadmium problem became known.
In response to the problem, a collaboration of central and regional governments, industry and researchers, implemented a strategy to manage cadmium.
Current fertilisers have much lower amounts of cadmium than in the past, but adding a small amount of cadmium in mineral phosphate fertilisers is unavoidable.
Once in the soil profile, it becomes insoluble. Attached to soil particles, it and leaches out only slowly.
Increased concentrations of cadmium are known to be linked to elevated kidney damage, and offal from livestock older than 30 months cannot be sold due to its cumulative effect in tissue.
Zinc levels are also known to have become elevated in pastoral soils over the past two to three decades, due to the metal’s presence in agricultural chemicals such as those used in managing facial eczema in the upper North Island.
Scientists took control soil samples from non-fertilised forest areas, and for comparison “spiked” samples of soil with cadmium, zinc and mercury.
They found the levels of antibiotic-resistant and heavy metal-resistant bacteria peaked in the samples with significantly higher heavy metal content.
Palmer acknowledged the scientists do not fully understand how the correlation came about, only that possibly the bacteria are using similar mechanisms to process the heavy metals as they are antibiotics in the soil.
“The work also showed that the same bugs (bacteria) that developed a resistance could pass on that resistance through a process known as horizontal gene transfer.”
However, Palmer stressed that the soil samples were spiked with levels of heavy metals that were generally greater than what occurs in pastoral soil.
“Whether they actually reach levels in soil to strongly select for antibiotic resistance is a moot point, but the results definitely suggest the higher levels make a difference.”
Soils in Waikato have been found to have cadmium concentrations five times higher than background levels after 70 years of superphosphate application. Zinc levels have doubled over the past 30 years.
The industry’s voluntary cadmium limit set in the late 1990s limited it to 280mg per kilogram of phosphate, still almost five times the level needed to avoid accumulation of cadmium.
A third of soils in New Zealand have too-high levels of phosphorous due to an oversupply of fertilisers, causing issues around water quality and toxic algae growth.
Palmer says at a broad policy level the research work highlights another reason over-application of phosphate is best avoided and is not sustainable.
“You have to commend the industry for setting up the working group on cadmium in the ‘90s. They realised there was a problem and tried to work to mitigate it.
“But going forward, it is possible to see how movements like regenerative agriculture provide other ways to farm that avoid these issues.”
He says it may be possible in future that new developments in science around treatments like endophytes may help make phosphate more plant-available, without having to apply it in a form that creates the issues the research identified.
Palmer says to build on the research it would be worthwhile exploring heavy metal levels in other pastoral areas including Canterbury, and possible links between glyphosate use and soil bacteria behaviour.
