Study examines cadmium uptake in New Zealand pastures

New Zealand’s pastoral landscapes are some of the loveliest in the world, but they also contain a hidden threat. Many of the country’s pasture soils have become enriched in cadmium—a toxic heavy metal that is readily taken up by grasses and then transferred to the cattle and sheep that graze them.

Pasture and sheep with mountains in background

The concern is that if cadmium concentrations rise to unsafe levels in meat and dairy products, human health and New Zealand’s agricultural economy could be jeopardized.

That so far hasn’t happened. However, because much of the cadmium in the nation’s pasture soils originates from mined phosphate fertilizers that farmers continue to use, New Zealand isn’t taking any chances. Farmers whose soils test high in the metal are being advised to apply low-cadmium fertilizers, for example. and reduce phosphorus applications overall.

Scientists, meanwhile, are trying to determine which soil factors most strongly affect soil cadmium concentrations, with an eye toward predicting where in the landscape pasture grasses are more likely to accumulate the metal. In a new study led by Brett Robinson of New Zealand’s Lincoln University, scientists found that soil pH, iron concentrations, and total cadmium levels were all excellent predictors of the potentially bioavailable fraction of soil cadmium. At the same time, they were relatively poor predictors of the actual cadmium concentrations measured in pasture grasses.

What this suggests is that the grasses themselves need to be investigated next. Different species of pasture grasses take up cadmium to different degrees. Their roots may also change soil chemistry in ways that affect cadmium’s availability. By refining models of soil-to-plant transfer of cadmium, people can better judge where livestock are at greatest risk of cadmium exposure in the future. The new knowledge should also enable the development of tools—such as pasture species selection—to reduce that exposure, the study’s authors say.

Their findings published online on Mar. 21, 2014 in the Journal of Environmental Quality.

How did New Zealand end up with so much cadmium in its soils? For many years, phosphate rock from Nauru Island—a tiny island nation in the South Pacific—was used to produce “single superphosphate” fertilizer for the country’s agricultural soils. Only later did people realize the phosphate contained high concentrations of cadmium, or roughly 100 mg per kg. Today, the average cadmium concentration in pasture soil has risen to 0.43 mg/kg, according to previous research—or more than two times background levels.

And in the current study, the average level across 69 sites was even greater: 0.89 mg/kg. (The authors think the higher number may stem from their focus on lands under intensive dairy farming, which tend to receive more fertilizer than grazing lands in general.) It’s certainly a troubling finding for a country where pasture covers nearly 40% of the total land area and animal production is a vital industry. But the problem is also not unique to New Zealand. Cadmium enrichment has been reported in soils worldwide, not only through use of cadmium-rich phosphate fertilizers, but also from atmospheric deposition of cadmium and land application of biosolids.

So the question now becomes, how can cadmium be managed? The metal is difficult to remove from soil once it’s there, so one important solution is to keep what's already present locked up and unavailable to plants. Robinson’s work indicates one way to achieve this. In the study, concentrations of plant-available cadmium rose as pH declined, suggesting that maintaining neutral or high soil pH levels whenever possible will reduce uptake of the metal by grasses—and ultimately by livestock.

Similarly, high iron levels were statistically correlated with lower concentrations of bioavailable cadmium, because iron oxides bind cadmium tightly and hold it in soil. Robinson is now working with the coal-mining company, Solid Energy New Zealand, and the Swiss Federal Institute of Technology to learn whether additions of the compound, lignite, from some coal mining operations, can increase the soil’s ability to bind cadmium, as well.