Due to water transport, the concentration of cadmium in soil and plants will decrease over time in most agricultural areas in Norway, even though cadmium levels in mineral fertilizers will increase to 137.4 mg per kilo of phosphorus. In many areas, however, the increase may lead to slightly more cadmium in the soil over time.
This is the main finding of an assessment of the risk of cadmium in mineral fertilizers, written by the Scientific Committee for Food and Environment (VKM). The Norwegian Food Safety Authority requested an assessment of the consequences an increased level of cadmium in mineral fertilizers may have on the environment and health.
Cadmium is a toxic trace metal that can be a risk to soil and water environment, plants, animals and humans. It is therefore desirable to have the least possible exposure to cadmium. Cadmium occurs naturally in phosphate-rich rock used for the production of mineral fertilizers, and is a source of pollution.
“It appears the concentration of cadmium in farmland is generally lower over time. The reason is that in many areas cadmium is leached from the soil through water in greater amounts than it is added,” explains Trine Eggen, who is responsible for academic quality of the assessment.
Eggen emphasizes that decreases in cadmium can vary between areas, depending on climate, soil quality and the current concentration of cadmium in the soil. High or low natural occurrence of cadmium also plays a role. High incidence results in greater water transport resulting in greater reduction.
The leaching can be particularly high in areas with high precipitation and good drainage in soil.
“Natural occurrence of cadmium is generally low in Norwegian agricultural land, but areas with alum shale can have significantly higher levels of cadmium," says Eggen.
This applies to some areas in Stange, Hedmark.
Contrary to previous belief, cadmium appears to be bound to soil, which also contributes to higher water transport.
“Our calculations of concentrations of cadmium in surface water indicate that cadmium in soil can pose a risk to aquatic organisms in watercourses that receive drainage water from cultivated soil,” tells Eggen.
VKM's calculations show that the effect of increasing the concentration of cadmium in mineral fertilizers will increase the supply of cadmium to the soil, but will not lead to accumulation of cadmium over time.
“The exposure is far below the level that can damage animal health,” Eggen adds.
There is uncertainty associated with variations in soil quality and climate, and the calculation models and equations used to estimate what happens to cadmium in the environment. Thus, exposure in soil, agricultural plants and water also becomes uncertain.
There is also uncertainty associated with changes in cadmium concentrations in plants and its effect on human intake. None of the cadmium concentrations used in our scenarios will affect dietary exposure to cadmium.
How VKM assessed the risk
In this risk assessment, VKM has calculated the mass balance for cadmium in soil. That is, we have compared the amount of cadmium supplied from different sources, with loss of cadmium. The mass balance indicates whether the concentration increases or decreases.
The concentration of cadmium in animal feed and food crops and surface water is calculated from the concentration in soil and added cadmium. Then the estimates are compared to cadmium concentrations known to have negative consequences for humans, animals and the environment. Four agricultural areas are used in the scenarios: Stange in Hedmark, Time in Rogaland, Ås in Akershus and Melhus in Trøndelag.
VKM's Panel on Animal Feed is responsible for the risk assessment.