PLANT VARIATION IN CD, PB, ZN AND AS ACCUMULATION AND BIOAVAILABILITY AND METHODS TO LIMIT RISK
Title: The Role of Crop Cd Bioavailability in Potential for Transfer of Soil Cd Risk to Humans and Wildlife
Submitted to: Meeting Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: October 20, 2006
Publication Date: November 9, 2006
Citation: Chaney, R.L., Reeves, P.G. 2006. The Role of Crop Cd Bioavailability in Potential for Transfer of Soil Cd Risk to Humans and Wildlife. Meeting Proceedings. On disk.
Cd is a common contaminant in soils affected by mining and smelting of Zn, Pb, Cu and Ag ores and where biosolids, composts and manures are applied. Zn is usually present at 100-200 times higher concentrations than Cd. Because of this relationship of Cd and Zn in ores and contaminated soils, Zn is phytotoxic before Cd could become phytotoxic; Zn is phytotoxic at 400-500 mg/kg dry leaves, while Cd usually reaches 20-100 mg/kg dry leaves before it is phytotoxic. Toxicity to soil organisms is also due to Zn rather than Cd. Although earthworms do bioaccumulate Cd, there is no record of Cd toxicity to earthworm consuming wildlife from mine waste affected soils with the normal ratio of Cd:Zn. Few locations with Cd contamination do not have the 100-200 fold higher Zn contamination which reduces the risk of adverse Cd effects.
In the last few years, we have shown an important role of crop species and crop Cd bioavailability in risk from Cd in contaminated soils. It is well known that large numbers of subsistence rice consumers in Asia who consumed rice home grown on mining contaminated soils for decades suffered renal tubular dysfunction due to excessive Cd intake. But at several locations where individuals consumed garden vegetables grown on much more highly Cd+Zn contaminated soils, no renal tubular disease was observed. At the same time, research on malnutrition in Asia found that subsistence rice diets promoted Fe and Zn deficiency in humans, causing important public health problems. Rice contains low concentrations of Fe and Zn, and much of the grain Fe and Zn is lost during polishing. Cd absorption by animals is inhibited by adequate dietary Fe and Zn, so the deficient supply of Fe and Zn from rice diets promoted risk from rice Cd. It has also been shown that Cd is largely transported into duodenal enterocytes on the ferrous transporter (DMT1). In our tests with rats fed diets with marginal or adequate levels of Fe-Zn-Ca (marginal animals did not have reduced growth rate), marginal supply increased Cd absorption by about 10-fold in rice diets, while diets with sunflower kernels contained phytate and higher Fe and Zn, and did not cause as high increase in Cd absorption with marginal diets.
In addition, rice accumulates Cd into grain without increase in grain Zn even when grown on soils with very high Zn+Cd contamination. Flooded soils produce sulfides and precipitate both Cd and Zn, but when the fields are drained at flowering, CdS is rapidly oxidized and Cd is absorbed and transported to grain while Zn is not. This pattern of high Cd without greatly increased Zn is very unusual, and is not observed in crops grown on aerobic soils.
Much of the concern about dietary Cd risks to humans is based on toxicological type studies in which Cd salts are applied to soils, or added to diets without the corresponding amount of Zn which would be present in the geogenic-contaminated environment. Zn inhibits Cd uptake by crops, inhibits Cd transport to grains and other storage organs of crops, and inhibits Cd absorption in the intestine of animals which consume the crops. In some research, added Cd without Zn actually induced Zn deficiency in the animal or plant, and caused adverse effects which simply are not relevant to environmental Cd+Zn contamination.
Thus, much of the concern about environmental Cd is based on misunderstanding of the food-chain transfer and bioavailability of Cd from usual contaminated soils. Cd in rice soils or tobacco fields do comprise risk to humans, but there is little evidence that Cd in other soils with the normal ratio of Cd:Zn in geogenic contaminated soils comprises risk even to highly exposed individuals.