PLANT VARIATION IN CD, PB, ZN AND AS ACCUMULATION AND BIOAVAILABILITY AND METHODS TO LIMIT RISK
Title: Cadmium and Zinc in Soils, Plants and Animals
Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: July 25, 2009
Publication Date: October 1, 2009
Citation: Scheckel, K.G., Chaney, R.L., Basta, N.T. and Ryan, J.A. 2009. Advances in assessing bioavailability of metal(loid)s in contaminated Soils. In: Sparks, D.L. editor. Advances in Agronomy. Volume 104. New York, NY. Academic Press. p 1-52.
Interpretive Summary: This chapter is a review of cadmium and zinc in soils, plants and animals. They are considered together here because nearly all cadmium contaminated land has 200-fold higher Zn contamination. This high soil zinc can cause toxicity to plants and prevent growth of plants with high cadmium levels. Crop cadmium is not a simple function of total soil cadmium. Uptake is affected by soil cadmium binding strength, soil pH and soil chloride, even by the preceding crop. Rice has unusual ability to accumulate Cd into grain during grain filling, and because growers commonly drain flooded fields at the beginning of flowering, the low cadmium availability of flooded soil is converted to high cadmium plant availability because Cd sulfide is oxidized, and oxidation of N, ferrous and manganous cause pH lowering which increases Cd plant availability. Sources of soil Cd contamination are summarized; except for rice soils, few other crops are at risk from usual Cd contamination sources (zinc mine wastes and smelter emissions). But high Cd phosphate fertilizers, high Cd biosolids, and mineralized soils affected by marine phosphorites cause Cd contamination with little Zn contamination so the Cd is much more phytoavailable and bioavailable. Rare cases of Cd contaminated agricultural products have raised Cd in some areas. It is important that risk from cadmium consider the nature of the Cd source, presence of normal Zn levels, and bioavailability of Cd in the crop. Rice is an exceptional case in that grain iron and zinc are insufficient to support human health, and these deficiencies promote cd absorption by humans. Because other crops do not raise the absorption of Cd by animals, the crop seldom causes Cd risk. Present international limits on crop Cd are based on the assumption that Cd has equal bioavailability or risk in all foods, and that Zn in the food has no affect on Cd bioavailability. These assumptions are in error, causing excessive concern about food Cd in some nations.
Cadmium (Cd) is well known for causing adverse health effects in subsistence rice farmers in Asia, and as a subject of food-chain concern, but is seldom important as a cause of phytotoxicity in the field. On the other hand, zinc (Zn) is commonly both a deficient and phytotoxic element in soils, the latter due to industrial contamination. Zn is associated with significant food-chain deficiency concerns, but not food-chain toxicity risk. In nearly all cases, soil Cd contamination occurs with 200-fold higher Zn contamination. Because these elements are usually co-contaminants, have similar properties in soils and plants, and are readily absorbed and translocated to plant shoots, Cd and Zn should be considered together. Further, their uptake and transport to plant shoots are competitive, therefore both elements need to be considered together to understand either Cd or Zn in detail. Thus the focus of this chapter is potential food-chain transfer of soil Cd risks, and remediation of Zn phytotoxicity. The interaction of soil Zn limiting Cd risk to food-chains is very important in understanding why rice caused essentially all Cd disease attributed to soil Cd.