Title: Cadmium in soils and its transfer to plants and the human food chain Authors
|Ryan, J -|
Submitted to: Symposium Proceedings
Publication Type: Proceedings
Publication Acceptance Date: March 15, 2013
Publication Date: April 9, 2013
Citation: Chaney, R.L., Ryan, J.A., Reeves, P.G. 2013. Cadmium in soils and its transfer to plants and the human food chain. Symposium Proceedings. pp. 175-212. Interpretive Summary: Cadmium concentrations in foods may be a limit in international sale of US crops to EU importers in the near future. This paper reviews aspects of cadmium and zinc in soils, plants, livestock, and humans to clarify under which situations soil cadmium may comprise a risk. Because of unique soil and plant chemistry, rice can accumulate dangerous levels of cadmium in grain but fails to accumulate increased Zn or Fe from soils which have 100-times more zinc contamination than cadmium contamination. We conclude that for normal “geogenic” (from ores and similar industrial contamination of soils) which have about 1g of Cd per 100g of Zn, only rice and tobacco are able to accumulate cadmium in the crop which will be absorbed by humans. The paper discusses sources of soil cadmium contamination, reactions of cadmium in soils, uptake of cadmium by plants in relation to soil variables, transport of plant cadmium to edible plant tissues, and absorption of cadmium from different foods in test animals and humans. These questions have become more important in recent times because the EU has adopted lower allowed daily intake of cadmium (in disagreement with international standards under the US/WHO Codex Alimentarius program), and proposed allowable cadmium concentrations in imported grains and other crops (significantly lower than Codex) that would interfere with sale of US agricultural crops (wheat, soybeans, sunflower kernels, flax, etc.). The irrationality of the proposed limits is summarized in terms of errors in estimating the allowable intake of dietary cadmium by humans, and the clear difference among foods in cadmium bioavailability. Although rice cadmium has caused human disease in many locations in Asia, no other crop or seafood (bivalve molluscs bioaccumulate cadmium), which normally have cadmium enrichment compared with wheat has been shown to cause cadmium human health effects. Research topics which could clarify the science where confusion about soil, crop and food cadmium risk is summarized.
Technical Abstract: Cadmium occurs naturally in all soils, but few soils contain higher than 1mg Cd kg-1. Most geogenic Cd is accompanied by 100-200 fold higher Zn except for marine shale or phosphorite derived soils which may have 1g Cd per 10g Zn or higher. Contamination by mining or smelter emissions of Zn-Pb-Cu industries has the usual low Cd:Zn ratio of geogenic Cd, while Cd industries may have very high Cd:Zn ratio emissions. This ratio is important because Zn phytotoxicity occurs when leaves contain about 400-500mg kg-1 dry weight, and plants accumulate Cd and Zn in about the ratio present in soils, thereby limiting plant shoot Cd to 5mg kg-1 when yields are substantially reduced by Zn phytotoxicity for geogenic Cd sources. When high Cd:Zn ratio occurs, plants may reach very high Cd levels and even Cd phytotoxicity. Plant Cd accumulation is also limited by higher soil pH, higher soil hydrous Fe and Mn oxides and organic matter, but promoted by acidic pH and high soil chloride levels. Foliar Zn is important both in limiting maximum plant Cd, and in inhibiting Cd transfer to storage tissues such as grain and fruits except rice. Essentially all human disease from soil Cd has resulted from rice or tobacco. Because rice is grown in flooded soils but these soils are drained during flowering causing soil oxygenation which allows soil CdS to be transformed back into Cd2+ bound to soil surfaces and soil pH to fall with oxidation of NH4-N, Fe2+ and Mn2+. This causes Cd to become much more phytoavailable and because of the nature of rice, Cd enters the grain with little accompanying Zn. High soil Zn does not protect against excessive Cd in rice grain in contrast to patterns of all other crops. In addition, rice grain is inherently low in bioavailable Fe, Zn and Ca. The low levels of total and bioavailable Fe, Zn and Ca in polished rice grain promotes Cd absorption by animals by 10-fold compared to diets with adequate levels of these nutrients. The high bioavailability of Cd in rice grain compared to other Cd-rich foods allows rice to transport soil Cd into human diets which can cause human disease in many rice consuming nations. The effect of crop Zn limiting risk from crop Cd, and Zn reducing the bioavailability of Cd in different crops needs to be taken into account in international regulation to prevent Cd risks. Because the EU has proposed regulations with lower allowable levels of crop Cd which do not take into consideration the science reviewed in this paper, it is important to the Zn-Pb-Cd industry to support additional feeding studies with additional crops grown on Cd-Zn contaminated soils to make this science stronger or suffer from the lower crop Cd limits.