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United States Department of Agriculture

Agricultural Research Service

Research Project: Plant and Soil Factors That Influence Bioavailability of Heavy Metals in Crops

Location: Crop Systems & Global Change

2012 Annual Report


1a.Objectives (from AD-416):
Objective 1: Characterize the influence of zinc and iron concentrations in edible crop tissues and crop species on the bioavailability of crop cadmium to animals (C1; PS 1.F). Objective 2: Characterize the potential transfer of soil lead, arsenic, and other trace elements by vegetable crops grown on element enriched urban and orchard soils and develop methods to prevent this transfer. (C1; PS 1.F). Objective 3: Characterize genetic resources and inheritance of grain Cd to reduce cadmium in durum wheat, flax and soybean. (C1; PS 1.F). Objective 4: Evaluate information about the risk from Cd in foods to support public decisions for foods of both plant and animal origin. (C1; PS 1.F). The ultimate goals of this Project Plan are to improve the science about risk of heavy metals in soils and crops in order to obtain improved regulatory limits for Cd in crops under Codex, and the information needed to provide improved advice about the risk of Pb in urban garden soils and crops. Essentially all human Cd disease from soil Cd has resulted from paddy rice grown on fields contaminated by mining or smelter emissions. Garden vegetables and other grains have not been found to induce Cd disease in highly exposed populations who grow crops on highly Cd+Zn contaminated soils. Some Europeans want to set crop Cd limits based on “attainable” levels rather than on the basis of potential risk to consumers. Such non-risk based standards will harm U.S. growers of durum wheat, sunflower kernels, flax, soybean, and some other crops. If crops must contain lower levels of Cd to win importation by other nations, both soil treatments, selection of soil series which produce lower Cd crops, and improved cultivars which accumulate lower amounts of Cd will contribute to growers needs. Providing a clear technical basis for the bioavailability of Cd in different crops appears to be the central issue which could change the demand for lower limits for crop Cd, and protect growers from unnecessary costs to produce lower Cd crops which may have no benefit. Only animal feeding tests can provide valid information about the bioavailability of crop Cd to animals, and the concentrations fed must represent levels in foods rather than toxic levels fed in most previous research. Because Zn is usually greatly increased in crops (except rice) when Cd is increased, the presence of Zn may substantially reduce the bioavailability of crop Cd, alleviating presumed risk of crop Cd. Because commercial carrots were found with higher than normal Pb concentrations when grown on historic orchard soils, U.S.-FDA requested that ARS examine the basis for carrot Pb accumulation and to learn if agricultural amendments can reduce carrot Pb when they are grown on high Pb soils. Certain root crops have xylem elements growing through the edible storage root, so if Pb is trapped within the xylem during normal growth of the crop, it will be in the edible root. But such Pb might have much lower bioavailability than the Pb-acetate used to establish diet Pb risk; 2-10% of food Pb is absorbed, while 60-80% of soluble Pb in water is absorbed by human volunteers.


1b.Approach (from AD-416):
Characterize the influence of zinc and iron concentrations in edible crop tissues and crop species on the bioavailability of crop cadmium to animals. Zinc incorporated in lettuce reduces weanling rat absorption of lettuce Cd. Using controlled chelator-buffered nutrient solutions (similar to Kukier and Chaney, 2002), Romaine lettuce will be grown to contain basal and sub-phytotoxic concentrations of foliar Zn (25 and 450 mg kg-1 DW), and the Codex Cd limit (4 mg Cd kg-1 DW); the high Zn represents maximum Zn levels normally present if lettuce is grown in acidic soils with geogenic Cd+Zn contamination at the beginning of yield reduction due to Zn phytotoxicity. Fe concentration in lettuce is tightly controlled genetically, but lettuce can supply higher bioavailable Fe than the marginal AIN diet, and thus plant Fe can reduce Cd bioavailability. The lettuce will be fed to weanling rats following the protocol of Reeves and Chaney (2004) in which American Institute of Nutrition (AIN) purified diets with marginal or adequate Fe-Zn-Ca were fed. The basal and high Zn lettuce will be mixed with both the marginal and adequate diets and fed for 28 days without radioisotope labeling used previously. At autopsy, the liver, kidney and duodenum will be removed for analysis. Tissues and blood will be tested to establish Fe status of the test animals. The tissues will be digested in HNO3, and Cd, Zn and Fe levels measured by ICP-Atomic Emission Spectrometry or ICP-Mass Spectrometry. Eight replicate rats will be fed each diet to accommodate the natural variation in such feeding tests (Reeves and Chaney, 2001). If the lettuce experiment shows a strong effect of crop Zn on crop Cd bioavailability, other crops may be studied using a similar approach (durum wheat; soybean).


3.Progress Report:
The project objectives fall under National Program 108 Action Plan, Component 1–Foodborne Contaminants, Problem Statement 1.F Chemical and Biological Contaminants, covering improving understanding and reducing risks from trace elements in agricultural products. Progress was made in each of the four objectives. Under Objective 1, a feeding test to evaluate the effect of zinc in lettuce on the bioavailability of cadmium in lettuce was designed and lettuce grown to prepare the test feeds. A new collaborator was required after changes in programs at Grand Forks, ND. Under Objective 2, cooperators in five states conducting cultivar field evaluations for agronomic performance, provided samples of mature grain of durum wheat or soybean cultivars grown on several soil series so that genotype-by-environment interaction could be assessed. Under Objective 3, the localization of lead in carrot xylem was identified as the mechanism whereby peeled carrots accumulate significant amounts of lead from contaminated soils in contrast with most other garden crops. In addition, a chemical extraction (or bioaccessibility) test was developed to correlate well with the outcome of human soil feeding tests to measure the bioavailability of soil lead from a field test using phosphate to remediate soil lead in Joplin, MO. Compared to earlier tests for soil lead bioaccessibility, the new method is better related to the bioavailability of soil lead to humans, less expensive to conduct, and reflects the influence of soil amendments which reduce soil lead bioavailability based on feeding tests. Under Objective 4, continuing evaluation of the literature and new evidence about dietary cadmium risk to humans was reviewed in order to provide advice to potentially impacted commodity groups and Foreign Agricultural Service staff who represent the U.S. in international food safety discussions. An attempt to sharply reduce allowable intake of cadmium in the European Union must be challenged to protect U.S. producers of durum wheat, soybean, sunflower, flax and other crops. Evidence from long exposures of farm families to excessive cadmium intake in Asia provides clear evidence that internationally accepted cadmium intake limits remain fully protective of humans.


4.Accomplishments
1. A simple and inexpensive test for bioaccessible garden soil lead. With the large increase in interest in gardening and agriculture in urban areas, the high levels of lead from historic paint and automotive exhaust emissions may comprise risk to children who could ingest garden soils either at the garden or when soil is carried to the home on tools and clothes of gardeners. And some crops may accumulate enough Pb to comprise risk (lettuce, carrot, etc.). Because common garden soil amendments can induce the formation of chemical forms of lead which have low bioavailability to humans who ingest soil, it is important that advice about urban gardens be based on bioavailable/bioaccessible lead rather than total lead. The soil test presently approved by U.S.-Environmental Protection Agency to estimate soil bioaccessible lead has been shown to over-estimate lead bioavailability in soils treated with phosphate or compost, and is prohibitively expensive for most gardeners. Thus a simplified test calibrated using soil samples from the Joplin field test of using amendments to reduce soil lead bioavailability. The phosphate treated soil had 69% reduction in soil lead bioavailability to humans fed the test soils. The U.S. Environmental Protection Agency (US-EPA) test with 0.4 M glycine-hydrochoride was modified for ease of operations and calibrated against the Joplin soils. Additional evaluation of the method was conducted during the FY, showing that the rate of shaking was a more critical factor than initially believed. The new method, conducted at pH 2.5 rather than the EPA method at pH 1.5, has nearly the same reduction in Pb bioaccessibility as observed in the human feeding test so estimated bioavailability can be measured from the correlation of bioavailability and bioaccessibility from the Joplin soils.

2. Working with U.S. producers to protect sales of grains with normal levels of cadmium. During the last year, an European Union panel recommended lowering of the Potentially Tolerable Weekly Intake of cadmium be lowered from 7.0 to 2.5 micrograms per kg body weight peer week, and then proposed lower levels of cadmium for important U.S. export crops. The Codex Alimentarius (World Health Organization) program did not support lowering the cadmium intake recommendation (previously 7 micrograms cadmium per kg body weight per week), and slightly lowered it in changing to a monthly intake recommendation in order to stress the long term nature of dietary cadmium risk to humans (Potentially Tolerable Monthly Intake to be 25 micrograms per kg body weight per month). Our previous research showed that the low iron, zinc and calcium levels of rice grain promote cadmium absorption by animals compared to animals fed adequate levels of these nutrients, but the EU committee did not take these important findings into account. Research findings such as these were brought to the attention of Foreign Agricultural Service and commodity group scientists who participate in international negotiations regarding food cadmium limits so they could use available scientific information to support the U.S. farm community. The EU Commission group dealing with the proposed lower limits for Cd in grains has decided not to adopt such regulations at this time, but have not formally rejected the proposal. Additional feeding trials would help settle these questions.

3. Cultivar variation of cadmium accumulation in U.S. soybean and durum wheat. Because grain cadmium can be a limit for marketing of crops, studies were begun or continued to assess cultivar variation in cadmium accumulation in grain of durum wheat and soybean. Breeding lower cadmium accumulating cultivars is one approach to protect markets for U.S. crops, but this requires knowledge of genetic variation and genotype-by-environment variation in crop cadmium. Cooperative studies were undertaken with plant breeders in Montana, Arizona and California to assess durum wheat genetic variation, and in Iowa and North Dakota to assess both genetic variation and soil series effects on cadmium accumulation in soybean. Crops grown during FY11 were analyzed, and the crops grown again during FY12 and will be analyzed. Both genotype and location affected grain Cd in soybean.

4. Flooding rice soils causes transformation of the chemical species of zinc and cadmium present. Previous studies reported on the chemical forms of Cd present in contaminated paddy soils at varied redox status. Additional studies examined the effect of soil redox on the forms of Zn, and the extractability of soil Zn in the highly contaminated paddy soil from Mae Sot, Thailand. The soils were alkaline because the farmers had added limestone to reduce yield reduction due to the massive Zn and Cd contamination of the soil. Extended X-Ray Absorption Fine Structure Spectrometry was conducted to characterize the forms of zinc present and changes during flooding and drainage. Flooding this soil caused no apparent change in the forms of Zn present, nor did draining the flooded soil to allow it to become aerobic. Interestingly, the two major forms of Zn present, the zinc-aluminum layered double hydroxide (hydrotalcite like),and Zn phyllosilicate are capable of holding Zn in forms which have little phytoavailability. The previous work showed that Cd and Zn were in different positions on solid surfaces in the flooded soil helping to explain how Zn and Cd in rice soils have such different phytoavailability.

5. Testing use of zinc fertilizer and ground rubber to reduce cadmium in durum wheat in Arizona. Durum wheat grain produced on Arizona soils rich in chloride may contain cadmium at levels which might affect marketing in the European Union. High soil chloride has been shown to interact with low soil zinc (Zn) availability to promote cadmium accumulation, such that application of zinc fertilizers may reduce grain cadmium to protect markets. A field test was designed in cooperation with scientists at Arizona State University to evaluate the effect of zinc sulfate and ground rubber application on cadmium in leaves and grain of two durum wheat cultivars with different cadmium accumulation potential. The plots were established in advance of planting to allow reactions of the amendments in the soils. All soils, plant tissues and irrigation waters were analyzed for trace elements and nutritional status. Wheat yield was not affected in these soils which were thus not zinc-deficient. The analyses showed that there was no reduction in grain Cd in response to the added Zn, in contrast with some soils which had true Zn deficiency without the high chloride in irrigation water. Other treatments such as foliar Zn spray during grain filling will be examined.


Review Publications
Khaokaew, S., Chaney, R.L., Landrot, G., Pandya, K., Sparks, D.L. 2012. Speciation and release kinetics of zinc in contaminated paddy soils. Environmental Science and Technology. 46:3957-3963.

Chaney, R.L. 2012. Food Safety Issues: Mineral fertilizers and soil amendments. In: Sparks, D.L., editor. Advances in Agronomy. New York, NY: Elsevier Press. p. 41-106.

Siebielec, G., Chaney, R.L. 2012. Testing amendments for remediation of military range contaminated soil. Journal of Environmental Management. 108:8-13.

Last Modified: 8/29/2014
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