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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

2011 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
Progress was made in each of the four objectives. Under Objective 1, a feeding test to evaluate the effect of zinc (Zn) in lettuce on the bioavailability of cadmium (Cd) in lettuce was designed and lettuce was 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-environmental interaction could be assessed. Under Objective 3, the localization of lead (Pb) in carrot xylem was identified as the mechanism whereby peeled carrots accumulate significant amounts of Pb 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 Pb from a field test using phosphate to remediate soil Pb in Joplin, MO. Compared to earlier tests for soil Pb bioaccessibility, the new method is better related to the bioavailability of soil Pb to humans, less expensive to conduct, and reflects the influence of soil amendments which reduce soil Pb bioavailability based on feeding tests. Under Objective 4, continuing evaluation of the literature and new evidence about dietary Cd 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 Cd 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 Cd intake in Asia provide clear evidence that internationally accepted Cd intake limits remain fully protective of humans.


4.Accomplishments
1. Effect of zinc and iron in Romaine lettuce on bioavailability of lettuce cadmium. Previous research showed that marginal zinc and iron in diets of rats strongly promoted cadmium absorption by rats compared to adequate levels. These earlier studies were conducted with sunflower kernels and rice grain, crops for which soil zinc supply has little effect on crop zinc levels, while soil iron supply has little effect on iron concentration in any crop. Lettuce represents the class of vegetable foods which can accumulate high levels of cadmium and zinc depending on soil supply, so an experiment was designed to grow Romaine lettuce to contain minimally adequate zinc and near excessive zinc, with fixed cadmium at the Codex Alimentarium (World Health Organization) limit for vegetable crops. Because the animal scientist cooperator in previous bioavailability testing is no longer available to conduct feeding tests, new collaboration was established with an experienced animal nutritionist at Beltsville as part of the new project. Lettuce has been grown to contain the goal levels of cadmium and zinc and the feeding test is scheduled to occur during FY12.

2. Effect of zinc fertilization of high cadmium soils on cadmium accumulation in Romaine lettuce in the field. Previous study showed that certain shale derived soils in Salinas Valley, CA, are naturally mineralized with cadmium but little zinc such that lettuce, spinach and some other crops grown on these soils contain levels of cadmium which do not comply with Codex Alimentarius limits. Further, greenhouse testing showed that application of a combination of limestone and zinc fertilizer could allow production of normal yields of Romaine lettuce and spinach with Codex compliant levels of cadmium. During FY11, a collaboration was initiated with a lettuce breeder at the USDA-ARS Lettuce research laboratory in Salinas, CA. During the FY, soils across the experiment station were evaluated and found to contain only normal levels of cadmium. Field tests of Romaine lettuce cultivars were sampled to test for genetic variation in lettuce cadmium to support selection of cultivars for the planned field test of zinc fertilizer and limestone to reduce cadmium uptake. Present results indicate that farmer’s fields in southwestern Salinas Valley will have to be sought for cooperating field tests.

3. Zinc from ground rubber tires can increase wheat yield and reduce wheat grain cadmium. Waste tires are a significant environmental problem; some tires are used to produce energy and others to produce ground rubber to mix with soil to reduce physical compaction in high traffic areas. Because tire rubber contains about 1.5% of highly purified zinc (very low in cadmium and lead) from use in the vulcanization of rubber, we tested use of ground rubber as a zinc fertilizer which could be used to inexpensively reduce cadmium accumulation by crops. One study tested the kinetics of release of zinc from ground rubber compared to standard zinc fertilizers; rubber served as a slow release source of zinc which kept plant zinc higher than did zinc sulfate. Zinc salts added to soils can be rapidly transformed to unavailable zinc minerals, while the slow release of the inexpensive recycled ground rubber as zinc fertilizer can correct zinc deficiency for many years. A field test using ash from burning ground rubber showed effective zinc fertilizer response in zinc-deficient bread wheat which caused a significant reduction in cadmium in wheat grain.

4. 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 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 potentials. The plots were established in advance of planting to allow reactions of the amendments in the soils. All soils, plant tissues and irrigation waters are being analyzed for trace elements and nutritional status. Wheat yield was not affected in these soils which were not zinc-deficient. The 2011 crop was harvested and is undergoing analysis.

5. Cultivar variation of cadmium (Cd) 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 protecting 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 have been grown during FY11 and will be analyzed in due course.

6. Flooding rice soils causes the transformation of the chemical species of zinc (Zn) and cadmium (Cd). Ingestion of home-grown rice grain with high levels of cadmium due to mine waste or smelter contamination has caused disease (renal tubular dysfunction) in several Asian countries. Cadmium concentration in rice plants remains low as long as the soil is flooded. But upon drainage, transformation of soil chemical species causes cadmium uptake to greatly increase from contaminated soils. Studies were undertaken to characterize the mineral species of cadmium and zinc present in highly contaminated Thai rice soil subjected to aerobic or flooded incubation. Extended X-Ray Absorption Fine Structure Spectrometry was conducted to characterize the forms of zinc and cadmium present, and micro-X-Ray Fluorescence was used to examine whether cadmium was co-localized with zinc as has been reported for contaminated aerobic soils. Because the soil was highly contaminated with zinc and cadmium, it had been limed to alkaline phosphorus (pH). Flooding this soil caused partial formation of cadmium sulfide (a highly insoluble and non-phytoavailable form of Cd) along with cadmium carbonate in the soil. But draining the anaerobic soil caused the cadmium sulfide to be transformed again to free cadmium bound to soil particles. One of the most significant findings was that rather than cadmium being co-localized with zinc in soil binding sites, cadmium and zinc were bound to different solid phases in the soil. Taken together, these findings help explain how rice can rapidly absorb soil cadmium when contaminated soils are drained to maximize rice yield, while grain zinc is not increased which promotes absorption of grain cadmium by consumers.

7. A simple and inexpensive test for bioaccessible garden soil lead (Pb). Many urban gardens contain high levels of lead from historic paint and automotive exhaust emissions and 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. 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 calibration using soil samples from the Joplin field test, was used to measure the reduction in soil lead bioavailability. The phosphate treated soil had 69% reduction in soil lead bioavailability to humans fed the test soils. Using this test calibrated on the Joplin soils, it was shown that most garden and orchard soils had only 5-10% absolute soil lead bioaccessibility rather than the 30% assumed by the U.S.-Environmental Protection Agency in developing of guidance for lead contaminated soils. A house-side non-garden soil had much higher fractional bioaccessibility than did garden soils. The test uses 0.4 molar glycine at pH 2.5, with 50 mL of extractant per 5 g of soil (screened < 2 mm rather than <0.25 mm), conducted at 20C for 2 hr. Common garden soil amendments which might reduce soil lead bioaccessibility are being evaluated.

8. Working with U.S. producers to protect sales of grains with normal levels of cadmium (Cd). During the last year, a European Union panel recommended lowering of the Potentially Tolerable Weekly Intake of cadmium, 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 Cd 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). In reviewing available information on human dietary cadmium risks, the most reliable data came from a study of Japanese farm families who ingested rice, drinking water and other foods grown on soils contaminated by mining wastes. In contrast with crops grown on aerobic soils, rice is commonly grown on flooded soils until flowering, and fields drained to improve yields and ease of harvest. But rice grown on mine waste contaminated soils can reach 7 mg cadmium per kg compared to the Codex Alimentarius limit of 0.4 mg cadmium per kg rice. 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. Rice is the principle human food with the ability to promote cadmium movement to edible grain and increased bioavailability due to low levels of iron, zinc and calcium. 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.


Review Publications
Gu, H., Qui, H., Tian, T., Zhan, S., Deng, T., Chaney, R.L., Wang, S., Tang, Y., Morel, J., Qiu, R. 2011. Mitigation effects of silicon rich amendments on heavy metal accumulation in rice (Oryza sativa L.) planted on multi-metal contaminated acidic soil. Chemosphere. 83:1234-1240.

Khaokaew, S., Chaney, R.L., Landrot, G., Ginder-Voget, M., Sparks, D.L. 2011. Speciation and release kinetics of cadmium in an alkaline paddy soil under various flooding periods and draining conditions. Journal of Environmental Science and Technology. 45:4249-4255.

Last Modified: 9/10/2014
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