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a U.S. Plant, Soil & Nutrition Lab., USDA-ARS, Tower Rd., Ithaca, NY 14853 USA
b Dep. Crop and Soil Sciences, Cornell Univ., Ithaca, NY 14853 USA
c Dep. Soil Science, ND State Univ., Fargo, ND 58105 USA
wan1{at}cornell.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMaterials and methodsNOTESResultsDiscussionREFERENCES |
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Abbreviations: Cddtpa, Nadtpa, and Zndtpa, DTPA-extractable Cd, Na, and Zn • Cdg, concentration of Cd in grain • CEC, cation-exchange capacity • Clw and SO4w, Cl- and SO2-4 extracted by water • CV, coefficient of variation • DTPA, diethylenetriaminepentaacetic acid • IC, inorganic C • OC, organic C • SO4–S, sulfate expressed as S • TC, total C
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMaterials and methodsNOTESResultsDiscussionREFERENCES |
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Crop plants differ in their tendency to accumulate Cd (Wolnik et al., 1983; Wagner, 1993; Chaney et al., 1996), but many crops accumulate Cd if it is available in the soil. Among crops that can accumulate Cd to levels that cause concern are leafy vegetables such as lettuce (Lactuca sativa L.) and swiss chard (Beta vulgaris L.), tuber crops such as potato (Solanum tuberosum L.), and seed or grain crops such as sunflower (Helianthus annuus L.), rice (Oryza sativa L.), and wheat (Triticum spp.). For reasons that are not well understood, many durum wheat cultivars accumulate two to three times as much Cd in grain as do most bread wheats (T. aestivum L.) (Chaney et al., 1996; Li et al., 1997).
Regulatory limits for Cd in plant foods have been discussed at Codex Alimentarius Commission meetings for several years. No final limits have been adopted for seed and grain, but the most recent Codex Commission recommended that 0.1 mg kg-1 serve as a guideline level for Cd in cereal grains, and proposed that 0.2 mg kg-1 be the maximum level (Codex Alimentarius Commission, 1999). While the need for such limits can be debated, it is clear that their adoption would restrict the acceptability and marketability of some wheat grain, especially grain from durum wheat grown in soils that are naturally rich in trace elements. Such limits would pose an agronomic challenge and an economic concern to several nations that produce and export durum wheat, including the USA and Canada.
Soil is the principal source of Cd accumulated by plants. The availability of Cd in soil is related to the soil characteristics that affect the availability of most trace metals. These characteristics include the concentration and form of metal in the soil, pH, organic matter content, clay content, interactions with other elements, and fertilizer practices. In addition, there appears to be an enhancement of Cd uptake in some crops by elevated salinity or chloride (Bingham et al., 1984; Li et al., 1994; McLaughlin et al., 1994, 1997; Smolders and McLaughlin, 1996; Weggler–Beaton et al., 2000). The results of these studies suggest that the enhancement of Cd uptake in the presence of salinity or Cl- may be a general phenomenon that occurs in many crops, including the major cereal crops.
This study was undertaken to discover if the Cd concentration in grain of a commonly grown commercial cultivar of durum wheat was related to soil characteristics at a location with known variability in salinity and expected variation in Cl-. The work was conducted in northern North Dakota, where most of the U.S.-grown durum wheat is produced. Many of the soils in this region are relatively rich in trace elements, having developed on glacial tills containing major amounts of marine shale (Moran et al., 1976).
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Materials and methods |
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TOPABSTRACTINTRODUCTIONMaterials and methodsNOTESResultsDiscussionREFERENCES |
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Three soils are found within the sampled area (M.D. Sweeney, unpublished data, 1989. Detailed soil map of Langdon Research Center (1:3960). Dep. Soil Science, NDSU, Fargo). These are the Miranda (fine, smectitic, frigid Leptic Natrustolls); Hamerly (fine-loamy, mixed, superactive, frigid Aeric Calciaquolls); and Svea (fine-loamy, mixed, superactive, frigid Pachic Hapludolls). These soils were developed on glacial till (Simmons and Moos, 1990). The Miranda is a somewhat poorly drained, very slowly permeable, alkali soil on typically flat sites. The Hamerly is a somewhat poorly drained, highly calcareous soil located on sites with a slope of 
1%. These two soils occupy the majority of the sampled area, but their distribution is complex as a result of microtopographic variation. Groundwater depth is shallow, and in the spring the Miranda and Hamerly soils are saturated to the surface. The Svea is present only at the eastern end of the sampled area. This soil is found on well-drained or moderately well-drained sites with a slope of about 3% or more.
Sampling was conducted on 14 Aug. 1997, when the durum wheat was close to maturity. Paired samples of durum grain and soil were collected at 124 sites distributed throughout the area. Each sample pair consisted of about seven heads of wheat and 1 kg of soil taken from the 0 to 15-cm depth beneath, or immediately adjacent to, the sampled wheat plants. Sample sites were distributed at intervals from about 1 to 10 m along five east–west traverses across the area.
Grain samples were physiologically mature at harvest, but some heads were not fully ripened, so all were oven dried at about 50°C to prevent spoilage. Dry heads were threshed by hand. Subsamples of 1 g whole grain were digested in Pyrex tubes (Corning, Corning, NY) in concentrated HNO3/HClO4 acids and then dissolved in 10 mL of 0.75 M HNO3 for analysis as described below.1
Soil samples were air dried, crushed, mixed, and passed through a 2-mm stainless steel sieve. Air-dried soil was used for all analyses, and results were expressed on an air-dry soil basis. (Soil moisture, measured on a group of representative samples, ranged only from about 25 to 35 g kg-1.) The soil pH in water, pHw(1:1), was measured by glass electrode in settled suspensions, after 30 min of intermittent stirring. Soil cation-exchange capacity (CEC) was determined by the sodium acetate (pH 8.2) method of Chapman (1965). Chelate-extractable elements were measured using the diethylenetriaminepentaacetic acid (DTPA) method of Lindsay and Norvell (1978), except that a 1:3 soil/solution ratio (g mL-1) was used to increase recovery of filtrate and decrease filtration time. After shaking for 2 h at 200 cycles per min on a horizontally-reciprocating shaker, the soil suspension was filtered through Whatman no. 42 cellulosic filter paper (Whatman, Clifton, NJ). The clear filtrate was collected and diluted 1:4 with 0.75 M HNO3 for analysis as described below. (Soils were analyzed also by digestion with HNO3/HClO4 to release elements not bound in acid-resistant silicate minerals, but these results will not be discussed here because their relationship to grain composition was poorer than for DTPA-extractable or water-soluble elements).
Chloride (Clw) and sulfate (SO4w) were extracted with water (0.25 g soil mL-1) by shaking at 200 cycles per min for one h. Suspensions were filtered through Whatman no. 42 cellulosic filter papers (Whatman) that had previously been rinsed with high-purity (18 M
) water to remove soluble contaminants, principally Cl-, and then dried before use. Clear filtrates were refrigerated and then analyzed by ion chromatography using an AS-11 anion column (DIONEX Ion Chromatograph, model DX300, Dionex Corp., Sunnyvale, CA) with isocratic elution by dilute carbonate/bicarbonate.
Acid digests of grain or soil samples and acidified DTPA-extracts were analyzed for Cd, Zn, Na, and other elements by inductively coupled, argon-plasma-emission spectrometry on a Thermo Jarrell-Ash model "Trace Analyzer" (TJA Solutions, Franklin, MA) with axial plasma. Generalized shifts in background emission were determined at off-peak wave length positions, and deducted from simultaneous measurements of intensities at on-peak wavelengths for each element. Sample and standard solution matrices were matched. Replicate samples were included routinely, and standard samples of soil and grain were analyzed occasionally. Recovery of method-of-addition increases in concentrations were between 95 and 105%, except for Na in DTPA extracts that gave recoveries in the range of 90 to 110%. Analyses on five occasions of National Institute for Standards and Testing wheat sample 1567a yielded Cd concentrations of 0.025 (±0.001) mg kg-1 in comparison to the certified value of 0.026 (±0.002) mg kg-1.
Soil total carbon (TC) and inorganic carbon (IC) were determined by the Soil and Water Environmental Chemistry Laboratory, Dep. Soil Science, NDSU. The TC was measured with a SKALAR CA-100 TOC Analyzer (Skalar Analytical B.V., DE Breda, The Netherlands), following standard methods (American Public Health Association, 1995). The IC was determined similarly by evolving CO2 with acid treatment. Organic carbon (OC) was determined as the difference between TC and IC.
Twenty-three samples of durum grain and soil were analyzed in duplicate to determine reproducibility. The variance between duplicates accounted for a negligible proportion of the total sample variance, and the coefficient of variation (CV) among duplicates was <5% for grain composition and DTPA-extractable elements. The percentage of total variance and the CV (%) for total and organic carbon analyses were <8%. Because most samples contained little or no IC, the variability among replicates was not established.
Statistical analyses and modeling were carried out with SAS using UNIVARIATE, CORR, REG, and NLIN procedures (SAS Institute Inc., 1988; Freund and Littell, 1991).
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Results |
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TOPABSTRACTINTRODUCTIONMaterials and methodsNOTESResultsDiscussionREFERENCES |
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Grain Cd was closely correlated with Cddtpa, Zndtpa (DTPA-extractable Zn), and the three characteristics related to soil salinity, Clw, SO4w, and Nadtpa (Table 2) . Grain Cd was significantly, but less closely, correlated to CEC and OC. The correlation with soil pH was relatively low. Correlations among the soil characteristics (Table 2) show that Cddtpa was very closely correlated with Zndtpa. Each of these concentrations was negatively correlated to pHw, and Zndtpa was more closely correlated to pHw than to any other variable. The three soil characteristics related to salinity were themselves closely related, with the correlation between Nadtpa and SO4w being especially close.
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 | (1) |
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The data set used for these regressions included a number of samples that appeared to be outliers in virtually all relationships tested. Excluding a group of 18 samples for which the absolute values of the Studentized residuals (Ott, 1993) were >1.96 increased the R2 values substantially, as would be expected, but changed the regression coefficients relatively little, e.g., for Clw
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Assuming that Cl- enhances Cd accumulation at least in part by forming complexes with soil Cd, it would be reasonable to expect that a multiplicative term involving Cddtpa and Clw might be especially closely related to Cdg. However, regressions including multiplicative terms involving Cddtpa, Clw, or their logarithms provided little improvement over relationships with Clw alone, and none were as successful as the simpler additive model (Eq. [1] or Eq. [2]).
Models based on the flexible growth model were also fitted with multiple nonlinear regression. These relationships were often as successful as those based on the logarithmic model, but were not pursued because they required an additional fitted parameter, convergence on stable parameter values was difficult to achieve, and the fit to the data was similar.
The results presented above include several correlations and regressions with reasonably high R2 values. Although these empirical relationships demonstrate a close association amongst the variables, it is important to recognize that correlation cannot prove causation. It is always possible that some unmeasured factor is responsible for an observed effect, and that this unmeasured factor is correlated simultaneously with measured variables. Nor should the functional form of these empirical models be viewed as especially important, because there is more than enough scatter in the data to accommodate a satisfactory fit by a number of flexible curvilinear models.
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Discussion |
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TOPABSTRACTINTRODUCTIONMaterials and methodsNOTESResultsDiscussionREFERENCES |
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In our study area, the salinity-related characteristics Clw, SO4w, and Nadtpa were well correlated with Cdg in the durum wheat cultivar Munich. Our data do not permit the relationship of Cdg to Clw, SO4w, or Nadtpa to be evaluated independently, because these soil characteristics were themselves so highly correlated. However, predictive relationships for Cdg based on logCl, or logCl and Cddtpa were the most successful, and stepwise regression involving all three salinity-related variables suggests that the correlations between Cdg and SO4w, or Nadtpa, were probably the result of multi-colinearity with Clw (Freund and Littell, 1991). More importantly, our results in combination with other information discussed below, lead us to conclude that elevated concentrations of Cl- in soil were primarily responsible for the increased concentrations of Cd in grain.
Chloride is well known for forming moderately stable complexes with ionic Cd2+, that is CdCl+ and CdCl02 (Smith and Martell, 1976; Lindsay, 1979). Simple stability calculations (e.g., Hahne and Kroontje, 1973; or Lindsay, 1979), indicate that formation of chloro-complexes of Cd should become significant when Cl- concentrations rise above approximately 10 mM, a range easily reached in the soil solution of salt-affected soils. Formation of complexes with Cl- tends to shift Cd from the solid to the solution phase, thereby enhancing solubility and mobility (Doner, 1978; Bingham et al., 1984; McLaughlin et al., 1997; Smolders et al., 1998). In addition to increasing transport to roots, these chloro-complexes of Cd may also be taken up directly by plant roots, but through mechanisms different than those responsible for uptake of unassociated Cd2+ (Smolders et al., 1998). Smolders and McLaughlin (1996) demonstrated that increasing Cl- in solution from 0.01 mM Cl- to 120 mM Cl- linearly increased the Cd concentration in both root and shoot of swiss chard in a solution culture system in which Cd2+ activities were well buffered by exchange resins. They noted that enhancement of Cd transport by Cl- complexation could be important within the root apoplast as well as in soils. Enhanced uptake by swiss chard and bread wheat has been demonstrated also in hydroponics or potted soil culture (e.g., Bingham et al., 1984; Smolders and McLaughlin, 1996; Weggler-Beaton et al., 2000). In the field, McLaughlin et al. (1994, 1997) showed that Cd concentrations in potato tubers were better related to concentrations of Cl- in soil solution than to activities of ionic Cd2+. Also in the field, Li et al. (1994) observed a weak but significant correlation between Cd in sunflower kernels and Cl- concentrations in subsoils. These studies suggest that the enhancement of Cd uptake in the presence of Cl- occurs in many crops, although the mechanisms are not yet fully understood.
Sulfate, too, forms a complex with Cd2+ (Lindsay, 1979), although it is of somewhat lower stability than those formed with Cl-. Evidence from studies of the effect of sulfate on Cd availability to plants suggests that this anion does not enhance Cd uptake or availability. For example, Bingham et al. (1986) found that Cd uptake by swiss chard was not increased by adding Na2SO4 to soil, which contrasted with their results for NaCl. Similarly, Li et al. (1994) reported that Cl- was much more closely related than SO2-4 to Cd accumulation in kernels of soil-grown sunflower. Very recently, McLaughlin et al. (1998) found only a small effect of SO2-4 on increasing Cd uptake by Swiss chard grown in soil, much less than that found for Cl- by Smolders et al. (1998).
Soil cations compete with one another for surface exchange sites, and there is the possibility that higher levels of salts contribute to Cd availability by displacing it from exchange sites. However, monovalent cations such as Na are relatively ineffective at displacing divalent cations such as Cd, even at relatively high concentrations, and Na seems unlikely to play any significant role in increasing Cd uptake (see also the discussion of this topic by Smolders et al., 1998). For these reasons, we are confident that the correlation between Cdg and Nadtpa does not imply a causal relationship, but is instead the indirect result of the correlation between Nadtpa and Clw.
Cadmium uptake by wheat and other crops has been reported to decrease as a result of Zn fertilization (Oliver et al., 1994; Grant et al., 1999). Our data do not suggest a similarly negative relationship between Cdg and the availability of soil Zn as measured by Zndtpa. This, however, is not surprising because the concentrations of Zndtpa were above levels where Zn-deficiency stress in crops would be expected. The absence of any Zn stress in these plants is supported also by analyses of the grain, which found concentrations of Zn to be in an adequate range (mean 26 mg kg-1, range 14-55 mg kg-1). The positive correlation noted between Cdg and Zndtpa (Table 2), was presumably an indirect result of the strong correlation between Cddtpa and Zndtpa.
The curvilinear relationship that we noted between Cdg and Clw in soil is qualitatively similar to that reported by McLaughlin et al. (1994, 1997) for Cd in potato tubers. The overall shape of these relationships is more important than the empirical models and quantitative coefficients used to describe them. These relationships show clearly that the rate of increase in plant Cd was greatest at relatively low Cl- levels. Responses to Cl- decreased substantially at higher concentrations, particularly in our data where Cd in grain appeared to approach a maximum. The specific mechanisms involved in shaping these responses to Cl- are not yet well understood and, especially in the field, are likely to involve several soil and crop characteristics. Nonetheless, it is important to note that the major stimulation of Cd uptake was obtained in a concentration range for soil Cl- that would not, on its own, place a soil in a particularly saline range. Thus, these results suggest that management of Cl- may need to be considered even in some non-saline soils, if Cl- levels are elevated and crop uptake of Cd is a concern.
Our results show a strong association of Cd in durum wheat grain with soluble Cl- in soil from the 0 to 15-cm depth of the root zone of the plant from which the grain was harvested. This close association, combined with research cited for other crops, suggests strongly that soluble Cl- in soils will enhance the accumulation of Cd in the grain of durum wheat, a major cereal crop.SAS Institute 1988
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NOTES |
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TOPABSTRACTINTRODUCTIONMaterials and methodsNOTESResultsDiscussionREFERENCES |
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TOPABSTRACTINTRODUCTIONMaterials and methodsNOTESResultsDiscussionREFERENCES |
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