Key words: cadmium, chloride, sunflower kernel. Abstract. Understanding soil factors related to cadmium (Cd) uptake and accumulation in plants is important for ...
Plant and Soi1167: 275-280, 1994. © 1994 Kluwer Academic Publishers. Printed in the Netherlands.
Effect of soil chloride level on cadmium concentration in sunflower kernels Y i n - M i n g Li 1, R u f u s L. C h a n c y 1 a n d A l b e r t A. S c h n e i t e r 2 I USDA-ARS, Environmental Chemical Laboratory, Beltsville, MD 20705, USA and 2Crop and Weed Sciences Department, North Dakota State University, Fargo, ND 58105, USA Received 25 May 1994. Accepted in revised form 25 August 1994
Key words: cadmium, chloride, sunflower kernel
Abstract
Understanding soil factors related to cadmium (Cd) uptake and accumulation in plants is important for development of agronomic technologies, and breeding strategy to produce low Cd crops. The objective of the study was to examine the effect of soluble salts (chloride and sulfate) and other soil factors on the Cd concentration in sunflower (Helianthus annuus L.) kernels. Commercial nonoilseed hybrid kernels and soils were sampled from 22 farmer's production fields in North Dakota and Minnesota. The sites sampled included saline and nonsaline variants from 7 soil series. Soils were sampled at four depths. Relationships between kernel Cd level and soil physical and chemical characteristics were examined. The soil pH covered a narrow range (7.3-8.1) at these sampled sites. Regression analysis showed that there was no correlation between kernel Cd and soil pH at any depth. The kernel Cd level was highly correlated with DTPA-extractable Cd in all 4 depths, and with clay content in sub-soils. Soil chloride and sulfate concentrations varied among soil series and within soil series. The absence of a statistically significant effect of soil sulfate level on kernel Cd concentration, indicated that soil sulfate levels did not affect Cd uptake by sunflower plants. However, soil chloride levels in sub-soil were correlated with kernel Cd. The most important soil factor was DTPA-extractable Cd. When chloride was included in the multiple regression equations, R square (R 2) values improved significantly. These results demonstrate that soil chloride concentration is another important factor related to Cd uptake in sunflower plants. Introduction
An important strategy to limiting cadmium uptake and accumulation in crops has been to seek gene(s), and to breed low Cd cultivars (Hinesly et al., 1982; Maiti et al., 1989; Li et al., 1994; Thomas and Harrison, 1991; Wagner and Yeargan, 1986), but this may take 5-10 years to be successful. In order to overcome the limitation of time and high cost in the genetic approach, utilization of agronomic technologies to produce low Cd crops has become more attractive (Chaney et al., 1993). Understanding soil factors related to Cd uptake in plants is important for development of agronomic technologies, and may be also important to breeding low Cd genotypes. Studies of the relationship between soil characteristics and plant Cd uptake have been carried out mainly in greenhouse experiments with Cd added as inorganic
salts, and with sewage sludge or contaminated soils (Alloway et al., 1990; Davis, 1984; Eriksson, 1989; Xue and Harrison, 1991). Knowledge of Cd uptake and accumulation in some major Cd-accumulating crops, such as sunflower, flax (Linum usitatissimum L.), durum wheat (Triticum durum L.), peanut (Arachis hypogaea L.), soybean (Glycine max L. Merr.) under varying soil conditions, is still limited. In general, increase in soil Cd content results in an increase in the uptake of Cd by plants (IPCS, 1992). The phyto-availability of Cd in soil depends upon plant species, soil pH, and a variety of other soil factors (e.g. clay, organic matter, chloride, sulfate). Soil pH has a profound effect on Cd solubility. Increasing pH clearly reduced Cd uptake in leaf lettuce (Lactuca sativa L.) (Chaney et al., 1982; Xue and Harrison, 1991), in rapeseed (Brassica napus L.) (Eriksson, 1989) and in many other crops. In recent work, we have shown that
276 heavy textured soils at North Dakota and Minnesota contained higher levels of total soil Cd and DTPAextractable Cd compared to light textured soils, caused significantly higher sunflower kernel Cd, but that soil pH had a smaller effect on kernel Cd levels (Li et al., 1994). Bingham et al. (1983, 1984, 1986) studied the potential of chloride and sulfate salinity to affect uptake of Cd by Swiss chard (Beta vulgaris 'Cicla'), a Cd accumulating leafy vegetable. They found that increased chloride levels caused a marked increase in soluble Cd, as well as in leaf Cd level, while sulfate had little effect. Recently, McLaughlin et al. (1994) reported substantially increased Cd accumulation in potato (Solanum tuberosum L.) tubers when high chloride irrigation waters were used in certain regions of South Australia. In sunflower Cd research, we found that the high salinity in Bearden silt loam soil (saline variant) in Grand Forks County, North Dakota apparently caused a large increase in kernel Cd concentration in a commercial nonoilseed sunflower hybrid (Sigco 954). Thus, besides soil Cd and pH, soil salinity could be another important soil factor in uptake of Cd by sunflower plants (Chaney et al., 1993). There have been few studies of the effect of soluble salts on Cd uptake by plants. Chloride and sulfate ions occur in all natural soils, and can reach high level in dryland soil. Chloride may be regarded as a selective ligand for Cd and other heavy metals. It is known that Cd 2+ can readily form stable complexes with chloride. Chlorides are highly mobile, and are persistent in dryland soils and therefore, under certain conditions, could be an important factor in the distribution of soil Cd (Hahne and Kroontje, 1973; Garcia-Miragaya and Page, 1976). The purpose of this study was to examine the effect of soluble salts (chloride and sulfate) and other soil factors on the Cd concentration in sunflower kernels.
Materials and methods Commercial nonoilseed hybrid (Sigco 954) kernels and soils were sampled from 22 production fields in North Dakota and Minnesota. The sites sampled included saline and nonsaline variants from 7 soil series, and displayed a wide range of physical and chemical characteristics (Table 1). Kernels and soils were sampled from the same location at each site. Soils were sampled at four depths (0-20, 20-30, 30---40, and 40-50 cm) because of the possibility that
soil factors could vary with depth, and interact with drought effects on rooting depth to cause variability in kernel Cd. Soil samples were collected using a stainless steel bucket auger, and placed in polyethylene bags to prevent metal contamination. Those samples collected at the depth of 30-40 cm and 40-50 cm are subsequently referred to as sub-soils. After air drying, soil samples were crushed with a stainless steel rolling pin, and sieved using a 2-mm stainless steel sieve. After thorough mixing of the sieved soil, aliquots were weighed out for different analyses. DTPA-extraction was conducted according to Lindsay and Norvell (1978), while pH was measured at a 1:1 (by volume) soil:water ratio, and total Cd was measured by aqua regia digestion as described by Li et al. (1994). Soil clay content was analyzed following the protocol of Gee and Bauder (1979). Soil chloride (C1-) and sulfate (SO ] - ) were extracted with a solution of 3 mM NaHCO3 and 1.8 mM Na2CO3 at a ratio of 5 g soil/25 mL extractant, shaking for 1 hour. Chloride and sulfate concentrations were determined following the protocols of Dick and Tabatabai (1979), and Tabatabai and Dick (1983), using an Ion Chromatograph. Reagent blanks, sample blanks and reference standards were analyzed with each batch of samples. Whole sunflower heads were harvested and threshed at maturity. Seeds were dried in a forced-draft oven, and threshed. The samples were hand picked to remove all trash and insect damaged seed. Seeds were dehulled using a mechanical dehuller made of stainless steel, aluminum and plastic materials which allowed no Cd contamination of kernel samples. Kernels (4 gram per sample) were dry ashed at 480 ° C overnight; the ash was wetted with 2 mL of concentrated HNO3, and heated to incipient dryness; 10 mL 3N HCI was added, the beakers heated to reflux for 2 hr, and then the solution was filtered into 25 mL volummetric flasks. Cd concentration was determined by flame atomic absorption spectrophotometry with deuterium background correction; an expanded scale was used. There were 10% duplicated samples in each batch. If duplicates disagreed by >_5%, samples were re-analyzed. If 2 replicates at a location differed by more than 0.25 fold, both were re-analyzed. Appropriate ground sunflower kernel analytical reference standards and sample blanks were analyzed with each batch of samples. As a further quality assurance measure, about 15 % of all samples were randomly selected for reanalysis, with good agreement with the original duplicate analyses. The relationship between kernel Cd level and soil factors was determined using linear regression pro-
277 Table 1. Description of the soil series of 22 high and low salinity sites from North Dakota and Minnesota farmer's fields where Hybrid 954 nonoilseed sunflower was sampled to evaluate kernel Cd concentration
Soil series
Number of sites
Description
Salinity
Bearden
4
Embden
2
High C1 and high SO4 Low C1 and low SO4 High SOl
Fargo
4
Hamerly
2
Ulen
4
Vallers
2
Wyndmere
4
Medium textured, Aeric Calciaquolls Coarse textured, Pachic Udic Haploborolls Fine textured, poorly drained, Typic Endoaquerts Medium textured, Aeric Calciaquolls Coarse textured, Aeric Calciaquolls Medium textured, poorly drained, Typic Calciaquolls Coarse textured, Aeric Calciaquolls
cedures through SAS to ascertain whether the degree of Cd uptake and accumulation in sunflower could be associated with soil factors at a specific soil depth.
Results and discussion There were large variations in kernel Cd concentration of hybrid 954 among 22 sampled sites for these four soil series. Kernel Cd concentrations ranged from 0.203 to 1.68 #g Cd g-1 DW, suggesting the importance of soil factors affecting the Cd level in sunflower kernels. Relationships between the kernel Cd concentration and soil physical and chemical characteristics examined were investigated by linear regression analyses (Table 2). Based on previous sunflower Cd studies (Chaney et al., 1993; Li et al., 1994) and other Cd research (Chaney and Hornick, 1977; Eriksson, 1989; Xue and Harrison, 1991), soil pH is usually an important factor controlling Cd availability in soils. Increasing soil pH was expected to reduce kernel Cd. However, the soil pH at these sampled sites covered a narrow range (7.3-8.1). Regression analysis showed that there was no correlation between kernel Cd and soil pH at each depth (Table 2). The kernel Cd concentration was highly correlated with DTPA-extractable Cd (p
