Pedosphere 27(3): 475–481, 2017 doi:10.1016/S1002-0160(17)60343-6 ISSN 1002-0160/CN 32-1315/P ⃝c 2017 Soil Science Society of China Published by Elsevier B.V. and Science Press

Predicting Cadmium Safety Thresholds in Soils Based on Cadmium Uptake by Chinese Cabbage

LU Junhe1, YANG Xinping1, MENG Xuchao1, WANG Guoqing2, LIN Yusuo2, WANG Yujun3 and ZHAO Fangjie1,∗ 1College of Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing 210095 (China) 2Nanjing Institute of Environmental Science, Ministry of Environmental Protection of the People’s Republic of China, Nanjing 210042 (China) 3Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008 (China) (Received February 26, 2017; revised April 20, 2017)

ABSTRACT Cadmium (Cd), a common toxic heavy metal in soil, has relatively high bioavailability, which seriously threatens agricultural products. In this study, 8 different soils with contrasting soil properties were collected from different regions in China to investigate the Cd transfer coefficient from soil to Chinese cabbage (Brassica chinensis L.) and the threshold levels of Cd in soils for production of Chinese cabbage according to the food safety standard for Cd. Exogenous Cd (0–4 mg kg−1) was added to the soils and equilibrated for 2 weeks before Chinese cabbage was grown under greenhouse conditions. The influence of soil properties on the relationship between soil and cabbage Cd concentrations was investigated. The results showed that Cd concentration in the edible part of Chinese cabbage increased linearly with soil Cd concentration in 5 soils, but showed a curvilinear pattern with a plateau at the highest dose of exogenous Cd in the other 3 soils. The Cd transfer coefficient from soil to plant varied significantly among the different soils and decreased with increasing soil pH from 4.7 to 7.5. However, further increase in soil pH to > 8.0 resulted in a significant decrease in the Cd transfer coefficient. According to the measured Cd transfer coefficient and by reference to the National Food Safety Standards of China, the safety threshold of Cd concentration in soil was predicted to be between 0.12 and 1.7 mg kg−1 for the tested soils. The predicted threshold values were higher than the current soil quality standard for Cd in 5 soils, but lower than the standard in the other 3 soils. Regression analysis showed a significant positive relationship between the predicted soil Cd safety threshold value and soil pH in combination with soil organic matter or clay content. Key Words: clay content, soil pH, soil organic matter, soil quality standard, transfer coefficient

Citation: Lu J H, Yang X P, Meng X C, Wang G Q, Lin Y S, Wang Y J, Zhao F J. 2017. Predicting cadmium safety thresholds in soils based on cadmium uptake by Chinese cabbage. Pedosphere. 27(3): 475–481.

INTRODUCTION been much public concern on the problem of Cd con- tamination in Chinese agricultural land (Zhao et al., Cadmium (Cd) is a common toxic heavy metal in 2015). According to the national survey (MEP MLR, the soil environment. Its relatively high bioavailabi- 2014), 19.4% of the soil samples from agricultural land lity in soil means that it can be absorbed and accu- in China exceed the national soil quality standards mulated by crop plants easily, which seriously threa- for one or more inorganic or organic contaminants. tens the quality and safety of agricultural products The main contaminants are heavy metals or metallo- (Gong and Pan, 2006; Bolan et al., 2013; Zhao et al., ids, accounting for 82% of the total contaminated soils, 2015). Chronic exposure to Cd through various path- among which Cd ranks the first with 7% of soil sam- ways including the dietary Cd intake can cause serious ples exceeding the soil quality standard. Mining is one health hazards (Karalliedde and Brooke, 2012). Itai- of the main factors causing soil Cd contamination. In itai disease, which was prevalent in the Jinzu Ri- southern and southwestern areas of China such as Yun- ver basin of the Toyama Prefecture of Japan during nan, Guangdong, Hunan and Guizhou, which are rich the 1930s and 1940s, was caused by consumption of in metallic mineral resources, soil Cd pollution caused Cd-contaminated rice produced from Cd-contaminated by mining and smelting is particularly serious. A nu- farmland (Nogawa et al., 2004). Recently, there has mber of studies showed that the edible parts of vegeta-

∗Corresponding author. E-mail: [email protected]. 476 J. H. LU et al. bles, such as sweet potato, cabbage, pepper, eggplant, appropriate (Zhao et al., 2015). For example, the natu- cucumber and spinach, grown in mining-impacted soils ral background levels of heavy metals are likely to vary frequently exceeded the Chinese food safety standards according to soil parent materials and pedogenetic pro- for Cd (0.2, 0.1 and 0.05 mg kg−1 fresh weight for the cesses (Zhao et al., 2015). Therefore, it is inappropriate leafy, rootstalk and other vegetables, respectively). Re- to set a single Class I value for the whole country. The cent surveys in some areas along the Xiang River basin Class II for Cd only considers the influence of soil pH in Hunan, China showed that large proportions (60%– on Cd bioavailability, just with two values separated 76%) of rice grain exceeded the 0.2 mg kg−1 Cd limit by pH 7.5. A recent study showed that soil pH in the in rice (Du et al., 2013; Zhu et al., 2016). range from 5.0 to 7.5 had a large influence on the Cd Vegetables are a main source of Cd ingestion by hu- transfer coefficient from soil to rice grain (Zhu et al., mans (Clemens et al., 2013). Because of the differences 2016). Other soil properties that may also influence in genetic characteristics and the edible organs, Cd Cd bioavailability, such as SOM and clay content (Ye accumulation varies substantially among different ve- et al., 2012, 2014; Ding et al., 2013), are not considered getable species (Kuboi et al., 1986; Yang et al., 2010). in the soil environment quality standards. Generally, Cd accumulation in the edible parts of ve- The objective of the present study was to determine getables follows the order of legumes (Leguminosae) < the Cd transfer coefficient from soil to a leafy vegetable melon vegetables (Cucurbitaceae) < alliums (Amary- in different soil types with contrasting soil properties llidaceae and Liliaceae) < root vegetables (Umbellife- collected from different regions of China. Chinese cab- rae and Cruciferae) < kale vegetables (Cruciferae) < bage (Brassica chinensis L.) was chosen because it is solanaceous vegetables (Solanaceae) < leafy vegetables one of the most important vegetables in China and is (Cruciferae, Compositae and Chenopodiaceae) (Kuboi known to be an accumulator of Cd (Kuboi et al., 1986; et al., 1986; Yang et al., 2010). Accumulation of Cd is Yang et al., 2010). The threshold values of soil Cd were not only determined by plant growth and genetic cha- estimated in order to avoid exceeding Cd safety limit racteristics, but also depends on the concentration and in leafy vegetables. The data allowed us to investigate bioavailability of Cd in soil. The bioavailability of Cd the influences of soil properties on Cd transfer from is influenced by soil properties including pH, soil or- soil to vegetable and Cd threshold values in soil. ganic matter (SOM), cation exchange capacity (CEC), soil texture, and other co-existing metals (Bolan et al., MATERIALS AND METHODS 2013; Ding et al., 2013; Liang et al., 2013; Ye et al., Soils and vegetable 2014). These factors interact and are difficult to distin- guish (Ding et al., 2013; Ye et al., 2014). In addition, In this study, 8 soils were collected from diffe- rhizosphere conditions also influence Cd bioavailability rent regions of China: Yingtan of Jiangxi Province, (Bolan et al., 2013). Zhaoqing of Guangdong Province, Changsha of Hunan The current National Soil Environment Quality Province, Dujiangyan of Sichuan Province, Weifang of Standard (GB15618-1995) of China was formally im- Shandong Province, Chongzuo of Guangxi Province, plemented in 1996. The standard specifies 3 classes of Guiyang of Guizhou Province and Baoding of Hebei benchmark values for heavy metals and metalloids in Province. Soil types and physicochemical properties soil. Class I is considered to represent the natural back- are shown in Table I. The vegetable used in the expe- ground, Class II is set up to protect agricultural pro- riment was Chinese cabbage cv. Shanghai Green. duction and human health via the food chain, whilst Pot experiment Class III is for the protection of crops or forests from phytotoxicity. For Cd, the Class I value is 0.2 mg kg−1, For each type of soil, 6 exogenous Cd concentra- whilst the Class II values are 0.3 and 0.6 mg kg−1 for tions were set as follows: 0.00, 0.25, 0.50, 1.00, 2.00 and soils with pH ≤ 7.5 and pH > 7.5, respectively. The 4.00 mg kg−1. This concentration range is considered Class III value is set at 1 mg kg−1 for soils with pH to be environmentally realistic compared to the range > 6.5. The standard values for soil Cd in China are of Cd concentrations reported for Chinese soils (Chi- stricter than the standard or guideline values in the nese Environmental Monitoring Station, 1990). There USA and EU countries (Zhao et al., 2015). were 3 replicates for each concentration. A total of 144 Although the present soil environment quality sta- pots were randomly arranged. Appropriate amounts ndards play an important role in soil environmental of CdCl2 (analytical grade) dissolved in deionized wa- protection and pollution control in China, there are de- ter were added to the soils and mixed thoroughly. Soil bates regarding whether the different class values are moisture content was raised to approximately 60% of PREDICTING CADMIUM SAFETY THRESHOLDS IN SOILS 477

TABLE I Selected propertiesa) of the soils collected from 8 different regions in China

Region Soil type Soil taxonomy pH Max WHC SOM CEC Clay Silt Sand C0,Cd % g kg−1 cmol kg−1 % mg kg−1 Yingtan Red soil Argi-Udic Ferrosol 4.69 56.5 37.9 14.5 13.8 51.3 34.9 0.41 Changsha Paddy soil Stagnic Anthrosol 4.62 65.7 43.6 6.9 20.0 67.0 13.0 0.48 Zhaoqing Latosol Rhodi-Udic Ferralosol 5.33 58.6 40.0 13.4 27.1 53.7 19.2 0.34 Dujiangyan Paddy soil Stagnic Anthrosol 7.03 54.4 31.7 26.1 13.6 61.8 24.6 0.42 Weifang Cinnamon soil Hapli-Ustic Argosol 7.03 44.6 7.9 18.0 10.4 57.7 31.9 0.26 Chongzuo Latosol Rhodi-Udic Ferralosol 7.43 44.9 33.1 16.5 41.3 29.5 29.2 0.97 Guiyang Yellow soil Ali-Perudic Argosol 7.60 45.5 75.9 9.9 48.1 41.4 10.5 1.14 Baoding Alluvial soil Ochri-Aquic Cambosol 8.69 42.0 5.7 10.9 13.1 61.5 25.4 0.27 a) Max WHC = maximum water-holding capacity; SOM = soil organic matter; CEC = cation exchange capacity; C0,Cd = Cd back- ground concentration. water-holding capacity (WHC) and the soils were left Air-dried soils were ground and passed through a to equilibrate for 2 weeks. Basal fertilizers (0.25 g 2-mm mesh sieve. Soil pH was measured at a 1:2.5 −1 −1 −1 kg urea, 0.10 g kg Ca(H2PO4)2 and 0.15 g kg (weight:volume) soil-to-water ratio. Soil organic matter K2SO4) were applied and mixed into the soils before (by K2CrO4-H2SO4 oil-bath heating), cation exchange sowing. Ten seeds of Chinese cabbage were sown to capacity (by 1 mol L−1 ammonium acetate leaching each pot and the seedlings were thinned to 4 per pot af- method at pH 7.0), and the contents of sand, silt and ter emergence. Soil moisture was maintained at around clay (by hydrometer method) were analyzed according 60% of WHC during plant growth with deionized wa- to the routine analytical methods of Lu (2000). ter. The experiment was carried out in a glasshouse, with natural sunlight supplemented with sodium va- Statistical analysis por lamps and temperature being maintained at 20–28 All the data were statistically analyzed using soft- ◦ C. Plant shoots (the edible part) were harvested 41 d ware SPSS 20.0. Data were shown as the means  after seedling emergence. The fresh plant samples were standard deviations. weighed, washed with deionized water and then oven- ◦ dried at 60 C to constant weight. The dry weights of RESULTS AND DISCUSSION plant samples were measured. Total Cd concentrations in the soils after Cd addi- Determination of Cd concentration and other soil pro- tions were determined. There was a linear relationship perties between the measured and expected values, with the Soil samples were air-dried, ground and passed relationship being close to 1:1 (data not shown), sug- through a 100-mesh sieve. Then 0.5 g soil sample was gesting that the doses of Cd added to the soils were weighed into a digestion tube and digested with 5 mL accurate. aqua regia (4:1 HCl:HNO3, high purity) in a heating Cd transfer from soil to Chinese cabbage block (McGrath and Cunliffe, 1985). After cooling, the digest was diluted to 20 mL with 2% nitric acid (high Compared with the control treatment, the addition purity). Blanks and certified reference materials (GB- of 0.25–4.0 mg kg−1 Cd into the 8 soils did not affect W07428) were included in each batch of digestion. the growth of Chinese cabbage. Even with the maxi- Dry plant samples were ground into fine powder in mum dose of Cd, no toxic symptoms were observed in a grinding machine. Then 0.25 g dry vegetable sample Chinese cabbage, indicating that the concentration of was weighed into a digestion tube and digested with 5 added Cd in the present study did not reach the level mL mixed acids (85:15 HNO3:HClO4, high purity) in a of phytotoxicity for Chinese cabbage. heating block (Zhao et al., 1994). The digest was dilu- Fig. 1 shows the relationships between Cd concen- ted to 20 mL with 2% nitric acid (high purity). Blanks tration (based on fresh weight) in the edible part of and certified reference materials (GBW10015) were in- Chinese cabbage and total Cd concentration in diffe- cluded in digestion. The Cd concentrations in soil and rent soils. Among the 8 soils tested, 5 soils showed plant digest solutions were determined using inductive- linear relationships between the Cd concentration in ly coupled plasma-mass spectrometry (NexION 300S, vegetable and the total Cd concentration in soil (sum PerkinElmer, USA) in the kinetic energy mode. of background and exogenous added Cd) over the en- 478 J. H. LU et al. tire range of soil Cd concentrations tested, suggesting soil Cd from regression analysis does not affect the pur- a constant ratio of phytoavailable Cd to the total soil pose of the present study, which was to estimate the Cd. Previous studies with carrot and spinach have also Cd threshold value in soil associated with the Cd safe- shown generally linear relationships between vegetable ty limit in vegetable; the Cd concentration in Chinese Cd and soil Cd concentrations with additions of rela- cabbage at the highest Cd dose was well over the Cd tively small doses of exogenous Cd (Ding et al., 2013; limit in vegetable. In the other 5 soils, linear regression Liang et al., 2013). In contrast, two latosols (Rhodi- was performed using all data points. Through linear Udic Ferralosols) from southern China (Zhaoqing and fitting, a regression equation of vegetable Cd concen- Chongzuo) and a paddy soil (Stagnic Anthrosol) from tration versus soil Cd concentration was obtained for Central South China (Changsha) exhibited a linear re- each of the 8 soils (Table II). The slope of the linear lationship for only the first 5 doses of Cd, followed by a regression equation represented average Cd transfer co- plateau at the highest Cd dose with the addition of 4.0 efficient (k) from soil to Chinese cabbage, reflecting the mg kg−1 Cd (Fig. 1). The reasons for this plateau may bioavailability of soil Cd to Chinese cabbage. A high be because the capacity of plant Cd uptake or root to k value indicates a high bioavailability of soil Cd. As shoot translocation was saturated, or the added high shown in Table II, the k value varied widely among dose of Cd became less phytoavailable owing to che- the different soils from 0.3 in Guiyang yellow soil (Ali- mical reactions in the soils. For these 3 soils, linear re- Perudic Argosol) to 1.63 in Yingtan red soil (Argi-Udic gression was performed with the first 5 concentrations Ferrosol), with more than 5-fold difference in the Cd of soil Cd only. Excluding the data point of the highest transfer coefficient.

Fig. 1 Relationships between Cd concentration (based on fresh weight, FW) in the edible part of Chinese cabbage and total Cd concentration (sum of background and exogenous added Cd) in different soils collected from 8 different regions in China. Vertical bars represent standard deviations of the means (n = 3). For the soils from Changsha, Zhaoqing and Chongzuo, linear regression includes only the first 5 doses of Cd.

TABLE II Fitted linear regression equations of Cd concentration in Chinese cabbage (y) versus Cd concentration in soils (x) collected from 8 different regions in China

Regiona) Soil taxonomy Fitted equation R2b) Predicted soil Cd threshold mg kg−1 Yingtan Argi-Udic Ferrosol y = 1.63x − 0.001 5 0.93 0.12 Changsha Stagnic Anthrosol y = 1.55x − 0.38 0.99 0.37 Zhaoqing Rhodi-Udic Ferralosol y = 1.28x − 0.15 0.99 0.27 Dujiangyan Stagnic Anthrosol y = 0.83x − 0.18 0.95 0.46 Weifang Hapli-Ustic Argosol y = 0.94x − 0.13 0.96 0.35 Chongzuo Rhodi-Udic Ferralosol y = 0.68x − 0.62 0.98 1.20 Guiyang Ali-Perudic Argosol y = 0.30x − 0.31 0.99 1.70 Baoding Ochri-Aquic Cambosol y = 0.82x − 0.13 0.99 0.40 a)For the soils from Changsha, Zhaoqing and Chongzuo, only the first 5 Cd doses were used for the linear regression. b)Coefficient of determination. PREDICTING CADMIUM SAFETY THRESHOLDS IN SOILS 479

Soil properties influencing Cd transfer clay and SOM, both of which are important factors influencing Cd availability (Chen et al., 2013; Ding et Multiple regression was performed to identify soil al., 2013). Indeed, in addition to soil pH, SOM and clay properties that significantly influence the k value. Soil content were also found to be significantly and nega- pH in combination with either SOM or clay content tively correlated with k, indicating that the bioavai- was found to be significant factors, as shown in the lability of soil Cd decreased with increasing SOM or following regression equations: clay content. k = 3.30 − 0.30·pH − 0.009 4·SOM In the control treatment, Chinese cabbage grown in the soils without additions of exogenous Cd also (R2 = 0.92, P < 0.001) (1) contained certain amounts of Cd, which was derived k = 2.87 − 0.24·pH − 0.013·Clay from the background Cd of the tested soils. The Cd transfer factors (the ratio of Cd concentration in the (R2 = 0.88, P < 0.01) (2) vegetable to that in the soil) were calculated for the It was clear from the regression equations that the control and + Cd treatments. The Cd transfer factor k value decreased with increasing soil pH, SOM and was 1.6–4.0 times higher in the + Cd treatments than clay content. As shown in Fig. 2, the k value decreased in the control (Fig. 3), indicating that the soil native more or less linearly with increasing pH from 4.5 to Cd had a lower phytoavailability compared to exoge- 7.5, indicating a strong effect on Cd phytoavailability, nous Cd. Despite this difference, the Cd transfer factor which has also been reported elsewhere (Adams et al., followed a similar trend with soil pH in both the con- 2004; Ye et al., 2012, 2014; Ding et al., 2013). Howe- trol and + Cd treatments (Fig. 3), suggesting that soil ver, the k value increased again when soil pH increased properties affect the phytoavailability of soil native Cd above 7.5 (Fig. 2). There are two possible explanations and exogenous Cd similarly. for this phenomenon. First, alkaline conditions could facilitate the dissolution of SOM, especially the humic acid component, and the complexation of Cd with the dissolved organic matter, resulting in a transfer of Cd from the solid phase to the soluble phase. This increase in soluble Cd may in turn increase Cd phytoavailabili- ty. Tyler and Olsson (2001) showed that the solubility of soil Cd presented a U-shaped pattern of decreasing initially and then increasing with the increase of soil pH, which is consistent with the observations of the present study. Second, the high pH alluvial soil (Ochri- Aquic Cambosol) from Baoding, which had a relatively high k value, also contained relatively low contents of

Fig. 3 Cd transfer factor from soil to Chinese cabbage in the control (without exogenous Cd) and the treatments receiving e- xogenous Cd (+ Cd). The soils collected from 8 different regions in China are arranged with increasing pH on the x-axis. Vertical bars represent standard deviations of the means (n = 3).

In the control treatment, the Cd concentration in Chinese cabbage did not correlate significantly (r = −0.26, P > 0.05) with soil background Cd concentra- tion in the 8 soils tested (data not shown). In contrast, vegetable Cd concentration correlated highly and ne- gatively (r = −0.90, P < 0.01) with soil pH. Chinese cabbage grown in the 2 soils with pH < 5.0 contained Fig. 2 Relationship between Cd transfer coefficient from soil to Cd concentration exceeding the Chinese food safety − Chinese cabbage and soil pH. standard (0.2 mg kg 1 FW), whereas in the other 6 480 J. H. LU et al. soils (pH > 5.0) vegetable Cd concentrations were be- Soil Cd threshold = 0.26·pH + 0.020·SOM − 1.82 low the limit even though Cd concentrations in the 4 (R2 = 0.81, P < 0.01) (3) soils exceeded the Class II of soil Cd standard. These results indicate that soil pH is a critical factor in de- Soil Cd threshold = 0.12·pH + 0.032·Clay − 0.94 termining whether vegetable Cd concentration would (R2 = 0.88, P < 0.01) (4) exceed the food safety. In the control treatment, com- bining soil pH with soil background Cd concentration, The above regression equations indicate that as for CEC, SOM or clay content did not improve the rela- Chinese cabbage planting, soil Cd safety threshold in- tionship with vegetable Cd concentration in multiple creases with increasing soil pH and SOM or clay con- regression analysis. tent. This finding is consistent with previous reports on rice, root vegetables and spinach (Zeng et al., 2011; Prediction of soil Cd safety thresholds Ding et al., 2013; Liang et al., 2013; Ye et al., 2014). Soil pH, SOM and clay content are known to influence The National Food Safety Standard (GB2762-20- − Cd adsorption in the solid phase and Cd speciation in 12) specifies a Cd limit of 0.2 mg kg 1 FW in leafy ve- the solution phase, thus impacting on Cd phytoavai- getables. This value is within the range of Cd con- lability (Ding et al., 2013). centrations determined by the linear models with the There are some limitations in estimating soil Cd results of the present study. Therefore, the regression threshold values in the present study. First, the exoge- equations in Table II could be used to estimate the nous Cd added to the test soils has a higher phytoavai- Cd threshold values in different soils in order to com- lability than the soil native Cd (Fig. 3). A aging period ply with the food safety standard. The predicted Cd longer than 2 weeks, which was used in our experi- threshold values for the 8 soils were in the range of − ment, may decrease the phytoavailability of exogenous 0.12–1.7 mg kg 1 (Table II), which decreased in the Cd. However, a study with 23 soils showed a very limi- following order of yellow soil (Ali-Perudic Argosol) ted aging effect on the added Cd after incubation for from Guiyang > latosol (Rhodi-Udic Ferralosol) from more than two years in soils with pH < 6.5 (Tye et Chongzuo > cinnamon soil (Hapli-Ustic Argosol) from al., 2003). A significant aging effect was observed on- Weifang > paddy soil (Stagnic Anthrosol) from Du- ly in soils with pH > 6.5 (Tye et al., 2003). Second, jiangyan > alluvial soil (Ochri-Aquic Cambosol) from Cd phytoavailability may also be higher in a pot ex- Baoding > paddy soil (Stagnic Anthrosol) from Chang- periment due to small rooting volume. Third, Chinese sha > latosol (Rhodi-Udic Ferralosol) from Zhaoqing > cabbage is known to have a relatively strong ability to red soil (Argi-Udic Ferrosol) from Yingtan. In 5 of the accumulate Cd. These factors could lead to an overes- 8 soils tested (yellow soil from Guiyang, latosol from timation of the risk of soil Cd to food safety. However, Chongzuo, cinnamon soil from Weifang and paddy soils it requires some safety margin to set soil environment from Dujiangyan and Changsha), the Cd safety thre- quality standards for considering diverse conditions en- sholds predicted in the present study were higher than compassing the worst case scenario. The experimental the Class II of the current soil Cd standard. This in- condition used in the present study could be consi- dicates that the current soil environment quality stan- dered to represent the worst case scenario in terms of dard could effectively guarantee the Cd content of Chi- the Cd phytoavailability in soil and the Cd accumula- nese cabbage not exceeding the food safety limit. In tion ability of vegetables. the other 3 soils (red soil from Yingtan, latosol from Zhaoqing and cinnamon soil from Baoding), soil Cd CONCLUSIONS thresholds obtained from this study were lower than the current soil standard (0.3 mg kg−1 at pH < 7.5 Eight soils with different soil types and contrasting and 0.6 mg kg−1 at pH > 7.5); i.e., the current soil en- soil properties were used to derive soil Cd safety thre- vironment quality standard could not guarantee com- sholds for leafy vegetable in compliance with the food pliance with the food safety limit if Chinese cabbage safety limit for Cd. The estimated soil Cd threshold was grown. varied from 0.12 to 1.7 mg kg−1. The threshold values Multiple regression was also used to examine the were lower than the Class II of the current soil envi- influence of soil properties on the estimated values of ronment quality standard for 3 soils, but higher than soil Cd threshold. Soil pH and SOM or clay content that for the other 5 soils. The Cd phytoavailability was were found to be significant factors in estimating soil mainly affected by soil pH, clay content and SOM. The Cd safety threshold, as shown in the following regres- transfer coefficient of Cd from soil to vegetable was sion equation: found to decrease with increasing soil pH from 4.7 to PREDICTING CADMIUM SAFETY THRESHOLDS IN SOILS 481

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