Geochemical cadmium anomaly and bioaccumulation

Science of the Total Environment 644 (2018) 624–634

Contents lists available at ScienceDirect

Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Geochemical cadmium anomaly and bioaccumulation of cadmium and lead by rapeseed (Brassica napus L.) from noncalcareous soils in the Guizhou Plateau Sha Zhang a, Jing Song b,c,⁎ a b c

Department of Plant and Soil Sciences, University of Delaware, Newark, DE 19716, United States Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, CAS, Nanjing 210008, China University of Chinese Academy of Sciences, Beijing 100049, China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• B. napus may uptake excessive Cd and Pb from naturally Cd-enriched noncalcareous soil. • Soil type dependent regional Cd anomaly (mean 0.43 mg kg−1) was characterized. • (Bio)availability of Cd, but not Pb is high in noncalcareous soil. • Reliable Cd soil-rapeseed relationships were derived using soil variables. • Field data suggest that surface contamination may overestimate Pb uptake in B. napus.

a r t i c l e

i n f o

Article history: Received 5 April 2018 Received in revised form 17 June 2018 Accepted 19 June 2018 Available online 11 July 2018 Editor: Xinbin Feng Keywords: Noncalcareous soils Guizhou province Geochemically reactive pool Plant uptake Phytotoxicity Region-scale soil criteria

a b s t r a c t The cruciferous crop, oil rapeseed (Brassica napus L.), may bioaccumulate excessive cadmium (Cd) and lead (Pb) as well, from Cd-enriched noncalcareous soils in Guizhou province of southwestern China. Field paired soil-rapeseed sampling and greenhouse experiment were performed to characterize the Cd anomaly in rapeseed-planting soils and to predict the bioaccumulation of Cd and Pb in raw seeds using soil variables. The results indicated that total soil-Cd concentrations averaged 0.43 mg kg−1 (range 0.11–1.41 mg kg−1) from field investigation; and a soil type dependent Cd anomaly was observed. Besides, cumulative frequency of total soil-Cd was plotted to be helpful in delimitation of regional Cd anomalies. Rapeseeds readily bioaccumulated Cd from soils as validated by greenhouse experiment and field data. Contrary results were observed in relation to rapeseed Pb levels measured from greenhouse experiment (very low) and field (very high) which was likely due to soil particle contamination as indicated by the considerably higher ratio of Pb to Cd level in seeds harvested from fields. Based on multiple stepwise regression analysis, reliable Cd soil−rapeseed relationships, but less reliable for Pb, were derived using either total or (bio)available metal concentrations and were further inversely used to derive local soil Cd criteria (e.g., total soil-Cd based, 2.5 mg kg−1) based on Hygienic Standard for Feeds (GB13082-2001). Although seed Cd levels (bfeed standard) observed in field data indicated a least human dietary risk, however, high (bio)availability of Cd, but not Pb, in Cdenriched acid soil still poses high environmental risks and may threaten food safety of other crops. © 2018 Elsevier B.V. All rights reserved.

Abbreviations: rsPb, Rapeseed Pb; NA-Pb, Nitric acid (0.43 M) extractable Pb concentration in soil; cePb, Calcium chloride (0.01 M) extractable Pb concentration in soil; SQSs, Soil quality standards; FQSs, Food quality standards. ⁎ Corresponding author at: Institute of Soil Science, Chinese Academy of Sciences, No. 71 East Beijing Road, Nanjing 210008, PR China. E-mail address: [email protected] (J. Song).

https://doi.org/10.1016/j.scitotenv.2018.06.230 0048-9697/© 2018 Elsevier B.V. All rights reserved.

S. Zhang, J. Song / Science of the Total Environment 644 (2018) 624–634

1. Introduction High levels of potentially toxic trace metal(loid)s in agricultural soils can contaminate human food chains (Song et al., 2016; Zhang et al., 2018d; Zhao et al., 2015). According to the 2014 National Soil Investigation, the exceedance percentages for Cd and Pb in Chinese soils were 7% and 1.5%, respectively. Cd is officially identified as the primary pollutant in Chinese soils (Ministry of Environmental Protection of the People's Republic of China, 2014). Elevated Cd in agricultural soil was mainly attributable to mining/smelting, followed by irrational use of sewage irrigation, fertilizers, pesticides, plastic film and other agricultural inputs, and livestock and poultry manure by-products (Luo, 2018). But in southwestern China (i.e., Guizhou Plateau), the excessive accumulation of Cd in agricultural soil (mean 0.659 mg kg−1) is substantially contributed by natural pedogenic processes such as the weathering of Cdenriched carbonate rocks (Luo, 2018; Zhang et al., 2018a). For example, in the karst area of southwestern China (i.e., Guizhou province), the parent rock is carbonate rock. During the process of soil formation, calcium carbonate, the main chemical constituent of the rock, is dissolved and leached, while arsenic (As), Cd, Pb, and mercury (Hg), even if very low in the rock, are retained and accumulated in topsoils gradually, which makes the concentrations of elements such as Cd, As, Hg, and Pb in topsoil 10–20 folds higher than that in the parent rock (Ministry of Land and Resources, 2014). Consequently, food crops, especially cruciferous crops, may readily bioconcentrate Cd from acid soils particularly. Soil pH is known as the most important influencing factor that controls the (bio)availability of Cd and other metals as well, in soils. Elevated soil acidification would enhance the Cd mobilization, thereby increasing the potential for plant uptake. This is especially true in Guizhou province of southwestern China as N52.3% of the agricultural soils has a soil pH b 6.5. Besides, given the facts that the highest background Cd level (mean 0.659 mg kg−1) in Guizhou soils (Luo, 2018); and a widespread contamination in agricultural soils caused by long-term historical Pb-Zn mining/smelting activities there (Zhang et al., 2018d; Zhang et al., 2017), the potential contamination in agricultural produce has increasingly gained attention of risk assessors and policy makers. The contamination status of some food crops such as leafy and root vegetables, maize, and potato in mining/smelting areas of Guizhou province had been investigated; and Cd and Pb were identified as the leading pollutants in foods (Zhang et al., 2018d; Zhang et al., 2017). Oil rapeseed (Brassica napus L.) was the leading oil crop in Guizhou province with a production of 90.25 million tons in 2016 (Guizhou Provincial Committee of Agriculture, 2017), but little information is available about its contamination status, which should be carefully examined especially since rapeseed, a cruciferous crop known for hyper-accumulating Cd, widely grows on acid agricultural soils across Guizhou province. A previous investigation that indicated Cd in rapeseed (rsCd) from Guizhou province averaged 0.12 mg Cd kg−1 (range 0.04–0.26 mg Cd kg−1, n = 15), which was much higher than that from other provinces of China (Wu et al., 2016), but few reported the uptake and bioaccumulation of trace metals by rapeseed grown there. Moreover, proper soil management in Guizhou province is still a challenge that current soil quality standards for agricultural land in

625

China (0.3 mg Cd kg−1, pH b 6.5) was unable to effectively identify those soils which were safe for growing food crops and those which are not (Zhang et al., 2018b; Zhang et al., 2018d), as there were no consistent results between exceedance percentage of trace metals in soils and agricultural produce. As mentioned earlier, naturally geochemical Cd anomaly and spatial mining/smelting activities resulted in a regional characteristic of Cd and Pb accumulation in agricultural soils across Guizhou province (Luo, 2018; Zhang et al., 2017). Cd levels in most Guizhou soils exceeded soil quality standards (SQSs, 0.3 mg kg−1, pH b 6.5), but Cd levels in agricultural produce were not necessarily exceeding Chinese food quality standards (FQSs). This inconsistency was attributable to the variation in the (bio)availability of metal in soil and the plantdependent uptake capacity. It was necessary to characterize the geochemical distribution and (bio)availability of metals in Guizhou soils. Additionally, reliable metal soil-plant relationships, accounting for the (bio)availability, would be useful to precisely determine which soils are safe for growing crops, thereby providing a better soil management. The study aimed to identify if oil rapeseed bioaccumulates excessive cadmium (Cd) and lead (Pb) as well, from cadmium-enriched noncalcareous soils in Guizhou province of southwestern China. We would characterize the distribution and the (bio)availability of Cd and Pb concentrations rapeseed-planting soils further in relation to the bioaccumulation of Cd and Pb in seeds. 2. Materials and methods 2.1. Soil collection and characterization A total of 5 agricultural soils to a depth of 20 cm were collected from Guizhou province (Table 1). A portable X-ray fluorescence analyzer was used to check Pb and Zn levels on site to ensure soils collected not apparently anthropogenically polluted. A grid sampling method (3 m × 3 m) was used (Zhang et al., 2018c). Soils collected from different locations of one particular cropland were mixed into one, representing a typical soil sample. Soil types included Ferri-Udic Ferrosols, Carbonati-Udic Argosols, Ali-Perudic Argosols, Ferri-Udic Argosols, and Purpli-Udic Cambosols, classified by Chinese Soil Taxonomy. Soils collected were air-dried, pulverized, homogenized, and stored for further determination of physiochemical properties and soil incubation experiment. Soil pH were measured in 0.01 M CaCl2 extracts (liquid (v): solid (w) ratio, 2.5:1) (Lu, 2000). Soil organic matter (OM) content was determined by using the acid dichromate oxidation method (Yeomans and Bremner, 1988). Cation exchange capacity, CEC, was determined by using the sodium acetate method (Lu, 2000). Soil clay content (b 2 μm) was determined using a Model LS 13320 Laser Diffraction Particle Size Analyzer (Beckman Coulter, Brea, CA). Briefly, all the 5 soils had loam-clay characteristics, with a pH b 6.2 and OM content b4.4% (w). Detailed information of soil properties was included in Table 1. 2.2. Cadmium and lead addition and soil incubation As indicated by Table 2, total soil-Cd levels from unpolluted soils were initially determined by ICP-MS (Thermo ×7, Thermo Fisher, Waltham, MA) as 0.21 (S1), 0.52 (S2), 1.39 (S3), 0.42 (S4), and

Table 1 Physicochemical properties of agricultural soils sampled from Guizhou province of China. Soils

S1 S2 S3 S4 S5 a b

Soil type

Ferri-Udic Ferrosols Carbonati-Udic Argosols Ali-Perudic Argosols Ferri-Udic Argosols Purpli-Udic Cambosols

Organic matter content. Cation exchange capacity.

Sampling coordinates

105.2529E 105.752380E 106.672867E 104.177704E 105.812050E

pH

24.5658 N 26.337854 N 26.402485 N 26.861269 N 28.558115 N

6.14 6.02 5.80 5.60 4.02

OMa

Clay

CECb

w, %

%

cmol(+) kg−1

1.4 4.4 2.7 2.5 2.0

58.4 44.5 51.9 34.7 37.9

17.8 25.4 20.7 18.8 11.8

626


Table 2 Total Cd and Pb levels in soils. Treatmenta

T0 T1 T2 T3 T4 a b

b

S1, Ferri-Udic Argosols

S2, Carbonati-Udic Argosols

S3, Ali-Perudic Argosols

S4, Ferri-Udic Ferrosols

S5, Purpli-Udic Cambosols

Cd

Pb

Cd

Pb

Cd

Pb

Cd

Pb

Cd

Pb

0.42 0.51 0.76 0.83 1.30

38 77 199 308 699

0.52 0.72 0.86 0.97 1.13

69 73 208 312 728

1.39 1.64 1.69 1.93 2.56

46 83 216 311 657

0.21 0.32 0.43 0.77 1.17

20 89 238 320 680

0.32 0.42 0.52 0.81 1.17

31 228 270 323 729

Unit, mg kg−1. T0 represents the soil without Cd or Pb addition.

0.32 mg kg−1 (S5). Initial total soil-Pb levels were 20 (S1), 69 (S2), 46 (S3), 38 (S4), and 31 mg kg−1(S5). Each type of soil was individually spiked with CdSO4·2.5H2O and Pb(NO)3, with varying in 4 treatments (T1-T4, Table 2). Soils without Cd and Pb treatments were included as controls (T0, Table 2). The doses of Cd and Pb added to soils were based on the national SQSs in China (GB 15618-1995). All the treatments were then aged for 3 months at 50%–60% of water-holding capacity. Afterwards, soils were air-dried again, re-pulverized, homogenized, and stored for subsequent greenhouse experiment. Subsamples of each soil were sieved through a 0.2 mm mesh sieve in preparation for total Cd and Pb analysis and soil extraction tests. Method of soil digestion for total metal measurement was described by Zhang et al. (2018d). The final levels of total soil-Cd and total soil-Pb were determined with ICP-MS and were listed in Table 2. The concentration of other metals of concern including As, Cu, Ni in soils were also determined in the purpose of investigating the influence on the bioaccumulation of Cd, Pb, and other metals in rapeseed under current Cd and Pb amendments. 2.3. Dilute acid and neutral salt solution extraction tests A dilute nitric acid (0.43 M), denoted by NA, was recently adopted to extract “the potential environmental available trace elements” as defined in ISO-17402 (ISO standard, ISO-17586:2016). The NA-extraction was also used for assessing leachability, (bio)availability (to plants), and (bio)accessibility (to human and animals) of trace metals in soils or solid matrix alike (Rodrigues et al., 2013). Reliability and reproducibility of NA-extraction were also validated by earlier studies (Groenenberg et al., 2017; Rodrigues et al., 2013; Rodrigues et al., 2010; Römkens et al., 2004). Therefore, NA-extraction was used in our study. Besides, calcium chloride (0.01 M) extraction was also adopted to enable an estimation of trace element concentration in the water phase with a result close to the actual soil porewater concentration (Houba et al., 2000). The CaCl2 (0.01 M)-extractable metal (i.e., ceCd and cePb) was operationally defined as short-term plant-available during the period of plant growth (Gerard et al., 2000). The extraction procedures were briefly described here. Soil samples (b0.2 mm) prepared in Section 2.2 were extracted with HNO3 (0.43 M), diluted by 30 ml concentrated HNO3 (w, 65%– 70%) with 1000 ml 18.2 MΩ cm ultrapure water and 0.01 M CaCl2 solution, prepared by dissolution of 1.11 g CaCl2 powder (w, 99%) in 1000 ml ultrapure water, respectively. After 6 h shaking, samples were centrifuged at 3500 ×g for 20 min. Supernatant from each tube was filtered through a 0.22 μm filter, acidified using 5% HNO3 solution, and stored at 4 °C. The levels of Cd and Pb in extract solutions were measured with ICP-MS. 2.4. Preliminary greenhouse experiment To ensure the success of greenhouse experiment for rapeseed growing on newly Cd- and Pb-spiked acid soils, a 20-d preliminary test was performed. The objective was to double check if seeds were normally germinated from all of the soils with differing contamination levels and if seedlings grew well without apparent phytotoxic symptoms. A

tray with 25 small pots filled with soils of 25 treatments as described in Section 2.2 was used. All soils were wetted with deionized water to about 80% of water-holding capacity. A total of 4 seeds was sown in each pot. Experiment was performed in well-controlled artificial climate incubator. Germination rate and growth condition were recorded during the experiment.

2.5. Greenhouse experiment Greenhouse experiment was performed in Nanjing Zhongshan Botanical Garden (118°22′–119°14′ E, 31°14′–32°37′ N). The cultivar of rapeseed used in our study is widely cultivated across Guizhou plateau and is adaptive to the unique plateau climate and soil conditions (e.g., pH b 6.5). Each soil was transferred to plant pots in triplicate, with every pot containing 5 kg soil with basal fertilizers (1.607 g of urea, 1.188 g of potassium chloride, and 1.438 g of potassium dihydrogen phosphate). Soils were wetted to a loose condition and seeds were sown. Four seedlings were selectively kept in each pot after a 30-d growth. When rapeseed growing to full maturity, seed pods containing seeds were harvested from each pot, then oven-dried to a constant weight, and peeled for seeds collection. Seeds were then rinsed thoroughly with deionized water and oven-dried again. Yield of seeds harvested from each pot was recorded. Seed samples around 0.5 g were directly digested with HNO3-HClO4 acids (10 ml: 0.25 ml) using a HotBlock system at 150 °C for 2 h. Concentrated nitric acid (Ultrapure) was then replenished until solid particles disappeared. Digested solution was vaporized to b0.5 ml, then diluted to 50 ml, and filtered through a 0.22 μm filter. The concentrations of trace metals including As, Cd, Cu, Ni, Pb, and Zn in diluted solution were analyzed with ICP-MS. Standard reference material of wheat GBW10014 (GSB-2) was used for analysis control, and measured Cd and Pb concentrations were 0.019 (±0.000, standard deviation, n = 3) mg kg−1 and 0.070 (±0.021) mg kg−1, respectively, which were close to 0.018 (±0.004) mg Cd kg−1 and 0.065 (±0.024) mg Pb kg−1 reported in the certified reference material. Detection limits were 2 μg Cd kg−1 and 0.5 μg Pb kg−1 in seeds.

2.6. Field paired soil-plant sampling across Guizhou province To obtain the information of Cd and Pb levels in rapeseeds and rapeseed-planting soils across Guizhou province, paired rapeseed-soil sampling was performed during May 2, 2015–May 15, 2015. Mature rapeseed pods and the corresponding rooting-zone soil (n = 66) were collected consistently in representative rapeseed-planting fields (Table A.1). Paired pods and soils were stored Zip-lock bags separately until back to lab for washing, drying, peeling, and weighing. Rootingzone soils collected from fields contained all of the soil types that were included into the greenhouse experiment. Pretreatment of seed and soil samples was described in Sections 2.5 and 2.1, respectively. Soil and rapeseed digestion and analysis of Cd and Pb levels referred to methods described in Section 2.4.


a

ð1Þ

Where [rsCd] represents the Cd level in rapeseed, mg kg−1, dry wt. [sCd] represents the metal concentration in soil, mg kg−1. [X]nI=1 represents some key soil properties such as soil organic matter contents, g kg −1 , cation exchange capacity, cmol (+) kg −1 , or clay contents, g kg−1. Parameters including k, a, b and c are coefficient estimates in the Eq. (1). To characterize the distribution of total soil-Cd and total soil-Pb levels, especially the total soil-Cd anomaly in rapeseed-planting soils across Guizhou province, a method of percentile (cumulative frequency distribution) was adopted (Yamane, 1973). IBM SPSS Statistics® 24 was used for statistical analysis and multiple stepwise regression analysis. Graphs were generated with Origin® 2017 for Windows.

3. Results 3.1. Characterization of the (bio)availability of cadmium and lead in soils Because soils used for NA-extraction soil test were noncalcareous, there was no such concern that carbonate consumed a large fraction of protons resulting in an insufficient extraction as found in calcareous soils (Houba et al., 1995). Geochemically reactive Cd and Pb pools, as indicated by HNO3-extractable levels, were strongly correlated with total levels (adj. R2 0.916 and 0.957 for Cd and Pb model, respectively, Fig. 1). However, the regression line was asymptotic to the 1:1 line with increasing total soil-Cd level (Fig. 1a) and total soil-Pb level (Fig. 1b), respectively. This indicated that the transformation of Cd and Pb from soluble to inert fraction, calculated by total subtracting geochemically reactive pool in soil, did not increase accordingly. It was therefore not surprising that the percentage of geochemically reactive to the total pool of Cd and Pb, respectively, was lower in unpolluted soil than in newly contaminated soil.

3.2. Phytotoxicity of cadmium and lead under the experimental condition Fig. 2 presented the 20-d nursery experiment using soils with different Cd and Pb treatments. Seed germination rate showed an insignificant difference (p N 0.5) between different treatments for each of soil types except Purpli-Udic Cambosols (Soil S5, Table 1). Seedlings died out gradually in Cd-spiked Purpli-Udic Cambosols, but seeds barely sprouted in Pb-spiked Purpli-Udic Cambosols. Because of this, older seedlings grown on other soils without Cd or Pb addition were transplanted to Purpli-Udic Cambosols for subsequent greenhouse experiment. But a significant stunting of rapeseed growth was observed in Purpli-Udic Cambosols whereas rapeseed plants grew well in other soils under the greenhouse condition (Fig. 2b, c, and d). Yield of seeds averaged 25.2 g pot−1 from Cd-spiked soils and 23.5 g pot−1 from Pbspiked soils. Lower yields of rapeseeds from Pb-spiked soils were likely attributed by Pb-induced phytotoxicity.

4

0.43 HNO3 extractable Cd

Previous studies preferred to use a log-transformed linear combination of soil contamination levels and some key soil properties such as pH and soil OM contents to improve the predictability of so-called soil-toplant transfer model that had a low R2 in the simple linear model (Franz et al., 2008; Rodrigues et al., 2012a; Rodrigues et al., 2012b; Römkens et al., 2009). In the present study, the extended-Freundlichtype equation was performed as Eq. (1) to predict plant uptake (Cd was used as an example).

Cd-spiked Soils Background Soils Regression line 1:1 line

2 1

0.2 0.1

Adj. R2 0.916 p < 0.001 n = 50

0.1

0.2

1

2

4

Total soil Cd, mg kg-1

b1000 0.43 HNO3 extractable Pb

2.7. Methodology and data analysis

log10 ½rsCd ¼ k þ a log10 ½sCd þ b ½pH þ c log10 ½X ni¼1

627

Pb-spiked Soils Background Soils Regression line 1:1 line

200 100

20

Adj. R2 0.957 p < 0.001 n = 50

10

10

20

100

200

Total soil Pb, mg kg

1000 -1

Fig. 1. Correlations of 0.43 M HNO3-extractable Cd and Pb levels, respectively, with total metal levels in soils. Square and circle signs represent data from spiked and unpolluted soils, respectively. Solid and dash line represent the regression line and the 1:1 line, respectively.

3.3. Bioaccumulation of trace metal(loid)s in rapeseeds harvested from greenhouse experiment As shown in Fig. 3, metal(loid) levels (mean ± standard deviation) bioaccumulated in seeds were 0.12 (±0.03) mg As (total) kg−1, 0.15 (±0.12) mg Cd kg−1, 4.7 (±0.54) mg Cu kg−1, 0.65 (±0.37) mg Ni kg−1, 0.09 (±0.10) mg Pb kg−1, and 56 (±7.62) mg Zn kg−1. Pearson correlation was performed, and results showed that rsCd was statistically significantly correlated with rsZn (r = 0.278, p = 0.043), which might indicate a synergistic effect between Cd and Zn in the process of uptake. Newly added Cd in soil didn't significantly influence the Pb uptake, but conversely, added Pb statistically significantly increased Cd uptake (r = 0.369, p = 0.06). Although total soil-Pb levels were hundreds of times higher than total soil-Cd levels added to soils (Table 2), seed bioaccumulated very limited Pb (Fig. 3). It was also worth noting that As levels were comparable to rsCd levels (0.11 mg Cd kg−1) from unpolluted soils. However, the concentration of As was generally found much higher than Cd concentration in edible vegetable rapeseed oil sold in China (Zhu et al., 2011), which indicated that As, unlike Cd, was more likely to be distributed in oil than in meals. Dietary intake risks for As in rapeseed oil should be further investigated, however, not discussed in our study.

628


Fig. 2. a. Germination test. b. Seedlings. c. Flowering. d. Fruit development stage. Seeds were all germinated but died out gradually in Cd-spiked Purpli-Udic Cambosols (Fig. 2a, S5, left). Seeds were barely germinated in Pb-spiked Purpli-Udic Cambosols (Fig. 2a, S5, right).

3.4. Characterization of paired rapeseed-soil samples collected from Guizhou province

Metals levels in seeds, mg kg-1 dry wt.

Soil pH averaged 6.0, ranging from 4.2 to 7.6 (90th percentile, 6.9) (Supplementary information, Table A.1). This was accordance to our earlier statement that rapeseed mainly grew on acid soils in Guizhou province. The total soil-Cd levels averaged 0.43 mg Cd kg−1 (range 0.11–1.41 mg kg−1, Table A.1) which was lower than the background level of 0.659 mg Cd kg−1 in Guizhou province. This might be attributable to the disproportionate field sampling to the actual distribution of soil types across Guizhou province especially since many Cd-enriched calcareous soils were not sampled. The total soil-Pb levels averaged 36.1 mg Pb kg−1 (range 8.5–84.5 mg kg−1) (Table A.1) which were close to the background level (35.2 mg Pb kg−1) reported by previous studies. Distribution of total soil-Cd level was soil type dependent (Fig. 4c). Carbonati-Udic Argosols (median 0.62 mg Cd kg−1) and Fe-leachi-

100

56

10

3.5. Prediction of cadmium and lead uptake by rapeseed

4.7 1

0.1

0.65 0.15

0.12

0.09

0.01

As

Cd

Cu

Ni

Stagnic Anthrosols (median 0.41 mg Cd kg−1) had higher background total soil-Cd levels than that other soil types such as Purpli-Udic Cambosols (median 0.18 mg kg−1). Soils with relatively higher Cd levels, e.g., points 1, 2, and 3 in Fig. 5 were not anthropogenically contaminated as there were no known contamination sources near the sampling sites. For example, the soil with highest Cd level (1.41 mg kg−1, geographical coordinates, 107.447327E, 27.233513 N) was a kind of paddy soil (soil type, Fe-leachi-Stagnic Anthrosols) which was enriched with phosphorus (P) minerals. Soils with 1.05 mg Cd kg−1 (point 2, Anshun region, 106.022011E, 26.274628 N) and 0.96 mg Cd kg−1 (point 3, Tongren region, 108.139565E, 27.878839 N) (soil type, Carbonati-Udic Argosols) were formed by natural weathering of carbonate-rocks. Enhanced weathering and leaching of P minerals and carbonate-rocks, especially under humid acid soil environment with intense radiation in Guizhou Plateau, had resulted in the excessive accumulation of Cd in soils. The range of rsCd from greenhouse experiment was comparable to that from fields (Fig. 4a). A total of 85% of field rapeseed data was b0.2 mg Cd kg−1 (range 0.01–0.61 mg Cd kg−1). However, virtually all the data of rsPb (mean 0.09 mg kg−1) from greenhouse experiment were significantly lower than that from fields (range 0.35– 3.02 mg Pb kg−1) (Fig. 3 and 4b). This inconsistency was likely caused by surface particle contamination and was discussed in Section 4.3.

Pb

Zn

Fig. 3. Metal(loid) levels in mature rapeseeds harvested from greenhouse experiments. Both data sets (n = 50) from including unpolluted and artificially spiked soils are included. Box chart includes 5th, 25th, median, mean (square signs), 75th, and 95th percentiles. Mean values (mg kg−1) are labeled beside boxes.

Based on Eq. (1), multiple linear (log transformed) stepwise regression analysis was performed to investigate Cd and Pb, respectively, soilrapeseed relationships. However, influencing factors such as pH were auto-excluded at the significance of p b 0.5 and coefficient of determination (R2) was not significantly increased as well. Fig. 5 presented Cd and Pb, respectively, soil-rapeseed relationships. The rsCd was strongly correlated with total soil-Cd or CaCl2 (0.01 M)-extractable Cd, denoted as ceCd, levels, respectively; and rsCd was satisfactorily predicted by total soil-Cd or ceCd, with an explanation of N92% of the variation in rsCd level. However, Pb soil-rapeseed relationships were less reliable since only b57% of the variation in rsPb level was attributable to soil Pb levels. The variation in rsCd level was also satisfactorily explained


a 1.2

2 3

n = 66

0.8

e

0.6 d

0.4 0.2 0.0

2.5 2.0

90 80 70 60 50 40 30 20 10 0

seed Pb total soil-Pb SQS 80 mg kg-1

3.0

1

Seed Pb, mg kg-1

Cd levels, mg kg-1

seed Cd total soil-Cd SQS 0.3 mg kg-1

1.4

n = 66

1.5 1.0 0.5

Total soil-Pb, mg kg-1

b

1.6

1.0

629

0.0 0

20

40

60

80

0

100

40

60

80

100

Cumulative frequency (%)

Cumulative frequency (%)

c 1.4 Range of tsCd, mg kg-1

20

25%~75% 5%~95% Median Line Mean

1.2 1.0 0.8 0.6

0.59 0.4

0.48

0.4

0.34

0.2

0.34

0.18

0.0

ols ols ls ols s ols h ro s ols Ant ic Argo ambos Ferros Argos Argoso c i n c d C i c g c i i U d c a d i t d iu hi-S onat rpli-Ud Ferri-U Ali-Per Ferri-U l e a c C a rb Pu FeFig. 4. Frequency distributions for (a) Cd and (b) Pb levels in rapeseeds (rsCd and rsPb, respectively) and rapeseed-planting soils (total soil-Cd and total soil-Pb, respectively). A double yaxis is used to in Fig. 4b for proper presenting rsPb and total soil-Pb, respectively. Solid lines (horizontal) represent soil quality standards (SQSs) for agricultural land (pH b 6.5) in China. (c) The ranges of total soil-Cd by soil types. The turning point d in Fig. 4a is estimated normal geochemical baselines and turning point e is used in delimiting regional anomalies.

by geochemical reactive Cd level with adj. R2 0.924 (Eq. (2)), which was comparable to total soil-Pb-based model. A slight increase in prediction of rsPb was observed using geochemical reactive Pb level (adj. R2 0.575, Eq. (3)). It worth noting that the (bio)availability of Pb, as indicated by cePb level, was very low in controls generally b0.005 mg kg−1 (Fig. 5d); and a considerable variation was found in rsPb level within the low cePb range. h i −1 log10 Rapeseed Cd; mg kg dry wt: ¼ 0:8065 h i −1 –0:5695adj:R2 ¼ 0:924; pb0:001; n ¼ 40 log10 Reactive Cd; mg kg ð2Þ h i −1 log10 Rapeseed Pb; mg kg dry wt: ¼ 0:4407 h i ð3Þ −1 –2:0558adj:R2 ¼ 575; pb0:001; n ¼ 40 log10 Reactive Pb; mg kg

4. Discussion 4.1. Lower soil pH increases labile cd and Pb levels Geochemically reactive Cd and Pb pool control the dissolved Cd and Pb concentrations in the soil solution, mainly through the processes of sorption/desorption and dissolution/precipitation (Groenenberg et al., 2010; Tipping et al., 2003); and soil pH was believed one of the most important factors influencing dissolved Pb2+ and especially Cd2+ ion concentration in the soil solution (Sauvé et al., 1997). Dissolved Cd2+ (as well as other metals such as Zn, Ni, and Pb) level increased rapidly with decreasing soil pH especially at an acid range (pH b 6.5), which was attributable to

the competitive effect of protons and a decrease of negative surface charges of soil minerals. Total dissolved Cd or free Cd2+ion activity (aCd2+) in the soil solution can be satisfactorily predicted using a competitive adsorption model, i.e., extended-Freundlich-type equation, with soil pH and reactive Cd pool or total soil-Cd level included (Groenenberg et al., 2010; Sauvé et al., 2000; Tipping et al., 2003). Soil variables such as soil OM content was often additionally considered in such model, as it can immobilize free metal ions by significant adsorption and complexation processes. In the present study, extended-Freundlich-type models (Eqs. (4) and (5)) incorporating with reactive concentration and soil pH satisfactorily predicted the exchangeable Cd and Pb levels (i.e., ceCd and cePb) in soils. Moreover, determination of coefficient (adj. R2) was only slightly improved to 0.901 and 0.940, when soil OM content was additionally incorporated into Eqs. (5) and (6), respectively. This was likely due to that adsorption of Cd2+ and Pb2+ by soil OM was greatly restricted especially in acidic soils with pH b 6.5 as indicated in our study. We also tried a different modeling method. As shown in Fig. 6, however, a polynomial surface model only considering reactive concentration and soil pH had already explained 90.8% and 98.5% of the variation in ceCd and cePb level, respectively. The performance of polynomial surface model was therefore better than extended-Freundlich-type model. This indicated that improvement of model performance was not only dependent on incorporation of more explaining variables but also determined by the method of modeling. h i −1 ¼ 0:913 log10 CaCl2 ‐extractable Cd; mg kg h i −1 –0:730 pH þ 2:676adj:R2 ¼ 0:878; pb0:001; n ¼ 50 log10 Reactive Cd; mg kg

ð4Þ

630


Rapeseed Cd, mg kg-1

a

b Newly Cd-spiked Unpolluted

0.4

Newly Cd-spiked Unpolluted

0.1

0.1

0.04

0.04 y=1.188 x - 0.750 Adj.R2 0.930, p