1. Introduction
With the rapid economic development occurring around the world, heavy metal pollution is common, and the area and degree of pollution is increasing, which poses a serious threat to the safety of agricultural production and human health [
1,
2]. Heavy metal pollution in crops has attracted significant attention as of late. The accumulation of heavy metals in soils is a major factor determining the high levels of heavy metals found in crops. The bioaccumulation of heavy metals—especially zinc (Zn), cuprum (Cu), lead (Pb), arsenic (As), chromium (Cr), nickel (Ni), and cadmium (Cd)—in the food chain presents a serious threat to human health [
3,
4]. For example, Pb poisoning can cause drowsiness, vomiting, irritability, loss of appetite, dizziness, and in severe cases, it can result in a coma or death [
5]. Long-term exposure to Ni can also have serious health consequences, including rashes, fatigue, headaches, dizziness, respiratory diseases, decreased lung function, and even fatal cardiac arrest [
6].
The potato is the most important non-grain food crop in the world. It is an important source of food owing to the energy, starch, vitamins, and minerals it contains [
7]. Due to the growing demand for the food crop, Chinese potato production and exports to the international market are increasing year by year. Known as the ‘Hometown of the Southern China Potato’, Weining county ranks first in terms of the potato production in Guizhou province. At an elevation of nearly 2200 m, Weining county is the highest county in the province. It possesses a low average temperature and long hours of sunshine that make it highly suitable for potato farming. Weining county has a potato cultivation area of 1100 km
2, with a total output of 2.7 million tons. The income earned from potato planting accounts for more than 20% of the average annual income of the county’s residents.
Located in western Guizhou province, Weining county is a typical karst landform composed of numerous types of limestone. It contains a large amount of Cd, As, Cu, Cr, and Zn in the carbonate rock and soil and is rich in mineral deposits [
8,
9]. According to the 2016 Bulletin on Geological Environment of Guizhou province, Guizhou had 4503 small-scale mines, including coal, phosphate, Fe, Zn, and Hg mines [
10]. When comparing the content of heavy metals in different agricultural soils (
Table S1), it is found that the average content of Cu, Cr, Zn, As, Pb, and Ni in Weining soils of karst landforms are generally higher than those in other regions [
1,
11,
12,
13,
14,
15,
16]. In Weining county and the adjacent Hezhang county, many handmade Zn mining areas have existed since the 17th century. Zn smelting activities nearly ceased in 2004 due to concerns related to food safety and environmental pollution [
17,
18]. The vegetable uptake of metals is one of the major pathways through which soil metals enter into the food chain. Potatoes are in close contact with the soil, and they are buried in the soil for several months before ripening. Therefore, the content of Zn, Cu, Pb, Ni, and other heavy metals in potatoes has always been the focus of attention. Related studies have also reported the content of heavy metals such as Zn, Cu, Pb, and Ni in potatoes and soils [
1,
19,
20,
21].
However, Weining county, the primary production area for potatoes in China, has a karst landform that gives it different characteristics from the other major potato-producing areas in the world. Our group has reported that the concentrations of Cd were 0.41 to 10.0 mg/kg in the soil and were 0.023 to 0.18 mg/kg in the potatoes in Weining county. A regression model to predict the concentration of cadmium in the potatoes based on soil properties was developed in Weining county [
22]. To date, no analysis has been performed regarding the heavy metal content (such as Cu, Cr, Zn, As, Pb, and Ni) of potatoes from Weining county. The correlation between the presence of heavy metals (such as Cu, Cr, Zn, As, Pb, and Ni) in potatoes and the environmental factors present in the karst landforms has not been investigated.
In this study, we analyzed and tested 56 samples from 15 towns and determined the Cu, Cr, Zn, As, Pb, and Ni concentrations in soils and potatoes in Weining county. The correlation between the concentrations of these heavy metals in potatoes and the environmental factors was analyzed. The bioconcentration factor (BCF), pollution load index (PLI), daily intake of metal (DIM), and health risk index (HRI) were obtained. Further safety assessments were carried out based on the daily consumption of potatoes cultivated in karst landforms.
3. Data Analysis
All the data were analyzed using SPSS 17.0. The K-means clustering algorithm analysis of the content of each metal was used to identify groups, then the significance analysis was evaluated between groups at p < 0.05. The Pearson correlation test was used to analyze the correlation between the content of heavy metals in the potatoes and soil organic matter (OM), soil pH, and elevation.
Other parameters, such as the BCF, PLI, DIM, and HRI, were also determined.
This section may be divided by subheadings. It should provide a concise and precise description of the experimental results, their interpretation, as well as the experimental conclusions that can be drawn.
3.1. Analysis of Soil pH and OM
The soil and water were mixed at 1:2.5 and the pH was measured with a Sartorius pH meter. Deoxidation was performed with K
2Cr
2O
7 and then the titration method was used to determine the content of organic matter in the soil [
25]. Each sample was made in three parallels during the process. In this study, the soil pH varied from 4.36~8.41, with an average value of 5.74. The amount of organic matter (OM) varied from 12.3~102 g/kg with an average value of 50.7 g/kg, as shown in
Table 1.
3.2. Bioconcentration Factor
To study the transfer of metals from the soil to the potatoes, the BCF values were calculated as follows [
4,
26].
Cpotato and Csoil represent the concentrations of heavy metals in the potato and soil (dry weight), respectively.
3.3. Pollution Load Index of the Soil
The extent of pollution by trace metals was assessed by employing the method based on PLI developed by Tomlinson et al. [
27], expressed by the following equations:
where C
soil represents the concentration of heavy metal in soil and C
PML represents the permissible maximum limit set by the World Health Organization for heavy metals in the soil. CF = contamination factor and n = number of metals [
28].
3.4. Daily Intake of Metals from Potatoes and the Associated Health Risk Index
A health risk assessment for consumers based on their intake of metal-contaminated crops was characterized using an HRI. The HRI value was calculated using the formula below [
29,
30,
31].
where DIM (mg/kg/day) is the daily intake of heavy metals via the exposure pathway through the ingestion of vegetables and R
fD is the reference dose. The DIM value was calculated using the following formula [
32,
33].
C
metal represents the concentration of heavy metals in the potato (fresh weight), D
food intake represents the average ingestion of potatoes per day (0.128 kg) in 2014 [
6], and the B
average weight represents the average body weight for adults, equal to 60 kg [
34]. The R
fD standard values recommended by the ‘Integrated Risk Information System’ for Zn, Cu, As, and Ni are 0.37, 0.04, 3 × 10
−4, and 0.02 mg kg
−1 day
−1, respectively [
35]. The R
fD value for Pb was 0.0035 mg kg
−1 day
−1 [
36] and Cr was 1.38 × 10
−2 mg kg
−1 day
−1 [
6].
5. Discussion
The concentration order of various heavy metals in soils varies from place-to-place owing to the different soil backgrounds and pollution conditions. Golia et al. [
1] has reported the order of concentrations of heavy metals in the soil of locally planted potatoes in Trikala was Zn (9.87 mg/kg) > Cu (0.925 mg/kg) > Pb (0.051 mg/kg) > Ni (0.024 mg/kg) > Cr (0.018 mg/kg) > Cd (0.015 mg/kg). Mehrdad et al. [
13] has reported that the order of concentrations of heavy metals in the soil of locally planted potatoes in the Hamadan province, western Iran, was Zn (55.6 mg/kg) > Cr (40.1 mg/kg) > Ni (32.9 mg/kg) > Cu (26.8 mg/kg) > Pb (10.4 mg/kg) > As (8.60 mg/kg) > Cd (2.20 mg/kg). In this study, the concentration order of the heavy metals in the potato cultivation soil was Zn (175 mg/kg) > Cr (119 mg/kg) > Pb (59.3 mg/kg) > Ni (50.8 mg/kg) > Cu (47.8 mg/kg) > As (25.8 mg/kg) > Cd (2.60 mg/kg) [
22]. It can be concluded that the content of heavy metals in the soil in the karst potato growing areas is higher than that in other potato growing areas. Since the 17th century, Zn smelting activities using indigenous methods have been carried out in Weining county [
18], which may be the reason for the excessive Zn content in some places. Currently, Weining is a major agricultural production county. The research has found that the pollution of Cr, Ni, and As in the soil was more serious than previously believed.
Golia et al. [
1] reported the concentration levels of heavy metals in potatoes (fresh weight) in the Trikala region: Zn (0.201 mg/kg) > Cu (0.101 mg/kg) > Cr (0.089 mg/kg) > Pb (0.077 mg/kg) > Ni (0.057 mg/kg) > Cd (0.008 mg/kg). Šrek [
19] reported the concentrations of heavy metals in potatoes (dry weight) in the Ruzyne region to be in the following order: Zn (16.1 mg/kg) > Cu (4.30 mg/kg) > Pb (0.60 mg/kg) > Ni (0.58 mg/kg) > Cr (0.19 mg/kg) > As (0.06 mg/kg) > Cd (0.04 mg/kg). Gebrekidan et al. [
20] reported that the concentrations of heavy metals in potatoes (dry weight) in the Tahtay region was Pb (2.58 mg/kg) > Cu (2.52 mg/kg) > Zn (1.40 mg/kg) > Cr (0.39 mg/kg) > Ni (0.25 mg/kg) > Cd (0.18 mg/kg). The average concentrations of heavy metals in Weining potatoes (fresh weight) were Zn (2.73 mg/kg) > Cu (0.675 mg/kg) > Cr (0.072 mg/kg) > Ni (0.055 mg/kg) > Pb (0.027 mg/kg) > As (<0.01 mg/kg), in our study. Evidently, the content levels of heavy metals in potatoes varied widely across the regions. The order of the levels of different heavy metals in potatoes is not the same in different agriculture soils.
Khan et al. [
21] reported the order of the BCF of the heavy metals (Mo, As, Se, Fe, Cu, Zn, Ni, and Pb) in potatoes irrigated by underground water in the Punjab region of Pakistan region as Zn (20.2) > Cu (4.63) > Mo (1.27) > Ni (1.09) > Fe (1.05) > Se (0.35) > Pb (0.19) >As (0.06). Zhang et al. [
22] reported that the average BCF of the heavy metal Cd in Weining potatoes was 0.24. The order of the BCFs of the metals (Zn, Cu, Pb, As, Cr, Cd, and Ni) in Weining County was Cd (0.24) > Zn (0.107) > Cu (0.088) > Ni (0.007 > Cr (0.003) ≈ Pb (0.003) > As (<0.001). In both studies, the ability of potatoes to absorb heavy metals from soil was found to be Zn > Cu > Ni > Pb > As; however, the ability of potatoes to absorb heavy metals was much larger in soils in central Punjab, Pakistan. Mehrdad et al. [
13] reported the order of the BCF of the heavy metals (As, Ni, Cd, Cu, Pb, Cr, and Zn) in potatoes from soils that overused phosphate fertilizers in western Iran to be As (0.46) > Ni (0.41) > Cd (0.40) ≈ Cu (0.40) ≈ Pb (0.40) > Cr (0.39) ≈ Zn (0.39), which seemed to suggest to suggest that due to the effect of phosphate fertilizer, the BCF of the potato to various heavy metals tends to be the same (about 0.4). It can be concluded that there were also differences in the BCFs of metals in different conditions. Therefore, it was necessary to study the concentration and bioconcentration of heavy metals in potatoes cultivated in karst landforms.
The content of heavy metals in plants has been proven to be influenced by many factors, such as the content of heavy metals in the soil [
4,
43], soil pH [
4,
5,
44], the amount of OM in the soil [
4,
22,
45], and elevation [
22]. Through the correlation analysis, it can be concluded that the content of metals found in potatoes cultivated in karst field environments of Weining were affected by many factors. In Weining, the high As content in the soil could increase the content of Pb in potatoes, the lower pH was beneficial to the bioaccumulation of Cr and Ni in potatoes, and the high altitude was detrimental to the bioaccumulation of zinc and copper in the potatoes.
6. Conclusions
Through an analysis of six heavy metals (Zn, Cu, Pb, As, Cr, and Ni) in the soil and potato samples, it can be concluded that the heavy metal pollution in Weining soil with a typical karst landform was caused by As, Cr, and Ni. The ability of the potato to absorb heavy metals from the soil was very small, so the concentration of heavy metals in potatoes was below the maximum threshold. There was a significant positive correlation between the content of As in the soil and the content of Pb in potatoes. The content of As in 70% of the soil samples exceeded the standard, which has a potential impact on the bioaccumulation of Pb in Weining potatoes. In this study, a lower pH was more favorable for the transfer of Ni and Cr to potatoes, and a high elevation was not a conducive factor for the bioaccumulation of Zn and Cu in the potatoes. The bioaccumulation of heavy metals (Zn, Cu, Pb, As, Cr, and Ni) in the potatoes and the HRI obtained by the DIM in this study were very low, further indicating that there is no potential health risk in the consumption of potatoes from the Weining karst landforms.