Heavy Metal Characteristics and Comprehensive Quality Index Evaluation of Soil-Crop System in 11 Cities of Yunnan Province, China

Yunnan province in China is a high background area of soil heavy metals, and agricultural planting and industrial and mining activities are relatively frequent, which aggravate soil heavy metal pollution. However, at present, there are few reports on the overall or large-scale soil-crop pollution and risk assessment of heavy metals in Yunnan Province. This study through 11 cities in Yunnan province of China farmland soil-crop systems of heavy metal lead, cadmium content, enrichment coefficient is analyzed, and using the method of potential ecological harm index, index of compressive quality to evaluate heavy metal pollution soil-crop system risk. Results showed that the average content of soil heavy metal Cd and Pb were 1.31 mg/kg, 64.17 mg/kg, which are higher than the background value of Yunnan province. The average contents of Pb and Cd in the edible parts of crops were 0.20 mg/kg, 0.08 mg/kg. The average content of heavy metals in crops in Diqing (Pb) and Nujiang (Cd) was 0.72 mg/kg and 0.148 mg/kg. The enrichment coefficients of heavy metals in edible parts of crops were the largest in Diqing (Pb) and Zhaotong (Cd). The average value of ecological risk index of Pb element in soil is 2.79, which indicates that the study area is in a slight ecological hazard, the average value agricultural products and promote the sustainable development of agriculture in Yunnan Plateau.


Introduction
Soil provides us with raw materials such as plants and animals for daily use, that is, "everything grows in soil". As a non-renewable resource, soil is the material basis for sustainable development (Zhang & Wu, 2018). The quality of soil environment is directly related to the quality and safety of agricultural products, even the ecological environment of cultivated land and human health. (Jing et al., 2016;Zhang, 2017;Jia et al., 2020) Cd is severely polluted in other regions, especially Europe, the United States and Hunan (Manfred et al., 2017;Li, 2020).
However, the overall situation of soil environment in China is not optimistic, and the environmental quality of arable land soil is worrying, with an over-standard rate of 19.4% (Bulletin of National Soil Pollution Survey, 2014). Xu Qisheng et al. (2018) found in their research on soil heavy metal pollution in central and southern China that: Pb and Cd exceeded the standard to varying degrees, Cd was the most serious, and Hunan province was the most serious area. , based on the statistics of literature data related to heavy metals in arable soil in China, found that the content of heavy metals in soil in southwest China was higher than that in other regions, and Pb in soil in Yunnan province had the highest exceeding background value (1.91 times). Cultivated soil in Hunan and Yunnan provinces had the most serious heavy metal pollution. Zeng Min et al. (2019) investigated the soil and crops in Jijie Town, Gejiu City and found that the average concentration of Pb and Cd in the soil exceeded the environmental quality standard, but Pb and Cd in only some crops exceeded the limit value of pollutants in food.
As a daily dietary necessity, crops contain nutrients needed by the human body, but with the gradual improvement of public living standards, the demand for pollution-free crops is also increasing. Huo Yanhui et al. (2021) analyzed and studied the reported heavy metal content of crops in China and showed that different crops have different absorption and enrichment abilities of heavy metal elements in soil. Leaf vegetable crops are easy to enrich Cd, legume crops are easy to enrich Pb, while onion, garlic, solanum, root and stem crops have weak ability to absorb and enrich heavy metals. Ran Jiwei et al. (2019) studied the surrounding area of Gejiu tin mine in Yunnan and showed that Pb and Cd in crops were 12.10 and 16.16 times higher than the food safety limit respectively.
Yunnan province is an important plateau characteristic agricultural production place, so the investigation and risk analysis of crop heavy metal pollution in At present, researches on the quantitative relationship of heavy metal content in soil-crop system are mostly carried out in greenhouse or field plot experiments, which are quite different from the actual field production (Zeng et al., 2013). In addition, the research scope of heavy metal pollution in agricultural products is small and single. Therefore this study in order to further understand the farmland soil heavy metal accumulation characteristics in Yunnan province and the pollution degree of risk, not only in Yunnan province typical farmland soil, also took its corresponding crop edible part related to the investigation and determination, through the analysis on risk evaluation method in Yunnan province farmland soil-crop systems of heavy metal pollution risk profile. In order to find the correlation between the heavy metal content in soil and the heavy metal accumulation in crops, and then put forward the agricultural pollution prevention and safety production countermeasures according to local conditions.

Study Area
Yunnan province is located in the southwest border of China, within the range of 21˚08' -29˚15'N and 97˚31' -106˚11'E, with a total area of 394,100 km 2 . It is a plateau mountainous province, and the mountainous area accounts for 88.64% of the total area of the province. The terrain of Yunnan province descends in a ladder form from north to south. The characteristics of high latitude and high altitude, low latitude and low altitude are consistent, make the climate between the north and the south show obvious vertical differences. There are 19 soil groups, 36 subgroups and 150 soil genera distributed in Yunnan. There are 458 known mineral areas in central Yunnan province. Yunnan is a large agricultural province with prominent regional characteristics in China and an important production base of plateau agricultural and sidby-products. The main crops include rice, corn, wheat, beans, potatoes, tobacco, sugar cane, tea, rubber, flowers, pineapple, banana, mango, coffee and so on.

Test Soil Samples
A total of 619 pairs of cultivated soil samples (0 -20 cm) and corresponding crop samples were collected from Qujing, Zhaotong, Kunming, Yuxi, Wenshan, Honghe, Lijiang, Dali, Nujiang, Diqing and Puer using five-point sampling method and three-point sampling method, and brought back to the laboratory.
Remove non-soil sundries such as stones and weeds in the soil, and then dry the soil naturally, pass through 0.15 mm, 1 mm and 2 mm sieve according to the measurement requirements of each index, and store for later use. The sampling points are shown in Figure 1.
The crop samples corresponding to soil in 11 counties were taken, the edible parts were washed with running water to remove impurities such as soil and yellow leaves, and the surface moisture was dried with absorbent paper after the  water was too pure. After weighing the fresh weight, the samples were put into the oven at 105˚C for half an hour, and then dried continuously at 75˚C, and the dry weight was weighed and ground for later use.

Test Items and Methods
The pH value was measured by glass electrode method, and the soil-water mass ratio was 2:5.
Flame atomic absorption spectrometer and graphite furnace atomic absorption spectrometer (Shimadzu-AA6880) were used in the detection.

Heavy Metal Pollution Assessment
1) Single pollution index method and index of compressive quality.
Single pollution index method: (Nazzal et al., 2021) relative impact equivalent, RIE; deviation degree of determination concentration from the background value, DDDB; deviation degree of soil standard from the background value, DDSB; n is to measure the oxidation number of element i; n is the number of measured elements; C i is the detected concentration of heavy metal element i; C si is the screening value of soil environmental quality of element i; C Bi is the background value of element i.
Quality index of agricultural products, QIAP CAPi corresponds to the content of agricultural element i at soil points; CLSi is the limit standard of heavy metal element i in agricultural products comprehensive quality impact index (IICQ): The number of soil X exceeding screening value; the number of soil Y exceeding background value; Z is the number of elements of IICQS; K is the background correction factor.

Data Processing Methods
Basic data processing was performed using Microsoft Excel, while significance variance, correlation, and clustering analysis were conducted by SPSS. Point import and Kriging spatial interpolation were analyzed by ArcGIS.

Accumulation Characteristics of Pb and Cd Heavy Metals in Soils of Different Administrative Regions
The soil pH value in the whole study area ranges from 4.23 to 7.86, with an average value of 6.84. The soil is acidic, and the coefficient of variation of pH is 9.15%, indicating that there is little difference in soil pH in the study area. Table 1 shows the heavy metal content in the soil of the whole sample in the study area. The contents of heavy metal Cd and Pb in soil ranged from 0.02 to 31.98 mg/kg and 2.68 to 889.92 mg/kg, respectively, with an average of 1.31 and 64.17 mg/kg, respectively. However, only the average content of Cd exceeded the screening value by 4.33 times (see Table 2 for screening values). The average Journal of Geoscience and Environment Protection  Table 2 and Table 3.
Cd content as shown in and Nujiang (Pb).

Accumulation of Cd and Pb in Edible Parts of Crops in Different Administrative Regions
The contents of Heavy metals in edible parts of all crop samples in the study area are shown in Table 4 The contents of Pb and Cd in edible parts of crops in different administrative regions vary greatly, and their contents are shown in Table 5 and    Table 5 and Table 6.
The accumulation of the two heavy metal elements is as follows:  Figure 2: among the five main crop types, the Pb content in edible parts of crops in descending order was legume > tubers > cereals > leafy vegetable > Brassica. The content of Pb in edible part of legumes was the highest (0.33 mg/kg), and that in leafy vegetables was the lowest (0.12 mg/kg). According to the enrichment coefficient, legumes (1%) > tubers (0.58%) > cereals (0.54%) > brassicas (0.51%) > leafy vegetables (0.34%).

1) Pb content as shown in
2) Cd content as shown in Figure 2: among the five main crop types, the content of Cd in edible parts of crops in descending order: leafy vegetables > tubers > Brassica > legumes > cereals; among them, the content of Cd in edible part of leaf vegetable was the highest (0.10 mg/kg), and that in cereal was the lowest (0.03 mg/kg). According to the enrichment coefficient, legumes (16.99%) > tubers (14.1%) > leafy vegetables (12.55%) > brassicas (11%) > cereals (4.8%).

1) Single pollution index method and index of compressive quality
Pb: The spatial distribution of potential ecological risk of Pb element in the surface soil of cultivated land in the study area is shown in Figure 3.  surface soil of cultivated land in the study area is shown in Figure 2 and Figure  3. The region with high value of potential ecological risk index of Cd element in soil is in the western part of Yunnan, and the whole range of eastern Yunnan is also affected to varying degrees. According to Hakanson potential ecological risk classification standard (Deng et al., 2019), the average value of Cd element ecological risk index in soil is 126.43, which indicates that the study area is in a relatively strong ecological risk, and the site of 30.69% is a slight ecological risk 30.86% of the sites were moderate ecological hazards, 21.97% were strong ecological hazards, 7.92% were very strong ecological hazards and 8.56% were extremely strong ecological hazards.
2) Index of compressive quality Figure 4 shows the comprehensive soil quality Impact Index (IICQs) and agricultural product Quality Index (IICQAP) in Yunnan Province. The parameters and standard values of soil background values are shown in Table 1. The contents of heavy metals in edible parts of crops are referenced, and the standard limits of GB 2762-2012, GB 15199-1994and GB 13106-1991. The mean value of IICQs and IICQAP in the study area was 2.97 and 0.14 respectively. The severely polluted soils (IICQS > 5) were mainly concentrated in qujing, Honghe and Nujiang rivers. Most of the crops were clean or slightly polluted (IICQAP < 2), and the highest value of IICQAP appeared in Zhaotong in eastern Yunnan and southern Nujiang in western Yunnan.
1) At 19.06%, both soil and crops were not polluted, and the soil environmental quality was good. 2) 1.62% indicates that the soil is not polluted, but the crops are polluted, and the comprehensive environmental quality of the soil is in the sub-health state. 3) At 49.27% of the sites, the soil was polluted, but the crops were in the limited quality standard, and the comprehensive environmental quality of the soil was in the state of sub-pollution, requiring close attention. 4) At 29.4% of the sites, the soil and crops were polluted to varying degrees. Figure 5 shows IICQ of cultivated land comprehensive quality impact index in the study area. The average value of IICQ was 3.12, indicating that the study   area was moderately polluted as a whole. 21.1% of the sites were clean, 0.48% were slightly polluted, 42.83% were mildly polluted, 49.81% were moderately polluted, and 14.65% were severely polluted.

Discussions
The cumulative pollution of heavy metal Cd in soil in the study area is the most serious, which is consistent with the investigation results of Zhang Xuhui et al. Yunnan province . Coefficient of variation is an indicator to measure the relative dispersion degree of each test value in a set of data and can reflect the average variation degree of the total sample value (Li et al., 2017). The two heavy metals Cd (195.15) > Pb (172.04) in the study area may have obvious regional differences in their spatial distribution (Zhao et al., 2022), which may be related to the complex geological background of Yunnan Province and the existence of mineral development and man-made source pollution in some areas.
Among the cities in this study, Pb and Cd pollution in the edible parts of crops to the problem of sample retention and placement, and their representativeness may be poor.

Conclusion
1) The average contents of Pb and Cd in soil were 64.17 mg/kg and 1.31 mg/kg, respectively. The average content of Pb and Cd has exceeded the background 2) The average contents of Pb and Cd in the edible parts of crops were 0.20 and 0.08 mg/kg, respectively. Among the 10 cities involved in the study area (except Puer), the highest average contents of heavy metals were found in Diqen (Pb) and Nujiang (Cd). The enrichment coefficients of heavy metals in edible parts of crops were the largest in Diqen (Pb) and Zhaotong (Cd), while Pb and Cd exceeded the standard in Nujiang and Diqen were the highest. The average content of Pb in legume crops and Cd in leafy vegetable crops is the highest, and the crop type with the highest enrichment coefficient of all elements in different crop types is legume. Therefore, the planting of legume crops should be reduced, while the planting of leafy vegetable crops should be reduced in areas with more serious Cd pollution.
3) The potential ecological risk assessment showed that the study area was at the critical level of low ecological risk and moderate ecological risk as a whole.
The potential ecological risk of heavy metal Cd in soil was the highest, while that of other elements was slight ecological risk. The average value of IICQ was 3.12, indicating that the study area was moderately polluted as a whole. 21.1% of the sites were clean, 0.48% were slightly polluted, 42.83% were mildly polluted, 49.81% were moderately polluted, and 14.65% were severely polluted.