Influence of heavy metal element in soil on safety of agricultural products

I. Research ideas and methods

Soil is the foundation of crop growth. Heavy metals in soil are partially accumulated in edible organs such as fruits and grains through the absorption of crop roots and the migration in plants, thus affecting the safety of agricultural products. Therefore, soil, air, water, pesticides and fertilizers are important factors for agricultural ecological environment and agricultural product safety.

Soil physical and chemical conditions such as soil organic matter, pH, redox conditions, texture and structure determine the existing forms of heavy metal elements in soil, which is an important soil environmental factor affecting the absorption and accumulation rate of heavy metal elements by crops. On the other hand, the content of heavy metal elements in crops is related to the types and varieties of crops, and the heavy metal elements absorbed by roots often need to travel long distances in plants to reach edible parts such as fruits and grains; Furthermore, heavy metal elements in soil are not the only source of heavy metal elements in crops and agricultural products. Heavy metal elements contained in dust deposited on leaves and fruits of crops and sprayed pesticides and fertilizers can also enter plants and agricultural products through foliar absorption. All these factors lead to the complexity of the relationship between edible parts and the content of heavy metal elements in soil. In this study, wheat was selected as the research object, and the supporting samples of root soil were collected at the same point, so as to eliminate the influence of various factors as far as possible, and the influence model of heavy metal elements in soil on agricultural products was studied, which provided scientific basis for the evaluation of ecological effects of heavy metal elements pollution in soil.

ii. relationship between soil element forms and heavy metal element contents in agricultural products

the correlation between heavy metal element contents in wheat seeds and corresponding root soil forms is shown in table 629. from table 5-1, it can be seen that the forms that affect the contents of Cu, Cd and As in wheat seeds are water-soluble, ion-exchanged and carbonate, and the other forms also have great influence on the contents of Cu and Cd in wheat (figure 5-11). The main form affecting the content of Ni in wheat seeds is ion exchange state; The main forms affecting the content of Zn in wheat seeds are humic acid state and iron-manganese oxidation state, and carbonate state and residual state also have great influence on Zn content. The main form affecting the content of Hg in wheat seeds is residue state. However, the contents of Cr and Pb in wheat seeds have no significant correlation with each form. Therefore, different forms of heavy metal elements have different biological activities and migration ability to wheat.

Table 5-1 Statistical Table of Morphological Correlation Coefficient of Heavy Metal Elements in Wheat Seed and Corresponding Root Soil

Note:? The confidence level is .5, * * is .1, and the number of samples is 86.

fig. 5-11 correlation diagram between Cu and Cd contents in wheat seeds and soil morphological contents in roots

III. Influencing factors of heavy metal elements migration and transformation in topsoil-crops

The ecological effect of heavy metal elements in soil on plants is controlled by many factors. The amount of heavy metal elements absorbed by plants from soil has a certain relationship with the total amount of heavy metal elements in soil, but the total content of heavy metal elements in soil is not a reliable indicator of plant absorption. The migration and transformation of heavy metal elements in soil-plant system are mainly affected by soil physical and chemical properties (pH, Eh, clay, organic matter, etc.), forms of heavy metal elements in soil and plant characteristics.

(I) Concept and evaluation criteria of safe crops

Crops can produce benign effects and functions on ecological environment, human health and biodiversity during eating and processing, and can be called safe crops or agricultural products. On the contrary, it can be called unsafe crops or agricultural products.

the so-called pollution-free food refers to pollution-free, non-toxic, safe and high-quality food, which is called pollution-free food or organic food, ecological food and natural food abroad, and also called green food in China. Pollution-free food (green food) is divided into AA grade and A grade, the main difference is that AA grade does not use any pesticides, fertilizers and synthetic hormones in the production process; Grade A allows limited use of pesticides, fertilizers and synthetic hormones.

Wheat is one of the important food crops under the "wheat-corn rotation" planting mode in the investigation area. In this study, 86 wheat seed samples and corresponding root soil were collected in Pingdu, Yantai and Wendeng, and the contents of As, Cd, Hg and Pb were analyzed.

See Table 5-2 for the standards of green food, pollution-free food and food hygiene limit of wheat in China. In this study, the safety evaluation scheme of wheat is determined as follows:

Table 5-2 Table w(B)/1-6

Note: The data source is pollution-free food standard: NY531—25；; Green food standard: NY/T421—2; Hygienic limit standard: GB 2715-25; GB1315—91； GB2762—94； GB1316—91； GB14961—94； GB15199—94； GB481—94； GB14935—94； GB1521—94。

1) The samples whose contents of As, Hg and Pb in wheat seeds are lower than the hygienic standard of green food are called green food; Samples greater than the standard of green food but lower than the hygienic standard of pollution-free food are called safe food; Samples with seed content greater than the hygienic standard of pollution-free food for urban residents are called over-standard food.

2) The standard limits of Cd in wheat seeds are consistent, so it is divided into two levels: the samples whose Cd content is lower than the standard are called safe green food; Samples that are higher than the standard are called over-standard foods. Samples whose contents of Zn, Cu and Cr are lower than the hygienic limit standard are called safe green food. Samples that are higher than the standard are called over-standard foods.

3) The hygienic limit standard of Se in wheat seeds is .3×1-6. Tan Jian 'an et al. (1989) pointed out that when the Se content in grain is between .25× 1-6 and .7× 1-6, its selenium content is on the marginal limit, so this study takes .4×1-6 as. Samples with a seed content of .7× 1-6 ~ .7× 1-6 are called selenium-rich foods, samples with a seed content of .7× 1-6 ~ .3× 1-6 are called selenium-poor foods, and samples with a seed content of < .4× 1-6 are called selenium-poor foods with a content of > .3× 1-6.

4) Comprehensive evaluation comprehensively examines the contents of As, Cd, Hg, Pb, Zn, Cu, Cr, Se in each seed sample, and the samples whose contents are all lower than the hygienic standard of green food are called green food; As long as one of the eight elements exceeds the hygienic standard of pollution-free food, it is called over-standard food, and the element content is between the two, it is called safe food.

(II) Contents of elements in wheat

1. Evaluation of wheat safety

The classification of As, Cd, Hg, Pb and Se in wheat seeds in the study area is shown in Table 5-3 and Table 5-4.

table 5-3 statistical table for grading As, Cd, Hg and Pb of wheat seeds unit: %

note: the number of samples in brackets means that there are no samples of this grade.

Table 5-4 Statistical Table of Se Grading of Wheat Seeds

Note: The number of samples in brackets means that there are no samples of this grade.

The statistical results showed that the contents of Cd, Zn and Cr in 86 wheat seeds exceeded the standard in different degrees, among which 1 samples of Cd and Cr exceeded the standard, accounting for 11.63% of the statistical samples. However, only 3 samples of Zn exceeded the standard, accounting for 3.49% of the statistical samples. As, Hg, Pb and Cu did not exceed the standard; But for Hg and Pb, 5.81% and 3.49% of wheat seeds are pollution-free food respectively. Generally speaking, Cd and Cr are the main factors that affect the safety of wheat seeds.

the Cd content of wheat seeds ranged from .18× 1-6 to .778× 1-6, and a certain proportion of wheat seeds in all regions exceeded the standard of pollution-free food. The Cr content of wheat seeds exceeds the standard, showing obvious regional differences (Figure 5-12).

Figure 5-12 Contents of Cd and Cr in wheat seeds from different regions

1— Green food; 2— Over-standard food < P > The Se content in wheat seeds is lower than the food limit standard, and the samples belonging to selenium-enriched products account for 22.9%, mainly distributed in Laizhou and Yantai. Some of the wheat seeds in the study area are poor in selenium, and the Se content in 5% of the wheat seeds is less than .4× 1-6 (Figure 5-13). The Se content of 27.91% wheat seed samples is between .4× 1-6 and .7× 1-6, which belongs to selenium-rich wheat.

fig. 5-13 diagram of Se content in wheat seeds from different regions

1-Se-enriched wheat; 2-Selenium-poor wheat; 3-Se-enriched wheat < P > The comprehensive evaluation results showed that there were 6 pollution-free foods from wheat seeds, accounting for 6.98% of the statistical samples, mainly distributed in Pingdu; There are 21 foods with wheat seeds exceeding the standard, accounting for 24.42% of the statistical samples, which are distributed in various regions. There are 59 green foods from wheat seeds, accounting for 68.6% of the statistical samples, which are distributed in various regions (Figure 5-14).

figure 5-14 histogram of comprehensive quality of wheat seeds in different regions (the greater the ratio, the worse the quality)

1-green food; 2-Safe food; 3— Distribution and distribution of over-standard foods

2.Cd, Hg and Pb in wheat roots, stems (leaves) and seeds < P > As, Cd, Hg and Pb absorbed during the growth of wheat plants have different distribution characteristics in different organs such as roots, stems, leaves and seeds. In this study, 28 sets of wheat stem (leaf) and seed samples were collected in Pingdu, Laizhou, Yantai and Wendeng, and 2 sets of root, stem (leaf) and seed samples were collected in Yantai and Wendeng to study the distribution characteristics of As, Cd, Hg and Pb in different parts of wheat plants.

the distribution characteristics of p>Cr, As, Cd, Hg and Pb in different parts of wheat in different regions are as follows: stem and leaf content > seed content (figure 515), and the average content of Zn in wheat in different regions is as follows: seed > stem and leaf; The contents of Cu and Se elements are slightly different, a few samples are seeds > stems and leaves, and most samples are stems and leaves > seeds.

Figure 5-15 Distribution of As, Cd, Hg, Pb, etc. in wheat stems (leaves) and seeds

White is the content of stems and leaves, and the color of patterns is the content of seeds

Theoretically, plants absorb heavy metal elements from soil solution, most of which accumulate in roots and stems near the ground, while the amount transported upward by transpiration is generally small. However, it was found that the contents of most elements such as Hg, Zn, Cd and Cu in wheat showed the distribution characteristics of stems and leaves > roots, and even the contents of Hg and Cu in stems and leaves were higher than those in root soil (Figure 5-16). The fact that heavy metal elements are mainly distributed in leaves and stems shows that a considerable part of heavy metal elements in wheat may come from atmospheric dry and wet deposition. In addition, the tillage method of returning straw to the field is easy to cause secondary pollution to the topsoil for heavy metal elements such as Hg and Cu.

(III) Enrichment coefficient of lead, cadmium, mercury and arsenic in wheat and its influencing factors

Enrichment coefficient refers to the ratio of the concentration of a substance or element in an organism to the concentration of the substance or element in the biological growth environment (water, soil and air). There are many factors influencing the absorption of harmful elements such As As and Cd by crop seeds, and the process is very complicated. Therefore, this section focuses on the absorption (enrichment coefficient) of harmful elements such as Cd, As, Pb, Hg and Cr by wheat seeds only from the statistical point of view, and establishes the quantitative relationship between the content of Cd in seeds and soil Cd, pH, OrgC or other indicators, in order to evaluate and study the ecological security on a regional scale.

Figure 5-16 Distribution of As, Cd, Hg, Pb, etc. in roots, stems (leaves) and seeds of wheat

1-seeds; 2-stem; 3-root; 4— Soil

1. Enrichment coefficient of elements in wheat seeds < P > As can be seen from Figure 5-17, the enrichment coefficients of elements are quite different, among which the enrichment coefficients of Cr, Pb, As and F are less than 1%, and that of F is the smallest (.5%); The enrichment coefficients of I and Ni are 2.79% and 2.5% respectively. The enrichment coefficient of other elements is more than 1%, among which Zn is the largest (51%), followed by Cd(34.31%). By comparison, the order of enrichment ability of wheat seeds to various elements is Zn > CD > Se > Cu > Hg > I > Ni > Cr > Pb > As > F.

Figure 5-17 Distribution of enrichment coefficients of As, Cd, Hg, Pb and other elements in wheat seeds

2. Effects of soil physical and chemical properties on enrichment coefficients of Pb, Cd, Hg, As and other elements

The enrichment coefficient of Cd in wheat seeds is relatively large, and it is generally high in Laizhou, with the greatest variability, and the enrichment coefficient of Cd in wheat seeds varies from 9.91% to 112. The absorption of Cd by wheat in Laizhou area is closely related to the Cd content and pH of soil.

the absorption of Cd by wheat seeds is significantly related to the total amount of Cd in the soil, and the Cd enrichment coefficient of wheat seeds is mainly affected by the soil pH. As can be seen from Figure 5-18, when the soil pH is in the neutral or alkaline range, the enrichment coefficient increases slowly with the decrease of pH, and when the soil pH is less than 5.5, it increases rapidly to 35%, which shows that the acidification of soil in the middle alkaline range can improve the Cd absorption rate. In addition, wheat seed Cd is significantly controlled by soil Cu and Zn contents (positive correlation).

fig. 5-18 relationship between Cd in wheat seeds and Cd in soil and Cd enrichment coefficient and pH

The enrichment coefficient of As in wheat is .4% ~ 1.14%, with an average of .35%, and the enrichment coefficient of As in most samples is less than 1.%. The enrichment coefficient of As is slightly different in wheat seeds from different regions, and Wendeng region is generally higher than other regions.

The absorption of As by wheat seeds is not significantly related to the total amount of As in the soil, but mainly controlled by clay content and pH value in the soil (Figure 5-19). With the decreAse of clay content, soil acidification and as enrichment coefficient of seeds show an upward trend, which shows that clay content and pH value in the soil play an important role in controlling the geochemical behavior of As.

Figure 5-19 Relationship between As enrichment coefficient of wheat seeds and clay and pH

The enrichment coefficient of Pb in wheat seeds is relatively small, except for Pingdu area, which is generally high (with an average of .56%). The enrichment coefficient of Pb in wheat seeds in other areas is generally less than .5%, and the enrichment coefficient of Pb in Wendeng area is generally lower than that in other areas, with an average of only .21%. The correlation coefficient between P b content of wheat seeds and enrichment coefficient can reach .85%. Only three of the 86 samples in the whole region are pollution-free wheat samples distributed in Pingdu area, others are green foods, and there is no food with Pb exceeding the standard in the whole region, which indicates that the low enrichment coefficient leads to the low content of Pb in seeds.

Pb absorption by wheat seeds is mainly related to soil organic matter and CEC. As can be seen from Figure 5-2, with the increase of soil organic matter and CEC, the enrichment coefficient will also increase.

Figure 5-2 Wheat Seed Pb