## 4 Results and Discussion
### Speciation Distribution Characteristics of Heavy Metals  
The speciation of various forms of heavy metals in soil and its distribution coefficient (that is, the ratio of the content of various forms of heavy metals to the total amount of heavy metals) is an important index to evaluate the environmental risk of heavy metals [36]. It was demonstrated that heavy metals in the water-soluble fraction and mild acid-soluble fraction easily migrate and transform in the soil, are absorbed by plants, and are more harmful to human beings and the environment; the reducible fraction is the form that is easier for plants to use; the oxidizable fraction is more stable and is the form that is harder to be used by plants, but it will be transformed under alkaline or oxidizing conditions and is potentially hazardous to organisms; and the residual fraction is originated from the soil minerals, which is stable in nature and can exist in the sediment for a long time, is not easy to be absorbed by plants, and is less potentially harmful to the whole soil ecosystem [37]. The distribution characteristics of heavy metal speciation forms in the soil of the study area are illustrated in Figure 3. Generally, there is little variation observed in the distribution characteristics of heavy metal speciation forms within the study area, the main components of Cr, Ni, Cu, Zn, As, Cd, Pb and Hg are all residual fractions. The residual fraction is a form of heavy metal that is not easily affected by changes in the physical and chemical properties of the soil and exists in the soil mainly in the form of layered silicates, which are blocked in the mineral lattice of primary and secondary minerals and are chemically very stable, making it difficult for them to be transported and transformed in the soil medium and have an impact on living organisms [38; 39]. The speciation characteristics of different heavy metals at each sampling point are illustrated in Figure 4. In the study area, Cr predominantly exists in a residual fraction, accounting for an average of over 90%, followed by an oxidizable fraction. This indicates the relatively stable properties of Cr in the study area. The speciation distribution of Ni in the 34 soil sampling sites was more consistent, with a high percentage of residual fraction, higher stability, higher unavailability in the soil, and low risk of contamination. The residual fraction still dominates the speciation form of Cu in the soil at each sampling point in the study area, while the proportion of oxidizable fraction for Cu is significantly higher compared to that of Cr and Ni. This may be due to the fact that the study area belongs to cultivated land, which is richer in organic matter, Cu has a strong affinity for organic matter, and organic matter available for exogenous Cu binding to form the oxidizable fraction is richer, causing it to exist in the form of an oxidizable fraction [40]. The speciation of Zn at the sampling sites is dominated by the residual fraction, followed by the mild acid-soluble fraction. Heavy metals in the mild acid-soluble fraction are mostly adsorbed on the surfaces of clay, humus and other minerals and microorganisms, and are released when environmental conditions such as pH change and can be directly utilized by biological organisms [41]. The pH value of the soil in the study area is in the range of 3.42-5.16, with an average value of 4.02, and the whole is acidic, which makes it easy to accelerate the dissolution and migration of the mild acid-soluble fraction, and there is a certain ecological risk of Zn. Among them, the proportion of mild acid-soluble fraction reached 17.46% and 21.76% in points 3 and 5, which are the two sampling points with the highest proportion of mild acid-soluble fraction in each sampling point in the study area, and they should be paid proper attention.