## 2 Bioavailability Assessment of Heavy Metals in the Soil Using DGT  
Inorganic pollutants (especially trace metals and quasi-metals) enter organisms through their environmental behaviors, and if they are enriched in large quantities, they will cause damage to organisms, toxic effects, and even threaten life and health. Heavy metals can enter the human body, and heavy metals proteins react with proteins, ribose, vitamins, hormones, and other substances in the body to form metal complexes or chelates, causing their original biochemical functions' disorder, which damages the kidney, liver, bone, immune system, reproductive system, and nervous system, even in pathological changes, and even death [35; 36]. Therefore, measures must be taken to reduce the exposure dose of heavy metals in the population to reduce their health risks to humans. The stress of most metals in the soil can reduce soil pH, bulk density, and cation exchange capacity (CEC), and also cause the lack of available potassium, alkali-hydrolyzed nitrogen, phosphorus, and other nutrients and their effectiveness [37]. The soil ability to provide nutrients for crops is weakened [38]. At the same time, the increase in the degree of metal pollution will destroy the active groups and spatial structure of enzymes, reduce the biosynthesis of enzymes, cause the reduction of enzyme activities [39], and inhibit the growth of microorganisms, resulting in a significant reduction in the number of soil microorganism populations and community diversity. Heavy metal intrusion made gradually decreased soil ammoniation, nitrification, and self-respiration inhibited. For different types of heavy metals, due to the differences in their physicochemical behavior and bioavailability, the migration and transformation rules in soil were obviously different. The complex pollution formed by heavy metals made the migration and transformation of heavy metals in the soil more complicated. Moreover, due to differences in experimental conditions and heavy metal types, different scholars often drew different conclusions, causing differences in the bioavailability assessment of metals. Hence, it was necessary to explore a stable, accurate, and efficient DGT technique for the determination of bioavailability. In 1998, DGT was used for the first time to measure the bioavailability of heavy metals in soil, then extending DGT technology to the field of soil [40]. During deployment of the DGT device, the average flux of available species in the soil transferred to the device can be directly measured, enabling the characterization of bioavailable metal concentrations in the soil. Therefore, in the early stage of the DGT technique, it was mainly used to evaluate the metal concentration in soil, and to investigate the correlation between it and the metal concentration content measured by other chemical analysis methods, such as CaCl\({}_{2}\) aqueous solution, soil pore water, BCR methods, etc. [41; 42; 43; 44]. Changes in soil metal concentrations measured by DGT over time scales also reflected the desorption of metal elements from the soil solid-phase composition and release to the liquid-phase composition, as well as changes in soil metal species. This promoted the research on the migration and transformation of metal elements in soil. A study using DGTfound that Cadmium (Cd), Copper (Cu), and Zinc (Zn) in soil under plastic sheds had relatively high bioavailability, and Cd exhibited the highest migration risk due to its largest available pool and stronger desorption capacity, which better reflected the migration of metals from soil to vegetables [45]. However, some studies had shown that although DGT was sensitive to changes in soil properties and the rhizosphere, and could better show the increase in metal mobility and availability, it could not successfully simulate the uptake of Cu by the aerial parts of tolerant plants [46]. In Cu-contaminated soils, the resupply of Cu from the soil solid phase to the solution was extremely slow, and the kinetics of Cu adsorption was also slow.