## 4 Discussion  
The soil of the tobacco growing region has a typical calcareous soil, geological background and the carbonate rock weathering effect releases metals into the soil, especially Cd [3,4], which increases the soil metal concentration and the environmental risk. HA reduces the bioavailability of metals by strong affinity and ability of forming stable chelate with metal ions, with carboxylic groups and phenolic-OH being the dominant binding groups in HA; this effect varies by metals [26; 27; 28]. Metals have different affinities for HA based on their stability constants, and Pb (14.8) has a stronger affinity than Cd(7.8), Cu(13.3), and Zn(8.1) [36]. It is worth emphasizing that the binding order of metals to HA in a typical multimetal contaminated soils is Pb \(>\) Zn \(\approx\) Cu \(>\) Cd [21]. The data processed by the Scatchard method has revealed two binding sites for Cu and Pb and one binding site for Cd and Zn in the sediments; thus, the complexing capacity order was Pb \(>\) Cu \(>\) Cd [37]. In this study, the reduction of available Pb (Figure 1, removal rate range 20% to 39%) in soil was higher than those of Zn (3% to 29%), Cu (9% to 18%), and Cd (23% to 37%). In addition, metals at low concentrations can be more effectively redistributed than those at high concentrations [38], and the fractions of metals are also critical factors affecting the removal of available metals [21; 39]. In this study, the total Cd in the soil was much lower than the amounts of other metals, while its exchangeable fraction content was higher. Conversely, the Pb and Cu exchangeable fractions were less than 20%, and the RES of Cu was up to 65% in the initial soil. Although some studies have shown that Cd has lower affinity for HA than Cu and Zn [26; 40], based on the above analysis, the reduction of available Cd (removal rate range was 23% to 37%) was more significant than that of available Zn and Cu (Figure 2 and Table 2). Using the chemical fraction results instead of the total metal concentrations provides better insight into the distribution and binding forms of metals for a better assessment of their mobility and bioavailability [22]. According to the BCR procedure, metal in the exchangeable fraction easily migrates and transforms in the soil and is accumulated by plants. Metal in the reducible fraction is associated with amorphous Fe and Mn (hydro) oxides, and these oxides strongly sorb metals, but according to soil environmental conditions, they are possibility desorbed. The oxidizable fraction is mainly organic and metal chelate, which has low mobility. Residual metal shows stronger stability and lower leaching potential in soil [21]. Therefore, the exchangeable fraction and reduction fraction are bioavailable, relatively soluble, and easily extracted when soil properties change, and these fractions are taken up by plants, constituting a greater environmental risk than those from OXI and RES [19; 41]. The majority of studies have found that the exchangeable fraction and reduction fraction can be transformed by organic amendments and converted into fractions (OXI and RES) scarcely taken up by plants [25; 42] because the oxidizable fraction and residual fraction bind with HA forming a chelate. Some complex mechanisms can occur during HA and metal interactions because the HA surface is rich in functional groups (carboxylic and hydroxyl groups) that easily forming complexes with metals. Range analysis (HA additions of 6.4, 10.3, and 14.8 kg:ha\({}^{-1}\)) showed that HA was the major factor that influenced the passivation of Cd, Pb, Cu, and Zn, in all treatments (Figure 1), and the Cu and Pb exchangeable + reducible fractions were bound and rendered unavailable (Figure 2, the oxidizable + residual fraction significantly increased). Combining these results with the Pearson correlation coefficient results (Table 1) indicated that the exchangeable fractions for Cd, Cu, Pb, and Zn and the reduction fractions for Pb and Cu were the main bioavailable metal fractions.