## 3 Results and Discussion
### Effects of Biochar and Humic Acid on Heavy Metal Content in Crops and Soil
#### 3.1.2 Heavy Metal Content and Morphology of Soils  
Biochar and HA, when applied at different dosages and ratios, exhibited varying remediation effects on the immobilization of different HMs in the soil. The results in Figure 2A showed that the Mn content of maize rhizosphere soil was significantly reduced by 12.02% and 11.08% in treatments YB3 and YB6, respectively, compared with Y treatment (\(p<0.05\)); the solidification and remediation efficiencies of Mn in the soil were further enhanced by the YB3H and YB6H treatments, which significantly reduced by 17.76% and 14.53%, respectively (\(p<0.05\)) compared with Y treatment. This indicates that an excessive application of remediation materials may have a negative effect on the immobilization of soil HM, thereby counteracting the original remediation benefits. This is mainly attributed to the fact that excessive remediation materials may disturb the balance of the soil and its microenvironment, and change the competition of HM ions for the active adsorption sites in the soil, resulting in easier desorption and release of HM ions from the soil, which in turn increases their mobility and bioavailability ([PERSON] et al., 2018; [PERSON] et al., 2018; [PERSON] et al., 2018).  
Figure 1: Aboveground and belowground HM contents in crops. Y: CK; YB3: 3%Biochar; YB6: 6%Biochar; YB3H: 3%Biochar + 1%HA; YB6H: 6%Biochar + 1%HA. Letters denote significance at \(p<0.05\), and differences in letters denote differences among treatments.  
Cu content of the maize rhizosphere soil in the YB3, YB6, YB3H, and YB6H treatments was significantly reduced by 16.15%, 15.29%, 16.42%, and 16.61% (\(p<0.05\)), respectively, compared with that in treatment Y, but there was no significant difference between the treatments of YB3, YB6, YB3H, and YB6H. This indicates that the different materials and ratios of treatments did not significantly affect the rhizosphere soil Cu. Meanwhile, the curing repair effects of biochar and HA on different types of HMs (Mn and Cu) are significantly different, which were mainly attributed to the fact that the chemical properties of Mn and Cu, especially the degree of filling of their d-electronic layer and the electron arrangement, determine the oxidation state and coordination chemistry of the HMs, which largely affect their interactions with biochar and HA [40].  
Biochar and HA not only affect the content of HMs in soil but also significantly affect their morphology. The results of the morphology of HMs in the rhizosphere soil of maize showed (Figure 2B) that in the treatments YB3, YB6, YB3H, and YB6H, the stable states (Residual and  
Figure 2: Soil HM content (**A**) and Percentage of soil HM by form (**B**). Y: CK; YB3: 3%Biochar; YB6: 6%Biochar; YB3H: 3%Biochar + 1%HA; YB6H: 6%Biochar + 1%HA. Letters denote significance at \(p<0.05\), and different letters denote treatment differences.  
Oxidizable) of Mn increased by 16.12%, 24.8%, 18.68%, and 20.65%, respectively, in comparison to the Y treatment. Conversely, the stable state of Cu showed a slight decrease compared to the Y treatment. These findings indicated that the application of biochar and HA not only effectively decreased the total Mn and Cu content in soil but also successfully immobilized a certain quantity of Mn, which was verified in most of the studies [41; 42]. Biochar and HA directly participate in HM immobilization through complexation and redox reactions in a soil micro-interaction environment. On the other hand, DOM released from biochar is involved in this process [43; 44] which collectively resulted in alterations in the quantity or morphology of HM within the maize rhizosphere soil.