*3.5. The Influence of Biochar on Soil and Plant Characteristics in Relation to Cd and Pb Contamination*

Biochar application improved soil properties and reduced above-ground rice plant metals concentrations, with direct or indirect pathways illustrated via PCA and correlation analysis (Figure 6). The PCA results shown that biochar application rates were positively related to soil properties (e.g., SOM, pH), soil enzyme activity and soil moisture factors in PC1 (32.9%). Negative relationships between biochar applications and Cd and Pb concentrations in the soil exchangeable phase, and Cd and Pb concentrations in crops, were also observed in PC1. The factors of PC2 mainly included organic and residual Cd and Pb fractions and microbial diversity, which were positively related to biochar application rates. The factors of PC1 and PC2 explained 72% of the variability for all factors, indicating that biochar was one of the key factors affecting soil properties, various soil Cd and Pb fractions, and microbial diversity. Similarly, Tang et al., found that rice straw biochar application was effective at improving soil pH and SOM, leading to decreased Cd availability and subsequent increases in soil enzymatic activity [61]. Xing et al., showed that a sludge-based biochar increased the ratio of immobilized soil heavy metals, which positively correlated to the presence of *V. fischeri*, the growth of wheat, and the activities of other soil microbes [39].

**Figure 6.** A principle component analysis.
