*3.1. Biochar E*ff*ects on Cd and Pb in the Soil Water Soluble Phase and Various Pools*

In-situ dissolved soil Cd and Pb concentrations may be considered bioavailable [31]. In-field, soluble soil Cd and Pb data, collected via the DMT, are presented in Figure S2. More often than not, increasing biochar application rate decreased paddy field soil water-soluble Cd and Pb concentrations over both the short- and long-term. Long-term Cd concentrations significantly decreased by 24.5–52.2% (2016), 32.1–73.2% (2017) and 37.1–80.8% (2018), while Pb concentrations decreased by 32.1–88.1% (2016), 15.7–82.4% (2017) and 10.3–46.2% (2018) (Figure S2A,C) with biochar applications compared to the control. Short-term Cd and Pb concentration were significantly decreased by 19.8–46.5% (2016), 14.5–67.4% (2017), 30.0–76.0% (2018) and 6.7–63.3% (2016), 17.6–65.5% (2017), 14.3–44.6% (2018) (Figure S2B,D), respectively, with biochar applications compared to the control. Greater short- and long-term Cd and Pb decreases were associated with greater biochar application rates. When soluble Cd increases were observed, they may have been related to increases in water soluble organic phases present that chelated Cd, as suggested by Fan et al. [32]. However, similar to most of our findings, Xu et al., also found that water-soluble Cd and Pb concentrations decreased by 59% and 13% with macadamia nutshell biochar amendment in a lab incubation study [33]. Comparable Cd and Pb results were found by Wang et al., when using rice straw biochar in a pot experiment [34].

Results of the BCR sequential extraction on soil Cd and Pb fractions are presented in Figure 1. The exchangeable Cd and Pb fractions (B1) were approximately 40% of the total Cd and Pb extracted, yet tended to decrease with increasing biochar application rate. When exchangeable Cd and Pb content decreased, it appears that increases were associated with the residual fraction (B4), indicating a reduction in Cd and Pb bioavailability and lower potential ecological risk associated with biochar amendment, as suggested by others [35]. In support of these findings, Chen et al., showed that biochar reduced Cd bioavailability by transforming the exchangeable fraction into the residual fraction, associating these changes to increases in soil pH [36]. Wang et al. [37] and Liu et al. [38] both

observed similar responses when using wheat straw or coconut shell biochar, respectively, to reduce Cd bioavailability. Sludge-based biochars have also been shown to decrease Pb and Cd bioavailability from 55.9% to 4.9% for Pb, and from 78.2% to 12.5% for Cd [39].

**Figure 1.** The effect of long- and short-term, increasing biochar application rates [0 (C0), 10 (C1), 20 (C2) and 40 (C3) t ha<sup>−</sup>1] on Cd and Pb fractions based on a European Community Bureau of Reference (BCR) sequential extraction ((**A**): Cd long-term; (**B**): Cd short-term; (**C**): Pb long-term; (**D**): Pb short-term). B1 = Exchangeable fraction; B2= Iron and manganese oxyhydroxides fraction; B3 = Organic fraction; B4 = Residual fraction.
