*3.2. Cadmium Bioavailability*

Overall, DTPA-extractable Cd concentrations were higher for the control and decreased as the rates of biochars increased [11]. Previous studies also suggested that biochars incorporation to soils effectively decreased the Cd availability as assessed by DTPA [33–38]. In this study, we correlated the DTPA-extractable Cd from the biochar-treated soils with Cd and accumulation in ryegrass plant parts (Table 4). Some studies revealed that the dilution effect caused by the increase of plant biomass also contributed to the decrease of Cd concentrations in plants cultivated in biochar-treated soils [33–35], which suggests that such decrease is not a consequence of biochar application. However, the positive correlations (and no correlations) found for DTPA-extractable Cd in the soil and Cd accumulation in ryegrass plant parts, when the biomass is accounted for, for both biochars, ensure the data reliability obtained in this study (Table 4). The absence of correlation suggests that DTPA-extractable Cd is better correlated with its uptake in plant parts, as shown in the previous study [11].

**Table 4.** Pearson correlations between DTPA-extractable Cd contents in the soil (*x*-axis) and Cd accumulation in ryegrass shoots and roots.


\*: *p* < 0.05. \*\*: *p* < 0.01. NS: non-significant (*p* > 0.05). SGB: switchgrass-derived biochar. PLB: poultry litter-derived biochar. SGB + PLB: the two biochar-treated soils.
