*2.1. Biochar Production and Characteristics*

The biochars used were converted from switchgrass (SG; *Panicum virgatum*) and poultry litter (PL) via slow pyrolysis at 700 ◦C. The feedstocks obtainment and the detailed description of their conversion into biochar products can be found in Antonangelo and Zhang [11]. Switchgrass- and poultry litter–derived biochars are referred to as SGB and PLB, respectively. The biochars coarse materials were ground with a mortar and pastel gently before sieved through 1 mm for further physicochemical analyses [24] and through 0.25 mm for the potting experiment. A full characterization of SGB and PLB including all physicochemical properties such as moisture, ash content, particle-size distribution, elemental composition, surface functional groups, chemical attributes, specific surface area, cation exchange capacity (CEC), and morphology can be found in [24]. For this study, it is useful to emphasize the ash, total carbon (TC), total nitrogen (TN), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S) contents, as presented in Table 1.


**Table 1.** Physicochemical properties of biochar derived from switchgrass (SGB) and poultry litter.

Numbers followed by "±" are the standard deviations of triplicates (n = 3).

#### *2.2. Soil Sampling and Analysis*

Cadmium contaminated soil samples were collected with a shovel (0 to 15 cm) from a residential yard near chat piles located in Picher, Ottawa County, Oklahoma. Soil sample preparation for analyses and for the potting experiment was described in Antonangelo and Zhang [11]. Before and after the pot experiment, soil samples were analyzed for DTPA–extractable Cd. Before the potting experiment, the total Cd content was determined by an inductively coupled plasma-atomic emission spectroscopy, ICP-AES (SPECTRO Analytical Instruments GmbH, Boschstr. 10, 47533 Kleve, Germany) after digestion by concentrated HNO3 and H2O2 using EPA method 3050B [25]. The TN, and total and DTPA-extractable Cd before the potting experiment were respectively 1.7 ± 0.1 g kg−1, <sup>9</sup> ± 0.6 mg kg<sup>−</sup>1, and 1.84 ± 0.03 mg kg−1. The total Cd content was about tenfold the maximum found in normal Oklahoma soils [26]. The DTPA-extractable Cd has been considered the most efficient method to predict the Cd phytoavailability to grasses, such as rice (*Oryza sativa*) [27] and perennial ryegrass [11]. Still, the threshold of DTPA-extractable Cd found by Wu et al. [27] was 0.03 to 0.16 mg kg−<sup>1</sup> depending on the soil organic matter (SOM) content, which is more than 10 times lower than the content found in the Tar Creek soil of this study.
