*2.1. Collection of Soil Samples and Preparation of Biochars*

Surface soil samples (0–15 cm) were collected from a field in Pingtung, Southern Taiwan (22◦31 57.9 N 120◦33 38.1 E). As of April 2016, pineapple (*Ananas comosus* (L.) Merr.) was the dominant crop on this land. The soil samples were air-dried, sieved through a 2-mm screen, and stored at room temperature. The biochar used in this study comprised Honduran mahogany (*Swietenia macrophylla*) wood sawdust obtained from the Department of Wood Design, National Pingtung University of Science and Technology. Two biochar materials were used in this study, namely WB300 and WB600, that were produced at pyrolysis temperatures of 300 and 600 ◦C, respectively. The biochar used in this study was supplied by the Industrial Technology Research Institute (ITRI) of Taiwan. Before being charred, the wood sawdust was dried at 60 ◦C for 24 h to < 10% moisture and cut to a particle size of 2 cm. For pyrolysis, the samples were placed in a tubular furnace (ITRI, Tainan, Taiwan) equipped with a corundum tube (diameter, 32 mm; length, 700 mm) and a N2 purging mechanism (flow rate, 1 L/min) to ensure an oxygen-free atmosphere. Heat treatments were performed at temperatures of 300 and 600 ◦C, with the heating rate being 5 ◦C min−1. The temperature was maintained for 2 h before cooling to an ambient temperature under an N2 flow. After the pyrolysis, the biochar materials were ground to pass through a 2-mm sieve, followed by homogenization through stirring.

#### *2.2. Preparations of Leaching Column*

Similar to the procedures applied by Lo [30], the biochar materials were thoroughly mixed with the collected sandy loam soil at application rates of 0% (Control, 0 tons ha<sup>−</sup>1) and 2% (40 tons ha−1) w/w for Bok choy (*Brassica rapa chinensis*) cultivation in Taiwan. Briefly, nutrient solutions of ammonium sulfate ((NH4)2SO4), calcium dihydrogen phosphate (Ca(H2PO4)2), and potassium chloride (KCl) were added to the soil at application rates of 2076, 227, and 191 kg ha<sup>−</sup>1, respectively (approximately 220, 30, and 100 kg ha−<sup>1</sup> for N, P, and K, respectively). The fertilizers were dissolved in deionized (DI) water and then mixed thoroughly with the soil samples. The volume of the nutrient solution applied was 60% of the water retention capacity of the treated soil samples. The treatment samples are outlined as follows: (1) control, comprising soil only (CK); (2) WB300, comprising soil to which 2% of the biochar pyrolyzed at 300 ◦C was added; and (3) WB600, comprising the soil to which 2% of the biochar pyrolyzed at 600 ◦C was added. A leaching experiment was conducted for each treatment in three replicates. As illustrated in Figure 1, a soil column with an internal diameter of 20.6 cm was constructed. The column was composed of two polyvinyl chloride (PVC) tubings of equal length, which were connected through a PVC fitting with a 5-cm interval. A nylon mesh (1 mm2) with filter paper (Whatman grade no. 42) above was placed between the joints to separate the soil in the upper part from the quartz sand (~2 mm in diameter) filled in the center PVC fitting. At a soil depth of 15 cm, the volume of the soil column was approximately 5000 cm3. All the columns were packed with the tested soil samples to obtain an initial bulk density of 1.2 g cm<sup>−</sup>3.

**Figure 1.** Schematic of the soil column constructed for the leaching experiment.
