*3.3. Properties of Leachate*

Both biochar-treated samples exhibited significantly smaller leachate volumes than that of the control for each flushing event (Table 4; Figure 4). On day 4 (the day of the first flushing event), the soil column with the control sample had a leachate volume of 530 mL, and both WB300- and WB600-treated samples retained approximately 150 mL more water than the control did (i.e., the leachate volume decreased by 28%). At the end of the experiment, the cumulative leachate volumes observed for the WB300- and WB600-treated samples were lower than that observed for the control by 9.2% and 13.7%, respectively.

Figure 5 displays the cumulative level of dissolved OC (DOC) in the leachate. This was highest in the control and lowest in the WB600-treated sample after the experiment (188 and 154 mg, respectively). After 42 days of incubation, the level of DOC leached from the soil column decreased by 6.50% and 20.0% in the WB300- and WB600-treated samples, respectively, compared with the control. The biochar materials contained low levels of OC. Accordingly, biochar introduces a negligible level of DOC into soils.


**Table 4.** Volume of the leachate from the soil columns after each flushing with DI water (n = 3).

The values followed by the same superscript letters within a column are not significantly different (*p* > 0.05) between the relevant treatments.

**Figure 4.** Cumulative leachate volume (VL) of treated samples (n = 3). Different letters above the bars for day 42 indicate significant differences between the relevant treated samples (*p* < 0.05).

**Figure 5.** Cumulative concentration of dissolved organic carbon (DOC) for different treated samples (n = 3).

The cumulative quantities of NH4 <sup>+</sup>-N and NO3 −-N leached from the soil columns are illustrated in Figure 6. WB300 remarkably reduced NH4 <sup>+</sup>-N leaching by 30.5% relative to the control (69.6 mg). Although the inhibitory effect of WB600 on NH4+-N leaching was relatively weak, it still reduced the total quantity of NH4 <sup>+</sup>-N leached from the soil by 10.6%, which was approximately one-third of that observed for the WC300-treated sample. The WB300- and WB600-treatments reduced the quantities of NO3 −-N leached from the soil samples by 13.8% and 16.4%, respectively, compared with the control (83.9 mg).

**Figure 6.** Cumulative quantities of (**a**) NH4 <sup>+</sup>-N and (**b**) NO3 −-N leached from the soil columns (n = 3).

The cumulative quantity of inorganic N (summation of the quantities of NH4 <sup>+</sup>-N and NO3 −-N) leached from the soil samples subjected to the different treatments differed significantly (Figure 7). The control exhibited the highest quantity of inorganic N leached from the soil (154 mg), and the WB300-treated sample exhibited the lowest quantity (33.3% lower than the control). Furthermore, WB600 treatment decreased the quantity of inorganic N leached from the soil by 13.7% only.

**Figure 7.** Cumulative quantity of inorganic N leached from the soil columns (n = 3).

Figure 8 displays the cumulative quantities of P leached from the soil columns. The total quantity of P leached from the WB600-treated sample decreased significantly (68.0%) compared with that from the control (12.2 mg). For the WB300-treated sample, the total quantity of P leached from the soil decreased by 45.2%. Compared with the control, the WB300 and WB600 treatments reduced the total

quantities of P leached from the soil by 29.71% and 7.70% (156 and 210 mg leached), respectively (Figure 9).

**Figure 8.** Cumulative quantity of phosphorus leached from the soil columns (n = 3).

**Figure 9.** Cumulative quantity of potassium leached from the soil columns (n = 3).
