*2.3. Soil Column Incubation and Leaching*

The soil columns were subjected to a 42-day incubation process conducted at room temperature (25–28 ◦C) and humidity (60%–80%) with repeated leaching in order to investigate the effects of biochar application on (1) the physicochemical properties of, (2) the hydraulic properties of, and (3) nutrient retention and leaching from the soils. Short-term duration of incubation period was chosen based on Yoo et al. (2013) [31], where the leaching experiment was finished within 60 days. Likewise, based on our previous studies, the variation of chemical and physical properties [13] and dynamic changes of N and P [7] after biochar addition might occur and finish within 8 weeks, therefore, a short-term experiment period (42 days) was selected in this study. Table 1 lists the analysed items.

According to the soil porosity and volume determined for the studied soil column, we used a leaching volume of 700 mL for each flushing process. A fine sieve was placed above the columns to minimize water disturbance to the soil surfaces during flushing. Throughout the experimental period, all columns were leached seven times (on days 4, 7, 14, 21, 28, 35, and 42) using DI water. The leachates were collected using 1000-mL measuring cylinders, and the volumes of the leachates were recorded. Leachates were then subjected to chemical analyses. After the final leaching event, the soil of each column was collected, air-dried, and ground to pass through a 2-mm sieve before further chemical analysis.


**Table 1.** Analysis items and relevant abbreviations for the leachates and soils.
