2.1.3. Experimental Method

First, the experiments were conducted using soil from different media to fill the columns, filled with distilled water and stabilized at a 30 cm scale at the initial water level. Then, potassium nitrate and ammonium chloride solutions were injected from the top of the column. Three days later, we took out the water sample, and adjusted the water level using a peristaltic pump, raising its height by 10 cm each time until the water level reached 50 cm; after that, we decreased the water level by 10 cm each time, until the water level

dropped to 10 cm. Finally, we raised the water level again, by 10 cm each time, until it reached 30 cm. The water levels during the experimental period were 30, 40, 50, 40, 30, 20, 10, 20, and 30 cm, respectively. Each water level was maintained for 3 d, and the change in the water level over time is shown in Figure 3, with the experiment using 24 d as a complete change period. The investigation was carried out for two cycles.

**Figure 3.** Time variation of water level.

Before each water level change, water samples were collected at four sampling ports using a diaphragm vacuum pump and a soil solution sampler to measure the concentrations of three forms of ammonium nitrogen, nitrate nitrogen and nitrite nitrogen.
