A soil sample bag with a height of 30 cm and an inner diameter of 10 cm was adopted. The soil required to fill a 21 cm soil column was calculated as 850 g, and the air-dried soil treated in the early stage was weighed and evenly filled to form a soil column. The soil was treated with water injection to ensure that the water content in the bag was 30%, 40%, and flooded, respectively, and stood for 24 h. Each soil water content was treated as follows: local farmers commonly use urea, ammonium bicarbonate, ammonium bicarbonate, and ammonium sulfate. Four nitrogen fertilizers (i.e., ammonium bicarbonate, ammonium sulfate, ammonium chloride, and urea) were treated with oxalic acid, DMPP, and NBPT, respectively (A1 ammonium bicarbonate, A2 ammonium bicarbonate + oxalic acid, A3 ammonium bicarbonate +DMPP, A4 ammonium bicarbonate +NBPT, collectively referred to as A treatment; B1 ammonium sulfate, B2 ammonium sulfate + oxalic acid, B3 ammonium sulfate +DMPP, B4 ammonium sulfate +NBPT, collectively referred to as B treatment; C1 ammonium chloride, C2 ammonium chloride + oxalic acid, C3 ammonium chloride +DMPP, C4 ammonium chloride +NBPT, collectively referred to as C treatment; D1 urea, D2 urea + oxalic acid, D3 urea +DMPP, D4 urea +NBPT, collectively referred to as D treatment). Each treatment was replicated three times. The mixture was applied to the surface of the soil column after resting. Then, the soil was covered at 135 g, the height was 3 cm, and the specific amount of water injected was calculated according to the quality of the covering soil. The mouth of the soil column was wrapped and sealed with a rope, and five small holes with a diameter of about 1 cm were pricked with needles to ensure ventilation, avoid an anaerobic environment, reduce water loss, and maintain soil water content during cultivation. Then, the soil column was placed horizontally indoors for 30 days.
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