Nitrogen and phosphorus leaching differs among cinnamon soil layers

Leached agricultural N and P are the most prominent groundwater pollutants. Five cinnamon soil layers (cultivation, leaching, calcium, clay, and parent) were analyzed via leaching tests to investigate N and P migrations. After five tests, the amount of soluble total N in the leaching solutions were 2412.63 mg·L^-1 in cultivation layer, 3028.94 mg·L^-1 in leaching layer, 244.16 mg·L^-1 in calcium layer, 3648.99 mg·L^-1 in clay layer, and 3356.51 mg·L^-1 in parent layer. The amount of soluble total N in the leaching, clay, and parent layers was significantly higher than that in the cultivation layer but that of soluble total N in the calcium layer was significantly lower than that in the cultivation layer. The amount of soluble total P in the cultivation layer leaching solution was 0.52 mg·L^-1, which was significantly higher than that in all other layers. In the 1st to 3rd leaching time, the leached amounts of nitrate nitrogen, soluble total N, and orthophosphate in the cultivation and leaching layers were significantly higher than those in the clay and parent layers. However, in the 4th and 5th leaching time, the leached amounts of nitrate nitrogen and soluble total N in the clay and parent layers were significantly higher than those in the other layers, and the leached orthophosphate amount did not differ among layers. The amount of leached ammonium nitrogen in the clay and parent layers was significantly higher than that in the other layers after each test, and that of soluble total P in the cultivation layer was always significantly higher than that in the other layers. Nitrate nitrogen was the primary form of leached N in the cultivation and calcium layers, accounting for 69.0% and 85.4% of the total amount of N, respectively; the nitrate nitrogen percentages in the other layers were 41.3% (leaching layer), 5.1% (clay layer), and 4.6% (parent layer). Inorganic orthophosphate was the primary form of soluble P, accounting for 75.9% of the total amount of soluble P. The soil organic matter content, cation exchange capacity (CEC), and clay content affected the migration and transformation of soil N and P. There was a significant positive correlation between organic matter and N and P leaching, and more organic matter content increased the leaching risk in the initial leaching stage. The CEC and clay content were negatively correlated with N and P leaching, and increased CEC and soil clay particles reduced the leaching risk. The physical and chemical properties and N and P leaching characteristics differed among soil layers, and leaching was affected by soil CEC, clay and organic matter contents.