## 3 Results and Discussion  
### Nitrogen Changes and Model Validation  
The process curves of the measured and simulated results of ammonia nitrogen, nitrate nitrogen and nitrite nitrogen in the coarse sandy soil column under the condition of water level fluctuation are shown in Figure 4.  
In the 15 cm and 25 cm sampling ports in the coarse sand column, NO3 -N showed a decreasing trend in the rising stage of the water level, with a decrease of 8.55% on average, the maximum decrease of 14.94% in the second cycle of the second rising stage of the water level, and the minimum decrease of 2.27% in the first rising stage of the second cycle of the water level. The concentration of NO3 -N showed an increasing trend in the declining water level stage, with an average increase of 9.10%, a maximum increase of 15.47% and a minimum increase of 4.62%, and the second fluctuation cycle was more obvious than the first fluctuation cycle of nitrate heel fluctuation. The average increase  
Figure 6: The process of nitrogen change in fine sand ((**a**) nitrate nitrogen; (**b**) nitrite nitrogen; (**c**) ammonium nitrogen). of NH\({}_{4}\)+-N concentration was 9.16% at the stage of the water level rise, and the average decrease of NH\({}_{4}\)+-N concentration was 14.355% at the stage of water level fall, and the fluctuation change was more obvious in the first cycle. The concentration of NO\({}_{2}\)\({}^{-}\)-N was much smaller than that of the NO\({}_{3}\)\({}^{-}\)-N and NH\({}_{4}\)+-N concentrations, and NO\({}_{2}\)\({}^{-}\)-N showed fluctuating changes and eventually stabilized. The process curves of the measured and simulated results of ammonia nitrogen, nitrate nitrogen and nitrite nitrogen in the medium sand soil column under the water level fluctuation conditions are shown in Figure 5.  
In the medium sand column, the trend of NO\({}_{3}\)\({}^{-}\)-N was similar to that of the coarse sand soil column, with insignificant changes in concentrations at the 35 cm and 45 cm sampling ports, and obvious fluctuation trends at the 15 cm and 25 cm sampling ports. The concentration of NO\({}_{3}\)\({}^{-}\)-N at the 15 cm sampling port decreased by 780.30% on average, especially during the first water level rise in the first cycle. The concentration of NO\({}_{2}\)\({}^{-}\)-N was fluctuating at the beginning and stabilized later. The process curves of the measured and simulated results of ammonia nitrogen, nitrate nitrogen and nitrite nitrogen in the fine sandy soil column under the condition of water level fluctuation are shown in Figure 6.  
In the fine sand soil column, the trend of the NO\({}_{3}\)\({}^{-}\)-N concentration changes at the 15 cm, 25 cm and 35 cm sampling ports were basically the same, and the decreases at 15 cm, 25 cm and 35 cm were 25.93%, 68.05% and 19.19, respectively, during the water level rise stage, and the increases were at 15 cm, 25 cm and 35 cm during the water level fall stage. The trends of NH\({}_{4}\)\({}^{+}\)-N concentrations at the 15 cm, 25 cm and 35 cm sampling ports were basically the same, and the concentrations at the 45 cm sampling port did not change much, and the increases at the water level rising stage were 32.44%, 23.39% and 27.03% for 15 cm, 25 cm and 35 cm, respectively, and the decreasing water level for the NO\({}_{2}\)\({}^{-}\)-N concentration was low and stabilized after fluctuating changes. Figures 4-6 show the actual measurement process curve of ammonia nitrogen, nitrate nitrogen and nitrite nitrogen under the conditions of water level fluctuation. When the water level rises, the dissolved oxygen content decreases, and the NH\({}_{4}\)\({}^{+}\)-N concentration should fall. Still, denitrifying bacteria become active and dominant under hypoxic conditions, promoting the increase in the NH\({}_{4}\)\({}^{+}\)-N concentration and the significant decrease in the NO\({}_{3}\)\({}^{-}\)-N concentration. When the water level drops, the dissolved oxygen content increases and nitrification plays a central role. The concentration of NO\({}_{3}\)\({}^{-}\)-N increases significantly.