*3.1. 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

of NH4 +-N concentration was 9.16% at the stage of the water level rise, and the average decrease of NH4 +-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 NO2 −-N was much smaller than that of the NO3 −-N and NH4 +-N concentrations, and NO2 −-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 NO3 −-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 NO3 −-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 NO2 −-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 NO3 −-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 NH4 +-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 NO2 −-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 NH4 +-N concentration should fall. Still, denitrifying bacteria become active and dominant under hypoxic conditions, promoting the increase in the NH4 +-N concentration and the significant decrease in the NO3 −-N concentration. When the water level drops, the dissolved oxygen content increases and nitrification plays a central role. The concentration of NO3 −-N increases significantly. However, due to the strong adsorption of the soil, the concentration of NH4 +-N in the free water of the soil solution decreases [43,44]. Nitrite nitrogen, in the three media, fluctuated and eventually stabilized, but its concentration was much smaller than the concentrations of NO3 −-N and NH4 +-N. Before the water level rises, when the dissolved oxygen is sufficient, the reproduction rate of nitrifying bacteria is slower than that of nitrosating bacteria; at this time, the nitrosation reaction dominates, causing the accumulation of NO2 −-N, resulting in an increased concentration of NO2 −-N in the soil solution during the rising water level stage. During the falling phase of the water level, nitrification dominates, a lot of H+ is produced in the solution, and the soil solution becomes weakly acidic, which strengthens the conversion of nitrite to nitrate. NO2 −-N no longer accumulates, and the concentration gradually decreases [29,45,46]. In the initial stage, the increase in NO2 −-N satisfies coarse sand, then medium sand, then fine sand. The more significant the particle size is, the more particle surfaces there are that microbial flocs can come into contact with, and more microbes can participate in the nitrification reaction. With a stronger microbial nitrification ability, it is understood that the nitrification ability of microbial flocs will increase with the increase in particle size [47].

## *3.2. Scenario Simulation*
