## 4 Discussion  
### Effect of Temperature and pH on Volatilization Process  
The process of ammonia volatilization from ammonium ion in solution can be divided into  
NH4+(liquid) <=> NH3(liquid) <=> NH3(gas). There is a dynamic chemical equilibrium process in the solution:  
NH4+OH-<=> NH3:H2O <=> NH3(liquid) + H2O  
The chemical equilibrium constant (\(K_{\rm c}\)) of this process is a constant for the equilibrium concentration of various ions in the solution system. In this solution, the \(K_{\rm c}\) is related to ammonium nitrogen, hydroxyl concentration, and temperature. Obviously, different fertilizers, ammonium bicarbonate or diammonium phosphate, will not affect the concentration of all ions in this chemical equilibrium, so it will not change the chemical equilibrium. This can explain the phenomenon that the values of \(\alpha\) between different fertilizers at the same temperature are almost the same. On the other hand, temperature will affect the process of ammonia molecules from liquid to gas. Ammonia molecules close to the liquid-gas interface break free from the shackles of the liquid molecules by molecule motion and finally escape into the air. As the temperature rises, the movement of ammonia molecules in the solution becomes more intense; meanwhile, the chemical equilibrium moves toward generating ammonia molecules, which leads to the phenomenon that the CIF value increases as the temperature increases. In contrast, the pH value plays an important role in this chemical equilibrium. The increasing hydroxyl concentration leads the chemical equilibrium to shift toward generating ammonia molecules, which is the source of ammonia volatilization. Theoretically, we know that when the pH value increases by one unit, the concentration of hydroxyl is increased by an order of magnitude. From the chemical equilibrium constant expression, the concentration of ammonia molecules is increased by the hydroxyl because the \(K_{\rm c}\) is constant in the same temperature. However, in this study, we found that the amount of ammonia that volatilized increased rapidly when the pH value increased by one unit, but it did not reach as much as 10 times, which may be due to the restraints of other ions on the ammonia molecules in the solution. Synergistic Effect of Temperature and pH on the Concentration Influence Factor and Initial Ammonia Volatilization Fluxes  
From the above analysis, we can see that the CIF and IAVFs, which are the coefficients in the linear equation of the ammonia volatilization rate and concentration, all showed a certain quantitative relationship with temperature and pH. The effects of temperature and pH on the CIF and IAVFs have a superposition effect, so it is not comprehensive to consider the effects of temperature or pH on the ammonia volatilization amount. In order to fully consider the effects of temperature and pH on a and b, binary, binary quadratic, and binary cubic equation models were used to quantify the CIF (\(a\)) and IAVFs (\(b\)) of SAB and SDP using MATLAB R2016a. These equations are shown in Tables 4 and 5. In accordance with previous research, we found that temperature and pH showed a superposition effect on the CIF and IAVFs. Meanwhile, the effect of temperature and pH on the CIF should be expressed as a binary cubic correlation, and their effect on the IAVFs should be binary quadratic correlated. Considering the results of the coefficient determination, as shown in Tables 4 and 5, a binary cubic model is suitable to express the quantitative relationship between temperature, pH, and CIF, and a binary quadratic model is suitable to express the quantitative relationship of temperature and pH on the IAVFs. Simultaneously, according to the 3D plot drawing using MATLAB R2016a, we found that the CIF of SAB (Figure 6) and SDP (Figure 7) showed a fast and non-linear tendency as the pH value increased. And, the change of \(\Delta pH/\Delta a\) increased rapidly as the pH increased, which shows consistency with previous projections.