## 3 Results and Discussion
### Effect of Antecedent Dry Periods on Pollutants Retention in bioretention Systems  
The results suggest that dry periods before rainfall have an important influence on the accumulation, migration, and transformation of nitrogen in the bioretention system. Overall, during the rainfall event,  
Figure 5: Concentration of NO3\({}^{-}\)-N using bioretention systems with the Control column, DS column, and SZ column under 1a (**a**), 5a (**b**), and 10a (**c**) rainfall intensities. Figure 6: Concentration of TN using bioretention systems with the Control column, DS column, and SZ column under 1a (**a**), 5a (**b**), and 10a (**c**) rainfall intensities. the effluent nitrogen concentration in the different bioretention systems showed a downward trend, followed by an upward drift. The difference in the effluent nitrogen concentrations of the different structures was found insignificant (Figure 7). The number of adsorption sites vacated by the media was not significantly different. The initial effluent concentration of NH4\({}^{+}\)-N in the SZ group was higher than that in the later stage. The main explanation is that the submerged layer is usually anoxic, which is unconducive to aerobic nitrification [24; 29; 32]. The effluent NH4\({}^{+}\)-N concentration in the bioretention system was lower than 0.2 mg/L in the experiment, which showed that the bioretention system had higher removal efficiency of NH4\({}^{+}\)-N in the rainwater runoff. Figure 8 shows the effluent rule of NO3\({}^{-}\)-N from different bioretention systems under different drought cycles. The removal efficiency of NO3\({}^{-}\)-N in the saturated system was higher compared with the non-saturated system. This suggests that the aquifer could create an anoxic environment and promote the denitrification to remove NO3\({}^{-}\)-N. With the outflow, the concentration of NO3\({}^{-}\)-N increased and then gradually stabilized. Also, the concentration of NO3\({}^{-}\)-N varied for the different drought cycles. Due to the role of the filter layer and submerged layer, the longer the antecedent dry period, the stronger the abiotic fixation and plant uptake of the upper soil. Additionally, the higher the denitrification of the lower layer, the greater the removal of NO3\({}^{-}\)-N.  
The effluent TN concentration also showed an upward trend. The difference in the effluent TN concentration in the different bioretention systems was not significant. The overall change in the trend was similar to that of the NO3\({}^{-}\)-N. The removal capacity of NH4\({}^{+}\)-N by the filter layer in the different systems exhibited minor differences. The longer the antecedent dry period, the more NO3\({}^{-}\)-N was removed by plants or by abiotic fixation. Given the combined effect of these two measures, the difference in TN removal by the different systems was found to be minimal. Although a longer dry period means that more denitrification could be carried out and would result in a lower NO3\({}^{-}\)-N  
Figure 8: The efficiency of NO3\({}^{-}\)-N removal using bioretention systems with the (**a**) Control column, (**b**) DS column, and (**c**) SZ column during different antecedent dry periods. Figure 7: Efficiency of NH4\({}^{+}\)-N removal using bioretention systems with the (**a**) Control column, (**b**) DS column, and (**c**) SZ column during different antecedent dry periods. content, the results show that the difference in the amount of NO3-N between the different systems was not significant (Figure 9). This means that the comprehensive effect of the filter layer and the aquifer is able to reduce the difference in TN removal rates [30]. Figure 10 shows the relationship between the dry period duration and TP removal. The TP concentration increased with a longer dry period. When the dry period was three days, the TP removal efficiency rates of the three systems was between 84.7% to 91.4%, and the effluent TP concentration was lower than 0.2 mg/L. From the 8 th to the 14 th day, the removal efficiency of TP decreased from 86.2% to 71.7%.