## 1 Introduction  
With the growth of urbanization, natural green spaces have been gradually replaced by impervious pavements, resulting in reduced nitrogen retention capacity in many urban areas [1; 2; 3]. Urban rainfall-runoff often carries high concentrations of nitrogen, phosphorus, nutrients, organic pollutants, and heavy metals [4; 5; 6]. Studies have shown that half of the nitrogen pollutants in urban rivers come from surface runoff formed during urban rainfall and the concentrations of metal elements in some runoff can exceed more than ten times the limitations for surface water quality [7; 8]. Nitrogen, heavy metals, and phosphorus present in the surface runoff lead to the functional degradation of about two-thirds of urban rivers, and the pollutants discharged from this surface runoff present a serious threat to the urban water system [9; 10; 11; 12]. In order to control the amount of nitrogen chemical oxygen demand (COD) and phosphorus in stormwater runoff and prevent the further deterioration of the urban water ecosystem, decentralized management of rainwater becomes critically important [13]. Many countries have adopted different rainwater management systems based on the existing urban structure, such as the Low Impact Development and Best Management Practice (BMP) in the United States, the Water Sensitive Urban Design in Australia, and the Sponge City Concept in China [14; 15]. Sponge city manages natural accumulation, infiltration, and purification of rainwater through the combination of natural and artificial facilities and releases rainwater when water is scarce, thus forming a good network of natural circulation. The occurrence of urban stormwater runoff is random and intermittent. Pollution sources are widely distributed and dispersed, and the concentration of the pollutants varies greatly. Thus, one of the keys to solving the problem of urban stormwater pollution is to understand how to collect, purify, and store urban stormwater efficiently [16]. Green infrastructure is considered as one of the BMPs for non-point source pollution, such as rainwater [17]. Among the various green infrastructures, the bioretention system has a strong capacity for runoff saturation and water purification. It has gradually become an essential means of ecological control of stormwater runoff [18; 19; 20]. The bioretention system has a multi-layer structure, which can maximize the functions of water-saturated utilization, and evapotranspiration. It involves complex physical, chemical, and biological processes. One of the technical problems in the bioretention system is the removal of nitrogen. The main reason is that the dissolved nitrogen, such as nitrite and nitrate, has high solubility and low adsorption capacity, while nitrogen removal is mainly carried out through plant absorption [21], negative charge adsorption in the soil medium [22], and microbial nitrification-denitrification process [23]. Due to the reoxygenation and the oxygen supply of plant roots, the surface layer of the bioretention system provides an excellent aerobic environment for the nitrifying bacteria and ammoniating bacteria, which promotes nitrification and ammoniation, respectively. The NO\({}_{3}\)\({}^{-}\)-N formed by the nitrification reaction and NO\({}_{3}\)\({}^{-}\)-N in the road runoff are partially absorbed by plants, while the other parts are denitrified to produce N\({}_{2}\) in an anaerobic environment and removed in the bioretention device [24]. Studies have confirmed that the mechanism of nitrogen removal by plants is different under saturated and unsaturated conditions of the bioconservation system [25; 26]. The phosphorus content in the road runoff is mainly divided into particulate phosphorus (PP) and dissolved phosphorus (DP). The PP can be removed by physical methods, such as precipitation and filtration [27]. DP is mostly found in the stormwater runoff, and its primary removal mechanism involves biological utilization, adsorption, and precipitation [28; 29].