## 1 Introduction  
In order to ensure the security of the global food supply, a large amount of nitrogen fertilizer has been utilized in the past few decades. For example, the average nitrogen utilization has achieved the rate of 305 kg/hm\({}^{2}\) in China [1]. Excessive bioavailable nitrogen such as nitrate nitrogen (NO\({}_{3}^{-}\)) and ammonia nitrogen (NH\({}_{4}^{+}\)) in rivers will lead to water eutrophication and, consequently, damage the ecological balance of river systems. Particularly in coastal areas, nitrogen pollution in rivers is generally serious due to dense population and active industry and agriculture production. If it cannot be maintained under effective control, the excessive nitrogen will continue to enter the ocean and further affect the health of the coastal ecosystem and the sustainable utilization of marine resources. Hyporheic zones of river ecosystem are water-saturated sediment (known as aquifer) below the riverbed and extending to the riparian areas on both sides. It is not only a key area for water exchange and solute migration between river and groundwater, but also an important place for microbial growth and metabolism. Mass exchange and energy transfer are frequent and biogeochemical reactions are complex within a hyporheic zone, which is of great significance to the structure, function, and health of river ecosystem [2; 3; 4]. Nitrogenis transported between rivers and their riparian zones through hyperheic exchange and transformed between different chemical forms (e.g., NH\({}_{4}^{+}\), NO\({}_{3}^{-}\), nitrite nitrogen (NO\({}_{2}^{-}\)), and N\({}_{2}\)) through biogeochemical reactions [5]. Therefore, to realize the effective control of nitrogen pollution in rivers, it is necessary to fully comprehend nitrogen migration and transformation in groundwater in hyperheic zones. Nitrogen in groundwater in a hyperheic zone varies geographically, and its form changes temporally, which forms a complex dynamic cycle [6; 7; 8; 9]. Some hydrological events (e.g., rainstorm, flooding, reservoir discharge, and tidal action) can change the recharge relationship between river water and groundwater, consequently influencing nitrogen migration in hyperheic zones [10; 11; 12; 13]. [PERSON] et al. carried out onsite monitoring to study the water exchange and the change in nitrogen concentration in the hyperheic zone during the rainstorm. It was found that the increase in nitrogen concentration in the hyperheic zone was mainly caused by the rainwater infiltration through the overlying unsaturated zone rather than the lateral seepage from the river [14]. The research conducted by [PERSON] et al. indicated that the flood process extended the mixing range of surface water and groundwater and had a significant impact on the mass cycle in the riverbed hyperheic zone [15]. [PERSON] et al. studied the impact of the reservoir operation on the lateral hyperheic exchange, and the results showed that the impact range could extend to 1-5 m away from the shore [16]. Unlike the hydrological events mentioned above, tides exhibit periodicity, and the tidal period is usually about 12 or 24 h. This makes the hyperheic exchange more frequent and the biogeochemical process in the hyperheic zone more complex [17]. In recent years, there has been growing concern over the hyperheic zones of tidal rivers. [PERSON] et al. quantified the tide-driven hyperheic exchange fluxes across the bank and the bed and deduced that the tidal bank storage in the sandy hyperheic zone might remove nutrients from rivers [18]. The geochemical measurements and numerical simulation performed by [PERSON] et al. showed that it was possible that the hyperheic zone with permeable sediment and low organic matter content could be a source of nitrate to the tidal river [19]. Hydrogeological conditions and ecological factors could affect the nitrogen cycle in hyperheic zones [20; 21; 22]. [PERSON] et al.