## 4 Discussion
Additionally, the riparian zone beyond 2 m away from the bank was covered with undisturbed shrubs. According to the research conducted by [PERSON] and [PERSON], the microbial communities in the soil under undisturbed forest ecosystems had the capacity to assimilate most of the NO\({}_{3}^{-}\) produced by nitrification [54]. Hence, the microbial assimilation of NO\({}_{3}^{-}\) could be considered as a reason for the NO\({}_{3}^{-}\) in the offshore groundwater being lower than that in the near-shore groundwater. During most of the study period, the river stage remained higher than the riparian groundwater level under tide action, which led to a continuous infiltration of river water into the riverbank. As the river water steadily flowed into the hyporheic zone, the NH\({}_{4}^{+}\) and NO\({}_{3}^{-}\) in the near-shore groundwater were continuously recharged; thus, the NH\({}_{4}^{+}\) and NO\({}_{3}^{-}\) concentrations showed overall upward trends in the near-shore zone over the 3 day study period. Accompanied by the seepage process, the riparian groundwater level had an uplift which made part of the oxygen in the upper vadose zone enter the groundwater [55]. Thus, the DO concentration in the riparian groundwater increased due to the lateral recharge from the river, as well as the vertical recharge from the upper soil, which contributed to a whole aerobic environment within 4 m from the bank. The increase in DO concentration in the offshore groundwater promoted nitrification [56]. Meanwhile, the fine particles of the silty clay sediment had strong adsorption capacity, which restricted the further migration of the NH\({}_{4}^{+}\) in the near-shore groundwater to some extent [57]. For these reasons, the NH\({}_{4}^{+}\) concentration in the offshore groundwater decreased over the study period. Although the further migration of the NO\({}_{3}^{-}\) in the near-shore groundwater and nitrification could have caused an increase in NO\({}_{3}^{-}\) concentration in the offshore groundwater, little change was actually observed. This might be due to the fact that the microbial assimilation of NO\({}_{3}^{-}\) was considerable in the offshore hyperboic zone under the shrubs. The variation range of the DOC concentration in the groundwater in each riparian well was significantly larger than that in the river. The DOC in the riparian groundwater mainly depended on the dissolution of particulate organic carbon (POC) in the sediment rather than the infiltration of the river water, which is consistent with the research of [PERSON] et al. [58]. Some previous studies reported the effect of DOC, pH, and temperature on the nitrogen cycle in the hyporheic zone. The DOC could play an important role in denitrification processes [59]. The reaction rate of nitrogen could be inhibited in an acidic environment (pH < 5.5), but there is an insignificant impact on nitrogen cycle in a neutral environment [60, 61]. The nitrification rate in soils gradually increases with temperature within the range of 15-35 \({}^{\circ}\)C [62]. However, in our research, there was an insignificant difference in DOC concentration in the riparian groundwater. Moreover, the pH remained neutral, and the temperature had a small change of 1-2 \({}^{\circ}\)C in the riparian hyporheic zone. Hence, DOC concentration, pH, and temperature may not be the dominant factors causing the spatiotemporal difference in inorganic nitrogen concentration in the groundwater in the riparian zone studied. Additionally, it should be noted that the time period involved in the research was a spring tidal period in October, and recharging the riparian aquifers from the river water was dominant during the period. However, the tide-driven hyporheic exchange process varies with tide strength and season. Since the hyporheic exchange greatly influences the nitrogen occurrence, the findings of the study have to be seen in light of some limitations.