**4. Discussion**

During the study period, the pH and the DO concentrations in the groundwater in the four riparian wells varied from 6.27 to 6.59 (neutral environment) and from 5.2 to 6.8 mg/L, respectively. It is generally considered that an environment is aerobic if DO concentration is greater than 3 mg/L [41]. Thus, the riparian hyporheic zone within 4 m from the bank was an aerobic and neutral environment in favor of nitrification. In past studies, denitrification was regarded as an anaerobic process, and it was considered that denitrification would no longer occur if DO concentration was greater than 4.5 mg/L [42,43]. However, recent research results showed that denitrification could also occur in aerobic zones in soils due to the existence of aerobic denitrifying bacteria which could live in the environment with a DO concentration of 3–9 mg/L and a neutral pH [44–46]. It was, therefore, deduced that aerobic denitrification and nitrification might coexist in the riparian hyporheic zone studied, and the combination of these two effects was conducive to the removal of nitrogen from the riparian zone.

In the investigation, the NH<sup>+</sup> <sup>4</sup> concentration in the near-shore groundwater was lower than that in the offshore groundwater, and the lowest value found at WB1 closest to the bank. It was reported by Pan et al. that the NH<sup>+</sup> <sup>4</sup> concentration in the riparian groundwater decreased with increasing the distance from the bank of the nitrogen-rich river [47]. This is inconsistent with the results of our research. By comparison, it was found that there were significant differences in NH<sup>+</sup> <sup>4</sup> concentration in the river water and type of the sediment. The NH<sup>+</sup> <sup>4</sup> concentration in the river water was 0.9–1.5 mg/L, and the sediment mainly consisted of coarse sand in their research, while the former was 0.4–0.7 mg/L and the latter was silty-clay soil in our research. Some studies indicated that a low permeability of the sediment could reduce flow velocity of the pore water and, consequently, prolong the solute residence time, which provided the sufficient reaction time for nitrogen in the hyporheic zone [48,49]. Thus, the lower-permeability sediment and the river water with the lower nitrogen concentration contributed to more consumption and less recharge of the NH<sup>+</sup> <sup>4</sup> in the near-shore groundwater, respectively. This could explain why groundwater with less NH<sup>+</sup> <sup>4</sup> was observed in the near-shore hyporheic zone. However, upon increasing

the distance from the bank, the DO concentration in the groundwater deceased gradually. The decline in DO concentration could lower the intensity of nitrification [50]. Furthermore, the mean TN content in the sediment at WB3 was highest among the four observation wells. The higher TN content could promote ammoniation, which is an important pathway for recharging the NH<sup>+</sup> <sup>4</sup> in the groundwater in hyporheic zones [51,52]. The greater recharge and lower consumption resulted in more NH<sup>+</sup> <sup>4</sup> existing in the groundwater in the offshore hyporheic zone.

The NO− <sup>3</sup> concentration in the riparian groundwater showed a downward trend as the distance from the bank increased along the test transect. This matches with the work of Zhang et al. [53]. Due to the strong nitrification reactions and the recharge from the river water, the NO− <sup>3</sup> was enriched in the near-shore groundwater. However, in the offshore hyporheic zone with the poorly permeable sediment, the recharge of NO− <sup>3</sup> from the river water was very slow and could take several hours or even more according to the delayed response time of the groundwater level to the river stage in the study. Moreover, due to the decline in DO concentration, a lower amount of the NO− <sup>3</sup> in the groundwater was produced by nitrification. Additionally, the riparian zone beyond 2 m away from the bank was covered with undisturbed shrubs. According to the research conducted by Stark and Hart, the microbial communities in the soil under undisturbed forest ecosystems had the capacity to assimilate most of the NO− <sup>3</sup> produced by nitrification [54]. Hence, the microbial assimilation of NO− <sup>3</sup> could be considered as a reason for the NO<sup>−</sup> <sup>3</sup> 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<sup>+</sup> <sup>4</sup> and NO− <sup>3</sup> in the near-shore groundwater were continuously recharged; thus, the NH<sup>+</sup> <sup>4</sup> and NO<sup>−</sup> 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<sup>+</sup> <sup>4</sup> in the near-shore groundwater to some extent [57]. For these reasons, the NH<sup>+</sup> 4 concentration in the offshore groundwater decreased over the study period. Although the further migration of the NO− <sup>3</sup> in the near-shore groundwater and nitrification could have caused an increase in NO− <sup>3</sup> 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 hyporheic 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 Peyrard 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 ◦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 ◦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. Future research will be performed to analyze nitrogen occurrence in groundwater in silty-clay hyporheic zones under the action of tides of different strength and its seasonal variation. Moreover, the variations of nitrogen forms and the influencing factors with depth will need to be investigated to reveal the vertical nitrogen cycle in hyporheic zones of tidal rivers.
