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A riparian zone, as the junction of surface water and groundwater, mainly controls the nitrogen cycle between surface water and groundwater through denitrification and other actions. Water conservancy projects will significantly change the hydrological environment of river regions, and affect the distribution and circulation of nitrogen in riparian zones. Exploring the influence mechanism of water conservancy projects on the riparian nitrogen cycle is of great practical significance for understanding the control and utilization of regional nitrogen. In this paper, the Xinglong Hydro-Junction is taken as the object, and three riparian zone sampling sections are set up in the upper and lower reaches of the project, with five sampling points in each section. TN, "tri-nitrogen" (refers to\begin{document}${\rm{NH}}_4^+ $\end{document}-N, \begin{document}${\rm{NO}}_2^- $\end{document}-N, \begin{document}${\rm{NO}}_3^- $\end{document}-N) and relevant soil physical and chemical properties are analyzed for the collected 150 soil samples. The results show that (1) the nitrogen content in the riparian sediments in the upstream of the water conservancy project is significantly higher than that in the downstream, and the average content of total nitrogen and "three nitrogen" in section A is 1.12-3.27 times that in sections B and C of the downstream. (2) The horizontal variation trend of contents of TN and "tri-nitrogen" in the riparian zones of the three sections is not consistent, showing that TN content in the same section is higher in the embankment, and the sampling point in the embankment close to the embankment is the abrupt change point (sharp increase or decrease) of "tri-nitrogen" content. (3) The vertical distribution pattern of TN and "tri nitrogen" is similar: nitrogen content decreases rapidly from 0 to 60 cm, and irregular change occurs below 60 cm. Nitrogen content decreases from top to bottom. The Xinglong Dam mainly affects its upper reaches. Through water storage, the groundwater level of the upstream riparian zone rises, and the sediments are submerged for a long time, leading to the decrease of denitrification capacity. Due to the difference in microgeomorphology caused by levee in the same section, the groundwater in the levee is deeper than that outside the levee, and the denitrification capacity of the sediments in the levee is weaker than that outside the levee.

In this study, the effects of five common oxidative decontaminating agents (H2O2 solution, ClO2 solution, NaClO solution, ozonated water and slightly acidic electrolytic water) on the bacterial decontamination of tilapia fillets and their influence on the texture, color and myofibrillar protein (MP) oxidation in tilapia fillets were investigated. The results showed that a decontamination rate of 80% was attained by immersion in 1 500 mg/L H2O2 solution for 8 min, 200 mg/L ClO2 solution for 10 min, 200 mg/L NaClO solution for 10 min, 9 mg/L ozonated water for 10 min, or 30 mg/L slightly acidic electrolytic water for 20 min. Compared with the control group, treatment with decontaminating agents increased the hardness and L* value of tilapia fillets, decreased the a* value, MP content, total sulfhydryl group content, and raised the content of carbonyl groups and the amount of surface hydrophobic groups. The degree of protein denaturation in the slightly acidic electrolytic water and NaClO solution groups was the highest; the fluorescence intensity of MP was decreased after decontamination treatments, with the greatest decrease being found in the slightly acidic electrolytic water-treated group; the relative contents of α-helix and β-turn in MP were decreased, while the relative contents of β-fold and random coil were increased, with the most significant changes in the secondary structure of MP being observed in the slightly acidic electrolytic water-treated group (P < 0.05). MP underwent degradation to different extents, and the degradation degree of the slightly acidic electrolytic water group was higher than that of the other treatment groups. In summary, the oxidation of fish MP occurred to varying degrees after oxidative decontamination treatments, with slightly acidic electrolytic water treatment having the strongest oxidative effect.

The study investigated the semi-hibernation and critical temperature of grass carp based on gradient cooling, and explore the effects of temperature (12–24 ℃) on the survival of grass carp and the concentration of ammonia nitrogen in the water. The temperature was reduced from room temperature (23–25 ℃) to the semi-dormant temperature (16 ℃) at cooling rate of 1, 3, or 5 ℃/h. The control group was reared at room temperature, and sampling was conducted after 0, 12, 24, 36, and 48 h. Serum biochemical and antioxidant parameters were measured to obtain the maximum cooling rate for grass crap. The results showed that the semi-dormant temperature of grass carp was 16 ℃, the critical temperature was 6–8 ℃, and the optimal cooling rate was 3 ℃/h. At 16 ℃, grass carp exhibited delayed stress response and decreased metabolism, and lower concentrations of ammonia nitrogen in the water. Except for aspartate aminotransferase level, the maximum values of other serum biochemical indexes at the cooling rate of 3 ℃/h, were significantly lower than those at 1 and 5 ℃/h (P < 0.05). The activities of superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSH-Px) exhibited a trend of initially increasing and then decreasing with survival time, whereas the content of malondialdehyde (MDA) showed an opposite trend, reaching a minimum value of 5.27 mmol/mg after 36 h. Compared to the control group, the contents of all biochemical and antioxidant indexes in the three cooling groups showed significant changes with survival time. Therefore, cooling grass carp at a rate of 3 ℃/h could reduce the stress response and allowed it to enter a semi-hibernation state at a more suitable time. The results from this study provide key parameters for the temporary cultivation and transport of grass carp, as well as useful guidelines for reducing the transport mortality.