*2.2. Sample Collection and Analysis*

In accordance with the sampling conditions of the study area and Technical Provisions for the Design of Water Quality Sampling Schemes (HJ495-2009), sampling points NYR, SGR, ZWR, BSR, and HJR were set up in the upper reaches of the five rivers, and sampling points S1, S2, S3, S4, and S5 were set up at the intersection of the rivers and reservoir areas. Three sampling points (S6, S7, and S8) were set up in the reservoir area, and one sampling point was set up at the outlet of the dam (BQ) (Figure 1), for a total of 14 sampling points. Water samples were collected from the study area in November 2020 and January and July 2021, representing the normal, dry, and wet periods, respectively. Water pH, water temperature (WT), electrical conductivity (EC), and dissolved oxygen (DO) were measured in the field by a WTW Multi3430 portable multiparameter water quality analyzer with an accuracy of 0.001 pH unit, 0.01 ◦C, 1 μs/cm, and 0.01 mg/L, respectively. The concentrations of HCO3 <sup>−</sup> and Ca2+ were titrated onsite using an alkalinity kit and calcium kit (Merck, Germany) with accuracies of 0.1 mmol/L and 2 mg/L, respectively. The collected water samples were filtered through a 0.45-μm filter membrane and loaded into polyethylene sampling bottles that had been precleaned with deionized water. Concentrated nitric acid was added to the water samples to pH < 2 for determination of cation concentration, and 2 drops of HgCl2 were added to the water sample to inhibit microbial activity for determination of dissolved inorganic carbon isotope (δ13CDIC).

Anions, cations, and δ13CDIC were measured at the State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, and δ15N-NO3 <sup>−</sup> and δ18O-NO3 − were measured at the Analysis and Testing Center of the Third Institute of Oceanography, Ministry of Natural Resources. The concentration of cations (K+, Na+, Mg2+) was determined by Inductively Coupled Plasma-Emission Spectrometer (VISTA MPX, Varian, USA), and the concentration of anions (NO3 −, Cl−, SO4 <sup>2</sup>−) was determined by ion chromatography (ICS90, Dionex, Sunnyvale, CA, USA). The limit of detection was 0.01 mmol/L. The method for the determination of water body δ13CDIC was to add 100% pure phosphoric acid into the injection bottle (vacuumized) and injection high purity helium gas, then inject 20 mL water sample into the injection bottle with a syringe, and heat it in a 60 ◦C water bath beaker. The CO2 produced by the reaction was separated by a cold trap and then loaded with helium into a Finnigan MAT253 gas isotope mass spectrometer for determination. The bacterial denitrification method was used for the determination of δ15N-NO3 <sup>−</sup> and δ18O-NO3 −. Denitrifying bacteria (ATCC 13985, DSM 6698) without nitrous oxide reductase activity were used to terminate the reaction after reducing NO3 − to N2O, thus obtaining nitrogen and oxygen in N2O from the NO3 − in the sample [28,29]. A GasBench continuous flow gas introduction instrument and MAT 253 stable isotope ratio mass spectrometer were used to determine the δ15N and δ18O contents in N2O. To ensure the accuracy of the obtained measurements, reference materials USGS34

(δ15N = −1.8%, <sup>δ</sup>18O = −27.9%), USGS32 (δ15N = +180%, <sup>δ</sup>18O = +25.7%), and IAEA-NO3 were used. The test accuracy of δ13CDIC was 0.2%, and the result is reported as parts pel mil (%) relative to the Vienna PDB reference standard. The test accuracy of δ15N-NO3 − and δ18O-NO3 − was 0.3%. Atmospheric nitrogen (N2) and Vienna standard mean seawater (V-SMOW) were used as references for the δ15N and δ18O results, respectively.

#### *2.3. Flux Calculation*

HCO3 − and NO3 − flux was calculated from the total water flow multiplied by the concentration of HCO3 − and NO3 − [30]. Flux is calculated using the equation:

$$flux\_{\mathbb{C}/N} = com\_{\mathbb{C}/N} \times Q \tag{1}$$

where *fluxC/N* refers to annual HCO3 − and NO3 <sup>−</sup> flux (t·a<sup>−</sup>1), *conC/N* is the concentration of HCO3 − and NO3 <sup>−</sup> (mg/L), and *<sup>Q</sup>* refers to the water discharge in unit time (m3·a<sup>−</sup>1).

#### **3. Results**

#### *3.1. Physicochemical Indices and Hydrochemical Characteristics of Water*

The physicochemical indices of the water body of Pingzhai Reservoir and its inflow river showed seasonal changes (Table 1). The water temperature ranged from 9.15 ◦C to 26.65 ◦C, with an average temperature of 16.92 ◦C. The pH value of water ranged from 7.89 to 10.67, with an average value of 8.69, generally showing the characteristics of weakly alkaline water. The EC of the water body varied greatly (196–578 μs/cm) and showed the temporal pattern dry season > normal season > wet season. In terms of the water DO concentration, the annual variation ranged from 6.39 to 11.51 mg/L, with an average of 8.5 mg/L. The water body was generally in an aerobic state, which was conducive to the occurrence of nitrification, and temporally, DO was the highest in the wet season and the lowest in the dry season.



Note: mean ± standard deviation (SD).

The total cationic equivalent concentration (TZ<sup>+</sup> = 2Ca2+ + 2Mg2+ + K<sup>+</sup> + Na+) in the Pingzhai Reservoir and its inflow rivers ranged from 1.92 to 14.40 meq/L, with an average value of 8.16 meq/L. The total anion equivalent concentration (TZ− = HCO3 − + NO3 − + Cl− + 2SO4 <sup>2</sup>−) ranged from 1.97 to 10.24 meq/L, with an average value of 6.10 meq/L. Taking the river and reservoir area together, the total cationic equivalent concentration of river water ranged from 3.41 to 14.39 meq/L, with an average of 8.91 meq/L. The total anion equivalent concentration ranged from 2.37 to 10.57 meq/L, with an average of 6.47 meq/L. The total equivalent concentrations of cations and anions in the reservoir were 1.97–9.45 meq/L and 3.24–6.01 meq/L, with averages of 5.71 meq/L and 4.63 meq/L, respectively. The Piper diagram can directly reflect the composition characteristics of the main ions in water (Figure 2). The predominant cations in the Pingzhai

Reservoir were Ca2+ and Na+, whose contents accounted for 68% and 21% of the total cations, respectively. The dominant anions were HCO3 − and SO4 <sup>2</sup>−, which accounted for 65% and 27% of the total anions, respectively. According to the Shukalev classification, the hydrochemical type in the study area was the HCO3-Ca type.

**Figure 2.** Piper diagram of hydrochemical types in the Pingzhai Reservoir basin.
