Study on the Carbon and Nitrogen Isotope Characteristics and Sources and Their Influence on Carbon Sinks in Karst Reservoirs

The hydrochemical analysis method was used to reveal the sources and spatiotemporal variations of carbon and nitrogen elements in the Pingzhai Reservoir, and the C–N coupling cycle and its influence on the karst carbon sink are discussed. The results show the following: (1) The hydrochemical type of the study area is HCO3-Ca. (2) From the river to the reservoir and then to the reservoir outlet, the values of HCO3− and δ13CDIC showed an opposite trend. The values of NO3−, δ15N-NO3−, and δ18O-NO3− were different in each stage of the river. (3) HCO3− mainly comes from the weathering of carbonate rocks and the oxidative decomposition of organic matter. Nitrate mainly comes from chemical fertilizers, soil organic nitrogen, sewage, and livestock manure. (4) The average proportion of HCO3− produced by HNO3 dissolving carbonate rock is 8.38%, but this part does not constitute a carbon sink. Compared with rivers, the proportion of HCO3− and (Ca2+ + Mg2+) produced by HNO3 dissolving carbonate rock in reservoir water is relatively large. The input of nitrate not only pollutes the water body with NO3− but also changes the carbon source/sink pattern of the water–rock interaction.