Anthropogenic exogenous nitric and sulfuric acids in karst plateau reservoirs and their impact on carbon sinks

Abstract

Carbonate rocks are the world’s largest carbon reservoir, and karstification is an important part of the global carbon cycle. They actively participate in the global carbon cycle at various time scales between the atmosphere, hydrosphere, and lithosphere and have significant carbon sink significance. Carbonated dissolution of carbonate rocks increases the concentration of HCO₃^–, which is considered a net sink of CO₂ in the atmosphere. However, with the involvement of nitric acid and sulfuric acid, carbonate rock dissolution may be a source of CO₂ in the atmosphere. This study determined the hydrochemical characteristics and multiple isotopes (δ¹⁵N_NO3, δ¹⁸O_NO3, δ³⁴S_SO4, δ¹⁸O_SO4, δ¹³C_DIC, and δ¹⁸O_H2O) of the Pingzhai Reservoir and investigated the effects of nitric acid and sulfuric acid on carbonate rock dissolution and carbon sink flux. The concentrations of Ca²⁺, Mg²⁺, and HCO₃^– indicate that the hydrochemical type of the Pingzhai Reservoir is controlled by carbonate rock dissolution. The ratio of [Ca²⁺+Mg²⁺]/[HCO₃^–+NO₃^–+SO₄^2-] is approximately 1, and the equivalent ratio of [Ca²⁺+Mg²⁺]/[HCO₃^–] and the δ¹³C_DIC value are distributed between carbonate rocks dissolved by carbonic acid and carbonate rocks dissolved by nitric acid and sulfuric acid, indicating that carbonic acid, nitric acid, and sulfuric acid jointly participate in the dissolution of carbonate rocks in the Pingzhai Reservoir. Geographical detection revealed that the sources of NO₃^– and SO₄^2- are influenced by both natural and anthropogenic factors. NO₃^– mainly comes from precipitation and NH₄⁺ nitrification in fertilizers (37.8 %), soil organic nitrogen (24.6 %), NO₃^– in fertilizers (23.5 %), and domestic wastewater and livestock manure (14.1 %). SO₄^2- is derived mainly from sulfide oxidation (82.1 %), soil organic sulfur (9.1 %), domestic sewage (5.5 %), and synthetic fertilizers (3.3 %). After removing sources that cannot produce nitric acid or sulfuric acid, the average contribution rates of nitric acid and sulfuric acid to the dissolution of carbonate rocks in the Pingzhai Reservoir were calculated to be 9.9 % and 52.2 %, respectively. The proportions of carbon sink overestimation during the dry season and wet season are 59.8 % and 64.4 %, respectively. The results emphasize that the interference of nitric acid and sulfuric acid on carbonate rock dissolution should be considered when assessing the carbon cycle.