Quantifying precipitation-induced changes in the proportion of nitrate inputs and their role in karst processes: Evidence from stable isotopes and water chemistry

Study region The study region is located within Shuanghe Cave National Geopark, China. Study focus Global warming has intensified extreme rainfall events, yet existing carbon cycle models generally overlook nitric acid weathering processes modulated by rainfall, leading to biased estimates of karst carbon sinks. In this study, conventional physicochemical parameters of water bodies, δ 15 N- N O − 3 , δ 18 O- N O − 3 and δ 13 C-DIC were measured simultaneously. Using the MixSIAR model, we distinguished the dynamic changes in nitrate source contributions before and after rainstorms, quantitatively estimated the proportion of carbonate weathering driven by nitric acid, elucidated the mechanism by which intensive rainfall-driven nitrogen input reduces the karst carbon sink, and corrected the biases inherent in traditional karst carbon-sink assessment methods. New hydrological insights After heavy rainfall, the proportion of anthropogenic nitrate increased markedly - by 37% in June and 22.4% in October - with higher nitrogen leaching intensity in June (agriculturally dominated) than in October (domestic-pollution dominated). Notably, the overall proportion of carbonate weathering attributed to nitric acid declined following rainstorms: from 8.24% to 7.14% in June, and from 4.96% to 3.56% in October. Under heavy storm conditions (daily rainfall ≥ 50 mm), the contribution of nitric acid to carbonate weathering is weakened by rainwater dilution. Therefore, for shallow karst watersheds, it is unwarranted to categorically conclude that rainfall promotes nitric-acid-mediated carbonate weathering; instead, the evaluation framework for weathering processes and carbon-sink effects should be further refined under different rainfall intensity scenarios.