中国北方典型岩溶河流硝酸盐来源解析及H2SO4和HNO3对流域岩石风化碳汇的影响OACHSSCD

Accurately quantifying carbon source-sink changes resulting from carbonate weathering under the influence of anthropogenic acids(HNO3 and H2SO4)in karst rivers of northern China is of great practical significance for supporting China's"Carbon Peak and Carbon Neutrality"goals.This study focuses on the Yufu River(YFR),a typical karst river in northern China,based on a surface water survey conducted during the dry season of 2019.By integrating hydrogeochemical and isotopic tracer methods(δ15NNO3,δ15NNH4,δ18ONO3 and δ13CDIC)with the Bayesian isotope mixing model(Simmr)and hydrochemical mass balance(forward modeling),we quantitatively identified the sources of nitrate and evaluated the roles of H2CO3,HNO3,and H2SO4 in carbonate weathering and CO2 sequestration.The Simmr model constrained by hydrochemical parameters and dual nitrogen isotopic data revealed that the main sources of nitrate in the Yufu River catchment were soil nitrogen(34.6％—39.3％)and ammonium fertilizer(27.7％—39.9％),followed by nitrate fertilizer(9.5％—16.7％),with relatively lower contributions from sewage and manure(7.1％—12％)and atmospheric deposition(4.4％—7.5％).The ion ratios and δ13CDIC values indicated that in addition to natural carbonic acid,externally derived acids such as HNO3 and H2SO4 from anthropogenic activities significantly participated in carbonate weathering.Hydrochemical mass balance calculations showed that CO2 consumption followed the order:Jinyangchuan(JYC)＞YFR main stream＞Jinxiuchuan(JXC)＞Erxian River(EXR).The amounts of CO2 released by sulfuric acid and nitric acid-mediated carbonate weathering decreased in the sequences EXR＞JXC＞JYC＞YFR and EXR＞JXC＞YFR＞JYC,respectively,which may be related to variations in agricultural fertilizer intensity across sub-basins.Notably,the net CO2 consumption rates across four monitoring sites exhibited significant spatial differences:JYC(+0.26 mmol/L),YFR(+0.01 mmol/L),JXC(-0.81 mmol/L),and EXR(-2.13 mmol/L).Among these,the JYC demonstrated a clear karst carbon sink effect with net CO2 consumption,YFR was near carbon neutrality with approximately balanced CO2 flux,while JXC and EXR functioned as net carbon sources releasing CO2 into the atmosphere.These spatial variations reflect the complexity of hydrogeochemical processes and their influence on the carbon cycle across the watershed.The findings highlight that intensive application of reduced nitrogen and sulfur fertilizers in agriculture leads to significant anthropogenic acid involvement in carbonate weathering,which is a key factor weakening the riverine carbon sink effect in northern China's karst regions.This study provides an important scientific basis for constructing river carbon cycle models and accurately assessing watershed carbon sinks in northern China and other temperate karst areas.