| Karst system is a particular terrestrial ecosystem, including vegetation, soil and water subsystems, and its related resources and environmental issues have been increasingly concerned by public. For the special geological background of karst catchment, the karst soil and water systems have unique characteristics of ecological environment, which exerts great influence on the local socio-economic development. Both carbon and nitrogen elements directly involved in biogeochemical role are the most important components of soil nutrient and groundwater solute. The isotopic fractionation during biogeochemical processes allows carbon and nitrogen isotopes to trace the migration and transformation process of carbon and nitrogen elements in soil and water systems, which provides an important means to study the soil erosion, carbonate dissolution and evolution of groundwater quality for karst catchment. In this thesis, Qingmuguan karst catchment in Chongqing, China, was taken as study area, aiming at using stable carbon and nitrogen isotopic techniques to characterize the contents and variations of carbon and nitrogen elements, identify their primary sources in karst soil and water systems and explore the impacts of natural and anthropic factors on them.The response of conduit stream discharge in Qingmuguan karst catchment to rainfall was very fast and dramatic, which leads to the strong dynamic variations of groundwater hydrochemistry. The geochemistry of both surface and subsurface water in the catchment were significantly controlled by the geological background. For instance, the geochemical type of lateral fissure water was the Mg-Ca-SO4 type, affected by the dissolution of dolomite and gypsum (higherδ34S value) and sulfide oxidation (lower pH value); while surface stream, karst fissure spring, cave fissure water and subterranean stream were alkalescent with the hydrochemical type of Ca-HCO3, belonging to karst water, primarily controlled by the dissolution of limestone. The values of hydrochemical indicators of subterranean stream ranged between those of fissure water and surface water, suggesting it was influenced mainly by fissure water and partly by fissure surface water. The soil water of shrub land and dry land was the Ca-HCO3 type, and that of afforestation farmland was the Ca-HCO3·SO4 type, while both grassland and coniferous forest had the Ca-SO4-type soil water. Hydrochemical variations of the karst surface and subsurface water were influenced by many factors, such as the geological background and karstification intensity, the flushing eluviation and dilution effects of rainfall, the land use and seasonal farmland fertilization and other anthropogenic inputs.The content and isotopic composition of soil carbon and nitrogen under different vegetation varied both laterally and vertically. Soil water soluble cabon and nitrogen were different and tended to decrease in the order as:grassland>coniferous forest land>afforestation farmland>shrub land>dry land. Both of them decreased from top to bottom in soil profile and were correlated negatively with soil pH value. NO3--N was the main species of dissolved inorganic nitrogen, and its content decreased from dry land, shrub land, afforestation land, grassland to coniferous forest land successively, but increased with soil depth increase, showing the cumulative effect. Surface soil organic nitrogen isotope (δ15Norg) showed positive relationship with those of covering plant leaves, which makes the soil organic nitrogen under defferent vegetations different. Soilδ15Norg values were lower and showed significant vertical disparities between 0 and 40 cm. Below 40 cm depth, soil 815Norg values increased significantly due to intensive mineralization and decomposition of residual soil organic matter. Apart from the 15N-depleted plant litterfall, soilδ15N was also affected synthetically by soil texture, structure, pH, organic matter content, C/N ratio, agricultural fertilizing activities, and so on. Soil water of shrub land, dry land and afforestation farmland had higher DIC concentrations andδ13CDIC values, with mean values of 230.66 mg/L and-9.62%o,158.09 mg/L and-9.93%o,115.46 mg/L and-10.5%o, respectively. Their DIC mainly originated from the dissolution of carbonate rocks and soil CO2. For these soil water,δ13CDIC values showed positive correlation with DIC concentration, and were higher in the rainy season but lower in the dry season. Howerver, soil water of grassland and coniferous forest land had lower DIC concentrations andδ13CDIC values, with mean values of 17.79 mg/L and-15.68%o,14.81 mg/L and-16.10%o, respectively. Their DIC mainly came from the dissolution of soil CO2.The lateral fissure water had lower DIC concentrations with average of 38.34 mg/L, while the cave fissure water, subterranean conduit stream, karst fissure spring and surface stream had higher DIC concentrations with mean values of 330.62 mg/L,326.74 mg/L,290.94 mg/L and 198.13 mg/L, respectively. DIC concentrations of both lateral fissure water and cave fissure water were higher in the rainy season than those in the dry season, while those of subterranean conduit stream and surface stream tended to decrease during the rainy season.δ13CDIC values of karst water ranged from-13.60%o to -5.89%o, suggesting the dissolved inorganic carbon mainly derived from carbonate rocks and soil CO2. During the rainy season,δ13CDIC values were generally 2%o-4‰lower than those in the dry season.decrease, indicating more DIC produced by soil CO2, but the relatively higherδ13CDIC values reflected that other acids (incl. organic acids, etc.) might involve in the weathering of carbonate rocks to some extent, which resulted in that the proportion of DIC from carbonate rocks increased. For karst fissure water, cave fissure water and subterranean conduit stream,δ13CDIC values showed negative correlation with DIC concentrations, and tended to be higher in the dry season but lower in the rainy season. However, there was no significant correlation betweenδ13CDIC values and DIC concentrations for surface stream,reflectingδ13CDIC was controlled by many factors.The mean NO3- concentration of atmospheric precipitation was 5.09 mg/L with meanδ15NNO3 value of 2.92%o, indicating its NO3-originated primarily from the dry precipitation or NOx from coal combustion. The NO3- of karst water generally associated with the human activities. With fewer influence of human activities,δ15NNO3 values were around 4%o, indicating NO3" mainly originated from the mineralized organic nitrogen. The input of sewage or waste could make the karst water have higher NO3- concentrations andδ15NNO3 values. In the dry season, surface stream had lower NO3- concentrations (mean,2.56 mg/L) andδ15NNO3 values (mean,4.74%o), suggesting its nitrate was mainly from soil organic nitrogen. However, it had higher NO3- concentrations (mean,7.87 mg/L) andδ15NNO3 values (mean,6.81‰) in the rainy season, indicating its main nitrate source was the mixture of inoganic and organic fertilizers. NO3- concentrations of subterranean conduit stream were in the range of 15.84-63.5 mg/L withδ15NNO3 values of 4.42‰～12.24%o. In the rainy season,δ15NNO3 values of it tended to be lower, reflecting its nitrate mainly came from the mixture of soil organic nitrogen and inorganic fertilizers. But its nitrate maily derived from the mixture of soil organic nitrogen and sewage or manure during dry season according to higherδ15NNO3 values.According to the geochemistry of groundwater, conbined withδ13CDIC,δ15NNO3 andδ34S, it can be inferred that the karstification pattern was the dissolution of carbonate rocks (mainly limestone) more significantly by carbonic acid but less by sulfuric and nitric acid in Qingmuguan karst catchment. Additionally, the organic acid from the soil system might facilitate the weathering of carbonic rocks, resulting in higher 813CDIC of groundwater. Based on the hydochemistry of groundwater in the Qingmuguan catchment, the chemical formula of dissolved carbonate rock could be calculated as (Ca0.85Mg0.15)C03. Dissolution rate of carbonate rock of Qingmuguan catchment had been esminated to be 115.31 t/(km2·a) using of hydrochemistry-runoff methods and the flux of atmospheric CO2 consumpution by carbonate dissolution was 46.07 t/(km2·a). Both dissolution rate of carbonate rock and its consumpution of atmospheric CO2 were much higher than the average of South China.
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