摘要
岩溶系统是陆地生态系统的重要组成部分,包括植被、土壤和水文等子系统,其资源环境问题越来越受到人们的关注。由于岩溶流域特殊的地质背景,使得岩溶水—土系统具有独特的生态环境特征,直接制约着当地社会经济的发展。碳氮元素是土壤营养物质和地下水溶质的重要组成部分,直接参与生物地球化学作用。生物地球化学过程中的同位素分馏使得稳定碳氮同位素能够示踪碳氮元素在水—土系统中的迁移转化,为研究岩溶流域水土流失、岩溶作用和岩溶水水质演变提供了重要手段。本文以重庆青木关岩溶流域为例,利用稳定碳氮同位素研究流域水—土系统碳氮分布、变化和来源,探讨自然与人为因素的影响。
青木关流域岩溶管道流的水文变化对降雨响应迅速,导致其水化学具有较强的动态变化特征。流域地表水和地下水的地球化学主要受地质背景控制,受白云岩溶蚀、石膏溶解(较高的634s值)和硫化物氧化影响(较低pH值)的侧向裂隙水水化学为Mg·Ca-SO4型,而受灰岩溶蚀控制下的地表径流、岩溶裂隙泉、洞穴裂隙水和地下河水等均呈弱碱性,水化学组成以Ca2+和HC03-为主,属于岩溶水。地下河水地球化学介于裂隙水和地表水之间,显示其受地表水和裂隙水共同补给,且以裂隙水为主。不同植被类型下的土壤水化学也不同,其中灌丛和旱地土壤水为Ca-HCO3型,退耕还林地土壤水为Ca-HCO3·SO4型,而草地和针叶林地土壤水为Ca-SO4型,它们同土壤理化性质密切相关。岩溶水地球化学的形成和变化既受地质背景和岩溶作用强度的控制,又受外界自然和人为因素的综合影响,包括降雨冲刷淋溶作用与稀释效应、土地利用类型和农田施肥时令以及其它人为因素。
不同植被下的土壤碳氮含量及其同位素均具有横向和垂向差异。土壤水溶性有机碳和有机氮含量为草地≈针叶林地>退耕还林地>灌丛地>旱地,并随着深度增加而降低,且与土壤pH值成反比。不同植被下土壤溶解无机氮以硝态氮为主,其大小为耕地土>灌丛土>退耕还林土>草地土>林地土,且随深度的增加,硝态氮含量升高,具有累积效应。表层土壤氮同位素与植物体氮同位素具有显著相关性,使得不同植被类型下的土壤氮同位素具有很大差异。土壤有机氮同位素垂直变异特征表现为,40 cm以上的土壤主要受植物残体影响而贫化15N,且变化较大；40 cm以下土壤受较强的矿化作用影响而富集15N,变化较小。除受植被影响外,土壤有机氮同位素还广泛受到土壤质地、结构、pH值、有机质含量、碳氮比和农业施肥活动等因素共同影响。灌丛地、旱地和退耕还林地土壤水具有较高DIC浓度和δ13 CDIC值,均值分别达230.66 mg/L和-9.62%‰，158.09 mg/L和-9.93‰,115.46 mg/L和-10.5‰,其DIC主要来自碳酸盐岩的溶蚀和土壤CO2的溶解。它们的δ13 CDIC值与DIC浓度却呈正相关关系,且雨季偏高,旱季偏低。针叶林地和草地的DIC浓度和δ13 CDIC值均较低,分别为17.79 mg/L和-15.68‰,14.81 mg/L和-16.10‰,其DIC主要来自土壤CO2的溶解。
除侧向裂隙水的DIC较低(38.34 mg/L)外,洞穴裂隙水、地下河水、岩溶裂隙泉和地表径流都具有较高的DIC浓度,均值分别为330.62 mg/L、326.74 mg/L、290.94 mg/L和198.13mg/L。侧向裂隙水和洞穴裂隙水的DIC浓度在雨季高于旱季,表现为土壤CO2效应。地下河水和地表径流的DIC浓度在雨季有所降低,表现为降雨稀释效应。岩溶水的δ13 CDIC值变化范围为-13.60‰～-5.89‰,指示其溶解无机碳主要来源于碳酸盐岩和土壤CO2。雨季,岩溶水的δ13 CDIC值较旱季偏低2‰～4‰,其DIC更多为土壤CO2成因。受其他酸(如有机酸等)参与岩溶作用影响,岩溶水的δ13 CDIC值会相对偏高,其DIC来自碳酸盐岩本身的比例增加。岩溶裂隙泉、洞穴裂隙水和地下河水δ13 CDIC值与DIC浓度具有反相关关系,雨季偏低,旱季偏高。地表径流δ13 CDIC值与DIC浓度没有明显的相关性,显示其受多种因素控制。
流域雨水的NO3-浓度为5.09 mg/L,δ15NN03为2.92‰,其硝态氮主要来自干沉降或煤炭等燃烧排放的NOx。岩溶水中的NO3-的形成主要与区内人类活动有关,在无人类活动影响下,岩溶水的δ15NNO3值约为4‰,其NO3-主要来自矿化的土壤有机氮；受污水或粪便影响的岩溶水则具有较高的N03-浓度和δ~(15)NNO3值。在旱季,地表径流的NO3-浓度和δ15NNO3值分别为2.56mg/L和4.7‰,指示其硝态氮主要来源于土壤有机氮；而在雨季,地表径流的NO3-浓度和δ15NNO3值分别为7.87 mg／L和6.81%o,其硝态氮主要来源为合成化肥和有机化肥的混合。地下河水的NO3-浓度变化范围为15.84-63.5 mg/L,其δ15NNO3值为4.42‰～12.24‰,总体上表现为雨季较低,其硝态氮主要来自矿化的土壤有机氮与合成化肥的混合；而旱季较高,其硝态氮主要来源为土壤有机氮和污水或粪便的混合。
根据青木关流域下水系统地球化学特征,结合δ13 CDIC、δ1NNO3口δ34S,判断流域岩溶作用方式以CaCO3—CO2—H2O岩溶动力系统溶蚀为主,而硫酸和硝酸参与流域碳酸盐岩溶蚀作用较弱。另外,流域碳酸盐岩溶蚀在一定程度上受到了有机酸的影响。根据流域水化学计算得出流域发生岩溶作用的碳酸盐岩的化学分子式为(Ca0.85Mg0.15)CO3,并利用水化学—径流量方法估算出了流域岩溶作用产生的碳酸盐岩溶蚀速率和大气CO2沉降率分别为115.31 t／(km2·a)和46.07 t/(km2.a),高于我国南方岩溶区碳酸盐岩溶蚀速率以及由此产生的大气CO2沉降速率。
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.
引文
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