摘要
本文尝试利用重庆丰都雪玉洞岩溶关键带观测站2015年7月~2016年6月大气N湿沉降、地下水化学以及δ15N-NO3-、δ18O-NO3-和δ13C同位素等数据，探讨岩溶关键带C-N耦合循环过程与环境效应。结果表明：1）观测站大气N湿沉降通量为20.9×103kg N/a，其中NH4＋-N和NO3--N分别占湿沉降量的52%和48%；2）地下水δ15N-NO3-和δ18O-NO3-分别介于2.5 ‰~5.6 ‰和3.3 ‰~15.6 ‰，大气沉降N和土壤N是地下水NO3-的主要来源；3）变化于-13.7 ‰^-10.4 ‰与0.59~0.62的地下水δ13CDIC与（Ca2＋＋Mg2＋）/HCO3-摩尔比证实岩溶关键带C-N耦合循环的存在，并控制碳酸盐岩的风化过程；4） C-N耦合循环输出的NO3--N和DIC-C分别为11.4×103kg N/a和287.1×103kg C/a，其中大气沉降与硝化过程形成的HNO3风化碳酸盐岩形成的DIC占DIC输出总量的3.5%。因此，岩溶关键带的C-N耦合循环不但导致地下水NO3-的污染，而且扰动了碳酸盐岩的风化过程。
Increase of sewage discharge from urbanization, use of N fertilizers and nitrogen deposition could have influenced the biogeochemistrical processes of the karst critical zone, but information on the magnitude and consequences of any change has been lacking. Xueyudong karst critical zone observatory is located in Fengdu County of Chongqing, Southwest China, and the geographical coordinate is 29°42＇~29°47＇N, 107°43＇~107°48＇E. The area of watershed is approximately 13.1km2and the elevation is between 230~460m above average sea level. The climate is subtropical monsoonal with annual mean precipitation of 1100mm and mean air temperature of about 16.5℃. The underground river is developed in the Triassic Feixianguan Formation （T1f）with a length of 10.5km, which consists of limestone with the thickness of about 150~250m. The thickness of soil is heterogeneous, varying from about 0 to 50cm. The types of land use are mainly forest land and dry land, and accounted for 81.7% and 18.3%, respectively. The vegetation is mainly composed of evergreen and broad-leaf forests, and shrubs. The nitrogen deposition, chemical and stable isotopic compositions of nitrogen and carbon from groundwater and rainwater, collected from July 2015 to June 2016, are used to discuss the C-N coupling cycle and evaluate its effects on the carbonate weathering in Xueyudong karst critical zone observatory. The results show that：（1）The concentrations of NH4＋ and NO3- in rainwater range from 0.3mg/L to 3.2mg/L with a mean value of 1.1mg/L and 1.2mg/L to 9.8mg/L with a mean value of 4.0mg/L, respectively. The total wet N deposition fluxes is 20.9×103kg N/a, of which NH4＋-N and NO3--N accounted for 52% and 48%, respectively; （2）While, the concentrations of NO3- in groundwater vary from 6.6mg/L to 13.5mg/L with a mean value of 10.6mg/L, and no any NH4＋ is found in groundwater; （3）The dominant dissolved ions and anions in groundwater are Ca2＋ （88mg/L）and HCO3- （237mg/L）, respectively. The hydrochemical type of the groundwater is HCO3-Ca; （4）The δ15N-NO3- and δ18O-NO3- range from 2.5 ‰ to 5.6 ‰ with a mean value of 4.0 ‰, and 3.3 ‰ to 15.6 ‰ with a mean value of 8.2 ‰ in groundwater, respectively, suggesting that the NO3- in groundwater is derived from the N deposition and soil N, and the nitrification is the dominant nitrate transformation process; （5）The δ13C values of limestone, plants and soil CO2 vary from 1.7 ‰ to 3.1 ‰ with a mean value of 2.4 ‰,-26.5 ‰ to -29.9 ‰ with a mean value of -27.9 ‰ indicating the vegetation in watershed is C3 dominant, and -22.6 ‰ to -25.7 ‰ with a mean value of -24.2 ‰, respectively; （6）The carbonate rocks are dissolved by CO2 from C3 vegetation under open system conditions which the δ13CDIC of groundwater has a value of around -15 ‰ with a molar ratio between （Ca2＋＋Mg2＋）and HCO3- of around 0.5 in groundwater, and by HNO3 introduced from N deposition and nitrification of soil N which the δ13CDIC of groundwater approaches a value of 2.4 ‰ with a molar ratio between （Ca2＋＋Mg2＋）and HCO3- of around 1 in groundwater. While the δ13CDIC vary from -13.7 ‰ to -10.4 ‰ with a mean value of -12.3 ‰, and the molar ratio between （Ca2＋＋Mg2＋） and HCO3- range from 0.59 to 0.62 with a mean value of 0.60 in groundwater, indicating that the carbonate rocks are not only attacked by carbonic acid, but also by the nitric acid introduced from N deposition and nitrification of soil N. The carbonate rocks are dissolved by HNO3 should be responsible for the elevated the δ13CDIC and the molar ratio of （Ca2＋＋Mg2＋）/HCO3- of groundwater; （7）The contributions of carbonate dissolution by HNO3 to total （Ca2＋＋Mg2＋）and HCO3- in groundwater vary from 4.9% to 6.9% with an average of 5.7%, and 3.0% to 4.2% with an average of 3.5%, respectively; （8）The total fluxes of NO3--N and DIC-C derived from C-N coupling cycle in Xueyudong karst critical zone observatory are 11.4×103kg N/a and 287.1×103kg C/a, respectively. Thus, this study indicates that the C-N coupling cycle not only impacts the groundwater quality, but also has a significant perturbation to the carbon cycling in karst critical zone.
出处
《第四纪研究》
CAS
CSCD
北大核心
2017年第6期1251-1261,共11页
Quaternary Sciences
基金
国家自然科学基金项目（批准号：41472321）和国家重点研发计划项目（批准号：2016YFC0502306）共同资助
