The Hydrogeochemistry And Carbon Sink Of Epikarst Zone Under Different Land Cover

The epikarst zone is in the convergence zone of atmosphere, lithosphere, biosphere and hydrosphere. And it is the important forward position about the water resources and energy conversion of karst dynamic system. The epikarst water which is in the epikarst zone is the importance water resources for production and living to the people in southwest karst mountains area. At the same time, because the carbonate rock-water-CO2-biology four-phase is dynamic balance, the epikarst system have a special carbon cycle, and as an important part of the atmospheric CO2sinking. But with the change of the land cover and the development of karst rocky desertification, the epikarst ecosystem which is sentitive to the change of the environment is changing. This includes not only the change of the epikarst water resources but also the change of the epikarst carbon sinking effect. Therefore, the change of epikarst water resources and karst carbon sinking effect which caused by land use and land cover change are needed to assess effectively.This not only can provide the security for the people's production and life in karst mountainous area, but also can provide a scientific basis for the global carbon cycle research.The author chose this as the topic of the dissertation. Three sites which had different land cover in Nanchuan city of Chongqing were selected for this research. They are:Boshuwan spring with shrud-grass land cover, Lanhuagou spring with agriculture land cover and Hougou spring with rocky desertification land cover. In the study, the hydrological geochemistry characteristics and carbon sinking effect of these there sites were researched. The following conclusions are obtained through this research.The chemical properties of epikarst springs under different land cover were studied. The results showed that water temperature, pH, EC and major ions in springs all exhibited obvious variations. The water temperature which affected by the air temperature had obvious seasonal variation. But the seasonal variation extent of epikarst springs was different. The pH of springs showed lower in the rainy season and higher in the dry season. And the pH of Hougou spring is slightly higher than the other two springs. Under the order of land cover from shrud-grass land, agricultural land to rocky desertification land, pH, EC and the contents of NO3-,SO42-,Cl-,Na+were significantly enhanced, while the content of HCO3-and the percentage of HCO3-+Ca+were markly decreased. With the changed of land cover and reduced of vegetation coverage, the purification and water conservation of soil decreased. At the same time, the contents of the major ions in epikaist springs were significantly enhanced. Therefore, all these indicate that the karst hydrochemical parameters, e.g., electronic conductivity, NO3-, SO42-, Cl-, Na+, HCO3-, can serve as indicators of different land use patterns and land cover change. And during the processes of protecting water resource and water environment, it has important significance to considering the change of land cover.The hydrological geochemistry of soil water in epikarst zone which had different land cover were studied. The results showed that: due to rainfall, vegetation and different soil physical properties and so on, the soil water quantity under different land cover was different. And the soil water quantity in different depth was also different. In the lime-stone soil environment that was rich in calcium, Ca2+was the major cation ion in soil water. By the effect of erosion intensity and soil leaching, the Ca2+and SO42-contents of soil water in Boshuwan and Hougou was increased with the increased of soil depth. But by the effect of fertilization, the the Ca2+and SO42-contents of soil water in Lanhuagou appeared opposite phenomenon. The Na+and Mg2+contents of soil water changed differently under different land cover. With the increase of soil depth, the contents of NO3-, K+, Total Fe, Al3+and Ba2+all reduced gradually. Under the order from Boshuwan spring, Lanhuagou spring to Hougou spring, the average contents of NO3-, SO42-and PO43-in soil water were significantly enhanced. According the correlation analysis between hydrological geochemistry of soil water and spring water, it showed that the hydrogeochemical elements of epikarst springs were affected by infiltration of soil water. But because of the different material sources in soil and the different morphology and motility of elements, the correlation between hydrogeochemical elements of soil water and epikarst spring was different.The diurnal variations of hydrological geochemistry characteristics of epikarst spring under different land cover were observed. The result showed the chemical dynamics of epikarst spring water were controlled by there main effects under rainfall condition. There were piston effect, dilution effect and water-rock-gas effect. But for the epikarst springs under different land cover, the effect which controled it was different.Through the research on the short time scale of hydrological and geochemical characteristics in Lanhuagou spring and Hougou spring, it further discussed the important role of land use and vegetation cover in improving the epikarst spring flow. At the same time, good vegetation and thick soil layer were benefit for the carbonate dissolution and the soil and water conservation. Under the condition of rainfall, for Lanhuagou spring, the carbonate rock dissolution and rainfall on soil leaching were the main sources of the water ions. While for Hougou spring, the rainfall on soil leaching and the erosion effect were the main sources of the water ions.The DIC concentrations and the δ13CDIC values showed seasonal variations, with lower DIC concentrations and δ13CDIC values in wet season. However, due to the different land cover of the spring catchments, the DIC concentrations and δ13CDIC values in springs were different. The relationship between DIC and δ13CDIC values showed positive in Boshuwan and Lanhuagou spring, while showed negative in Hougou springs.Through the correlation analysis between hydrochemistry and δ13CDIC values of springs, the dissolution of carbonate rock by carbonate is the major source of DIC in springs. Sulfuric acid and nitric acid also took park in the dissolution of carbonate rock. But the springs under different land cover affected by different factors. So the ratios of nitric and sulfuric acid in carbonate rock dissolution was different. The ratios of nitrate and sulfate in carbonate rock dissolution in Boshuwan spring, Lanhuagou spring and Hougou spring ranged from2%to7%,9%to13%,10%to46%, respectively. In addition, the HCO3-of springs from the soil was not like that the karst carbonate dissolved HCO3-in springs involved50%of soils or atmospheric CO2consumption and50%of the carbonate rock dissolution. In Boshuwan spring, the HCO3-of spring had53%from the soil CO2. While in the Lanhuagou and Hougou spring, the HCO3-of spring had45%and40%from the soil CO2respectively. So, as the increase of farming area and fertilizer input in the spring catchments, the ratios of the water HCO3-from the soil CO2decreased gradually. Vegetation restoration is conductive to enhancing the karst carbon sinking, while the cultivated land and rocky desertification will reduce the carbon sequestration.The annual atmospheric CO2deposition of Boshuwan,Lanhuagou and Hougou spring were2303Kg/a,1284Kg/aand176Kg/a, respectively.