| Land use and land cover change, which can produce a series of direct or indirect impacts on the environmental changes, is one of the important research fields of current global change. The changes can influence the direction and intensity of karstification by affecting the physical and chemical properties of soil in karst areas. Furthermore, studies in recent years have shown that karst dissolution can produce a considerable number of relatively stable carbon sink.Qingmuguan karst valley, which is located in the junction of Beibei District, Shapingba District and Bishan County, Chongqing, was selected as the study area. the field investigations, soil sampling and analysis, and the observation of the hydrologic and hydrochemical indexes were carried out in a whole hydrological year, after analysizing the background information, such as the conditions of geology, geomorphology, meteorology, hydrology and land cover etc, and the results of previous studies. The study proposes to find out the sensitivity of the karstification responsing to the environmental change and its carbon sink effect by analysing the physical and chemical properties of the soil, the dissolution amount of standard limestone tablets at different depths in different seasons under different land use, as well as the hydrologic and hydrochemical characteristics of the water samples taken at the exit of underground river each month of the whole hydrological year.After a whole hydrological year’s field monitoring, laboratory test and data analysis, the following conclusions are drawn:(1) The soil properties are influenced significantly by the land use change. The soil porosity, soil moisture, soil pH, soil organic matter content, soil CO2concentration vary among different land use types in different seasons. Indicators changed in the vertical direction also in the same type of land use. Generally, soil porosity, soil organic matter content reduce with depth increasing. While the soil organic matter mainly concentrates between the surface and20cm depth, and reduce rapidly under this depth with the depth increasing. Soil moisture, soil pH, soil CO2increases with depth, the former may be the result of migration of water to deeper under gravity; the medium may be associated with the reduction of organic matter content and closer to the alkaline carbonate rocks related to parent rock with the depth increasing; while the latter may due to the larger molecular weight of itself that make the migration to deeper layer easily, and the CO2produced by the respiration of biological activity in soil.(2) The dissolution amounts of the soil under different land use types are tested by standard limestone tablets method and it was found that significant differences of the dissolution rate occurred in different land use types in soil. Overall, the amount of dissolution in decreasing order is:tilled land (178.67mg·m-29d-1), paddy (156.86mg·m-2·d-1), woodland exchanged from tilled land (162.37mg·m-2·d-1), fallow land (149.20mg·m-2·d-1), woodland mixed with Chinese fir-bamboo (137.67mg·m-2·d-1), naked rock surface (70.32mg·m-2·d-1), bamboo forest (36.46mg·m-2-d-1).In the vertical direction, the soil dissolution rate differs in the same land use type, and increase with the increase of depth generally. The average dissolution rate of various land use types in the depth of0cm,20cm,50cm,80cm are121.77mg·m-2·d-1,130.64mg·m-2·d-1,149.47mg·m-2·d-1,187.08mg·m-2·d-1respectively. In the time scale, there are also differences in the dissolution amount of the same type of land use under different seasons. The average soil dissolution amount of each season under different land use types are as follows:spring,168.17mg·m-2·d-1, summer,185.82mg·m-2·d-1autumn,104.04mg·m-2·d-1,winter,55.44mg·m-2·d-1. It can be seen that the dissolution amounts of spring and summer are equivalent, but much larger than that of autumn and winter. The dissolution amount of summer is about1.79times and3.35times larger than that of autumn and winter respectively.The variation features of karst dissolution in different time and space scale show that the karstification is fast, and is very sensitive to the environmental change, which participate in the short time scale carbon cycle process actively and play an important role in the recovery of atmospheric CO2.The main reasons which bring about the differences in time and space of the soil dissolution amount under different land use types may be the change of a series of soil properties, including soil moisture, soil organic matter, soil pH and other physical and chemical properties, which resulted from the land use types change can influence the dynamic condition of karstification, such as CO2and hydrodynamics and the intensity of biological effects and then the intensity of dissolution is changed.(3)In order to obtain more accurate flow data converted by level data recorded by the automatic water level monitor indirectly, on-site monitoring related to the water level and velocity of flow was carried out at Jiangjia spring during the rainy season, a large number of first-hand data was obtained, with which the stage-discharge relationship equationas is calculated as follow: Q=2.5274H2.0642, R2=0.9669(4) Both of the standard limestone dissolution tablets method and hydrochemistry-flow method are used to study the karstification dissolution rate and its carbon sink effects. The total dissolution of carbonate rocks of the study area measured by the former method is about599.78t/a, the corresponding amount of CO2consumption and carbon sinks are about263.90t/a and71.97tC/a respectively. While HCO3-flux measured by the latter method is about24.14×106mol/a(1472.24t) within a hydrological year of the study area, the corresponding amount of CO2consumption and carbon sinks are approximately530.97t/a, and144.81tC/a respectively.In short, the properties of soil under different land use types in the study area, karstification dissolution rate and groundwater hydrochemistry indexes all showed the variations in time and space (horizontal and vertical direction) scale, which indicate that the karst system is sensitive to environmental change, and the conclusion that the geological processes including karst processes are significant in global carbon cycle not only in long-time scale but also in short-time response. |
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