| Soil water is a link contacted the surface water, groundwater and biogebchemical cycles, which is also a carrier of mass transport and transfer, and plays a signicifance role in the conversion of "four kinds of water " and nutrient cycling. In karst areas, soil water is an important form of rainfall to underground water, some of the rainfall seep into soil and migrate to karst aquifer through the pores in soil. Organic and inorganic solutes in soil system have been dissolved by hydrone in this process, which controls the geochemical characteristics of the karst water system. Thus, soil water serve as a vital part in the process of eco-hydrological, and it has a great significance to do some research about soil water which is a carrier of nutrients transport and migrate. Carbon plays a vital role in karst ecosystem. A proportion of carbon dioxide produced by microbial activities and respiration of roots can be dissolved and carried by the infiltration water and then dissolve carbonate materials in the karst soil system. This process can produce a lot of dissolved inorganic carbon (DiC) into the soil water and groundwater. The stable carbon isotope composition of DIC (δ13CDIC) can be used as a tracer for studying biogeochemical processes in soil water because the potential biogenic, atmospheric and geologic sources of C have distinct δ13C signatures.This article has selected Qingmuguan karst catchment,Chongqing, SW China, as a study area. We selected five typical different vegetations(grass land Q1, dry land Q2, shrub land Q3, afforestation land Q4, bamboo groves), soil water sampler have been settled according to the soil genetic horizon and soil samples were collected at10cm intervals under all vegetations. Chemical composition, DOC, carbon isotope of soil water had been tested in Lab. The main physical and chemical properties of the soil, such as the soil mechanical composition, carbonate content, moisture, pH value, water soluble organic carbon and inorganic nitrogen have been tested, and then discussed the impact of human activities, vegetations, geological background, climate and soil physical and chemical properties on hydrochemical composition of soil water. The DIC concentrations and δ13CDIC values of soil water were analyzed in order to reveal the characteristics of temporal and spatial variations of DIC fluxes, which can help us to get an insight into the carbonate dissolution and its related carbon cycling. The main results showed that:(1) The physical and chemical properties of soil, like water content, organic carbon, texture, carbonate content and so forth have a significance influence on the hydrochemical properties and the transformation form of ion in soil water. The pH value of soil is mainly affected by parent material, soil development stage, overlying vegetations and human fertilization. The pH value of different soil profile and pH value of soil water with correspond sampling points has the similar trends:All of them increased from top to bottom. The spatial variation showed:shrub land> dry land> afforestation land> bamboo groves> grass land,which reflects the control effect of soil on soil water. WSOC of soil and DOC of soil water in different soil profiles of top layer were higher than that of the bottom layer, while they are inversely related in the spatial variation. Soil nitrate has been greatly affected by agricultural fertilization activity. Due to the high solubility of soil nitrate, the content of soil nitrate is consistent with the NO3-in soil water, which is dry land> shrub land> afforestation land> bamboo groves> grass land. As the soil clay has a very strong adsorption to cations, the content of soil clay in soil profile shows inverse correlation with the concentration of K+NaT in soil water. There is a good correlation between the concentration of DIC in soil water and the content of carbonate in different soil profile. The concentration of DIC in different sampling points was shrub land> dry land> afforestation land> grass land> bamboo groves, The DIC in soil water and the content of carbonate in different depth also showed a good consistency.(2) The hydrochemical characteristics of soil water under different vegetations are different, while they are similar in different depth under the same vegetation. Ca2+and HCO3-are the main ions in soil water resulted from calcium accumulation and alkaline of soil environment in karst area. The soil water of Q1and Q5was the Ca-SO4type, and Q4was the Ca-HCO3·SO4type, while both the Q2and Q3had the Ca-HCO3·SO4type soil water. The temperature of soil water under different sampling points is changed significantly with the seasons, the average value of conductivity in soil water were:569.28μs/cm(dry land)>547.89μs/cm(shrub land)>397.33μs/cm(afforestation land)>209.16μs/cm (bamboo groves)>131.46μs/cm (grass land).The ion concentration showed different characteristics affected by human activities, geological background, precipitation, land use and soil physical and chemical properties.(3) DIC concentrations and δ1CDIC of soil water showed distinct temporal and spatial variations in the study area. Soil water derived from grass land and bamboo groves had lower DIC concentrations and δ13CDIC values, with a mean value of56.12mg/L and-17.22‰,27.15mg/L and -16.56‰, respectively. The δ13CDIC indicated that these DIC were mainly derived from the dissolution of soil CO2. Soil water derived from the dry land, shrub land and afforestation land had higher DIC concentrations and δ13CDIC values, with the averages of153.88mg/L and-12.2‰,221.82mg/L and-11.9‰,127.11mg/L and-11.65‰, respectively. Meanwhile, these δ13CDIC values of soil water were generally4‰～5‰higher in the rainy season than those in the dry season. Moreover, these DIC values showed positive correlations with corresponding carbonate content in soil. These indicated that DIC in these soil water were primarily derived from dissolution of carbonate materials by soil CO2, suggesting that great carbonate dissolution occurs in the karst soil system which could contributes to the reduction of soil CO2released to the atmosphere to some extent. |
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