Ecohydrogeochemical Effects Of Karst Vegetation In Guangxi

The deterioration of ecosystem in karst areas in Guangxi is very serious, which hinders local economic development and environmental improvement, and becomes one of key problems in the province. How to reconstruct the ecosystem is also one of the major projects in Guangxi today. It is necessary to understand ecohydrogeochemical effects of karst vegetation. Moreover, we should study the physicochemical characteristics of water samples which are affected by vegetation, and compare the physicochemical characteristics of water samples under different vegetation types. In this paper, the main types of karst vegetation in Guangxi, including secondary forest mainly formed by ever-green broad-leaf wood after more than forty years of closing the land for reforestation, and scanty shrub cluster after twenty years of closing the land for reforestation is studied. Some indexes of water samples, including pH value, electricity conductivity, acidity, alkality, negative ions content,positive ions content are discussed.The research shows: The total content of chemical compositions in rainfall are increased through vegetation, different elements show different variation. Ions list in throughfall according to leaching coefficient shows: K⁺ï¹¥Mg²⁺ï¹¥Cl^-ï¹¥HCO₃^-ï¹¥Ca²⁺ï¹¥Na⁺,Cl^-, HCO₃^-, Na⁺ and Ca²⁺ shows negative leaching, K⁺ and Mg²⁺ shows positive leaching, ions list in stemflow shows: K⁺ï¹¥Mg²⁺ï¹¥HCO₃^-ï¹¥Ca²⁺ï¹¥Cl^-ï¹¥Na⁺,except for Cl^- and Na⁺, others in stemflow assembled more than in throughfall. Dissociative CO₂, total carbon, dissolved inorganic carbon, dissolved organic carbon, acidity are increased, pH values are declined and electricity conductivities rose in both water samples.In spite of the same outer factors, soil water in secondary forest mainly formed by ever-green broad-leaf wood is more than which in scanty shrub cluster, but natural water content of soil in secondary forest is less than which in scanty shrub cluster. The response of different vegetation-soil systems are similar: the total content of chemical compositions are increased,but different from each other: Content of Ca²⁺, Mg²⁺and HCO₃^- in soil water of Nongla are lower than Dongwang, contents of Cl^-, SO₄^2-, K⁺, Na⁺ and dissociative CO₂ are higher than Dongwang. Spring Landiantang's physicochemical characteristics are stable, its chemical contents'variation coefficients are smaller than rainfall, throughfall and stemflow; moreover, its total ions content, pH value, electricity conductivity, combined CO₂, total rigidity and total alkality are higher than Spring S25 in Yajicun in Guilin; its Ca²⁺, HCO₃^-, total carbon, total inorganic carbon and total organic carbon are higher than Spring Shangnongla and Spring Dongwang; its water temperature is affected remarkably by air temperature; From the diurnal physicochemical variations in a normal day with no rainfall, water temperature varies with air temperature, electricity conductivity and pH value of the spring are influenced by the vegetation's photosynthesis and respiration besides air temperature; From the monthly physicochemical variations, water temperature still varies with air temperature, conductivity varies the same with water temperature, pH value varies different from water temperature; From the total year's physicochemical variations, response of the vegetation's photosynthesis and respiration is not obvious.A lot of dissociative CO₂ of rainfall after vegetation can break karst dynamics system's balance and strengthen dissolution rate, accelerate carbon cycle; some chemical contents are carried into karst ecosystem by rainfall after vegetation, which can increase some elements'effectivity, provide more nutrition to vegetation so as to improve vegetation's growth and succession.All the upper research is needed further study and discussion. But a basic conclusion can be drawn: Karst vegetation not only has the same functions as the non-karst vegetation but also have unique functions. So people must restore and rebuild karst vegetation, it is the key to keep karst ecosystem's healthy cycle.