The origin and distribution of carbonic anhydrase (CA) which could accelerate Karst processes were explored in this paper. The soil samples used in the experiment were collected from Jinfu Mountain of Chongqing,Misuga of Liu Panshui, Nongla of Guangxi and the Yaji Karst Experimental Site of Guilin, representing different Karst ecosystems of southwest China. The CA activities of the soil samples were determined according to the pH decline method. The results indicated that the CA activity could be detected not only in surface (10~20cm depth) soils of different karst ecosystems, but also in the control plot on a shale and sandstone area. The facts showed that CA, which is widespread in animals, plants and prokaryotes, is present in the environment such as soils where the living organisms exist, through the decomposing and secreting of the living organisms. Because CA can remarkably catalyze the reversible reaction between CO_2 and HCO~-_3, so it might have influence on the surrounding environment. For example, CA might have effects on atmospheric CO_2 precipitation or carbonate rock dissolution. However, these presumptions require further studies. The comparative analysis showed that the CA activity varied obviously among the soils in different karst ecosystems. Of the four kinds of Karst area, the mean CA activity of the surface soil in Misuga of Liu Panshui, which had the lowest rate of covered vegetation, was the lowest at 0.02 U/g dry soil. Nongla and Jinfu Mountain, where there is abundant plant biodiversity and healthy vegetation, had higher mean CA activity in the surface soil with 3.83 U/g dry soil and 3.13 U/g dry soil respectively. The Karst Experimental Site, where the vegetation is not growing as well as at the Nongla and Jinfu Mountain sites, had lower mean CA activity in the surface soil with 2.61U/g dry soil. Moreover, there were certain differences in CA activities in the soils between different kinds of Karst landscape in the same kind of Karst ecosystem. These results suggest that the activity of CA from soil correlated highly with the characteristics of Karst ecosystems as represented by their different karst geochemical background, Karst landscape as well as plant species and growth status.
On the other hand, higher CA activity could be detected in rhizosphere soils, such as the soils in plots NL4#, NL5# of Nongla and JFS3# of Jinfu mountain. Furthermore, the variation of CA activity in soils with season was almost in accordance with seasonal changes in the quantities of soil microorganisms in Karst ecosystems. These facts implied that plant roots and soil microorganisms serve as important sources of CA. Meanwhile, the bacteria that can produce CA were screened through special plates containing calcium carbonate, and the activity of extracellular or intracellular CA was measured. In addition, the CA activities of predominant bacteria from two different types of Karst areas were also measured. The results indicated that most of the screened strains and the predominant bacteria produced detectable activity of intracellular or extracellular CA. There were obvious differences in intracellular and extracellular CA activities of soil bacteria among different karst ecosystems with different conditions of vegetation. NLCa602, isolated and screened from NL5# soil sample of Nongla where the vegetation is abundant, had the highest intracellular CA activity of 13.52 U/mg protein. While GLCa102, isolated and screened from a soil sample from doline No.1 covered with flourishing bush cluster in the Karst Experimental Site, had the highest extracellular CA activity of 1.12 U/mg protein. These results suggested that the activity of CA from soil bacteria in the two different karst areas also correlated highly with characteristics of Karst ecosystems featured by their different landscape and vegetation.