Karst dissolution rates of carbonate rocks in north-south geographical boundary of China—the Qinba Mountain Area

The karst area in the south of China is one of the three major karst distribution areas in the world. But the economic development of the karst mountainous area is severely restricted by the fragility of its environment and unreasonable disturbance of human activities. As quantitative data reflecting the intensity of karstification, karst dissolution rates can be studied to facilitate the restoration of karst ecosystem and control of rocky desertification. Especially in recent years, research on rocky desertification control and ecological restoration in karst areas of Southwest China has been achieved with fruitful results, and hence is widely concerned by many domestic scholars on karst ecological characteristics and vulnerability, rocky desertification control and other issues. The Qinba area of Shaanxi Province is an important dividing line between the north and south of China in a climate of subtropical continental monsoon. It is also known as the "central water tower" and has a large sinkhole group at high latitude. Its special geographical location obviously differentiates the climate and ecological environment between the north and south. At the same time, carbonate rocks are widely exposed in the Qinba area, especially in places such as Zhen'an, Shanyang, etc. Unreasonable human activities have destroyed the karst ecological environment, making rocky desertification prominent. By analyzing dissolution rates from the aspects of vegetation, climate, and human factors, we explore the impact of regional environmental changes on karst formation, which may provide data support for ecological restoration, karst carbon sequestration, and rocky desertification control, and may also fill the gap in the study of karst formation in the Qinba area of Shaanxi Province.The three representative vegetation types—woodland, shrub and grassland—were selected in field dissolution tests in the study area. In this study, we used standard dissolution specimens (square test pieces with the side length of 4 cm and thickness of 0.3 cm) of crystalline limestone from the Late Triassic Wujiaping Formation (P3w) in Xiaonanhai town, Hanzhong City. Each vegetation type was divided into 5 layers (100 cm in the air, surface, 20 cm under the soil, and 50 cm and 100 cm under the soil). Three standard dissolution test pieces were places in each layer. After a full hydrological year from March 31, 2021 to October 20, 2022, a total of 806 dissolution specimens were retrieved, and the amount and rate of dissolution of each specimen were obtained. The effects of vegetation types, depths, rainfall and temperatures on the karst process and the relationship between karst dissolution rates and degrees of rocky desertification were comprehensively discussed.The results showed that there were significant differences in subsurface dissolution rates among vegetation types. The average underground dissolution rate of forest land was the highest, followed by that of shrub land. The rate of grassland was the lowest. It was found that the dissolution rate of forest land in the same layer was higher than those of shrub and grassland, and the underground dissolution rate of shrub was higher than that of grassland. The organic carbon contents of different vegetation types and the changes of soil physical and chemical properties by vegetation are the fundamental factors that affect the direction and intensity of the karst process. At the same time, we analyzed the influence of rainfall and temperatures on dissolution rates in different regions. The results showed that there was a significant positive correlation between rainfall and dissolution rates (R=0.84), indicating that rainfall plays a key role in karstification. Rainwater absorbed CO2 from the air during the process of falling to the surface. During infiltrating from the surface to the soil, rainwater combined with CO2 released by plant root respiration and produced by microbial metabolism to form carbonic acid after it absorbed CO2 in the process of falling to the surface. Consequently, the continuous dissolution of carbonate rocks by both surface water and groundwater containing carbonic acid led to the development of rocky desertification. The correlation coefficient between temperatures and corrosion rates is 0.45, which indicates that the temperature is not an important factor affecting the dissolution rate.The formation of rocky desertification is the result of the joint action of natural factors and human factors, and unreasonable human activities are the main factors. A comparative study on dissolution rates of different degrees of rocky desertification in the Qinba area shows that dissolution rates increase with the increase of rocky desertification degrees as follows, severe rocky desertification>moderate rocky desertification>mild rocky desertification. This phenomenon is more obvious in areas with mild rocky desertification, for example, the dissolution rates of Beiyang Mountain in Zhen'an are three times as much as those of the areas mostly distributed with mild rocky desertification. Besides, the aggravation of rocky desertification is often accompanied by extensive agricultural production patterns and severe ecological environment damage. It can be seen that in densely populated areas of karst mountainous areas, people's transformation of the karst environment is the main reason for the aggravation of rocky desertification, and the higher the dissolution rate is, the higher the degree of rocky desertification becomes.The study of dissolution rates in the Qinba area of Shaanxi Province shows that with the forward succession of vegetation, dissolution rates of carbonate rocks will increase. Rainfall can promote the dissolution of carbonate rocks, which is one of the important factors affecting karstification. Moreover, rocky desertification is the result of the interaction of carbonate rock dissolution and human disturbance, and human factors play the main role.