The unique karst ecosystem of Southwest China is an example of a typical vulnerable ecosystem and key eco-region in China. Rocky and soil desertification in the karst environment are two of the most serious problems, and unsuitable land use patterns will intensify the desertification. Increasing attention has been paid to the impact of land-use types on karst processes and consequently to investigate the best way to restore the ecosystem in rock and soil desertification areas, it is necessary to study the influences of changes in the soil environment on karst processes under different land use patterns. Soil physical and chemical variations result from land cover, and the resulting special karst micro-environments could intensively affect karst processes under different land use patterns.
A case study was conducted at Mt. Zhongliang, Chongqing, in order to analyze and investigate the dissolution rate in a karst area and the factors which influence it under different land use patterns. Standard limestone tablets were buried in the field, soil CO₂ collection devices were set up, and tracer experiments using a bright blue dye were carried out. The dissolution rate of carbonate rocks, soil CO₂ concentrations, and dissolved organic carbon (DOC) contents were measured in different soil layers at depths of 20 cm and 50 cm, as well as land use patterns and soil properties, such as moisture, porosity and pH.
The average contents of soil DOC from the highest to lowest were in the order: woodland>grassland>vegetable land. There was a significant difference in soil DOC content in the upper soil horizon (0-20 cm) between woodland and vegetable land, as well as in the lower horizon (20-50 cm) among woodland, vegetable land and grassland. The diversity of soil water content and porosity indicates that woodland and grassland are beneficial for maintaining water content. Use of the dye indicated an obvious distribution of soil preferential flow in woodland and grassland, but not in the vegetable land. Soil CO₂ concentrations and pH values at 50 cm depth were higher than at 20 cm, and were present in the order: grassland>woodland>vegetable land. Woodland and grassland have greater vegetation coverage, more plant residues, and greater microbial activity, which may be the reason for the variations in soil physical and chemical properties under the different land use patterns. The study showed that soil physical and chemical properties were affected by different land use patterns, forming special karst micro-environments and resulting in different karst processes in different soil layers. Dissolution rates in the dry season were in the order: grassland>vegetable land>woodland. Soil CO₂ concentration is an important driving force affecting the dissolution rate in woodland and grassland during the dry season; soil moisture and water-supplying capacity are key factors influencing the dissolution rate under the different land use patterns in the dry season; the average dissolution rate in the vegetable land is higher than that in the woodland as more acid materials are produced in the vegetable land which has the lowest pH; the DOC of soil has a strong transforming ability with water infiltration, which is one of the reasons why the dissolution rate of forest land 50 cm beneath the soil is higher than that at 20cm. This research provides a theoretical basis and data reference for the mechanism of karst dissolution and the karst carbon sink in southwestern China.