Temporal and spatial variations of soil CO₂ and δ¹³${\mathrm{C}}_{{\mathrm{CO}}_2} $ from different land uses in typical dolomite areas of Southwest China

Shuanghedong national geopark is a prominent example of dolomite on a global scale. Soil CO₂ in the dolomite areas is an important driving force of karstification and has great significance for the global carbon cycle. However, the mechanism of soil carbon cycle in dolomite areas has yet to be clarified. In order to investigate the spatial and temporal variations and influencing factors of soil CO₂ and δ¹³$ {\rm{C}}_{{\rm{CO}}_2} $ in different land uses in typical dolomite areas and to clarify the mechanism of soil carbon cycle in these areas, this study selected Shuanghedong National Geopark, a typical dolomite area in Southwest China, as the study area. Soil CO₂ and δ¹³$ {\rm{C}}_{{\rm{CO}}_2} $ were sampled from six typical land use types in Shuanghedong national geopark. These samples were monitored outdoors and subjected to laboratory experiments over the course of one year. Additionally, the data were systematically analyzed with mathematical and statistical methods.

The study results indicate as follows, (1) Soil CO₂ concentrations of different land use types were observed in the following order: cropland>irrigation grassland>abandoned land>scrubland>forested land>fallow land. Soil temperature and water content, overlying vegetation, organic carbon, and human activities were important factors influencing the changes of soil CO₂ in different land use types. (2) The values of soil δ¹³$ {\rm{C}}_{{\rm{CO}}_2} $ of different land use types were ranked as follows, scrubland>fallow land>cropland>irrigation grassland>abandoned land>forested land. Differences in diffusion rates of soil ¹²C and ¹³C, decompositional conversion of organic carbon and isotopic fractionation, plant photosynthesis and carbonate rock dissolution led to changes of soil δ¹³$ {\rm{C}}_{{\rm{CO}}_2} $ in different land use types. (3) In terms of temporal changes, soil CO₂ concentrations of different land use types were high in the rainy season and low in the dry season, while concentrations of soil δ¹³$ {\rm{C}}_{{\rm{CO}}_2} $ exhibited opposite seasonal patterns, showing small values in the rainy season and large in the dry season. Oxidative decomposition of soil organic matter, metabolic activities of microorganisms, respiration of plant roots and dissolution of carbonate rocks are the main sources of soil CO₂ in dolomite areas. Therefore, oxidation and decomposition of organic matter, dissolution of carbonate rocks, photosynthesis of plants, and metabolism of microorganisms store part of CO₂ in soil to form carbon sinks. The other part of soil CO₂ in the dolomite area are primarily attributed to the higher partial pressure of soil CO₂ compared to that in the atmosphere. This difference in pressure facilitates diffusion, allowing CO₂ to escape into the atmosphere as a carbon source. In addition, the study also indicates that different land use types, soil temperature, water content, overlying vegetation, human activities, and organic carbon all influence the intensity of carbon sources and carbon sinks in the soil carbon cycle in dolomite areas to varying degrees. The research findings have deepened the understanding of soil CO₂ in dolomite areas of northern Guizhou, revealed the spatiotemporal differences and influencing factors of soil CO₂ and δ¹³$ {\rm{C}}_{{\rm{CO}}_2} $ in different land use types, clarified the mechanism of soil carbon cycle in typical dolomite areas, and provided a reference for the study of soil carbon cycle in these areas.