Short-Term Soil CO2 Concentration Responses to Precipitation Events in Karst Land with Diverse Vegetation Coverage in Southwestern China

Exploring the short-term fluctuations in soil carbon dioxide (CO2) concentrations is critical for understanding the terrestrial carbon (C) cycle, especially in karst soil due to its vast C reserves and fragile eco-environment. In this study, we investigated the rainfall-event-driven CO2 concentration variations in the soil profiles of cropland (CR), abandoned cropland (AC), grassland (SG), and secondary forest (SF) in a typical karst watershed by a sensor technique to understand the short-term soil CO2 dynamics and their controls at different recovery stages. Our results showed that the CO2 concentrations in all studied soil types responded to rainfall quickly and decreased significantly during rainfall. CR had the largest decrease (1350 ppm), followed by AC (912 ppm), with a response time of approximately 150 min. By contrast, the decreases in soil CO2 in SG and SF were smaller, with decreases of 428 and 155 ppm and shorter response times of approximately 45 min. The soil CO2 concentration showed an obvious diurnal variation pattern consistent with the soil temperature, but there was a lag effect of soil CO2. After rainfall, the diurnal difference in soil CO2 concentration decreased, which may be due to a decrease in temperature difference or may be related to soil CO2 entering groundwater or being consumed by the chemical weathering of carbonates. Our data indicated that the sensitivity of both soil C production and loss to environmental changes such as temperature and precipitation varies in soils with different vegetation recoveries in karst areas. As vegetation recovers, CO2 drops less during rainfall, suggesting that vegetation recovery enhances soil C stability, thereby helping to sequester atmospheric CO2 and contribute to the enhancement of C sink functions in karst ecosystems.