Spatial distribution and origin of soil CO2 in Andeok area, Jeju

Abstract

CO₂ flux measurements and soil gas samples were collected to determine the origin and distribution characteristics of soil gas discharged at Andeok area, Jeju, South Korea. Additionally, this study aims to discuss agricultural activities that may contribute to increased soil CO₂ emissions. CO₂ flux was measured at 127 points, while soil gas sampling for gas components and carbon-13 (δ¹³C_CO2) analysis was performed at 56 points. The measured CO₂ fluxes in the study area ranged from 0.8 to 83.8 g/m²/d (mean: 14.3 g/m²/d), which was lower than CO₂ fluxes observed in areas with geogenic CO₂ discharge (e.g., near active volcanoes, faults, and CO₂-rich water sources). The soil gas analysis revealed that the CO₂ concentration and δ¹³C_CO2 ranged from 491 to 23,722 ppmv (mean: 3,205 ppmv) and from −25.2 to −10.7‰ (mean: −19.4‰), respectively. In the cumulative probability diagram, the threshold values for CO₂ flux and soil CO₂ concentration were 41.3 g/m²/d and 6,693 ppmv, respectively. Samples exceeding the CO₂ flux threshold were affected by urea fertilizer application based on land use nearby, and did not overlay with samples exceeding the soil CO₂ threshold, indicating different transport mechanisms. Results from a detailed survey near a soil gas sampling location close to high soil CO₂ showed that soil CO₂ concentrations increased with depth, especially during summer when atmospheric temperatures increased. Spatially, higher soil CO₂ concentrations were observed in areas with accumulated plant debris from tangerine orchards. The δ¹³C_CO2 and relationships among CO₂-O₂, N₂-CO₂, and N₂/O₂-CO₂ indicated that all collected samples had a biogenic origin. Although geogenic soil gases were not observed in this study, our findings are useful for future research on Jeju Island, as they provide baseline values for distinguishing geogenic soil gas from those with natural and agricultural origins. Moreover, this result provides insights into agricultural activities such as urea fertilization and plowing and their contribution to soil CO₂ increase, and inspires future research on negative soil CO₂ flux.