Volcanic degassing releases large amounts of hydrothermal CO₂, which influences long-term global climate. Soil hydrothermal CO₂ is an important form of volcanic degassing, making the corresponding output crucial for understanding the solid Earth degassing and climate change. We investigated the soil gases from the Changbaishan volcanic area, the most active volcano in China, focusing on their gas compositions and carbon isotopic compositions, and soil CO₂ fluxes. Our results showed that the soil gases from diffuse degassing structures had higher CO₂ concentrations and isotopic compositions closer to hydrothermal CO₂ compared to gases from volcano-affected structures. We developed an isotopic mixing model to determine the fractions of atmospheric CO₂, biogenic CO₂, and hydrothermal CO₂ in soil gases, providing a more accurate constraint of carbon sources. Our findings revealed that hydrothermal CO₂ was the primary carbon source for soil CO₂ in diffuse degassing structures, whereas biogenic CO₂ predominated in volcano-affected structures. We integrated the reported data to calculate the local carbon budget for the Changbaishan volcanic area, demonstrating that the study area acts as a carbon source. Additionally, we collected soil hydrothermal CO₂ data from 80-degassing volcanoes and estimated the soil hydrothermal CO₂ output from global volcanic areas to be approximately 150 Mt yr⁻¹. Our study highlighted the effectiveness of using δ¹³C-CO₂ and Δ¹⁴C-CO₂ to identify hydrothermal CO₂ and emphasized the importance of accurately estimating soil hydrothermal CO₂ emissions.

We collected 31 soil gas samples following CO₂ flux measurements in October 2023 in the Changbaishan volcanic area. Gas chromatography-quadrupole time-of-flight mass spectrometry (GC/Q-TOF) was used to measure gas components. The ratio of ¹³C/¹²C was analyzed by GC combined with isotope ratio mass spectrometer (IRMS). The ratio of ¹⁴C/¹²C was analyzed by a 0.5 MV accelerator mass spectrometer (AMS). The soil CO2 fluxes was measured  by accumulation chamber method in 85 sites in the Changbaishan volcanic area.