Simulation of physical sorption of CO<sub>2</sub> of volcanic ash soil in closed-chamber methods

Volcanic ash soils have a high capacity to absorb CO2 because of their porous structure, which can lead to underestimation of soil respiration rates when measured using the static closed-chamber method. This study aimed to quantify the impact of CO2 adsorption on soil respiration measurements in volcanic ash soils using computational modeling based on adsorption data reported by Tabata et al. (2025). The Freundlich isotherm was used to calculate the adsorption ratio in two situations: (1) when the CO2 concentration was changed from 0 to 5% at any gas/soil ratio (volume/weight: v/w), and (2) for the gas/soil ratio at a certain concentration. Soil respiration rates in the simulated static closed-chamber method was estimated under conditions such as a vial as a container (volume of 136 mL) with a soil amount of 10 g per dry soil. The estimated soil respiration rates showed that the adsorption ratio at 5°C reached 0.92 at a CO2 concentration of 5000 ppm and a gas/soil of 10. The amount of CO2 emitted by soil respiration was calculated under an interval of 600 seconds. The results showed that the adsorption ratio increased with increasing CO2 concentration or decreasing gas/soil ratio. This study highlights the importance of considering CO2 adsorption when measuring soil respiration in volcanic ash soils using the closed-chamber method, and provides insights for developing more accurate and reliable measurement methods.