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Simulation of physical sorption of CO2 of volcanic ash soil in closed-chamber methods
2025 Volume 81 Issue 3 Pages 147-151
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2025 Volume 81 Issue 3 Pages 147-151
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.
Accurate quantification of soil respiration is essential for assessing the global carbon cycle, and researchers continue to measure it (e.g., Ballantyne et al., 2017; Desai et al., 2022). Soil is considered the most uncertain factor for improving the accuracy of future projections of climate change (Varney et al., 2022; Nissan et al., 2023), and it is important to quantify soil respiration, which plays a role in the carbon cycle in soil. Although global soil respiration can be viewed as having increased over the past decade (Hashimoto et al., 2015), the detailed causes of this increase are unknown. Therefore, understanding the various mechanisms underlying soil respiration is required.
The chamber method is mainly used to measure soil respiration and can be divided into dynamic (flow-through chamber) and static (closed-chamber) methods (Luo and Zhou, 2006; Bond-Lamberty et al., 2024). In the chamber method, the soil is covered with a chamber in the field; on the other hand, the sample is placed inside the chamber in a laboratory experiment. In the flow-through chamber method, the carrier gas flows at a constant flow rate and was introduced into the analyzer. In the closed-chamber method, CO2 concentration is measured as it changes in response to soil respiration using gas chromatography or nondispersive infrared CO2 sensors. The amount of CO2 derived from soil respiration was calculated using the flow rate or volume of the chamber. The flow-through-chamber method allows for continuous measurement of soil respiration, but its disadvantage is that the construction of the experimental system is complicated and difficult. In contrast, the closed-chamber method cannot be applied to processes that depend on gas concentration (e.g. photosynthesis) (Yonemura et al., 2019a).
Volcanic ash soils are formed by the weathering and deposition of volcanic ash released by volcanic eruptions, and they cover less than 1% of the global land area (Soil Survey Staff, 1999). Volcanic ash is characterized by its porous structure and large surface area, which allow easy absorption of organic matter (McBratney et al., 2014). As a result, volcanic ash soils have a higher carbon storage capacity than other soils, and store approximately 5% of the world’s total soil carbon content (Eswaran et al., 1993). Volcanic ash soil is thought to play an important role in the terrestrial carbon cycle. Tabata et al. (2025) reported that agricultural soils derived from volcanic ash in Tsukuba, Ibaraki Prefecture, Japan, physically adsorbed CO2. The mechanism of physical adsorption is generally regarded as van der Waals forces (Atkins et al., 2018), and previous studies on CO2 adsorption by biochar have reported that the amount of CO2 adsorbed is determined by the specific surface area, pore size distribution and pore volume (e.g. Huang et al., 2015; Jang et al., 2018). Thus, the adsorption of CO2 by volcanic ash soils is a common physical phenomenon, but it has not been recognized in the fields of agricultural meteorology and soil science. Physical adsorption of CO2 is a phenomenon that is considered to affect measurements of soil respiration.
The objective of this study was to quantify the effect of CO2 absorption on soil respiration in volcanic ash soils using the closed-chamber method. The numerical calculations were performed based on the Freundlich equation reported by Tabata et al. (2025). The results of this study are expected to provide new insights into the study of respiration in volcanic ash soils using the closed-chamber method, and to help develop more accurate and reliable measurement methods.
The data used to evaluate the effect of CO2 adsorption in volcanic ash soils on soil respiration measurements using the closed chamber method were based on the methods and results of Tabata et al. (2025). The experimental method and analysis results were summarized here in this paragraph. Topsoil (0-15 cm) and subsoil (15-20 cm) were collected from an experimental field (36°01′N, 140°07′E) in Tsukuba, Ibaraki, Japan. CO2 adsorption data obtained from air-dried topsoil were used. The C content, N content, pH, and soil particle density of the topsoil were 36.3 (SD: 0.2) g kg-1, 2.7 (SD: 0.03) g kg-1, 6.47 (SD: 0.03), 2.37 (SD: 0.01) g cm-1. The amount of CO2 adsorbed was measured using a gas exchange measurement system (Yonemura et al., 2019b). The soil samples were placed in a chamber in an incubator (IG420, Yamato Scientific Co., Ltd., Tokyo, Japan), through which a carrier gas flowed at a constant flow rate of 200 mL min-1. The carrier gas was dehumidified and directed to a CO2 analyzer (LI820, Licor Inc., Lincoln, NB, USA) after passing it through the chamber. CO2 concentration of the carrier gas was controlled stepwise in the range of 0-2920 ppm, and the concentration was changed at intervals of at least 48 h. These experiments were performed at 5°C and 25°C. The CO2 concentration of the carrier gas was controlled in steps from 0 to 2920 ppm. The total amount of adsorption and desorption was calculated by the CO2 emission during the 48-hour period after the concentration change. Based on these calculated adsorption values, regression analysis using the Langmuir, Brunauer-Emmett-Teller (BET), and Freundlich equations was performed (Atkins et al., 2018). The Freundlich equation showed a high correlation at both 5°C and 25°C, and best described the adsorption phenomenon of volcanic ash soil (Tabata et al., 2025). For example, the coefficients of determination for topsoil were 0.54, 0.71, and 0.91 for Langmuir, BET, and Freundlich, respectively at 25°C. Therefore, the Freundlich equation was used in this study.
2.2. Calculation of adsorption ratio in 2 situationsWe considered adsorption/desorption equilibrium in a hypothetical closed system.
where rg⁄s is the ratio of the gas phase (v) to the soil phase (w), Vg is the volume of the gas phase (cm-3), and Ws is the soil mass (g). Considering a closed space, the equilibrium state at any concentration when the gas and soil phases exist in the ratio given in equation (1) was calculated. The CO2 adsorbed by the soil in a closed system at any concentration was calculated using the Freundlich equation (Atkins et al., 2018)
where n is the amount of adsorbed CO2 (mol g-1),
Table 1. Empirical constants of Freundlich isotherm in this study.
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where ns is the amount of adsorbed CO2 into any soil weight (mol). The total amount of gas in the gas phase was calculated using the ideal gas equation of state.
where P, Vg, nt, R and T are the pressure, volume of the gas phase, amount of substance in the total gas, gas constant (8.31 Pa m3 K-1 mol-1), and temperature (K), respectively. The amount of gas phase CO2 was calculated by multiplying it by its concentration.
where ng is the amount of CO2 (mol) in the gas phase. The proportion of CO2 adsorbed by soil was calculated using the results of a series of calculations.
where rs is the adsorption rate. The calculations above were performed for the two cases. Equation (6) was used for the adsorption ratio when the CO2 concentration was changed from 0 to 5% at any gas/soil ratio, and Equation (7) was used for the gas/soil ratio at a certain concentration.
2.3. Estimation of the effect on soil respiration measurementWe simulated head space concentrations under conditions such as using a 136 mL chamber (No. 8 wide-mouth vial, Maruemu Co., Osaka, Japan) with soil amount was 10 g (dry weight) and the static closed-chamber method is applied to the simulated head space concentrations. For the estimation, the soil respiration rate was referred to Tabata et al. (2025) that measured with gas exchange measurement system based on flow-through-chamber method (Yonemura et al., 2019b) and was assumed to occur at a constant rate as 0.7 and 7.2 pmol g-1 s-1 at 5 and 25°C. Moisture was not included in the study. The amount of CO2 released by soil respiration was calculated on a time interval of 600 second, and the accumulated CO2 release amounts on days 15, 30, 60, and 90 were calculated.
where
If the adsorption rate at concentration (
By substituting Equation (10) for Equation (9) and rearranging, the amount of CO2 in the gas phase (ng' ), taking adsorption into account, can be derived.
If there is no adsorption effect,
Our calculations show that the absorption ratio in soil decreases with increasing CO2 concentration and gas/soil ratio, and that the absorption ratio increases with decreasing temperature (Fig. 1 and 2). For example, at gas/soil ratios of 10 and 25°C, the adsorption ratio was 0.982 at a CO2 concentration of 100 ppm and 0.944 at 5% (Fig. 1, (A)). When the temperature was lowered to 5°C at a 10 of gas/soil ratio, the adsorption ratio was 0.994 at a CO2 concentration of 100 ppm and 0.963 at 5% (Fig. 1 (C)). As the gas/soil ratio increased, the adsorption ratios tended to decrease at all CO2 concentrations (Fig. 2). At 25°C, the adsorption ratios were 0.982, 0.847, and 0.357 for the same CO2 concentration of 100 ppm as the gas/soil ratio was increased to 10, 100, and 1000, respectively (Fig. 2 (A)). Similarly, at 5°C, the adsorption ratios were 0.994, 0.945, and 0.631 for gas/soil ratios of 10, 100, and 1000, respectively, which were higher than those at 25°C (Fig. 2 (B)).
Fig. 1. Adsorption ratio to soil varying by CO2 concentration at several gas/soil ratio. (A) and (B) are simulated at 25°C, on the other hand, (C) and (D) are simulated at 5°C. Additionally, gas/soil ratio (v/w: ml g-1) are (i) 1, (ii) 10, (iii) 100, and (iv) 1000.
Fig. 2. Adsorption ratio to soil varying by gas/soil ratio with each CO2 concentration at (A) 25°C and (B) 5°C. CO2 concentration are (i) 100 ppm, (ii) 1000 ppm, (iii) 5000 ppm, (iv) 1%, (v) 2% and (vi) 5%.
These results indicate that the CO2 concentration in the vial and gas/soil ratio have a significant effect on the amount of CO2 adsorbed in a closed system. To reduce the influence of adsorption, the measurements should be performed at the highest possible temperature and with the largest possible headspace. Even with such measures, however, it is difficult to completely eliminate the effect of adsorption, and at least 14.3% of carbon dioxide was adsorbed into the soil (Fig. 1, (B)), even if the gas/soil ratio was set to 1000, and the measurement was performed at 25°C.
The effect of adsorption on the measurement of soil respiration was significant, with only approximately 9% of the original CO2 emission at 25°C and 4% at 5°C when adsorption was considered under 90 days incubation conditions. At 90 days of incubation, the estimated soil respiration without adsorption effect was 55.8 μmol g-1, whereas it was 4.8 μmol g-1 when affected by adsorption (Fig. 3, (A)). In addition, after 90 days of incubation at 5°C, the estimated soil respiration was 5.5 μmol g-1 without adsorption effect and 0.2 μmol g-1 with adsorption effect.
Fig. 3. CO2 adsorption by volcanic ash soil affect to measurement of soil respiration using static-closed-chamber-method at (A) 25°C and (B) 5°C. Soil respiration without consideration of adsorption (i) and with consideration of adsorption (ii).
These calculations quantitatively showed that the application of the closed-chamber method to volcanic ash soils, using CO2 as a tracer, underestimated the respiration rate. If this method is used, it is necessary to correct for the adsorption effects. However, the amount of adsorption varies considerably depending on the incubation temperature, ever-changing CO2 concentration, and gas/soil ratio. In addition, the calculations in this study were based on the assumption that the equilibrium state was reached instantaneously; however, there may actually be parameters related to the rate of absorption. Therefore, it is extremely difficult to perform accurate correction. In the future, more detailed parameters affecting adsorption and information on the absorption capacity of various volcanic ash soils are required.
We appreciate to three anonymous reviewers who provided us helpful comments to improve the manuscript. This study was supported by a Grant-in-Aid from KAKENHI (Grant No. 23310017 and 23HP2004).
