Comparison of the closed-chamber and gas concentration gradient methods for measurement of CO2 and N2O fluxes in two upland field soils

Abstract

We measured nitrous oxide (N2O) and carbon dioxide (CO2) fluxes from Gray Lowland soil (onion field) and Andosol soil (maize field) using the closed-chamber method and the concentration-gradient method based on Fick's law (gradient method). Measurements of gas concentration (at a depth of 0.05 m) and relative gas diffusion coefficients (D/D 0) (0–0.05 m depth) in the soil were carried out every week during the snow-free season (May–October) each year for 6 years in the Gray Lowland soil (1995–2000) and for 3 years in the Andosol soil (1998–2000). The seasonal pattern of N2O and CO2 fluxes using the chamber method was similar to those using the gradient method, and there were significant positive correlations between the fluxes using the chamber and gradient methods when extremely high N2O flux values were excluded (Smirnov–Grubbs’ outlier test, P < 0.01). There were no significant differences in N2O fluxes between the two methods, but CO2 flux using the chamber method was higher than that using the gradient method. As the gradient method could not measure the production, consumption and gas diffusion in the surface soil above the soil-air sampling tube (upper 0.05 m), differences in extremely high N2O and CO2 fluxes between the two methods resulted when the production and consumption of these gases were active in the soil above the installed location of the soil-air sampling tube. Measurements of gas concentration and D/D0 in the soil were required at every measurement during the investigation period because these values showed large seasonal variation. The measurement of CO2 flux was more influenced by plants than the N2O measurements. Therefore, it is necessary to consider the distance between the instruments (chambers and soil-air sampling tubes) and nearby plants. Our results suggest that the gradient method could lead to under or over estimation of CO2 flux and to extremely high N2O flux measurements. In contrast, the gradient method could be used for N2O flux measurement, excluding extremely high fluxes, and to understand seasonal patterns in CO2 flux. The gradient method is useful because it can estimate gas fluxes both in the soil and from soil to the atmosphere at the same time.