Assessing Soil CO2 Efflux Using Continuous Measurements Of CO2 Profiles In Soils With Small Solid-State Sensors

This paper describes a new method to monitor continuously soil CO₂ profiles using small solid-state CO₂ sensors buried at different depths of the soil. Based on the measurement of soil CO₂ profile and a gaseous diffusivity model, we estimated soil CO₂ efflux, which was mainly from heterotrophic respiration, and its temporal variation in a dry season in a Mediterranean savanna ecosystem in California. The daily mean values of CO₂concentrations in soils had small variation, but the diurnal variation was significant and correlated well with soil temperature. The daily mean CO₂ concentration remained steady at 396 μmol mol⁻¹ at 2 cm depth during the dry summer from days 200 to 235 in 2002. Over the same period, CO₂ concentration decreased from 721 to 611 μmol mol⁻¹at 8 cm depth, and from 1044 to 871 μmol mol⁻¹ at 16 cm. The vertical soil CO₂concentrations changed almost linearly with depth up to 16 cm, but the gradient varied over time. Based on the soil CO₂ gradient and the diffusion coefficient estimated from the Millington–Quirk model, continuous soil CO₂ efflux was calculated. The daily mean values of CO₂ efflux slightly decreased from 0.43 to 0.33 μmol m⁻² s⁻¹ with a mean of 0.37 μmol m⁻² s⁻¹. The mean diurnal range of CO₂ efflux was greater than the range of daily mean CO₂ efflux within the study period. The diurnal variation of soil CO₂ efflux ranged from 0.32 to 0.45 μmol m⁻² s⁻¹ with the peak value reached between 14:30 and 16:30 h. This pattern corresponded well with the increase in soil temperatures during this time. By plotting CO₂ efflux vs. soil temperature, we found that CO₂ efflux correlated exponentially with soil temperature at the depth of 8 cm, with R² of 0.86 and Q₁₀ of 1.27 in the summer dry season. The Q₁₀ value increased with the depth of soil temperature measurements. The high correlation between CO₂ efflux and temperature explains the diurnal pattern of CO₂ efflux, but moisture may become another factor driving the seasonal pattern when moisture changes over seasons. The estimated CO₂ efflux using this method was very close to chamber measurements, suggesting that this method can be used for long-term continuous measurements of soil CO₂ efflux.