Assessing soil CO 2 efflux using continuous measurements of CO 2 profiles in soils with small solid-state sensors

Abstract

This paper describes a new method to monitor continuously soil CO 2 profiles using small solid-state CO 2 sensors buried at different depths of the soil. Based on the measurement of soil CO 2 profile and a gaseous diffusivity model, we estimated soil CO 2 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 CO 2 concentrations in soils had small variation, but the diurnal variation was significant and correlated well with soil temperature. The daily mean CO 2 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, CO 2 concentration decreased from 721 to 611 μmol mol −1 at 8 cm depth, and from 1044 to 871 μmol mol −1 at 16 cm. The vertical soil CO 2 concentrations changed almost linearly with depth up to 16 cm, but the gradient varied over time. Based on the soil CO 2 gradient and the diffusion coefficient estimated from the Millington–Quirk model, continuous soil CO 2 efflux was calculated. The daily mean values of CO 2 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 CO 2 efflux was greater than the range of daily mean CO 2 efflux within the study period. The diurnal variation of soil CO 2 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 CO 2 efflux vs. soil temperature, we found that CO 2 efflux correlated exponentially with soil temperature at the depth of 8 cm, with R 2 of 0.86 and Q 10 of 1.27 in the summer dry season. The Q 10 value increased with the depth of soil temperature measurements. The high correlation between CO 2 efflux and temperature explains the diurnal pattern of CO 2 efflux, but moisture may become another factor driving the seasonal pattern when moisture changes over seasons. The estimated CO 2 efflux using this method was very close to chamber measurements, suggesting that this method can be used for long-term continuous measurements of soil CO 2 efflux.