What actually controls the minute to hour changes in soil carbon dioxide concentrations?

Abstract

The monitoring of carbon dioxide (CO2) in anthrosol showed CO2 concentrations ([CO2]) up to 10,000 ppmv in dependence on external conditions. During dry season, [CO2] oscillated in a diurnal cycle with mean amplitude about 1520 ppmv. [CO2] was strongly positively correlated with soil temperature, T(soil), (correlation coefficient r ~ 0.92). However, T(soil) lagged behind [CO2] by 55 min. Due to the phase shift, the [CO2]/T(soil) dependence showed typical hysteresis loop with a counterclockwise rotation. A simple model of two oscillating signals indicates that this direction of rotation would mean violation of causality. The lag of T(soil) behind [CO2] would be conceivable if heat and CO2 were transported to the point of measuring from soil top layer and the CO2 transport was faster than heat transport. An effect of photosynthesis on [CO2] via root respiration is not too probable at dry season because it works on a longer time scale. Nevertheless, the correlation of [CO2] with the illumination (IL) in spectral range of 380–720 nm did not rule out such possibility (correlation coefficient r = 0.63 at 4-hour lag of [CO2] behind IL).Wet season was simulated by artificial soil sprinkling: adding water to soil induced the strong/immediate increase of [CO2] which was attributed to enhanced heterotrophic respiration. The dependence [CO2] = f(WEx) where WEx is water excess in L m−2 was almost linear, but its slope increases exponentially with temperature. Based on this finding, the relation SH(z) = b1 × exp(b2 × T(soil)(z) / T0) × (θ(z) / Φ) + b3 (where SH(z) is heterotrophic respiration [mol m−3 s−1], T(soil)(z) is soil temperature [K], T0 is standard temperature [K], θ(z) is moisture [m3 m−3], Φ is soil total porosity [m3 m−3], z is vertical coordinate, b1, b2, b3 are parameters) was proposed. A participation of root respiration on immediate fluctuation of [CO2] is less probable. This would be possible only in case of pressure propagation through plant xylem/phloem system.