Diurnal hysteresis between soil temperature (T-s) and both CO2 concentration ([CO2]) and soil respiration rate (R-s) were reported across different field experiments. However, the causes of these hysteresis patterns remain a subject of debate, with biotic and abiotic factors both invoked as explanations. To address these issues, a CO2 gas transport model is developed by combining a layer-wise mass conservation equation for subsurface gas phase CO2, Fickian diffusion for gas transfer, and a CO2 source term that depends on soil temperature, moisture, and photosynthetic rate. Using this model, a hierarchy of numerical experiments were employed to disentangle the causes of the hysteretic [CO2]-T-s and CO2 flux T-s (i.e., F-T-s) relations. Model results show that gas transport alone can introduce both [CO2]-T-s and F-T-s hystereses and also confirm prior findings that heat flow in soils lead to [CO2] and F being out of phase with T-s, thereby providing another reason for the occurrence of both hystereses. The area (A(hys)) of the [CO2]-T-s hysteresis near the surface increases, while the A(hys) of the R-s-T-s hysteresis decreases as soils become wetter. Moreover, a time-lagged carbon input from photosynthesis deformed the [CO2]-T-s and R-s-T-s patterns, causing a change in the loop direction from counterclockwise to clockwise with decreasing time lag. An asymmetric 8-shaped pattern emerged as the transition state between the two loop directions. Tracing the pattern and direction of the hysteretic [CO2]-T-s and R-s-T-s relations can provide new ways to fingerprint the effects of photosynthesis stimulation on soil microbial activity and detect time lags between rhizospheric respiration and photosynthesis.
2015. Vol. 120, no 8, 1605-1618 p.