The hysteresis response of soil respiration and soil CO2 concentration to soil temperature

Abstract:

Diurnal hysteresis between soil temperature (Ts) and both CO2 concentration ([CO2]) and soil respiration rate (Rs) 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 layer-wise mass conservation 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]-Ts and CO2 flux-Ts (i.e., F -Ts) relations. Model results show that gas transport alone can introduce both [CO2]-Ts and F-Ts hysteresis, and also confirm prior findings that heat flow in soils lead to [CO2] and F(z) being out of phase with Ts, thereby providing another reason for the occurrence of both hysteresis. The area (Ahys) of the [CO2]-Ts hysteresis near the surface increases, while the Ahys of the Rs-Ts hysteresis decreases as soils become wetter. Moreover, a time-lagged carbon input from photosynthesis deformed the [CO2]-Ts and Rs-Ts 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]-Ts and Rs-Ts relations can provide new ways to fingerprint the effects of photosynthesis stimulation on soil microbial activity and detect the corresponding time lags.

Key words: Hysteresis; Photosynthesis; Soil CO₂ concentration; Soil respiration; Soil temperature; Soil moisture