Response of abiotic soil CO2 flux to the difference in air-soil temperature in a desert

Highlights: • We measured hourly F_a, T_s, T_a and T_a-T_s from June to October 2016 in a desert, China. • The mean hourly F_a during this period was −0.08 μmol m⁻² s⁻¹. • F_a was weakly related to T_s or T_a but strongly and positively correlated with T_a-T_s. • Abiotic CO₂ exchange between soil and atmosphere depends on temperature gradient. • Thermal convection driven by T_a-T_s may play a major role in generation of negative F_a. Abiotic soil CO₂ flux (F_a) has been observed in drylands worldwide, while its generation mechanism is not determined. The remarkable features for the occurrence time of F_a, that the positive F_a (CO₂ emission from soil) and negative F_a (CO₂ absorption by soil) generally occurred in the daytime and nighttime, respectively, suggested that the generation of F_a is related to temperature. Previous studies have focused on the sole effect of soil temperature (T_s) or air temperature (T_a) on F_a, but very little is regarding to the overall response of F_a to both T_a and T_s in the air-soil system. In the Mu Us Desert, China, we continuously measured the hourly F_a, T_s, T_a and the difference between T_a and T_s (T_a-T_s) at 4-day intervals from June to October 2016. During this period, soil abiotically absorbed atmospheric CO₂ at a rate of 0.32 ± 0.19 g m⁻² d⁻¹. F_a was not correlated with T_s and was weakly correlated with T_a, however, it was strongly and positively correlated with T_a-T_s. A quadratic model adequately fitted the relationship between F_a and T_a-T_s, and similar relationships were also observed between positive F_a and T_a-T_s and between negative F_a and T_a-T_s. The results of F_a measurement highlight that desert soils can absorb CO₂ from the atmosphere through abiotic processes. The relationships of F_a with T_s, T_a and T_a-T_s indicate that, abiotic CO₂ exchange between soil and atmosphere depends on air-soil temperature gradient in desert. Furthermore, the thermal convection driven by T_a-T_s may play a major role in the generation of nocturnal negative F_a. Our findings provide a new possible perspective for revealing the reasons of F_a generation in drylands.