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

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