Response of deep soil CO₂ concentration to precipitation events in semi-arid areas

In arid and semi-arid areas, soil moisture strongly influences the balance between respiration and diffusion, altering soil CO₂ concentration and surface flux. Numerous studies have focused on the relationship between surface soil CO₂ flux changes and rainfall events. Subsoil carbon constitutes a large fraction of the total carbon stock, but it is unclear how rainfall events influence subsoil CO₂ concentration dynamics. We continuously monitored CO₂ concentrations at 10, 50, and 100 cm in the soil profile from 2019 to 2021, and analyzed the various responses of subsoil CO₂ concentration to rainfall events. In this study, soil temperature showed apparent seasonal characteristics. As the air temperature changed, the soil temperature of different depths also changed from 100 cm < 50 cm < 10 cm to 10 cm < 50 cm < 100 cm. The soil moisture content of different layers was in the order of 10 cm < 100 cm < 50 cm, and a significant fluctuation was found at 10 cm. The soil CO₂ concentration gradually increased with the increase of the depth in the order of 10 cm < 50 cm < 100 cm, with mean values of 0.66×10⁴, 0.87×10⁴, and 1.04×10⁴ μmol∙mol⁻¹, respectively. On sunny days, the soil CO₂ concentrations at 10, 50, and 100 cm showed apparent diurnal variations and could be expressed as a single-peak curve. However, rainfall events significantly affected the change trends of CO₂ concentrations. Approximately 78% of the rainfall events quickly altered the soil CO₂ concentration in 10 cm layer. When the rainfall amount was exceeded 25 mm, the CO₂ concentration at 50 and 100 cm decreased after 91 and 121 hours. When the soil moisture status changed from drying to wetting phases under rainfall events, > 25 mm precipitation promoted an increase in soil CO₂ concentration at 10 cm by 30% which then began to decrease. The soil CO₂ concentrations at 50 and 100 cm decreased by 16.3% and 10.9%, respectively, with an increase in soil moisture. In arid and semi-arid areas, rainfall negatively affects the changes in soil CO₂ concentration at 10 cm depth under lower soil moisture content. This is because the decrease in gas diffusivity led to an increase in CO₂ concentration. Soil CO₂ concentrations at 50 and 100 cm depths decreased under rainfall events, although the soil moisture was higher than the field capacity. This was caused by the high soil moisture content, which inhibited microbial respiration. The responses of soil CO₂ concentration at different depths to rainfall differed and largely depended on the soil moisture content.