The dynamic processes of CO2 migration as influenced by moisture and temperature in the arid sandy shrubland vadose zone

Abstract

Desert ecosystems have recently garnered significant attention from scientists due to their potential to reduce carbon sink deficits and mitigate climate warming. However, limited attention has been given to the migration of CO₂ in the subsurface of these ecosystems. In this study, continuous monitoring of CO₂ concentrations, volumetric soil water contents (VWC), and soil temperatures (T_s) was performed at depths of 20, 50, and 100 cm within the root zone of Salix psammophila. The SOILCO2 module integrated into HYDRUS-1D was utilized to assess spatiotemporal variations in soil CO₂, its dynamic processes, and responses to extreme meteorological conditions. The results indicate that soil CO₂ concentrations, production rates, and fluxes throughout the soil profile followed clear temporal trends related to air temperature (T_a) and soil temperature (T_s) during the growing season. They began to rise in May, peaked in July and August, and then gradually decreased afterward. Moreover, rainfall regulated the real-time fluctuations of CO₂ concentrations, production rates, and fluxes throughout the soil profile. Droughts and extreme rainfall events primarily impact soil CO₂ fluxes due to changes in gas diffusion and microbial activity within the soil. Increases in rainfall-induced soil moisture stimulate soil CO₂ generation (via microbial activity) but impede its upward migration (via gas diffusion pathways restricted by decreased soil aeration), thereby enhancing CO₂ concentrations. The soil-atmosphere CO₂ exchange was primarily affected by processes occurring in the 0–50 cm soil depth, mainly the CO₂ production. This study highlights the importance of understanding the water-heat-carbon coupling mechanisms in the root zone, which is crucial for elucidating soil carbon emission dynamics.