Abiotic CO₂ cycling in a desert soil: Linking surface fluxes and subsurface dynamics across seasons

Abiotic processes in the subsurface produce and consume CO₂ and may significantly contribute to the soil CO₂ flux (F_c) in arid regions. However, their contribution to F_c in these environments is poorly understood. We measured the surface and subsurface dynamics of temperature, water vapor, and CO₂ during a field experiment over two years in the Negev Desert, Israel. We observed a regular diurnal pattern of soil-atmosphere water vapor exchange over the dry season, which was significantly coupled with F_c. Simulations of F_c using a mechanistic model revealed that diurnal fluctuations in water content, originating from adsorption-desorption cycles, played a major role in driving F_c. This finding suggests that atmospheric water vapor, adsorbed to soil particles at night, supplies water to dissolve gaseous CO₂ and can explain the diel pattern of CO₂ exchange reported in dry periods. The subsurface measurements show that geochemical processes produce and consume soil CO₂ both during the dry summer season and after rainfall, and that this process is concentrated in horizons of carbonate accumulation, which act as hotspots for CO₂ redistribution. Our findings further reveal that summer heat waves were associated with net CO₂ emissions, suggesting that increases in minimum daily air temperature, predicted to be exacerbated by climate change, may amplify CO₂ emissions from alkaline soils. Overall, pulses of increased microbial CO₂ production in the biocrust domain following wetting dominated the total yearly carbon loss of 24.26 gC m⁻², while geochemical CO₂ consumption act as an important moderator of respiration rates in desert ecosystems.