Water and Carbon Dioxide Interactions in the most unlikely places: The hidden dynamics of the Sahara Desert soils

Soil CO₂ efflux is primarily attributed to the metabolic activity of soil organisms and is a major component of the global carbon balance. The carbon balance of deserts, such as the Sahara Desert, the largest desert on Earth, is considered neutral as low soil moisture inhibits biological activity and reduces the soil CO₂ efflux to its lower limit. Studies in the last decades challenge this paradigm, reporting a mysterious nocturnal CO₂ uptake by desert soils, which in some cases leads to a net gain of carbon by the soil. While the factors controlling this phenomenon are still under debate, it is clear that the presence of water is essential. How, then, can nocturnal CO₂ uptake occur in the driest soil conditions when no apparent water is available to drive the process? We embarked on a field expedition in the Sahara Desert, southwest Morocco, during the summer of 2022 to explore the dynamics of water and carbon in this presumably “stagnant” ecosystem. We discovered nocturnal water vapor adsorption, driven by atmospheric water vapor transported from the Atlantic Ocean and penetrating hundreds of kilometers inland where the vapor is captured in the soil’s top layer. Changes in soil water content were determined from soil relative humidity (measured using a profile of relative humidity sensors) and soil-specific vapor sorption isotherms (measured using a vapor sorption analyzer). With this novel method, we were able to detect a daily increase of 0.3 mm of water even at a distance of 250 km from the Ocean. Concurrent measurements of CO₂ fluxes (measured using manual and automatic flux chamber systems), confirmed that small atmosphere-to-soil CO₂ fluxes occurred during the night, coinciding with downward water vapor fluxes. This indicates that the atmosphere provides a consistent water source and may initiate soil CO₂ uptake. Simultaneous measurements of water vapor and CO₂ fluxes at a second site suggested that the quality of the correlation between the two fluxes depends on soil properties. Overall, the daily CO₂ cycle was unbalanced (net uptake of 0.08 g m^-2) implying that the soil acted as a carbon sink. This sink is small, but considering its occurrence even in inland desert ecosystems and the fact that arid and hyper-arid regions occupy 26% of Earth’s terrestrial surface, the effect of atmospheric water capture by desert soils on CO₂ exchange may play a significantly larger role in the global carbon balance than previously thought.