Pioneering Evidence of the Dynamics of Water Vapor and CO2 Fluxes in Sahara Desert Soils

Soil CO₂ flux (F_c) is a major component of the global carbon balance. It is often overlooked in arid regions as low soil moisture restricts microbial and root respiration. Studies in the last decades challenge this paradigm, reporting an anomalous diurnal cycle of F_c in arid regions. This diurnal cycle is thought to be initiated by geochemical reactions in the soil. This hypothesis is controversial since F_c occurs even during the driest soil conditions when no apparent water is available to dissolve the soil CO₂. We used a static chamber system, as well as profiles of temperature and relative humidity sensors accompanied by meteorological measurements, to provide the first evidence of water vapor and CO₂ fluxes in dry soils of the Sahara Desert, Morocco. We show that water vapor advected inland from the Atlantic Ocean is diffused into the dry soil. Using soil-specific vapor sorption isotherms, we show that the soil water content increased nightly by 0.7%–1% as water vapor was adsorbed to soil particles. The water vapor flux was significantly correlated to F_c (r² = 0.76, p-value < 0.001), supporting the hypothesis that water vapor adsorption (WVA) provides the water source to initiate nocturnal soil CO₂ uptake. The quality of the correlation and the magnitude of F_c were affected by the soil's specific surface area, salinity, and reactive CaCO₃ content. These findings suggest that WVA initiates chemical reactions that consume soil CO₂ even in the driest environments.