5) Mr. Nadav Bekin

Soil CO₂ efflux (Fc) is the largest terrestrial source of atmospheric CO₂ and is typically attributed to biological respiration. In arid environments, however, Fc is often assumed to be negligible due to limited microbial activity under dry conditions. Here, we present two years of high-resolution field observations from the Negev Desert, Israel, capturing the dynamics of surface and subsurface temperature, relative humidity, water vapor, and CO₂. We show that under dry conditions, atmospheric water vapor adsorbed to soil particles at night provides sufficient moisture to dissolve CO₂ in the uppermost soil layer. This process creates a downward CO₂ gradient and explains the frequently observed diel pattern of nocturnal CO₂ uptake and daytime efflux in desert soils. Using the sfDBM model, we estimate that water vapor adsorption can explain over 50% of the nocturnal CO₂ uptake. We also show that heatwaves, which elevate nighttime air temperatures, suppress CO₂ dissolution and enhance net CO₂ efflux. While abiotic processes contribute to CO₂ dynamics year-round—including during wet periods—they are secondary to CO₂ pulses driven by biocrust activity following rainfall. Our findings suggest that water vapor adsorption plays a critical but climate-sensitive role in regulating soil–atmosphere CO₂ exchange in deserts.