Thermally driven CO₂ adsorption/desorption and its effect on soil respiration

Soil respiration counterbalances global photosynthesis, yet several aspects of this process remain unexplained despite decades of research: (i) nighttime negative soil carbon flux in drylands and (ii) diurnal temperature-dependent hysteresis patterns. We propose that abiotic CO₂ adsorption-desorption in the soil is the cause or contributor to these phenomena. Laboratory isotherms were conducted on desert soil at various diurnal temperatures and CO₂ partial pressure levels. These results, combined with field data from the Mojave Desert, demonstrate that adsorption causes negative nocturnal CO₂ fluxes, while desorption during the day leads to positive fluxes. In addition, by accounting for thermally driven physical CO₂ exchange, the apparent temperature hysteresis commonly observed is reversed. The annual CO₂ adsorption in the thermally active soil layer ranges from 16.7 to 19.3 grams of CO₂-C per square meter per year, indicating a possible abiotic exchange of up to 2.2 to 2.5 billion tonnes of CO₂-C with the atmosphere globally. Our findings suggest that gaseous adsorption should be included in the global carbon cycle to more accurately represent land-atmosphere CO₂ exchange.