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.