Unravelling the main mechanism responsible for nocturnal CO₂ uptake by dryland soils

Soil respiration, or CO₂ efflux from soil, is a crucial component of the terrestrial carbon cycle in climate models. Contrastingly, many dryland soils absorb atmospheric CO₂ at night, but the exact mechanisms driving this uptake are actively debated. Here we used a mechanistic model with heuristic approaches to unravel the underlying processes of the observed patterns of soil-atmosphere CO₂ fluxes. We show that the temperature drop during nighttime is the main driver of CO₂ uptake by increasing CO₂ solubility and local water pH of a thin water film on soil particle surfaces, providing favourable conditions for carbonate precipitation. Our data demonstrate that the nocturnal inorganic carbon absorption is a common soil process, but often offset by biological CO₂ production. The uptake rates can be impacted by different successional stages of biocrusts that consume or produce CO₂ and modify the pH of the soil water film, which can be maintained by non-rainfall water inputs, such as pore space condensation. Annual estimates of nocturnal carbon uptake, based on in situ continuous measurements at the soil level in drylands are still very scarce, but fluxes of up to several tens of g C m^-2 y^-1 have been reported, potentially accounting for a considerable fraction of the global residual terrestrial carbon sink.