Mechanisms of nocturnal soil CO2 uptake influenced by a succession of biological soil crusts in drylands

Many dryland soils absorb atmospheric CO₂ at night. Despite the relatively small annual carbon (C) uptake, ranging locally from 1 to 10 g C m^-2, it may have large-scale effects, as drylands cover almost 45% of the Earth’s land surface. This process might contribute to the global missing C sink of 3.1 ± 0.9 Pg C year^-1. As dryland soils have high inorganic C contents compared to organic C, mechanisms of the nocturnal CO₂ uptake likely involve both biotic and abiotic processes that are tightly coupled to water availability. Biological soil crusts (hereafter, biocrusts) cover about 30% of global drylands and provide favourable physico-chemical hotspots for this C exchange mechanism. In this study, we present a mechanistic model of inorganic C binding that is enhanced by the activity of biocrust communities. The model results show good agreement with field measurements of the soil-atmosphere CO₂ exchange dynamics under contrasting conditions of water availability and temperature in the Tabernas Desert (Spain). We further show that inorganic C sequestration rates at night vary, depending on the successional stages of biocrusts. Our findings have the potential to substantially improve the often-overlooked processes of dryland systems in the global C cycle. By unravelling the mechanisms occurring along the succession of biocrusts, this study highlights the roles of soil biological agents in mitigating CO₂ emissions in a drier future.