Water vapor adsorption by dry soils: A potential link between the water and carbon cycles

Water vapor adsorption (WVA) by soil is a potential contributor to the water cycle in drylands. However, continuous in-situ estimates of WVA are still scarce and the understanding of its coupling with carbon cycle and ecosystem processes remains at an incipient stage. Here we aimed to (1) identify periods of WVA and improve the understanding of the underlying processes involved in its temporal patterns by using the gradient method; (2) characterize a potential coupling between water vapor and CO₂ fluxes, and (3) explore the effect of soil properties and biocrusts ecological succession on fluxes. We assumed that the nocturnal soil CO₂ uptake increasingly reported in those environments could come from WVA enhancing geochemical reactions involving calcite. We measured continuously during ca. 2 years the relative humidity and CO₂ molar fraction in soil and atmosphere, in association with below- and aboveground variables, over the biocrusts ecological succession. We estimated water vapor and CO₂ fluxes with the gradient method, and cumulative fluxes over the study. Then, we used statistical modelling to explore relationships between variables. Our main findings are (1) WVA fluxes during hot and dry periods, and new insights on their underlying mechanisms; (2) a diel coupling between water vapor and CO₂ fluxes and between cumulative fluxes, well predicted by our models; and (3) cumulative CO₂ influxes increasing with specific surface area in early succession stages, thus mitigating CO₂ emissions. During summer drought, as WVA was the main water source, it probably maintained ecosystem processes such as microbial activity and mineral reactions in this dryland. We suggest that WVA could drive the nocturnal CO₂ uptake in those moments and discuss biogeochemical mechanisms potentially involved. Additional research is needed to monitor soil water vapor and CO₂ uptake and separate their biotic and abiotic components as those sinks could grow with climate change.