Water from Air: an Overlooked Source of Moisture in Arid and Semiarid Regions

Abstract:

Understanding water sources, use, and availability in Earth’s arid and semiarid lands is fundamental to understanding and forecasting dryland ecosystem function. While it is accepted that drylands can obtain water from sources other than precipitation (e.g., from dew), little is known about how liquid water supplied to dryland soils via water vapor adsorption (WVA) helps to regulate dryland communities and their activity. WVA – the adsorption of water vapor and subsequent formation of liquid water within soil – occurs at times when the relative humidity in the soil atmosphere is lower than the relative humidity of the overlying air. Surprisingly little research has been focused on WVA, though environmental conditions are such that this phenomenon is likely to occur quite regularly in dryland regions throughout the world. Here we used an observational study in a semiarid grassland and a series of laboratory experiments to explore the role of WVA in providing soil moisture and regulating soil microbial activity (assessed as CO₂ efflux rates). Through the introduction of ¹⁸O liquid water into the atmosphere of a closed system, we were able to trace the movement of water from the atmosphere to the soil via WVA under a range of temperature conditions. Both in the field setting and in the laboratory, elevated rates of soil respiration were observed with WVA-induced increases in soil moisture. Taken together, these results indicate that vapor adsorption is occurring under a variety of common conditions and provide compelling evidence that this water source may strongly influence the activity of soil microorganisms, including fundamental aspects of the soil carbon cycle.