Wet–dry cycles on sandy and loessial Negev soils: Implications for biocrust establishment and growth?

Long-term field campaigns in the Negev observed wet–dry cycles (WDCs) following rain events, principally on sand. Aiming to assess the relevant mechanism (whether dewfall, water capillarity or distillation derived by temperature-induced vapor flux [TIVF] which stems from the soil), a set of outdoor experiments was conducted on sand and loess. They included nonrainfall water (NRW) and temperature measurements during the rainless dry seasons (summer, fall) and during the winter, which was divided to a wet (following rain) and dry periods. Dew measurements were conducted on cloths, whether directly attached to the soil or located at 0.5 cm above ground, aiming to eliminate capillarity and assessing the possible contribution of dewfall and TIVF-induced distillation. Our findings show that (a) WDCs were only observed following rainstorms, particularly on sand, (b) WDCs could not have been explained by capillarity (they continued at the presence of an air gap), (c) significantly lower minimum surface temperatures (averaging 0.8°C) characterized the sand, which following rainstorms resulted in higher NRW due to TIVF, that is, visibly noted WDCs. With sand being characterized by a lush cover of biocrusts in the Negev dunefields, while being largely absent from the loessial playas scattered within the dunefield, it is suggested that WDCs may play an important role in the establishment and growth of biocrusts over sand dunes of the Negev and possibly in other arid dunes.