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

Abstract

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.

One‐year‐long evaluation of non‐rainfall water available to soil biocrusts in the Negev Highlands

“…Indeed, no net photosynthesis was recorded by crustose lichens inhabiting the soil at the open landscape following dewfall during 2 years of measurements in the Negev (Wilske et al, 2008), implying that soil-dwelling lichen biocrusts may principally (or solely) rely on rainwater. They may be activated by rain that wets the ground directly or indirectly (following temperature-induced wet-dry cycles that may result in distillation) during several (or more) mornings following the rain event (Kidron et al, 2022). In fact, all the reports that claimed to register net photosynthesis by soil biocrusts following NRW were not conducted under natural dewfall conditions (Ouyang & Hu, 2017) or on intact soil (Chamizo et al, 2021) and therefore should be cautiously considered.…”

“…In order to eliminate any involvement of distillation, that is, vapour that stems from the wet soil after rain (Monteith, 1957), measurements were only confined to dry days and to days in which the upper 3 cm of the soil was dry (Kidron et al, 2022).…”

“…Furthermore, while photosynthetic measurements confirmed the ability of dew to yield net photosynthesis by the crustose chlorolichens (hereafter lichens) that cover rocks and cobbles (Lange et al, 1970), no net photosynthesis was yet reported from the Negev soil biocrusts (Wilske et al, 2008). On the other hand, soil biocrusts were shown to benefit from distillation, that is, vapour that stems from the wet soil following rain and which, as a result of a temperature gradient within the soil profile, condenses during the night and the early morning at the surface (Kidron et al, 2022).…”

“…On the other hand, soil biocrusts were shown to benefit from distillation, that is, vapour that stems from the wet soil following rain and which, as a result of a temperature gradient within the soil profile, condenses during the night and the early morning at the surface (Kidron et al, 2022).…”

Mornings with the highest non‐rainfall water during 2 years of measurements in the Negev–Ecological implications

“…During the current analysis we focused on three thresholds (a) the 0.03 mm threshold that serves to mark the minimum value allowing for organism activity, that is, respiration of chlorolichens, and is therefore considered as the biologically‐dependent threshold of NRW (b) the 0.05 mm threshold that allows for net photosynthesis (i.e., growth) of chlorolichens, and (c) the 0.1 mm threshold, which allows for net photosynthesis by cyanobacteria. To avoid the involvement of distillation, that is, vapour that stems from the wet soil (Monteith, 1957), and therefore cannot be regarded as NRW (Kidron et al, 2022), only the dry seasons or days that were at least 2 weeks away from rain events (and may therefore not be impacted by the rain) were considered.…”