Pseudo soil in hyper-arid ephemeral stream channel beds: Geo-ecological implications of the fluvio-pedogenic unit (FPU)

The type of stratum in dryland ephemeral channel beds (wadis) determines its hydrological characteristics, which impact vegetation productivity. The objective of this study was to assess some key properties of a cemented, fine-grained bed stratum, previously characterized as a fluvio-pedogenic unit (FPU), as well as those of the overlying non-cohesive gravel bed stratum, known as a fluvially active unit (FAU). While the latter is an outcome of regular fluvial processes of recurring scour and fill, the former is characterized by horizonation, calcium carbonate deposition, oxidation, and accumulation of fine-grained material, defining it more as a soil or pseudo soil than as a sediment. The study was conducted in the hyper-arid southern Negev and Arava Valley of Israel, where the FAU and the underlying FPU were investigated, focusing on geo-ecological functioning. The results revealed considerable differences between the two types of strata. The contents of hygroscopic moisture, total organic carbon, and calcium carbonate were fourfold, 57%, and 36% greater, respectively, in the FPU than these in the FAU. Above all, the fine particle contents, including the clay and silt fractions, were almost eightfold and almost sixfold greater, respectively, for the FPU than those for the FAU, characterizing the former as loamy fine sand and the latter as fine sand. Altogether, these properties determine the stratum's field capacity and permanent wilting point, which were 73% and 50% greater, respectively, in the FPU than those in the FAU. In turn, the last two properties determine the available water capacity, which was almost twofold greater under the FPU than that under the FAU. Considering the (i) FPU's mean thickness of 0.6 m; (ii) FPU's horizonation into A, B, and C horizons; and (iii) a 70–90% decrease in available water capacity between FPU's A and B horizons, as well as between its B and C horizons, then the total available water capacity throughout this stratum's thickness is 20–87% greater under the FPU than that under the FAU. In addition, the FPU's cemented nature seems to provide a safe stratum that remains stable during floods, sustaining the establishment and growth of deep-rooted perennial vegetation. It is concluded that the FUP's properties have positive implications for vegetation growth and survival under the harsh climatic conditions across the region.