Reg Soils Research Articles

Increased root zone oxygen by a capillary barrier is beneficial to bell pepper irrigated with brackish water in an arid region

Limitations to growth and biomass production are expected under high (>30°C) soil temperatures due to low soil oxygen levels and insufficient oxygen transport to roots. These limitations will be exacerbated when irrigation with brackish water dictates large amounts of water application for leaching salts. We hypothesized that the introduction of an artificial capillary barrier (CB), in the form of a gravel layer at the lower root-zone boundary, can increase yields of irrigated horticultural crops due to better soil aeration and improved oxygen supply to roots. The specific objectives of the study were (1) to isolate scenarios where insufficient oxygen concentrations may limit pepper plant growth, (2) to measure oxygen concentrations in media under varied scenarios of CB placement below the active root zone and (3) to understand the environmental factors leading to sub-optimum oxygen concentrations in horticultural soils in a desert environment where supra-optimal soil temperatures are prevalent. At high root-zone temperatures oxygen flux to the roots was lower than the potential uptake rate and therefore soil oxygen concentrations were sub-optimal. These conditions led to reduced plant biomass and fruit yield. Fruit yield was found to decrease by 1% for every soil oxygen concentration decrease of 700ppm. In a fine textured Reg soil, allowing roots to penetrate into the capillary barrier gravel layer increased oxygen concentration in the root zone by 5% and improved biomass and fruit yield by 16% and 18%, respectively.

Transition from arid to hyper-arid environment in the southern Levant deserts as recorded by early Pleistocene cummulic Aridisols

The time at which deserts established their current arid or hyper-arid conditions remains a fundamental question regarding the history of Earth. Cosmogenic isotope exposure ages of desert pavement and welded, calcic–gypsic–salic Reg soils that developed on relatively flat alluvial surfaces ∼2 Ma ago in the Negev Desert indicate long geomorphic stability under extremely dry conditions. Over a short interval during their initial stage of development between 1–2 Ma, these cumulative soils are characterized by calcic soils reaching maximum stage III of carbonate morphology. This interval is the only period when calcic soil horizons formed on stable abandoned alluvial surfaces in the southern Negev Desert. Since ∼1 Ma pedogenesis changed toward more arid soil environment and the formation of gypsic–salic soil horizons that were later followed by dust accumulation. The dichotomy of only moderately-developed calcic soil (stages II–III) during a relatively long time interval (10 5–10 6 years) indicates an arid environment that does not support continuous development but only occasional calcic soil formation. The very low δ 18O and relatively high δ 13C values of these early pedogenic carbonates support soil formation under arid climatic conditions. Such an environment was probably characterized by rare and relatively longer duration rainstorms which occasionally allowed deeper infiltration of rainwater and longer retention of soil moisture. This, in turn enabled the growth of sparse vegetation that enhanced deposition of pedogenic carbonate. At ∼1 Ma these rare events of slightly wetter conditions ceased and less atmospheric moisture reached the southern Negev Desert leading to deposition of soluble salts and dust deposited in the soils. The combination of long-term hyperaridity, scarcity of vegetation and lack of bioturbation, salts cementation, dust accumulation and tight desert pavement cover, has protected the surfaces from erosion forming one of the most remarkably stable landscapes on Earth, a landscape that essentially has not eroded, but accumulated salt and dust for more than 10 6 yr.

The role of rare rainstorms in the formation of calcic soil horizons on alluvial surfaces in extreme deserts

Soils in similar geomorphic settings in hyperarid deserts (< 50 mm yr − 1) should have similar characteristics because a negative moisture balance controls their development. However, Reg soils in the hyperarid southern Negev and Namib deserts are distinctly different. Soils developed on stable alluvial surfaces with only direct input of rainfall and dust depend heavily on rainfall characteristics. Annual rainfall amount can be similar (15–30 mm), but storm duration can drastically alter Reg soil properties in deserts. The cooler fall/winter and dry hot summers of the southern Negev Desert with a predominance brief (≤ 1 day) rainstorms result in gypsic-saline soils without any calcic soil horizon. Although the Namib Desert receives only 50–60% of the southern Negev annual rainfall, its rainstorm duration is commonly 2–4 days. This improves leaching of the top soil under even lower annual rainfall amount and results in weeks-long grass cover. The long-term cumulative effect of these rare rain-grass relationships produces a calcic-gypsic-saline soil. The development of these different kinds of desert soils highlights the importance of daily to seasonal rainfall characteristics in influencing soil-moisture regime in deserts, and has important implications for the use of key desert soil properties as proxies in paleoclimatology.

Permanent Quaternary hyperaridity in the Negev, Israel, resulting from regional tectonics blocking Mediterranean frontal systems

Although various episodes of wet Quaternary climates have been suggested in studies of the Negev Desert, here we demonstrate that Reg soils, developed on flat alluvial surfaces and sensitive to minor changes in precipitation, indicate that the southern Negev has been permanently hyperarid at least since the middle Pleistocene. The wetter episodes were restricted to the northern Negev, currently mildly arid. Gypsic-saline Reg soils that de- veloped on stable surfaces during .200 k.y. in the southern Negev are cumulative and polygenetic, but none exhibits even the weakest development of calcic horizons. Under current climate conditions in the Negev, calcic soils are widespread in areas with rainfall .80 mm yr 21 . The fact that the hyperarid (,50 mm yr 21 ) southern Negev lacks any calcic horizons indicates that it never experienced an average rainfall of $80 mm yr 21 during the middle-late Pleistocene, whereas the northern Negev was wetter. This Negev dichotomy is explained by the main physical features controlling rainfall over the Negev, i.e, the southward-decreasing depth of the atmospheric boundary layer with distance from the Mediterranean, and the altitude of the central Negev Highland (1000 m). The interaction between these two features often prevents the passage of rain clouds into the southern Negev. The Holocene and Pleistocene gypsic-salic soil distribution across the Negev closely matches the current circumstances. We suggest that hyperaridity has prevailed over the southern Negev since the last stages of the uplift of the central Negev Highland in the late Pliocene-early Pleistocene.

Luminescence dating of fault-related alluvial fan sediments in the southern Arava Valley, Israel

Infrared stimulated luminescence (IRSL) from coarse-grained alkali feldspars was used to date alluvial and colluvial deposits from Nahal Shehoret, situated in the hyperarid southern Arava Valley. Israel. Late Pleistocene gravelly and sandy wadi deposits, associated with fault-related colluvial sediments, were exposed in trench 17 (T-17) which was excavated across a distinct fault scarp. Deposition of the lower alluvial beds in T-17 took place between 90 and 100 ka. This was followed by the first period of' tectonic activity and by erosion. The age of the upper alluvial bed ranges from 75 to 60 ka. A period of stability and reg soil formation was then followed by two scarp-forming faulting events resulting in colluviation. The colluvium deposited after the second event was dated to 36 ka. Modern colluvial sediments from Nahal Shehoret have residual IRSL ages of a few hundred years, whereas modem alluvial sediments have ages of a few thousand years. Colluvial sediments may therefore be more suitable for luminescence dating in arid regions.

THE MICROMORPHOLOGY OF GYPSUM AND HALITE IN REG SOILS—THE NEGEV DESERT, ISRAEL

Gypsum and halite are the most common salts in reg soils developed on alluvial parent material under extremely arid conditions in the Negev Desert, Israel. The aim of this paper is to document the changes in the micromorphology of these salts at different stages of Reg soil development on two alluvial fan chronosequences. The micromorphological analyses included thin section observations and scanning electron microscope and electron microprobe analyses. In this arid soil environment, gypsum and halite possess a variety of crystal forms which may change with depth in a single profile and/or between profiles of different ages. The variety of crystal forms results from changes in the microenvironmental conditions that occur in desert reg soils over time. Poikilitic lenticular gypsum is found in all Reg soils and is distributed throughout the profiles. The conditions needed for such crystals to form are high ionic impurities and deposition in a void system where space is not limiting. Microcrystalline alabastrine gypsum is only found in mature Reg soils and is crystallized when the profile has high amounts of fine material and a well-developed desert pavement. In a well-developed Reg soil, profile indicators, such as a well-developed desert pavement and high amounts of fine earth, limit the leaching depth and cause gypsum deposition from supersaturated soil solutions under high evaporation rates close to the surface. Prismatic and fibrous gypsum are less common. Low amounts of prismatic gypsum are found in young and mature soils while fibrous gypsum is found only in mature soils in re-cemented shattered gravel. The halite crystal form is mainly cubic with low amounts of host material incorporated into the crystal. It occurs predominantly in mature Reg soil profiles through the crystallization from supersaturated soil solutions at the depth of maximum water penetration. Although the alabastrine, prismatic and fibrous gypsum and cubic halite are deposited in a displacive manner, no correlation was found between their occurrence and the distribution of shattered gravel in the soil profile.

Soils as a tool for estimating ages of Quaternary fault scarps in a hyperarid environment — the southern Arava valley, the Dead Sea Rift, Israel

On the alluvial fan of Nahal Shehoret in the extremely arid region of the Negev desert, Israel, Reg soils developed on stable alluvial surfaces in colluvium along terrace risers and in colluvial units on a complex fault scarp were compared. The degree of soil profile development provided an age estimate of the surface that was faulted, a maximum age estimate of the scarp or terrace riser, and an estimate of the maximum recurrence intervals of faulting events. IRSL dates of the tectonically displaced alluvial surfaces and colluvial units at the same site enabled us to re-evaluate the use of Reg soils for relative dating. The major faulting event started at 34.8 ± 4.3 ka. It displaced a late Pleistocene surface of the Nahal Shehoret alluvial fan dated at 56 ± 10.8 ka. Renewed fluvial activity and deposition of alluvium on the downfaulted block had almost terminated by 13.6 ± 2.3 ka. The scarp was formed during the latest Pleistocene and was followed by smaller faulting events which had only minor effects on the fault scarp morphology. The similarity of age estimates from IRSL dating and the degree of soil development in this environment indicates that soils can be used to date surfaces and colluvial units.

Stages and rate of the gravel shattering process by salts in desert Reg soils

Gravel shattering by salts is a common process in gravelly young to mature desert Reg soils. The shattered gravel is found on the surface and in the B and C horizons down to a maximum depth of 50 cm. Field studies on the process of salt weathering in deserts are scarce. Therefore, the objectives of this study are to present the stages of development of the gravel shattering process by salts in desert Reg soils and to discuss factors influencing the rate of development under natural conditions prevailing in the Negev Desert. The studied Reg soils were developed on two alluvial fan chronosequences in the Negev desert: the fan of Nahal Ze'elim and the fan of Nahal Shehoret. The development of the shattering process can be divided into five stages (A–E) based on chemical, physical and micromorphological features of the Reg soils. Stages A–D (very young yo young Reg soils) represent the early Holocene to the present, some 14,000 years; salinity values are between 0.27 – 16 mmho/cm and the percentage of the shattered gravel is between 20% and 70%. The shattered gravel, however, still maintains its original shape. Stage E (mature Reg soil) represents the middle Pleistocene to early Holocene, or 500,000 years. Salinity values at this stage reach 62 mmho/cm. Although part of the gravel does not keep its original shape and starts to disperse in the profile, the percentage of shattered gravel is only 75–80%. It was found that the process of salt shattering in Reg soils does not depend on concentrations or mineralogy of the salts but on the micro-environmental conditions of the soil profile. The process requires extreme and rapid variations of temperatures and moisture conditions, such as found close to the soil surface, to cause salt crystallization in a confined system. With time the sealing of the soil surface, through the development of desert pavement and a gravel-free B horizon, results in a change in the soil micro-environment. This change impedes the process irrespective of the amounts, concentrations and types of salt minerals which exist in the profile. The fact that in extreme arid regions the gravel shattering process develops primarily close to the Reg soil surface, suggests that it can be used to identify buried Reg soils and to reconstruct climatic and micro-environmental conditions of paleosols.

Rates and modes of dust accretion and deposition in an arid region—the Negev, Israel

Summary Rates of dust accretion and deposition are dependent on the amount of available dust and the trap efficiency of a particular site. Several types of dust-trapping terrains are widespread in deserts: (1) Gravelly (Serir) surfaces that turn with time into Reg soils; (2) vegetated surfaces in the desert fringe that may turn into löessial terrains; (3) stabilized sand dunes; (4) playa surfaces. Loessial terrains exhibit a high rate of dust accretion during the late Pleistocene—0.07–0.15 mm a −1 on the interfluves and ≤0.5 mm a −1 along the flood plains. Gravelly surfaces usually trap about 0.1 mm a −1 of dust initially but the rates decrease to several μm a −1 due to plugging with dust and salts and may ultimately remain constant as a gravel-free B horizon develops. The amounts of imported dust, from both local and distant sources, have changed during the Quaternary due to climatic fluctuations. Roofless ancient building—most efficient dust traps—show that although large amounts of dust were available (much of it from local sources) during the late Holocene, there was not intensive dust accretion during this period due to increasing aridity and decreasing trap efficiency. Wash and gullying led to destruction of the once widespread efficient trapping terrains.

Fluviatile deposits and morphology of alluvial surfaces as indicators of Quaternary environmental changes in the southern Negev, Israel

Summary Quaternary environmental changes in the southern Negev are recorded in the alluvial deposits and the geometry of depositional surfaces in the Timna Valley. There are some distinct differences between the alluvial deposits of the Holocene to modern flood plains and earlier (Pleistocene) ones. The Holocene to modern alluvium is characterized by sand, fine- to medium-sized gravel and very coarse gravel in large gravel bars. The sediments indicate environments ranging between extremely arid and semi-arid dominated by floods of low to moderate magnitude but with occasional short large floods. The Pleistocene sediments include: (1) debris-flow deposits indicating talus activity under a moderately arid to semi-arid climate; (2) alternating fine-grained and gravelly layers reflecting floods of low to moderate magnitudes. The nature of the soils supports these conclusions. Thick and argillic Reg soils are typical of the Pleistocene surfaces whereas shallow loamy ones are characteristic of the Holocene terraces. The alluvial surfaces usually converge downstream. The intersection points migrate upstream, following stages in the recession of a major escarpment. Broad pedimented surfaces and bajadas are characteristic of the Pleistocene period, whereas long narrow terraces of gentle declivity were formed during the Holocene. The latter suggest depletion of the available debris and in the effectiveness of large floods.

The evolution of holocene reg (gravelly) soils in deserts: An example from the dead sea region

Reg soil is an aridisol developed upon a stable alluvial surface composed of sand to coarse gravel. The objectives of the present study were: (1) to examine the initial evolution of Reg soil properties with time; (2) to assess and explain the rates of development of different properties; (3) to establish the suitability of different soil properties for relative-age dating. The time-dependent soil properties examined were: (1) Mechanical weathering of surficial rocks and desert pavement evolution; (2) Chemical and biological weathering of surficial rocks; (3) The thickness of soil profiles and soil horizons; (4) Soil texture in the different horizons; (5) Salts and gypsum distribution and concentration in different soil horizons; (6) Degree of mechanical weathering of gravel in C horizons. Fifteen successive surfaces of the Nahal (= wadi) Ze'elim sequence of alluvial fans were selected for study (Dead Sea region, Israel, under an extremely arid climate). The sequence is of Holocene age — 14,000 years B.P. to recent. The main conclusions are: 1. Reg soils develop rather fast in several 10 3 years since their initiation. However, the various soil properties developed at different rates. 2. The rates of change of three soil properties decrease with time. These are the texture in different horizons (change in added airborne and rain washed dust), salinity within the soil profile (change in added salts and gypsum) and the amount of mechanically shattered rocks. 3. Soil thickness is established rather rapidly. Young and older Holocene soil have practically the same depth. Surficial weathering rates appear to remain fairly constant with time. 4. In coarse gravel the evolution of Reg soils is dependent on added fines and salts of airborne origin and the resultant changing hydrologic regime in the soil profile. The run-off/infiltration ratio changes as more fines and salts are added to the soil profile and as desert pavement develops. 5. Surficial features and fine clay in the soil profile may be used for approximate dating of otherwise undatable Holocene alluvial surfaces. Salts and gypsum content should be cautiously used in a rather general way.

Evolution of reg soils in Southern Israel and Sinai

Reg soils (mostly Camborthids and Gypsiorthids) cover some 15% of the Negev and Sinai deserts. Analytical data of seven profiles on depositional surfaces of increasing age in the Negev show clear relationships between ages of surfaces and soil profile features, exemplified in the development of the cambic, salic and gypsic horizons.The youngest soil, a Coarse Desert Alluvium soil (Typic Torriorthent) of dry wadi beds, up to a few thousand years old, has little profile differentiation and is generally non-saline or slightly saline. On the higher terraces connected with the Lisan Formation (about 70 000–12 000 years B.P.) clear profile differentiation and the beginning of development of cambic horizons can be discerned, evident in their colour-textural p differences. The soils are already saline and somewhat gypsiferous. Both soils are not yet defined as Regs because of their negligible or restricted profile differentiation.Reg soils on the older and higher geomorphic surfaces in the Arava Rift Valley, several hundred thousand years old, have well-differentiated profiles, with cambic, salic and gypsic horizons. Below their stony desert pavements, typical Reg soils have light coloured vesicular horizons, over mellow, very pale brown loam, frequently with laminar structure, almost completely stone-free layers. The reddish brown loamy to clay loam B horizons are very saline and show intensive salt weathering of gravels. Gypsic and petrogypsic horizons occur at depth. Such Reg soils represent the stable surfaces and soils of deserts and were developed over a long period of desert weathering.