CHARACTERISTICS OF NUBIAN SANDSTONE-DERIVED SOILS

Nubian sandstone exposures in sub-humid, semi-arid, and arid environments have given rise to red, sandy but in other respects very different soils. Soils have a fairly well developed profile only in the sub-humid zone, including a textural B horizon and are free of soluble salts and carbonates. In the semi-arid and arid zones, profile differentiation is weak or non-existent. Soils are shallow and contain carbonates, and in the arid zone also soluble salts, including gypsum. Kaolinite is the only clay mineral which is common to all the nubian sandstone parent materials. It is the major clay mineral in the sub-humid zone soil. In the semi-arid soils smectite is a second major clay component. In the arid zone both smectite and palygorskite, in minor amounts, accompany kaolinite. Both smectite and palygorskite are probably pedogenic neoformation products. Material of aeolian origin has probably been introduced into the silt and fine sand fractions of both the semi-arid and arid soils. Some contamination of the clay fractions may have also occurred.     

Soil organic matter and land degradation in semi-arid area,Israel

Soil organic matter (SOM) was monitored at five research sites along a climatic transect extending from the Judean Mountains (mean annual rainfall 700 mm; annual mean temperature 17 °C) to the Dead Sea (mean annual rainfall < 100 mm; annual mean temperature 23 °C) in Israel. At four sites, representing four climatic regions, Mediterranean (site GIV), semi-arid (site MAL), mildly arid (site MIS) and arid (site KAL), four to eight soil samples were taken four times a year, in January, March, May and September, from 1992 through 1993 and 1994 and in April and August 2000. In the last 2 months soil samples were also taken from another site (MAB) in the semi-arid area. Comparison between the sites along the climatic transect shows that, except for site MAB, SOM increased significantly in both 0–2 cm and 2–10 cm, from the arid site, through the mildly arid site and the semi-arid site, to the Mediterranean site. Analysis of SOM temporal patterns of the two semi-arid sites (MAL and MAB) shows significant change from the normal SOM pattern in both the regional scale and the soil profile scale in one site (MAB). The a-normal pattern of SOM and the low soil aggregate stability at MAB indicates land degradation and it is attributed to overgrazing.     

Genesis and transformation of minerals in the formation of red (Alfisols) and black (Inceptisols and Vertisols) soils on Deccan basalt in the Western Ghats, India

Six noncalcareous pedons from the basaltic terrain of the Western Ghats in Maharashtra, India, were identified for the present study. Of these, two red-soil pedons (Typic Haplustalfs) and one black-soil pedon (Vertic Ustropept) are from the Bhimashankar plateau at an elevation of 1000 m above mean sea level, experiencing a humid (>5000 mm rainfall) tropical climate. The other three pedons of black soils (Typic Chromusterts and Typic Ustropept) are from the semi-arid zone (500–1000 mm rainfall), at an elevation of 825–893 m. Clay mineralogical investigation indicated that interstratified smectite-kaolin (Sm/K) is dominant in red soils whereas smectite is dominant in black soils. The Sm/K is formed by the transformation of montmorillonite, the first weathering product of Deccan basalts in a humid tropical climate. We suggest that the interstratification of kaolin with chloritized smectite may also be an important ephemeral stage during the transformation of smectite to kaolinite. The presence of zeolites provided sufficient bases to prevent the complete transformation of Sm/K to kaolinite. The presence of smectites and zeolites made the formation of black soils possible in microdepressions even in a tropical humid climate. The genesis of Sm/K and smectite in red and black soils, respectively, suggests that these soils formed through a progressive landscape reduction process. The presence of both Sm/K and smectite in black soil clays of semi-arid climate suggests that the smectite of these soils was formed in an earlier humid climate.     

Clay mineral evolution along a soil chronosequence in an Alpine proglacial area

As a consequence of global warming, additional areas will become ice-free and subject to weathering and soil formation. The most evident soil changes in the Alps will occur in proglacial areas where young soils will continuously develop due to glacier retreat. Little is known about the initial stages of weathering and soil formation, i.e. during the first decades of soil genesis. In this study, we investigated clay minerals formation during a time span 0-150 years in the proglacial area of Morteratsch (Swiss Alps). The soils developed on granitic till and were Lithic Leptosols.Mineralogical measurements of the clay (< 2 μm) and fine silt fraction (2-32 μm) were carried out using XRD (X-ray Diffraction) and DRIFT (Diffuse Reflectance Infrared Fourier Transform). Fast formation and transformation mechanisms were measured in the clay fraction. The decreasing proportion of trioctahedral phases with time confirmed active chemical weathering. Since the start of soil formation, smectite was actively formed. Some smectite (low charge) and vermiculite (high charge) was however already present in the parent material. Main source of smectite formation was biotite, hornblende and probably plagioclase. Furthermore, irregularly and regularly interstratified clay minerals (mica-HIV or mica-vermiculite) were formed immediately after the start of moraine exposure to weathering. In addition, hydroxy-interlayered smectite (HIS) as a transitory weathering product from mica to smectite was detected. Furthermore, since the start of soil evolution, kaolinite was progressively formed. In the silt fraction, only little changes could be detected; i.e. some formation of an interstratified mica-HIV or mica-vermiculite phase.The detected clay mineral formation and transformation mechanisms within this short time span confirmed the high reactivity of freshly exposed sediments, even in a cryic environment.     

Mineral Composition and Weathering of Soils Derived from Xiashu Loess

Mineralogical, physical and chemical analyses of the soils derived from Xiashu loess were carried out. The primary minerals of these soils were found to be mainly composed of light minerals, such as quartz, feldspar and mica, with traces of heavy minerals. Clay minerals, more complicate in composition, were dominated by hydromica, accompanied by smectite, vermiculite, chlorite, kaolinite, 2:1/1:1 randomly interstratified minerals and small amounts of quartz, goethite, lepidocrocite and hematite, Clay minerals were characterized by low crystallinity and fine particle size. In light of the quartz/feldspars ratio of the 0.01-0.05mm silt fraction, and the clay mineral composition, the freeness of iron oxide, and the silica/sesquioxide and silica/alumina ratios in < 0.002mm clay fraction, it is concluded that the weathering intensity of these soils was lower than those of red soil and yellow earth, but higher than that of brown earth, and that the soil allitization, depotassication and hydroxylation of cl     

Mineralogy of soil kaolinites from Cameroon

Different particle-size fractions of soil clays from the semi arid north and the humid tropical south of Cameroon have been characterized with reference to their chemical composition, clay mineralogy and kaolinite crystallinity (Hinckley indices). Selected samples were also examined by Mössbauer spectroscopy. Hinckley indices of kaolinites, which were a major component of the coarse clays, varied considerably (< 0.1 - 0.69) and differed significantly as a function of the geographic and landscape positions of the soil profiles. The Hinckley indices averaged 0.31 in soils from southern Cameroon and 0.30 and 0.07 in soils from high and low landscape positions in northern Cameroon, respectively. Kaolinite crystallinity is therefore considered to vary as a function of transport and/or depositional environment of the kaolinite-containing material. Mössbauer spectra showed that kaolinite-dominated Vertisol coarse clays contained higher relative amounts of Fe²⁺ than the corresponding fine clay, which is dominated by smectite. It is conceivable that the Fe²⁺ content of the kaolinite reflects the redox environment of the samples.     

Dispersion and migration of fine particles in two palygorskite‐containing soils of the Jordan Valley

Migration of different mineral particles within columns of soil‐sand mixtures containing 10 or 20 mass % of soil was investigated by establishing differences in the mineral suite between the ”︁bulk clay” and the ”︁mobile fine material” fractions. The ”︁bulk clay” fractions of all soils contained smectite, palygorskite, kaolinite, quartz, feldspar, and calcite. The soils were saturated with sodium by leaching with NaCl solution, and then leached with distilled water. Clay dispersion and particle migration occurred in the columns. Values of SAR (sodium adsorption ratio) of the effluent decreased with time due to carbonate dissolution. At a certain SAR value, the clays apparently formed aggregates, and as a consequence particle migration stopped in the column. In addition to clay‐sized particles (< 2 μm), very‐fine‐silt‐sized particles (2— 5 μm) were able to migrate in the soil‐sand mixtures, too, and to some extent fine‐silt‐sized particles (5—10 μm) as well. Average size of mobile particles decreases with increase of soil content in the soil‐sand mixtures. The mineralogical composition of the ”︁mobile fine material” changed during the experiment. At the beginning of the experiment, the ”︁mobile fine material” was enriched in the non‐phyllosilicates (especially in calcite, and in some cases in quartz, feldspar and dolomite) and contained low concentrations of phyllosilicates (smectite, palygorskite and kaolinite). At the end of the experiment, the proportion of non‐phyllosilicates decreased, and as a consequence, the proportion of phyllosilicates increased. Among the non‐phyllosilicates, calcite was the most mobile mineral. Among the phyllosilicates, palygorskite was preferentially mobilized in topsoil horizons. In subsoil horizons, on the other hand, kaolinite was preferentially mobilized. This difference was explained by the different nature of carbonates in the topsoil and subsoil horizons. Palygorskite is preferentially occluded within the soil carbonates of lacustrine origin over smectite and kaolinite. These carbonates are present mainly in the subsoil horizons. As a consequence, the presence of these carbonates in the subsoil horizons decreases the migration of mainly palygorskite.     

b-Fabric of some benchmark Vertisols of India in relation to their mineralogy

At present precise information on the b-fabric of Vertisols is inadequate for gaining a comprehensive knowledge about such soils formed in alluvium of basic igneous and metamorphic rocks and spread in varying agro-climatic zones of peninsular India. The aim of the present study, on five benchmark Vertisol series (Sarol, Aroli, Nimone, Bellary and Kovilpatti), was to assess the possible reasons for differences in the b-fabric of the subsoils despite their common shrink-swell properties, as evidenced by the presence of sphenoids and/or slickensides, and similar clay contents and amounts of fine clay smectite. In Sarol and Aroli soils from the sub-humid region, the b-fabric is porostriated, whereas in Nimone and Kovilpatti soils of the semi-arid and Bellary soils of the arid regions the fabric is either mosaic-speckled or granostriated, indicating weak plasma separation, a result of restricted swelling of clays. The related distribution pattern of the s-matrix in all these soils was open porphyric. Recent literature has suggested that weak plasma separation is an effect of dissolution and crystallization of calcite. However, generally low concentrations of soluble calcium (c. 1 mmol dm^?3) in soil solutions of Vertisols in arid and semi-arid agroclimates suggest that the presence of calcite does not guarantee the presence of sufficient soluble calcium to impair swelling of clays by contracting the diffuse double layer. Weak plasma separation in Bellary and Kovilpatti soils compared with Sarol and Aroli soils is explained in terms of a decrease in the internal surface area of fine smectite, as evident from a very high degree of chloritization in smectite interlayers, and smaller proportions of coarse smectite. In Nimone soils the weak plasma separation is primarily due only to a decreased internal surface area of fine smectite.     

Chemical characteristics of humic acids from five soils in Kenya

Humic acids (HAs) isolated from five soils (Luvisols, with 460–600 mm rainfall, Nitosols and Ferralsol, with 1060–1950 mm rainfall) from different ecological regions of Kenya were characterized by ¹³C NMR and IR spectroscopy, elemental analyses and optical measurements. The data suggest that excessive rainfall in sub-humid regions may lower the molecular weights of the HAs as well as their aromaticities. Concentrations of aliphatic components, including aminoacids and carbohydrates, in the HAs from the sub-humid regions were higher (40–42 wt/wt %) than those in the HAs isolated from soils of the semi-arid regions (29–32 wt/wt %). Chemical characteristics of the soils from the sub-humid regions of Kenya resembled those of the HAs from temperate regions but differed from those of HAs originating from arid regions. Annual rainfall and vegetation appeared to affect the composition of HAs but effects of elevation and temperature were less clear.     

Formation of clay minerals in soils developed from basic volcanic rocks under semiarid climatic conditions in Lanzarote,Spain

Clay formation in soils was studied in four landscapes of different ages in Lanzarote (Canary Islands). Landscape IV_B is about 250 years old, IV_A subrecent, III early young pleistocene, and landscape I, late tertiary. The soils have developed from basic to ultrabasic pyroclastic fall deposits or basalt.Clay formation in soils of landscape IV_B is in an initial stage having a clay content of 1–2 kg/m², whereas in landscape IV_A 30–40 kg/m² and in landscape III 100–200 kg clay/m² are accumulated. In polygenetic soils of landscape I more than 3000 kg clay/m² can be found. In the youngest soils IV_B, smectite is the most prominent clay mineral, whereas IV_A-soils are dominated by “amorphous” clay. The clay fraction in older soils contains illite, smectite as well as kaolinite in smaller quantities.The chemical composition of the clay fraction of soils with different ages changes significantly according to the mineral composition.     

尼日利亚热带雨林地区的内陆山谷中土壤发育和肥力特征: 矿物成分和粒度分布

The particle-size distribution and mineralogical composition of the clay (< 2 μm) and fine-sand (0.25--0.10 mm) fractions in soils of two inland valleys in Abakaliki and Bende, Southeast Nigeria, were investigated to provide basic information on soil-forming processes and agricultural potentials. These soils were silty or clayey, deriving from Cretaceous or Tertiary shale materials. The particle-size distribution and its computation on a clay-free basis revealed relatively remarkable lithologic breaks in a couple of pedons. The effect of lithologic discontinuities on soil mineralogical composition was not, however, conspicuous. Petrographic investigation revealed that quartz predominantly comprised the fine-sand fraction in the soils at both study sites. Nevertheless, the clay mineralogical composition of the soils was a mixture of kaolinite, irregularly interstratified smectite-illite intergrades (S/I), hydroxyl-Al interlayered 2:1 type clays (HICs), vermiculite, smectite, halloysite and illite along with fine-sized quartz in Abakaliki. The soils of Bende predominantly contained smectite, which was partially interlayered with hydroxyl-Al and kaolinite. It is suggested that seasonal floodwater has slowed the disintegration of weatherable clay minerals inherited from the shale, while quartz originating from the sandstone is predominant in the fine-sand fraction. Additionally, a possible soil-forming process observed at the both study sites was ferrolysis, which was indicated by a clear decreasing pattern of HICs downward in the soil profiles. The entry of S/I and vertical distribution patterns for a couple of clay minerals in the pedon suggested that the soils in Abakaliki have developed under the significant influence of aeolian dust delivered by the harmattan. The findings might describe a site-specific deposition pattern of harmattan dusts as well as hydromorphic soil-forming processes in the wetlands of the inland valleys.     

Clay mineral composition of river sediments in the Amazon Basin

Clay minerals are important in evaluating the maturity of suspended sediments, weathering intensity and source area. However, there are processes that can change the mineral assemblage such as river transportation, deposition, remobilization and tributary inputs. In terms of water discharge and sediment yield, the Amazon is one of the largest rivers in the world. Most of the suspended sediments come from the Andes, crossing the lowlands before reaching the ocean. This study measures the spatial distribution of clay mineral assemblages over the entire Amazon basin. The results obtained show the main features of the Amazon River main stem and larger tributaries from their sources to their confluence. Clay mineral composition highlights the evolution of the Madeira and Marañón–Solimões River, which start in the Andes with high illite + chlorite content. Downstream, smectite contents increase. Moreover, all shield tributaries show high kaolinite content. The lower Amazon River is characterized by relative high smectite content, different from the Andean sources. The clay mineral results show that suspended sediments of the Amazon River have three main sources: 1) the Andes mountains; 2) the Amazon shields and 3) the Piedmont basins, especially the Pastaza alluvial megafan and the Fitzcarrald Arch basin. Lateral bank erosion plays also a significant role, by the introduction of more mature sediments into the river, enriched in smectite.     

Clay mineral distribution in two toposequences of tropical soils of India

The clay fractions of the soils of two toposequences derived from gneissic rocks (Peninsular gneiss) in southern India consist chiefly of kaolinite and amorphous ferrialuminosilicates (AFAS). The cation exchange capacity of the amorphous constituents, ranging between 21.4 and 107.6 mequiv./100 g, has been explained on the basis of a structural model. Considerable amounts of smectite were present in the clays of soils on the toeslopes. Two hypothetical pathways for the formation of the clay minerals are proposed.     

The impact of long-term irrigation on the degree of aggregation and the mineralogical composition of the clay fraction in dark chestnut soils of the Transvolga region

Dark chestnut soils of the Ershov Experimental Station in the Transvolga region are characterized by the even distribution and aggregation of clay minerals in the profile. Hydromica, chlorite, kaolinite, and smectitic minerals predominate in the clay (<1 μm) fraction. The smectitic phase consists of randomly ordered mixed-layered minerals of the following types: mica-smectite with a low (<50%) content of smectite layers, mica-smectite with a high (>50%) content of smectite layers, and chlorite-smectite. In some horizons, the smectitic phase occurs in the superdispersed state. The long-term irrigation of these soils with fresh water of the Volga River has led to certain changes in the composition and properties of the clay particles. The weakening of bonds between them has taken place. As a result, the content of water-peptizable clay has increased by two times, and the content of aggregated clay of the first category (AC1) has increased by 1.5 times at the expense of a decrease in the contents of tightly bound clay (TBC) and aggregated clay of the second category (AC2). Also, the redistribution of organic matter bound with clay particles has taken place: its content in the AC1 fraction has decreased, whereas its content in the AC2 and TBC fractions has increased. In the topsoil horizon, the amount of the smectitic phase has lowered, whereas the contents of hydromica, kaolinite, and fine-dispersed quartz in the clay fraction have increased. In general, some amorphization of the clay material has occurred. The periodic alkalization of the soil solutions upon irrigation has led to the conversion of the smectitic phase into the superdispersed state in the entire soil profile.     

Differential hydrological response of biological topsoil crusts along a rainfall gradient in a sandy arid area: Northern Negev desert,Israel

Drylands are regarded as highly sensitive to climatic change. The putative positive relationship between average annual rainfall and runoff, assumed for areas between 100 and 300 mm ignores the fact that climatic change in drylands is not limited to climatic factors alone, but is often accompanied by a parallel change in surface properties. Data on rainfall, runoff and soil moisture regime were collected at five monitoring sites in a sandy area, along a rainfall gradient from 86 to 160 mm. Despite the uniform sandy substratum the frequency and magnitude of runoff declined with increasing annual rainfall. Under wetter conditions a thick topsoil biological crust develops. This crust is able to absorb and retain large rain amounts, limiting the depth to which water can penetrate, and therefore water availability for the perennial vegetation. In the drier area, the thin crust can absorb only limited rain amounts, resulting in surface runoff and deeper water infiltration at run-on areas. Our findings demonstrate the important role played by different types of biological soil crusts along the rainfall gradient considered, and question the generally held belief that higher rainfall necessarily leads to deeper water infiltration in sandy arid areas; and higher water availability for the perennial vegetation.     

Residue cover and rainfall intensity effects on runoff soil organic carbon losses

Soil cover and rainfall intensity (RI) are recognized to have severe impacts on soil erosion and an interaction exists between them. This study investigates the effect of rainfall intensity (RI) and soil surface cover on losses of sediment and the selective enrichment of soil organic carbon (SOC) in the sediment by surface runoff. A field rainfall simulator was used in the laboratory to produce 90 min rainfall events of three rainfall intensities (65, 85 and 105 mm h^− 1) and four cover percentages (0%, 25%, 50% and 75%) on soil material at 9% slope. A strong negative exponential relation was observed between cover percentage and RI on sediment loss under 85 and 105 mm h^− 1 of rain, while under RI of 65 mm h^− 1, the highest sediment loss was observed under 25% cover. Overall, higher RI and lower cover produced higher sediment and consequently higher nutrient loss, but resulted in a lower SOC enrichment ratio (ER_SOC) in the sediment. The amount of runoff sediment rather than the ER_SOC in the sediment was the determinant factor for the amount of nutrients lost. The values of ER_SOC were high and positively correlated with the ER values of particles smaller than 20 µm (p < 0.01). Although the sediment contained substantially more fine fractions (fine silt and clay, < 20 µm), the original soil and runoff sediment were still of the same texture class, i.e. silt clay loam.     

Negative and positive effects of topsoil biological crusts on water availability along a rainfall gradient in a sandy arid area

Drylands are regarded as highly sensitive to climatic changes. A positive relationship between rainfall and environmental factors (water availability for plants, productivity, species diversity, etc.) is often assumed for areas with an annual rainfall of 100–300 mm. This assumption disregards the fact that a climatic change in arid and semi-arid areas is not limited to climatic factors. This change is often accompanied by a parallel change in surface properties. The alteration of surface properties may have opposite effects on the water regime and ecosystem characteristics. Data on rainfall, runoff, soil moisture regime, and vegetation cover were collected at five monitoring sites in a sandy area along the Israeli–Egyptian border, where average annual rainfall varies from 86 to 160 mm. Data obtained show a decrease in water availability for perennial plants with increasing annual rainfall, clearly expressed by a lower survival of perennial plants in the wetter area. Results obtained cast doubt of the prevailing idea regarding the positive relationship between average annual rainfall and ecosystem characteristics. The findings are attributed to the decisive role played by the non-uniform properties of the topsoil biological crusts along the rainfall gradient considered. Under wetter conditions a thick topsoil biological crust develops. This crust is able to absorb large rain amounts, limiting thus the depth of rainwater infiltration and water availability for the perennial vegetation. A better water regime was found in the drier area, where a thin crust allows deeper water infiltration and water concentration by surface runoff. The process described may be regarded as a desertification process with increasing annual rainfall.     

广西水耕人为土黏粒矿物组成及其空间分布特征

黏粒矿物影响着土壤理化性质,可指示成土因素特征和土壤发生发育过程/强度,也是中国土壤系统分类的基层单元土族矿物学类型划分的重要依据。本研究选择了广西不同纬度和成土母质的18个代表性水耕人为土的剖面,应用X射线衍射(XRD)方法分析了其典型水耕氧化还原层(Br层)的黏粒矿物组成及其空间分布特征,并确定了其中"黏质"剖面的土族控制层段矿物学类型。结果表明:①供试土壤的黏粒矿物主要包括高岭石、伊利石、三水铝石、1.42 nm过渡矿物、蒙脱石和蛭石等,依次分别出现在100%、88.9%、72.2%、61.1%、44.4%和38.9%的剖面中。②黏粒矿物组成在纬度空间分布上具有明显规律性特征。随着纬度降低,土壤黏粒中的高岭石增加,伊利石、蒙脱石、1.42nm过渡矿物逐渐减少;纬度>23°N区域内,成土母质对黏粒矿物组成影响明显。③纬度23°N是黏粒矿物组成和土族矿物学类型分界线,<23°N区域,黏粒矿物均以高岭石为主,是"黏质"剖面的土族控制层段的主要矿物学类型;>23°N区域,黏粒矿物组成以高岭石、蒙脱石、伊利石或1.42 nm过渡矿物为主,因成土母质不同而异,"黏质"剖面的...     

Erodibility of a soil drainage sequence in the loess uplands of Mississippi

The susceptibility of loess soils in the lower Mississippi to runoff and erosion losses varies as a function of landscape position and mapping units. This study was conducted to determine the effects of soil drainage on physical and chemical properties that influence erodibility through their control of aggregate stability. Soil samples were collected from the A- and B-horizons of the five representative pedons in the Memphis catena whose drainage class varied from well-drained to poorly-drained. The fine earth fraction (< 2 mm) of each soil was characterized for a range of basic soil physical and chemical properties. Additional sub-samples (< 8 mm) were placed in a rainfall simulator pan (0.6 m × 0.6 m test area) and subjected to simulated rainfall at an intensity of 64 mm h^− 1. Soil erodibility was assessed by the use of an aggregation index (AI) computed from water dispersible clay (WDC) relative to total clay contents. The data show that as soil drainage classes became wetter, the percentage of sediment < 53 µm increased with a decrease in soil AI resulting from a loss of Fe, Al, and Si oxide cementing agents. These results suggest that cementing agents responsible for soil aggregate stabilization are mobilized under conditions of relatively low redox potentials which increase soil erodibility.     

Soluble salts dynamics in the soil under different climatic conditions

The dynamics of soluble salts concentration in the soil was investigated at seven research stations in Israel that represent four climatic regions: Mediterranean, semi-arid, mildly arid and arid. Measurements were taken in different seasons from soils that were developed on hillslopes, which are consisted of hard calcareous rocks. The relationship between the soluble salts content and rainfall was found to be non-linear. An abiotic threshold, which is characterized by a sharp change in the soluble salts content, exists around 200 mm isohyet: sites that receive less than 200 mm rainfall are characterized by significantly high soluble salts content whereas sites that receive more than 200 mm are characterized by very low soluble salts content. Each side of this abiotic threshold expresses typical behavior that can be defined as “environmental signature”. At the “dry” side of the threshold temporal heterogeneity, rate of change, potential of change and differences between layers are higher than those at the “wet” side of the threshold.