Clay Illuviation Research Articles

Soil Characteristic Response Times and Pedogenic Thresholds during the 1000-Year Evolution of a Paddy Soil Chronosequence

A paddy soil chronosequence consisting of five profiles derived from calcareous marine sediments with cultivation history from 0 to 1000 yr was studied to assess the dynamic changes in soil properties and major elemental mass balance during soil evolution and to understand the response rates of soil properties at different time scales. The threshold concept was applied to increase our understanding of paddy soil genesis processes. Results showed that 50 yr of paddy cultivation induced measurable accumulation of soil organic C (SOC) in the surface horizon and marked reduction of magnetic susceptibility (MS), soft isothermal remanent magnetization (IRMs), and Ca, Mg, and Na elemental concentrations. Complete removal of CaCO3 from profiles, measurable profile differentiation of free Fe oxide (Fed) and total Fe, clay illuviation, reduction of hard isothermal remanent magnetization (IRMh), and net losses of elemental P and Mn occurred during the 300- to 700-yr time period. Soil clay mineral composition and Si, Al, and K elemental concentrations showed little change in 1000 yr. The rapid changes in SOC, MS, IRMs, Ca, Na, and Mg within 50 yr primarily resulted from anthropogenic activities, which are extrinsic thresholds. The noticeable changes in horizon differentiation of Fed, clay illuviation, and the decrease in IRMh during the 300- to 700-yr time period indicate threshold values for carbonate, which is an intrinsic threshold. In conclusion, our data demonstrate that different soil components and properties have different transformation processes and associated rates influenced by anthropogenic activities and the soil carbonate status of the original soil.

Polygenesis of volcanic paleosols in Armenia and Mexico: Micromorphological records of climate variations in the quaternary period

The Pleistocene volcanic paleosols of Armenia and Mexico combine the features of both arid and humid pedogeneses. They were studied in order to reconstruct the environmental conditions during the period of the initial settlement of humans in these areas. The main attention was paid to the micromorphological analysis of the pedogenic features. The pedocomplexes studied in Armenia have been forming since the Early Pleistocene. They bear evidences of humid (weathering, clay illuviation, and gleying) and arid (calcification) pedogeneses alternating in different soil layers and pointing to climatic cycles. The nonuniform lithology of the soil profiles is related to pulsating volcanic activity. Mexican paleosols of the Late Pleistocene period are similar to the lower layers of the soils in Armenia with respect to the features characteristic of the humid and arid phases of pedogenesis. This allows us to suggest that the early stages of the formation of the Armenian pedocomplex proceeded under warmer paleoclimatic conditions similar to those of a tropical climate. It is shown that the arid pedogenesis may erase the features produced during the previous stage of humid pedogenesis. The studied paleosols are characterized by the destruction of the features inherited from the humid phase (the clayey groundmass and illuviation coatings) by the newly forming calcite. Paleopedological records point to the fact that the paleoenvironmental conditions during the periods of the initial settlement of humans in both regions differed significantly from the modern environmental conditions in these regions.

Microstructure of genetic horizons in automorphic soils of the Timan Ridge

Data are presented on the physicochemical composition and specific macro-, meso- and micro-morphological features of automorphic soils formed on silty loams in the northern and middle taiga subzones of the Timan Ridge. These soils have a texture-differentiated profile and are well aggregated, which is manifested at all levels of structural organization of the soil mass. The morphological structure of the middle soil horizons is characterized by the presence of specific nongleyed cryometamorphic horizon CRM. Its formation is due to the development of long-term seasonal cryogenic processes in relatively deep (up to 40 cm) light and medium loam deposits under conditions of the northern and middle taiga subzones. The processes of cryometamorphism, combined with Al-Fe-humus and textural differentiation, result in the formation of podzolic, iron-illuvial, cryometamorphic, and textural horizons (O-E-BF-CRM-BT). The textural horizons have a set of micromorphological features indicating that recent clay illuviation is a weak process.

Soil genesis, morphodynamic processes and chronological implications in two soil transects of SE Sardinia, Italy: Traditional pedological study coupled with laser ablation ICP-MS and radionuclide analyses

Soils are characterized by an intrinsic variability through time and space at different scales of observation and are often affected by morphodynamic processes strictly interacting with pedogenesis, under changing (paleo)environmental/climatic conditions. We focus on exploring this interplay in modern soils and paleosols of southeast Sardinia (Italy), integrating two innovative techniques to a traditional pedological study: (i) laser ablation ICP-MS spot analyses applied to detect microscale distribution of trace elements in discrete soil features of organic-mineral (A) and argillic (Bt) horizons in thin sections, and (ii) radioactive disequilibria as tracers and chronometers of pedogenetic alteration and geomorphic dynamics. The Pleistocene paleosols developed mainly during interglacial periods, with important clay illuviation and rubification. The other soils are essentially characterized by organic matter incorporation into the profiles. Field discontinuities described within soil profiles, related to phases of sediment aggradation and/or surface erosion, are often supported by laboratory results, mainly micromorphological and geochemical data. LA-ICP-MS data show an overall trend of increase of most trace elements (REEs included) from skeletal rock fragments to both clayey and organic-rich soil matrix to clay coatings. This pathway is clearly controlled by the degree of weathering of primary minerals and by pedogenetic processes, where element adsorption onto reactive sites of organic matter and clay particles and clay illuviation play prominent roles. Recent trends of the studied soils/paleosols in terms of geomorphic stability (equilibrium) or morphodynamic processes (erosion/accumulation rates) are estimated using radionuclide disequilibria in the order of 1 cm/a or lower.

Spatial arrangement of soil mantle in Glacis de Buenavista, Mexico as a product and record of landscape evolution

Factors controlling spatial soil organization have recently attracted the attention of soil scientists because they can contribute to regional management of soil resources. We present here a case study in the Glacis de Buenavista (Mexico), an extended volcanic piedmont, in order to develop an integral understanding of its soil cover as a product and, at the same time, as a record of the main stages of landscape evolution. Three phenomena were considered: Luvisol and Vertisol type soils, and pedosediments, all of them analyzed in terms of their development degree (clay content, macro and micromorphology, selective extractions of Fe (Fe d, Fe o), silicon (Si o) and aluminum (Al o), weathering index and clay mineralogy). Luvisols, represented in the Ahuatenco, Mexicapa and Buenavista sections, are located in the northern and central part of the area. They are red-clayey, polygenetic soils that show strong weathering (with high kaolinitic clay content and high weathering index values), clay illuviation, and reductomorphic processes, combined with vertic features in some parts of the profiles. Vertisols dominate the central and southern portion of the Glacis. They exhibit cracking, angular blocks with wedge-like shapes, slickensides and stress-cutans. The pedogenesis of the Vertisol type is clearly associated with the presence of smectitic minerals in the clay fraction as a product of neoformation. Factors controlling this pedodiversity are: relief, which affects bioclimatic differentiation within the studied landsurface and defines the lateral redistribution of moisture and dissolved substances; and time. Luvisols represent a longer pedogenetic phase that started in the Late Pleistocene, according to the age of buried paleosols, while Vertisols mainly originated in the Late Holocene (according to the age of buried organic horizons). Pedosediments are located in the central area of the Glacis. Here, past and present geomorphic processes interact to produce the greatest soil diversity. The recent human-induced erosion partly destroyed polycyclic Luvisols and exposed ancient subsurface pedosedimentary strata. At the same time it produced patches of unconsolidated pedosediments consisting mostly of redeposited Luvisol materials.

Micromorphology and development of loess-derived surface and buried soils along a precipitation gradient in Northern Iran

The northern footslopes of the Alborz Mountains and an extensive hill area in eastern Golestan Province are covered by thick (∼70 m) loess deposits, divided by different types of interstadial and interglacials paleosols. The area shows a precipitation gradient of about 800 mm over 80 km north-south. Along this gradient, eight pedons of modern soils and six paleosols were selected in order to investigate the types of modern and past soil forming processes and to compare the degree of soil development of surface and buried soils. The soils were described and sampled using standard sedimentological/pedological laboratory analysis. The soil pH and calcium carbonate contents show a decreasing trend with precipitation, while soil organic carbon, clay content and cation exchange capacity increase with rainfall. The silt content, however, shows a decreasing trend with rainfall. Clay mineralogy of parental loess is illite > chlorite > smectite > kaolinite. From north to south, the relative proportion of smectite in soil horizons increases, reaching almost dominance in regions with typic xeric soil moisture regime (SMR; ∼600 mm of rainfall). In the areas with udic SMR, vermiculite increases and sometimes dominates. Micromorphology provides evidence for the formation of Bt horizons and intensified decalcification and increase of clay mobilisation as indicated by b-fabrics with increasing rainfall. Soils within the xeric and udic SMRs are either Alfisols or Mollisols, both showing clay illuviation features, whereas clay mobilisation is very limited in the arid part of the area. The paleosols show differential weathering degrees indicated by trends of increasing carbonate depletion, clay mobilisation, and clay enrichment with assumed paleo-precipitation. Pedogenesis, micromorphological properties and clay minerals in the last interglacial paleosols suggest similar climate controlled trends to those reflected in the modern soils.

Micromorphological approach to polycyclic pedogenesis on the Messak Settafet plateau (central Sahara): Formative processes and palaeoenvironmental significance

The Messak Settafet is a wide plateau located in the Libyan central Sahara (SW Fezzan); it is cut into the Nubian sandstone and interpreted as relic of a Tertiary peneplain, heavily affected by aeolian deflation. Notwithstanding degradation phenomena, discontinuous, thin soil bodies are locally preserved below the hamada. Several profiles, located in correspondence of different geomorphological units (the hamada, the endorheic depressions, and the southern escarpment), were described and sampled for micropedological analyses. Micromorphology outlined that the main characteristics of the residual B horizons from each geomorphological units are (i) high concentration of ferruginous nodules, (ii) multiple generations of clay illuviation, (iii) fragmentation and deformation of textural pedofeatures, and (iv) calcite mobilization and redeposition. According to horizon macro- and micromorphological characteristics, these pedologic bodies do not seem to be in equilibrium with the present-day climatic conditions and should be regarded as paleosols generated by processes regulated by large water availability and warm temperature, such as rubification, clay translocation, and calcite redistribution. The development of horizons acted mainly since the Early/Middle Pleistocene during subsequent pluvial periods and was interrupted by several erosional and aeolian input phases (arid periods). Pedological processes ended at the onset of the severe dry environmental conditions at the Mid-Late Holocene transition. The development of this kind of paleosols in the central Sahara is also discussed in comparison with pedogenetic processes occurring in temperate regions during interglacial and interstadial intervals.

Soil structure transformations from ferralic to nitic horizons on a toposequence in southeastern Brazil

The soil structure transformation from ferralic to nitic horizons was studied in a toposequence on quaternary red clayey sediments and diabase in Piracicaba (SP), Brazil. Morphological and micromorphological studies, image analysis, soil water characteristic curves and monitoring of (total) soil water potential head were used. The presence of polyconcave vughs, clayskins and planar voids shows that the vertical and lateral transition and structural transformation from ferralic to nitic horizons is given by the coalescence of the microaggregates, probably due to tensions created in a drier period in the past. Changes to a more humid climate with a defined dry season and alternate drying and wetting cycles resulted in the fissuration of the previously coalesced material, forming polyhedral aggregates and microaggregates. Simultaneously, clay illuviation filled the voids and together with the compacting action of the biological activity of these soils contributed to the coalescence of microaggregates.

A geophysical analysis of hydro-geomorphic controls within a headwater wetland in a granitic landscape, through ERI and IP

Abstract. Wetlands are undergoing considerable degradation in South Africa. As interventions are often technical and costly, there is a requirement to develop conceptual process models for these wetland systems so that rehabilitation attempts will be successful. This paper presents an approach using the geophysical methods of Electrical Resistivity Imaging (ERI) and Induced Polarization (IP) to delineate sub-surface hydro-geomorphic controls that maintain equilibrium disconnectivity of wetland-catchment processes, which through gully erosion are increasing the catchments connectivity through loss of water and sediment. The findings presented here give insight into the geomorphic processes that maintain the wetland in an un-degraded state, this allows for the development of a conceptual model outlining the wetland forming processes. The analysis suggests that sub-surface clay-plugs, within an otherwise sandy substrate are created by illuviation of clays from the surrounding hillslopes particularly at zones of valley confinement.

Factors Impacting Overweight Adolescents' Underestimation of Their Actual Weight Status

In the northwestern part of the Indo-Gangetic Plains (IGP), India, non-sodic and moderately sodic soils occur on microhigh (MH) and highly sodic soils on microlow (ML) positions under a semi-arid climate. The main soil-forming processes have been clay illuviation, deposition of pedogenic calcium carbonate and concomitant development of sodicity. The microlows are repeatedly flooded with surface water during brief high-intensity showers, so the soils are subject to cycles of wetting and drying. This provides a steady supply of alkalis by hydrolysis of feldspars, leading to precipitation of calcium carbonate at high pH and development of subsoil sodicity. This impairs the hydraulic conductivity of soils and eventually leads to the development of Natrustalfs with exchangeable sodium percentages increasing rapidly up the profile. The semi-arid climate and topography interact to facilitate greater penetration of bicarbonate-rich water in microlow than microhigh positions. Thin sections show deformational pedofeatures such as cross and reticulate striation of plasmic fabric, disruption of clay pedofeatures and carbonate nodules and elongation of voids as a result of tectonic activity during the Holocene. In view of the persistent build up of stresses in the IGP in general, and the study area in particular, it seems that the neotectonics have caused the microtopographical variations (0.5–1 m) that control the formation of sodic soils.

The role of tree uprooting in Cambisol development

The role of tree uprooting in soil formation was studied in a natural forest in a Cambisol soil zone on both the fine pit-mound spatial scale and on the coarse forest ecosystem scale. The effect of a lack of pit-mound dynamics in managed forests was also assessed. Properties of pit-mounds were studied in a 10.8 ha plot. From a total of 1562 pit-mounds, a representative sample of 51 was chosen for dendrochronological dating. Ages were determined to be between 9 and 191 years. The development of soils was studied for 14 pit-mounds of all ages. A total of 210 samples were taken from microsites at mounds, pits, and currently non-disturbed ground, from the depths 0–10, 15, 30, 50 and 100 cm. In addition, samples were taken from 9 profiles in managed forests in which tree uprooting dynamics have been prevented for at least 200 years (45 samples). Each sample was analyzed for 38 chemical and physical soil characteristics. Multidimensional statistical methods were used to evaluate the significance of (i) sampling depth, (ii) microsite, and (iii) age since the last disturbance on soil properties in the natural forest. Depth explained 12.1% of the variability in soil characteristics (p < 0.001), while microsite and age explained 7.5% (p < 0.001) and 1.8% (p = 0.048) of the variability, respectively. The highest values of Ca, Mg, C and CEC were found in pits, whereas mounds had the highest values of labile Al and exchangeable acidity. Currently non-disturbed soils had values close to the average between mound and pit values. Despite generally higher values of sorption complex characteristics, pits showed leaching of the sorption complex, leaching of C and mild clay illuviation up to about 100 years of age. Mounds showed increasing CEC values over time, as well as an increasing proportion of humic acids, with significant changes in the proportions of Fe (and Al, Si) forms. Some soil characteristics had a unimodal time course. Unlike the natural forest, the managed forest soils had considerably higher contents of the crystalline forms of Fe, Al and Mn, reflecting a more advanced stage of soil formation. However, the different disturbance regimes did not affect the course of clay illuviation, sorption complex leaching, or the content of organic matter.

Geochemical characterization of placic horizons in subtropical montane forest soils, northeastern Taiwan

Well‐developed placic horizons have been found in subalpine forest soils with large clay contents in Taiwan. We investigated their formation processes in five profiles in a subalpine ecosystem of northeastern Taiwan, using scanning electron microscopy (SEM), energy‐dispersive spectrometry (EDS), electron probe microanalysis (EPMA), differential X‐ray diffraction (DXRD) and chemical extractions. The placic horizons, ranging from 3‐ to 17‐mm thick, always occurred above argillic horizons with abrupt changes in pH and texture between the two horizons. When fully developed, the placic horizons were clearly differentiated between upper and lower sub‐horizons. EDS and chemical extractions revealed that the cementing materials in both were predominantly inorganic Fe oxides. However, contents of aluminosilicates and organically complexed Fe and Al were greater in the lower than in the upper placic sub‐horizon. Results of EPMA indicated that interstitial fine materials in the upper placic sub‐horizon were composed mainly of Fe oxides, whereas Fe oxides were codominant with illuvial clay in the lower sub‐horizon. These analyses identified the migration of Fe and clay as major formation processes in both sub‐horizons. We hypothesize that there is a pedogenic sequence that starts with clay illuviation, followed by podzolization. The resultant textural and permeability differentiation reinforces the tendency to profile episaturation that is already inherent from the heavy rainfall and clayey surface soils. Topsoil Fe is therefore reduced and mobilized, and then illuviated with clay and organically complexed Fe/Al to initiate the lower placic sub‐horizon. The poor permeability of this layer reinforces the moisture conditions in the surface soils, and the further reduction, illuviation and deposition of inorganic Fe to form the upper placic sub‐horizon.

Factors of paleosol formation in a Late Cretaceous eolian sand sheet paleoenvironment, Marília Formation, Southeastern Brazil

The Marília Formation, which crops out in southeastern Brazil, is interpreted as a Late Cretaceous eolian sand sheet area. The sedimentary succession, ca 110 m thick, is characterized by alternating strata of eolian deposits and paleosols. The paleosols constitute more than 66% of the thickness, and are an important element in the interpretation of the paleoenvironmental controlling factors which affected the soil formation in this eolian sand sheet. In this paper six paleosol profiles are described and assigned to two pedotypes: Itaja and Apore. The Itaja pedotype is constituted of a sequence of five superimposed polygenetic profiles which show different phases of clay illuviation and carbonate precipitation, and it has been classified as Aridisol. The Apore pedotype overlies the Itaja pedotype. This pedotype shows an increase in leaching, CIA-K ratios, reddening, and illuvial clay features in respect to Itaja, and has been classified as Alfisol. The analysis of the factors which controlled the soil formation revealed that both pedotypes formed on a stable landscape probably covered by a community of low stature plants, in which the soils had sufficient time to develop very mature profiles. The changes in soil-forming processes were driven principally by variations in available soil moisture from precipitation. Using depth-to-carbonate functions from Bk and CIA-K proxy from Bt horizons of the Itaja pedotype, mean annual precipitation (MAP) estimates range from 240 to 1078 mm/yr, respectively. These contrasting climatic conditions resulted in the superimposition of arid or semi-arid with more humid climates and determined the considerable change in the pedogenic features, with many horizons showing the interlacing of calcite and clay features. In the Apore pedotype CIA-K proxy from Bt horizons estimates that averaged MAP was around 900 mm/yr, and the prevalent humid conditions can be attested by soil properties and abundance and depth of root traces. Our results show that the diversity in pedotypes is mainly attributed to differences in paleoclimatic conditions during Maastrichtian time, and that eolian sedimentation was restricted to periods of harsh arid conditions, as in semi-arid climates, with MAP estimates around 240 mm/yr, the formation of soils with well developed calcic horizons was possible.

Atributos químicos, mineralógicos e micromorfológicos de horizontes coesos de latossolos e argissolos dos tabuleiros costeiros do estado de Alagoas

O caráter coeso é um atributo característico de horizontes subsuperficiais que, quando secos, apresentam consistência muito dura e extremamente dura, passando a friável ou firme quando úmidos. A formação desses horizontes, entretanto, ainda é um assunto polêmico, não estando completamente esclarecida. O objetivo deste trabalho foi realizar uma caracterização química, mineralógica e micromorfológica de solos coesos, visando entender a sua gênese e definir características que possam complementar a definição do caráter coeso no Sistema Brasileiro de Classificação de Solos. Quatro perfis de solos foram coletados na região dos tabuleiros costeiros de Alagoas, envolvendo um Argissolo Amarelo, um Argissolo Acinzentado e dois Latossolos Amarelos. Os solos foram descritos morfologicamente, e as amostras dos horizontes coesos e não coesos foram coletadas para execução de análises químicas de Fe, Al e Si por extração com DCB, oxalato, CaCl2 e água quente, análises mineralógicas por DRX e caracterização micromorfológica. Os solos apresentaram baixos teores de Fe, com domínio das formas de baixa cristalinidade e predominância de caulinitas com moderado a alto grau de desordem estrutural em todos os horizontes. Não foi observada, nos horizontes coesos, tendência de aumento dos teores de Al e Si extraídos com DCB e oxalato, indicando que a sua gênese não se deve à presença de agentes cimentantes. Os resultados das análises mineralógicas e micromorfológicas sugerem que a gênese do caráter coeso apresenta duas fases distintas, sendo formado inicialmente pelo entupimento dos poros decorrente da iluviação de argila fina, havendo posteriormente uma perda de Fe na parte superior, que colapsa a estrutura e provoca um ajuste face a face da caulinita.

Differences of stabilized organic carbon fractions and microbiological activity along Mediterranean Vertisols and Alfisols profiles

This study examined the chemical and structural properties of humic substances and microbiological activity in order to verify differences in carbon dynamics along soil profiles in two Vertisols ( Typic Haploxerert and Xeric Epiaquert) and two Alfisols ( Mollic Haploxeralf and Ultic Haploxeralf) developed under Mediterranean climate in Italy. Humification parameters, thermal methods, including differential scanning calorimetry (DSC) and thermogravimetry (TG), together with Fourier transform infrared (FT-IR) and fluorescence spectroscopies were used to characterize humic acids (HA). Microbiological activity of soils was assessed by basal respiration, metabolic quotient ( qCO 2) and C mic:C org ratio. FT-IR spectra and thermal analysis DSC/TG of HA extracted from the upper horizons showed a higher aliphatic character, whereas HA extracted from the lower horizons had a higher content of aromatic structures and polysaccharides. Moreover, the fluorescence index of the HA (HIX flu) showed a higher degree of aromatic polycondensation in the subsoils. The C mic:C org ratio was negatively correlated with the HIX flu of HA ( P < 0.05), while qCO 2 was positively correlated with the labile components of HA (Exo1/(Exo2+Exo3)) ( P < 0.05). These results suggest reduced C availability in deep horizons where HA structures resulted in complex and thermally more stable molecules. Following discriminant function analysis all the chemical and microbiological properties with the exception of labile thermal fraction of HA and soil pH largely varied between B and the upper horizons. In deep horizons of both Vertisols and Alfisols the microbial biomass was nutritionally stressed with a low efficiency in C turnover. No correlation was found between specific soil processes, such as argilliturbation or clay illuviation and humic substances properties or soil microbial biomass and activity.

Pedosediments of karstic sinkholes in the eolianites of NE Yucatán: A record of Late Quaternary soil development, geomorphic processes and landscape stability

The pedogenetic and geomorphic processes controlling soil development in tropical karstic landscapes, are still poorly understood. We have discovered and studied thick pedosediments in karst pockets, developed in Late Pleistocene lithified calcareous dunes (eolianites), in the NE coast of the Yucatán peninsula. The morphological geochemical and mineralogical characteristics of the pocket fills, were interpreted as a record of pedogenesis and geomorphic dynamics, that shaped the soil mantle during the Holocene. Sand fraction mineralogy and the Zr/Ti ratio, indicated a mixed origin of the soil parent material; volcanic and granitic/metamorphic components, redeposited in the lithoral and coastal environments were identified. Within the soil matrix, primary minerals were transformed by weathering and caused desilicification, accumulation of clay (hydroxy-interlayered vermiculite and kaolinite) and dithionite–citrate–bicarbonate extractable iron oxides (Fe d). Reworked pedofeatures indicative of clay illuviation and redoximorphic processes were observed. This set of processes points to a continuous phase of humid pedogenesis, several thousand years long in the Early–Middle Holocene. However, the development of thick recalcified pedosediments in the pockets, contrasting with the thin background Rendzinas requires active soil redeposition, during a late phase of geomorphic activity. The available Accelerator Mass Spectrometry (AMS) radiocarbon date indicates that the switch of the erosion/sedimentation processes could be related to ancient Maya land-use impact.

Role of deforestation and hillslope position on soil quality attributes of loess-derived soils in Golestan province, Iran

Conversion of natural land resources into croplands, which is triggered by the rapid population growth, causes serious soil degradation. A loess hillslope located in eastern Golestan province of Iran was selected to study the role of deforestation and slope position on soil quality attributes. Surface (0–30 cm) and subsurface (30–60 cm) soil samples were taken from five slope positions (summit, SU, shoulder, SH, backslope, BS, footslope, FS and toeslope, TS) of forest (FO) and adjacent deforested cultivated land (DEF) in a factorial trial with completely randomized design. Ten pedons were also investigated and undisturbed soil samples were taken from different horizons for micromorphological studies. The texture of the original loess is silt loam. The soil textural class varies from silty clay loam in FO to silt loam in DEF, mainly due to the loss of finer particles as a result of soil erosion followed by deforestation and long-term cultivation. Mean weight diameter (MWD) of aggregates decreased following deforestation (0.88 mm compared to 1.49 mm in FO), as a result of considerable losses of organic carbon (OC) and breakdown of aggregates. Bulk density (BD) increased and soil infiltration rate decreased by about 50% in DEF. Reduction of annual organic matter input to soil as a result of deforestation and also rapid oxidation of organic matter in DEF were responsible for a significant decrease (>70%) in OC and total nitrogen (TN). Soil microbial respiration (SMR) also decreased significantly, following deforestation. Carbon, and N contents and population of fungi were significantly higher in all hillslope positions of the FO than the DEF. Changes in soil quality attributes were not significant in different slope positions of FO, which might be related to the stability of forest landscape. Effect of different slope positions on soil quality attributes was more pronounced in the DEF. The SH and BS were found as the most susceptible positions to erosion in DEF. Soils of the FO were mainly classified as Alfisols and Mollisols with evidences for clay illuviation compared to the weakly developed Inceptisols formed in the DEF. Micromorphological investigations revealed that the FO soils had strong granular and crumb microstructure with a high porosity indicating the presence of high amount of organic matter. The high microbial and faunal activity was confirmed through the presence of excremental pedofeatures in the topsoil of the FO. Lower organic matter and consequently microbial activity in the topsoil of the DEF have resulted in the massive microstructure with little porosity.

SEM image analysis in the study of a soil chronosequence on fluvial terraces of the middle Guadalquivir (southern Spain)

SummaryA Quaternary fluvial chronosequence (Guadalquivir River, southern Spain), consisting of five soil profiles with estimated ages of 300 years (Haplic Fluvisol), 7000 years (Haplic Calcisol), 70 000 years (Cutanic Luvisol), 300 000 years (Lixic Calcisol) and 600 000 years (Cutanic Luvisol), was studied. Increasing soil age was associated with increases in: reddening, development of structure, clay content, dithionite‐extractable iron (Fed) and aluminium (Ald) and strengthening of X‐ray diffraction (XRD) peaks for phyllosilicates and iron oxides; there were also decreases in pH and percentage of carbonates in the fine earth and lower XRD peaks for calcite and dolomite. These changes indicate that the principal pedogenic processes were weathering, clay illuviation, rubefaction and the weathering and leaching of carbonates. We have further characterized the pedogenetic chronosequence by quantification of ultramicrofabrics of ped interiors using image analysis (IA) techniques on images obtained with scanning electron microscopy (SEM). We have estimated morphometric ultramicrofabric parameters for particle clusters, skeleton grains and pore space. These are closely related to analytical, mineralogical and macromorphological properties. In the principal component analysis, the first two principal components of the combined morphological, analytical and mineralogical data accounted for 78% of the total variance. The first component (48%) is loaded by variables associated with clay illuviation, relative accumulation of iron and aluminium sesquioxides and the weathering and leaching of carbonates. The components are related to ultramicrofabric development trends. We tested several chronofunctions derived from analytical and morphometric attributes. The logarithmic model fitted best, and we interpret this as indicating pedogenetic processes that are converging towards a steady state.

Clay accumulation and argillic-horizon development as influenced by aeolian deposition vs. local parent material on quartzite and limestone-derived alluvial fans

Where precipitation is ample (udic moisture regimes) and evidence of illuviation clear, argillic-horizon genesis is often easily understood. In drier climates (aridic to ustic), argillic horizons are often interpreted as relict features of the Pleistocene. However, in high montane, western landscapes where climate falls in between these two extremes, the role of aeolian clay inputs vs. in-situ weathering contributing to clay formation is of debate. Two mollisols, one developed in quartzite alluvium (Charkiln series — 3 pedons) and one in limestone alluvium (Troughspring series — one pedon) were examined to determine whether atmospheric dust or local geologies contribute to their development, and whether Bt horizons in both pedons were argillic horizons. We hypothesize that both soils have argillic horizons and that clays are derived from site parent materials. Thin-section specimens of Bt horizons, and sand and silt fraction grains were prepared and examined using a petrographic microscope. Regional dust XRD mineralogic data were compared to XRD rock fragment and soil mineralogic data. Clay films, while seen in all field pedons, were very few and faint in the Troughspring series. Illuviation of clays was evident in thin sections of the Charkiln pedons but not the Troughspring pedon. In the Charkiln series, illuviation is observed as clay-depleted A horizons, clay-accumulating B horizons, clay bridging, clay films on grains and in channels. In the Troughspring pedon, clay-depleted A horizons and clay-accumulating B horizons, very few clay films on grains and some pressure features in packing voids are seen, but grain bridging and coatings on channels are not observed. While local geologies influence soil mineralogy in both series, aeolian dust deposition is as significant and its affect is further enhanced by organic-acid weathering from an overlying pinyon-juniper woodland and the current ustic climate regime. While evidence of an argillic horizon is seen in the Charkiln series, excess Ca in soils is believed to be inhibiting illuviation in the Troughspring series and thus preventing argillic-horizon development.

Soil Developmental Stages of Layered Cambisols and Calcisols on Socotra Island, Yemen

Abstract Semiarid tropical regions are very suitable to study the polygenesis of layered substrates and soils. To determine relic and recent soil features of tropical soils, pedons in the northern part of Socotra Island, Yemen, were investigated. The aim of the study was to create developmental stages of widely distributed soils, which in particular are layered Cambisols and Calcisols. In most layers of those investigated soils, denudative components occur, which clearly indicate the relocation of coverloams and coverclays with an incorporation of extraneous materials, such as small pieces of relic clayey loams with different shapes. These fragments indicate strong chemical weathering, preceding erosion, and redistribution and/or embedding into younger sediments. Calcium carbonate found at a depth of up to 30 cm in 52% of the layered soils is very typical of an increasing aridity in parts of the semiarid tropics, particularly in areas with high groundwater levels and interflow. In addition, oxidation features and clay illuviation in noncalcareous substrates were detected. At the end, two major soil developmental stages of layered Cambisols and Calcisols were defined and demonstrated at two selected profiles. We assume that patterns of soil polygenesis depend not only on the composition of the respective substrates but also on changing climate conditions. Geochemical and soil physical methods as well as micromorphological analysis were used to describe and classify the soils.