Differential weathering of granitic stocks and landscape effects in a Mediterranean climate,Southern Iberian Massif (Spain)

Mineralogical and structural features control the weathering processes and landform development of two different crystalline stocks in the Iberian Massif, Spain (the Santa Elena and Linares stocks). The Santa Elena stock shows irregular saprolite profiles and boulder landforms whereas the Linares stock develops broad and plain landscapes with uniform saprolites and less boulder forms. The Santa Elena stock is more closely jointed and fractured than the Linares body. The main secondary minerals are kaolinite, and illite; illite/smectite interstratification was only observed in deep samples from the Linares profile. The Linares profile is feldspar-enriched whereas in Santa Elena plagioclase is almost absent and clay mineral content, especially kaolinite, is higher. Feldspars show dissolution channels developed along cleavage planes filled by clay minerals. Biotite–kaolinite intergrowths have fanned-out textures of epitaxial disposition. Weathered materials are enriched in Al and H₂O and Fe, and depleted in Ca, Na, Mg, Ti, P. The Santa Elena weathered materials are richer in H₂O than those from Linares and have lower Si/Al ratios. Mineralogical and geochemical evidence indicate the Santa Elena materials are more intensely weathered. High fracturation and high Ca-richer plagioclase contents are key factors producing the pervasive Santa Elena stock weathering. Fluvial erosion removed the alteration products in incised tectonically controlled streams resulting in boulder accumulation.     

小兴安岭山地暗棕色森林土粘土矿物学特性

本文研究了温凉湿润地区不同地形条件下花岗岩母质上的山地暗棕色森林土土壤链的矿物风化.结果表明,由阳坡岗顶、中部到阴坡下部,酸性淋溶由强变弱.原生铝硅酸盐矿物蚀变为次生的粘土矿物:黑云母酸-弱酸性→二八面体Al(层间)蛭石弱酸-中性→Al绿泥石;微斜长石→蒙皂石;斜长石弱酸性→高岭石或蒙皂石一高岭石类矿物的混层物.在针阔混交林作用下,A层腐殖酸与铝、铁络合,铝蛭石化过程弱.AB和B层黑云母和长石蚀变强,矿物晶层间羟基铝积累,阳坡岗顶Al(层间)蛭石和蒙皂石化,形成了较高的土壤潜在酸度;阴坡下部除Al蛭石外,并有成土Al绿泥石形成,潜在酸度低.含有蒙皂石的C层,层间铝有积累.矿物蚀变和晶层Al转移,是山地暗棕色森林土酸度的主要来源.红松林类型和林木生长情况与土壤粘土矿物学特性及其化学环境条件密切有关.     

Mineralogical study of weathering products of granodiorite at Shinshiro City (I)

Rock weathering has long been a subject of study for geologists, mineralogists, chemists and soil scientists since the dawn of this century. In methods for investigating rock weathering, three aspects seem to be present. The one is a chemical aspect in which weathering process is considered by comparing chemical composition of fresh parent rock with that of the weathered rock, the difference being attributed to gains or losses of chemica,l elements with respect to a supposed immobile element, usually aluminum. This aspect can elucidate the chemical behaviour of rock, that is, of material as an assemblage of constituent minerals in the environment of weathering. Among many such studies mentioned. PoLYNov's “Cycle of Weathering” (6) is one of the most comprehensive and fruitful acheivements. The second aspect is a mineralogical one, in which interests are directed toward skeletal minerals surviving against severe attacks of weathering. It is commonly observed that some of the original constituent minerals still remain in weathered material after others have been extinguished. The former minerals are more stable than the latter. On the basis of these observations, the sequence of resistantiability or stability to weathering can be determined for many rock forming minerals. GOLDICH's study of rock-weathering (3) is a representative one in this aspect. The third aspect is concerned with clay mineralogy. Primary rock-forming minerals are weathered into very finegrained materials most of which had been believed amorphous until techniques now used in clay mineralogical reserch proved their crystalline state. Besides primary skeletal minerals, weathered materials are now known to consist mainly of both amorphous and crystalline secondary minerals, mostly appearing in minus two micron fraction and being objects of interest in clay mineralogy. Any study of rock weathering hitherto performed stood more or less on the three aspects above mentioned but, all the abovementioned seem to fall into the common tendency of dealing with materials as bulk mass. That is, they were concerned much more with fresh rock versus its weathered end products as a whole rather than with the process or mechanism by which fresh rock changed into weathered material. Thus, in the temperate to subtropical humid region, it is known that, for example, kaolinite minerals, gibbsite, and some of 2: 1 type clay minerals are found in weathered materials of rocks and further that Na, K. Mg. Ca, and Si are leached away, while H, Al, and Fe are concentrated in the weathered products, but it is scarcely understood from what constituent minerals of the parent fresh rock any of the clay minerals now present in the weathered material were derived. Though, a mineralogical or chemical tracing of the courses of decomposing minerals from their initial phases to subsequent modified phases was already pioneered by STEPHEN (8). such a trend is believed, by the author, to be a fourth aspect necessary for further thorough understanding of rock weathering. This way of study may also serve in bridging between experimental data on chemical reactions of specific minerals with reagent solutions on the one hand and observations of mineralogical interrelation of parent minerals to resultant weathering products on the other hand. Granitic rock offers a suitable situation for this fourth aspect because of the ease in picking up mineral grains at various weathering stages due to the coarseness of its constituent minerals and also to its unique mode of physical disintegration.     

Podzolization as a deferralitization process: a study of an Acrisol–Podzol sequence derived from Palaeozoic sandstones in the northern upper Amazon Basin

Morphological, geochemical and mineralogical studies were carried out in a representative soil catena of the low‐elevation plateaux of the upper Amazon Basin to interpret the steps and mechanisms involved in the podzolization of low‐activity clay soils. The soils are derived from Palaeozoic sandstones. They consist of Hydromorphic Podzols under tree savannah in the depressions of the plateaux and predominantly of Acrisols covered by evergreen forest elsewhere. Incipient podzolization in the uppermost Acrisols is related to the formation of organic‐rich A and Bhs horizons slightly depleted in fine‐size particles by both mechanical particle transfer and weathering. Weathering of secondary minerals by organic acids and formation of organo‐metallic complexes act simultaneously over short distances. Their vertical transfer is limited. Selective dissolution of aluminous goethite, then gibbsite and finally kaolinite favour the preferential cheluviation of first Fe and secondly Al. The relatively small amount of organo‐metallic complexes produced is related to the quartzitic parent materials, and the predominance of Al over Fe in the spodic horizons is due to the importance of gibbsite in these low‐activity clay soils. Morphologically well‐expressed podzols occur in strongly iron‐depleted topsoils of the depression. Mechanical transfer and weathering of gibbsite and kaolinite by organic acids is enhanced and leads to residual accumulation of sands. Organo‐metallic complexes are translocated in strongly permeable sandy horizons and impregnate at depth the macro‐voids of embedded soil and saprolite materials to form the spodic Bs and 2BCs horizons. Mechanical transfer of black particulate organic compounds devoid of metals has occurred later within the sandy horizons of the podzols. Their vertical transfer has formed well‐differentiated A and Bh horizons. Their lateral removal by groundwater favours the development of an albic E horizon. In an open and waterlogged environment, the general trend is therefore towards the removal of all the metals that have initially accumulated as a response to the ferralitization process and have temporarily been sequestrated in organic complexes in previous stages of soil podzolization.     

ANDESITE WEATHERING

Petrographic and quantitative mineralogical analyses of two andesites and their saprolite (weathered rock) from the Cascade Range in California reveal a mineral weathering sequence in the rocks related to crystal size and composition and to weathering environment. Both the hypersthene andesite and the olivine andesite studied have been subjected to moderate to intensive leaching by acid solutions percolating through the superjacent soil bodies. Although the two parent rocks differ in chemical and mineralogical composition, their weathering to saprolite has followed a similar progression. During early stages of weathering of both rocks, relatively large hypersthene phenocrysts are most resistant. Following in decreasing order of resistance in both cases are plagioclase phenocrysts and finegrained mafic minerals, olivine, and glassy matrix material. Quartz is relatively stable in the earliest weathering stages, but it decreases rapidly with increased weathering. Free iron oxides and clay increase with increased weathering. Amorphous clay dominates the early weathering stages, but as weathering progresses, kaolin increases relative to amorphous clay.     

A climosequence of soils on late quaternary volcanic ash in highland Papua New Guinea

Some chemical, physical and mineralogical properties of three soils developed in volcanic ash at altitudes of 1040 m, 1720 m, and 2350 m in Papua New Guinea's Enga Province are presented. Silt-fraction mineralogy and total chemical analyses show that fresh ash occurs in the upper approximately 30 cm of profile at each site. This fresh ash is probably only a few thousand years old and overlies an older weathered ash (Tomba Tephra;more than 50000 years old). At the lowest site the majority of the primary minerals (predominantly amphiboles, volcanic glass and felspar, and some pyroxenes) have been decomposed by weathering to produce a high clay content. With increasing altitude, greater proportions of silt-sized, unweathered and partially altered primary minerals are recognised and molar ratios of calcium, magnesium, sodium and potassium to aluminum increase accordingly. In the clay fractions, allophane with Al/Si ratios of approximately 2.0 is dominant at the highest site, whereas with decreasing altitude lower proportions of allophane occur, Al/Si ratios decline and halloysite becomes dominant. Gibbsite is found in all three profiles     

Sand mineralogy and related properties of some soils in South‐Western Nigeria

Abstract

Five representative soil profiles were excavated along a toposequence selected in the Itagunmodi area of South‐Western Nigeria. The soils were subjected to physical, chemical, and mineralogical analyses. The results indicated soils with high fine sand and clay contents, but low silt content. The soils were found to vary from slightly acid to strongly acid (pHH₂O = 4.0 to 6.2). Organic C, available P, and Kjeldahl N contents decreased with increasing depth. Cation exchange capacity (CEC) ranged from 3.11 to 28.75 cmol(+)/kg soil. Base saturation was low (<51%). From a total elemental analysis, Si was found to be the dominant element, followed by Al, and then Fe. Extractable P, and exchangeable K, Mg, and Ca were quite low. The dominant minerals in the fine sand fraction were quartz, feldspar, zircon, hornblende, tourmaline, and opaque ores. The variation in the zircon/tourmaline (two resistant minerals) ratios with depth suggests a stratification of the parent material. The change in the quartz/feldspar ratios was an indication that the degree of weathering in the soil profile is not uniform.     

Weathering of gneiss in Calabria,Southern Italy

The effects of weathering in a Mediterranean climate on the mineralogy and microfabric of Paleozoic gneiss of the Sila Massif, Calabria, southern Italy, have been studied. Field observations show highly weathered rock forms a residual soil. Micromorphological and mineralogical properties of bedrock and saprolite show that the weathering process is characterized by at least two major stages, having two distinct rock microfabrics. In the first stage, the morphological features of the original rock are preserved and weathering is manifested mainly by microfracturing, and large portions of the rock remain unaltered. The second stage of weathering involves further development of microcracks and progressive chemical attack on the minerals. This latter stage occurs along both compositional and microstructural discontinuities, with etch pitting of feldspar, and neoformation of clay minerals and ferruginous products replacing feldspar, biotite, and iron-bearing garnet. The determination of quantitative petrographic indices provides a measure of the various stages of weathering.     

Intensive kaolinization during a lateritic weathering event in South-West Spain: Mineralogical and geochemical inferences from a relict paleosol

This study documents the mineralogical and geochemical record of a lateritic weathering event during the Pliocene in South-west Spain. The paleoweathering profile derived from arkosic sands and comprises a white sandy clayey saprolite, a red mottled clay zone overlain by a thick soft layer, and a ferruginous pisolitic hardcap partially dismantled by erosive processes. Kaolinite, quartz and degraded K-feldspars are the major minerals forming the saprolite, along with Al-goethite (mottled zone) as well as gibbsite and boehmite (soft layer), while the pisolitic duricrust is essentially composed of quartz grains embedded in a matrix of Fe oxy-hydroxides (hematite, goethite, and/or maghemite). Besides quartz, the phases most resistant to alteration were a variety of heavy minerals (mostly titanium oxides) that are present as minor impurities in the residual kaolin deposit.     

Surface morphology of quartz grains from tropical soils and its significance for assessing soil weathering

The surface morphology of quartz grains can indicate the degree of weathering of soil material. We have compared two methods of assessing the relative weathering of soils on the basis of differences in the surface morphology of quartz sand grains in a catena of soils in Rwanda. One method is based on the presence or absence of surface features indicative of weathering or freshness, while the other uses the frequency and size of dissolution etch pits. A ranking of relative weathering could be obtained using the first technique for the slightly and somewhat weathered soils but not for the weathered soils. On the other hand, weathering trends and differences between the horizons studied were detected in weathered soils using the second method. The introduction of more specific definitions of the weathering classes used in the latter method leads to clear improvement of the inter-observer reproducibility of the weathering classification. The surface features on the quartz grains suggest that the soil at the summit is less weathered than the other soils of the sequence. Quartz grains from the well-drained soils on the slopes, which are subjected to more intense leaching and thus to stronger chemical weathering, have more triangular etch pits and chatter marks. In the imperfectly drained soils in the valley bottom quartz grains are less etched because dissolution is inhibited by the oversaturation in silica of the drainage waters.     

Effect of natural organic soil solutes on weathering rates of soil minerals

The rate at which minerals in the soil weather is affected by pH and concentration of organic solutes (DOC). The rates of release of Al, Ca, Fe, K, Mg, Na, P and Si from a mineral soil sample to solutions of natural organic solutes and HCI (control) were determined at pH 3 and 4 for up to 17 weeks. Soil solutions were collected by centrifuging materials of O horizons from various soil types under four tree species (spruce, birch, oak, beech) and passed through a cation-exchange resin to yield H⁺-saturated organic solutes. The acceleration of the elements' release by the organic solutes was shown directly by the relative ligand effect (RLE), that is, the release rate in the organic solute solution divided by the release rate in the HCI solution (control) at the same pH. The RLE was greater at pH 4 than at pH 3, and it decreased for the elements in the order Fe > Ca > Mg > Al ≈ Si > K ≈ Na. This indicates that natural organic solutes are more important weathering agents at higher than lower pH and for weathering of mafic minerals rich in Ca, Fe and Mg than of felsic minerals such as K- and Na-feldspars. For all elements and at both pHs, RLE was strongly correlated with the concentration of DOC, which was also closely correlated with titratable acidity of the organic solutes. The important effect of soil type and tree species in producing weathering-promoting organic solutes therefore seems to be expressed through the concentration and not the composition of the organic soil solutes.     

Mikrosonden- und Röntgenbeugungs-Untersuchungen an Tonbelägen von lessivierten Böden aus Löß

Microprobe and x-ray diffraction studies of clay coatings in leached soils derived from loess Point analyses of Si, Al, K, Mg, Ca, Fe, Mn, as well as of Zn and Pb of clay coatings in soil thin sections from illuvial horizons of four leached soils were carried out using an electron microprobe (EMA). The K, Mg, Si, and Al content, the Si/Al-molar ratio of coatings and XRD-analyses indicate that the clay coatings consist mainly of illitic clay minerals. This suggests either a mineral-specific displacement of illite or the formation of illitic clay coatings from migrated smectite or expanded illite (“illitization”) through subsequent K uptake. The clay coatings show, with the exception of Fe, Mn, Zn and Pb, only little variation in their chemical composition within the same horizon. Between the horizons of the investigated soils, however, distinct differences in the total element content, especially in the K content may occur. The strong vertical variation of the K content in clay coatings in some horizons is probably due to acidic conditions and lateral infiltration which lead to an increased weathering of illite and the depletion of K. The EMA results show that Pb is predominantly bound in Fe-Mn-Oxide coatings on the surface of clay minerals or in Fe-Mn-Oxide precipitates within clay coatings. Microanalyses of K depleted clay coatings indicate on the other hand an increased fixation of Zn in expanded illites.     

Evaluating the degree of weathering in landslide-prone soils in the humid tropics: The case of Limbe,SW Cameroon

This study analyses the behaviour and mobility of major and some trace elements during the physical and chemical development of landslide-prone soil profiles in Limbe, SW Cameroon. The soils result from in situ weathering of Tertiary basaltic and picrobasaltic rocks. Textural and chemical characterisations, together with two mass balance models are applied to understand the mobility and redistribution of elements during the weathering of pyroclastic cones and lava flows. Weathering indices are used to estimate the extent of weathering. The chemical composition of the samples is evaluated by Inductively Coupled Plasma–Optical Emission Spectroscopy (ICP–OES) and their mineralogical composition by X-Ray Diffraction (XRD) analyses. It is observed that intensive weathering results in thick meta-stable soils in which significant loss of Ca, K, Mg, Na and Sr has taken place. There is a noticeable relative enrichment in all analysed trace elements (Ba, Zr, Y, Sc, V, Ni, and Co). Ti, Fe, Al, Mn, P and Ce tend to be leached in some horizons and concentrated in others. Zr, Ti, and Ce concentrations are greater in the soils than in the bedrock but show slight fluctuations in the soil and saprolites hence cannot be used as immobile elements for mass balance evaluations. Y increases progressively with advanced weathering. Major secondary mineral phases developed through weathering are a mixture of expanding (smectites) and non-expanding clays (kaolinite, halloysite and mica). The profiles show the presence of textural heterogeneities that can be exploited as slip surfaces. Data plotted in Si–Al–Fe diagram point out that the most advanced stage of weathering noted in these profiles is the kaolinisation stage.     

Composition of characteristic soils on the raised atoll Bellona,Solomon Islands

Knowledge is scarce about the composition of soils on Bellona and other Pacific atolls. Such knowledge is important as it is closely related to sustainable land use of these special soils formed on carbonatic/phosphatic materials. Therefore, the micromorphological, chemical and mineralogical composition of samples from genetic horizons in three dominant Bellonese soils (Malanga, Kenge Ungi and Kenge Toaha) and underlying rock (Tanahu) were investigated. Tanahu mainly consists of dolomite, but this mineral is absent in the three soils, which are dominated by phosphate-containing minerals. The Malanga soil is strongly dominated by Ca and P with minor amounts of Al, F and Fe present in fluorapatite, hydroxyapatite and crandallite. In contrast, Al, Fe and P dominate in the Kenge Ungi and Kenge Toaha soils in accordance with a mineralogy consisting of crandallite together with aluminum oxides (gibbsite/boehmite) and iron oxides, mainly goethite with minor contents of hematite. The observed carbonate for phosphate substitution in the apatites and crandallite is important as it indicates an increased phosphate availability in the soils. All three soil samples contain ≤ 1% Si and very little K and Mg. Total contents of essential microelements are considered adequate, but the rather high contents of Sr and U, especially in the Kenge Ungi soil may be problematic. Although the composition of the soils suggests substantial fertility and resilience, the lack of K-containing weatherable minerals (silicate minerals) is in line with a very low K (and low Mg) content and explains why fertilization may be needed to sustain future cultivation of these special soils. Due to a possible low bioavailability of Fe, Mn and maybe other micronutrients at the circumneutral pH of these carbonatic/phosphatic soils as well as the rather high contents of Sr and U, it may be recommended to test element availability using appropriate chemical soil tests supplemented by plant experiments to ensure safe and sustainable (optimal) soil use. However, according to the local farmers, the Malanga, Kenge Toaha and Kenge Ungi soils are considered well suited for production of the preferred crops.     

Effect of north and south exposure on weathering rates and clay mineral formation in Alpine soils

A comparison was made between two soil climosequences on north- and south-facing slopes in northern Italy to determine the influence of slope aspect on soil processes. The climosequences span an elevational gradient ranging from moderate (1200 m a.s.l.) to high alpine (2420 m a.s.l.) climate zones on surfaces having an age of about 15 000 years. The soils were investigated with respect to organic C, oxalate and dithionite extractable Fe, Al and Si, elemental losses (Ca, Mg, K, Na, Fe, Al, Si, Mn) and clay minerals. The stocks of soil org. C as well as of oxalate-extractable Fe and Al was greatest in the subalpine zone near the timberline. There are no clear differences in organic C content between the soils on north- and south-facing sites. Fe-oxalate and to a lesser extent Al_o-stocks were, however, greater on north-facing sites, indicating that weathering is greater there. Eluviation and illuviation of Al and Fe within the soil profile, typical for podzolisation, was more distinctly expressed on the N slopes. The probability of ITM (Imogolite-type-material) formation in the soil seemed to be greater on south-facing sites. On the north-facing sites, element leaching was most intense in the subalpine zone close to the timberline while on the south-facing sites this was only the case for the base cations. The N slopes exhibited higher leaching of elements which generally indicates a higher weathering intensity. On south-facing sites, typical podzolisation processes were measurable only above 2000 m a.s.l. The development of smectites is also a reflection of the weathering intensity; smectite was discernible in the surface horizon at all sites on N slopes but the highest amount was detected in the sub-alpine climate zone. For the south-facing sites only in the alpine climate zone could smectite be detected. Higher temperatures and an increased number of freeze-thaw cycles on south-facing slopes should theoretically enhance rates of chemical weathering. This could, however, not be confirmed with our measurements. The degree of chemical weathering increases from the south- to the north-facing sites that are characterised by lower temperatures, lower evapotranspiration and consequently by a higher humidity. Although precipitation in Alpine regions is abundant, the availability and flux of water through the soil is the prime factor in weathering intensity.     

Clay minerals,oxyhydroxide formation,element leaching and humus development in volcanic soils

A weathering sequence with soils developing on volcanic, trachy-basaltic parent materials with ages ranging from 100–115,000 years in the Etna region served as the basis to analyse and calculate the accumulation and stabilisation mechanisms of soil organic matter (SOM), the transformation of pedogenic Fe and Al, the formation and transformation of clay minerals, the weathering indices and, by means of mass-balance calculations, net losses of the main elements. Although the soils were influenced by ash depositions during their development and the soil on the oldest lava flow developed to a great extent under a different climate, leaching of elements and mineral formation and transformation could still be measured. Leaching of major base cations coupled with a corresponding passive enrichment of Al or Fe was a main weathering mechanism and was especially pronounced in the early stages of soil formation due to mineral or glass weathering. With time, the weathering indexes (such as the (K + Ca)/Ti ratio) tend to an asymptotic value: chemical and mineralogical changes between 15,000 and 115,000 years in the A and B horizons were small. In contrast to this, the accumulation of newly formed ITM (imogolite type materials) and ferrihydrite showed a rather linear behaviour with time. Weathering consisted of the dissolution of primary minerals such as plagioclase, pyroxenes or olivine, the breakdown of volcanic glass and the formation of secondary minerals such as ITM and ferrihydrite. The main mineral transformations were volcanic glass ? imogolite ? kaolinite (clay fraction). In the most weathered horizons a very small amount of 2:1 clay minerals could be found that were probably liberated from the inner part of volcanic glass debris. The rate of formation and transformation of 2:1 clay minerals in the investigated soils was very low; no major changes could be observed even after 115,000 years of soil evolution. This can be explained by the addition of ash and the too low precipitation rates. In general, soil erosion played a subordinate role, except possibly for the oldest soils (115,000 years). The youngest soils with an age < 2000 years had the highest accumulation rate of organic C (about 3.0 g C/m²/year). After about 15,000 years, the accumulation rate of organic C in the soils tended to zero. Soil organic carbon reached an asymptotic value with abundances close to 20 kg/m² after about 20,000 years. In general, the preservation and stabilisation of SOM were due to poorly crystalline Al- and Fe-phases (pyrophosphate-extractable), kaolinite and the clay content. These parameters correlated well with the organic C. Imogolite-type material did not contribute significantly to the stabilisation of soil organic matter.     

Stoichiometry of base cations and silicon during weathering of a deep soil profile derived from granite

Evaluation of the stoichiometry of base cations (BCs, including K⁺, Na⁺, Ca²⁺, and Mg²⁺) and silicon (Si) (BCs:Si) during soil mineral weathering is essential to accurately quantify soil acidification rates. The aim of this study was to explore the differences and influencing factors of BCs:Si values of different soil genetic horizons in a deep soil profile derived from granite with different extents of mineral weathering. Soil type was typic acidi-udic Argosol. Soil samples were collected from Guangzhou, China, which is located in a subtropical region. To ensure that the BCs and Si originated from the mineral weathering process, soil exchangeable BCs were washed with an elution treatment. The BCs:Si values during weathering were obtained through a simulated acid rain leaching experiment using the batch method. Results showed that soil physical, chemical, and mineralogical properties varied from the surface horizon to saprolite in the soil profile. The BCs:Si values of soil genetic horizons during weathering were 0.3-3.7. The BCs:Si value was 1.7 in the surface horizon (A), 1.1-3.7 in the argillic horizon (Bt), and 0.3-0.4 in the cambic (Bw) and transition (BC) horizons, as well as in horizon C (saprolite). The general pattern of BCs:Si values in the different horizons was as follows: Bt > A > Bw, BC, and C. Although BCs:Si values were influenced by weathering intensity, they did not correlate with the chemical index of alteration (CIA). The release amounts of Si and BCs are the joined impact of soil mineral composition and physical and chemical properties. A comprehensive analysis showed that the BCs:Si values of the soil derived from granite in this study were a combined result of the following factors: soil clay, feldspar, kaolinite, organic matter, pH, and CIA. The main controlling factors of BCs:Si in soils of different parent material types require extensive research. The wide variance of BCs:Si values in the deep soil profile indicated that H⁺ consumed by soil mineral weathering was very dissimilar in the soils with different weathering intensities derived from the same parent material. Therefore, the estimation of the soil acidification rate based on H⁺ biogeochemistry should consider the specific BCs:Si value.     

The dissolution of micas by fulvic acid

Three micas commonly occurring in soils, that is, biotite, phlogopite and muscovite, were shaken with 0.2% (W/V) aqueous fulvic acid (FA) solution for 710 h at room temperature. Proportions of major constituent elements extracted (Fe, Al, Mg, K and Si from biotite, Al, Mg, K and Si from phlogopite and Al, K and Si from muscovite) were determined.Depending on the type of mineral and its Fe content, substantial amounts of Fe, Al, Mg, K and Si were brought into aqueous solution by the FA under mild conditions. The ease with which the micas were attacked by the FA decreased in the following order: biotite > phlogopite > muscovite. IR and ESR analyses indicated the formation of metal—FA complexes as a major reaction mechanism by which the minerals were dissolved. Our data show that aqueous FA solution can dissolve from micas relatively large amounts of metals and Si, which may then become more available to plant roots and microbes, active in soil genesis processes, as well as participants in the synthesis of new minerals.     

Early detection of the effects of compaction in forested soils: evidence from selective extraction techniques

Purpose

Soil compaction resulting from mechanisation of forest operations reduces air permeability and hydraulic conductivity of soil and can result in the development of hydromorphic and/or anoxic conditions. These hydromorphic conditions can affect physico-chemical properties of the soils. However, early detection of these effects on mineralogical portion of soils is methodologically difficult.

Materials and methods

To analyse the effects of soil compaction on iron minerals in loamy Luvisol, three compacted and three non-compacted soil profiles up to the depth of 50 cm were collected from an artificially deforested and compacted soils after 2 years of treatment. Soil was compacted with the help of 25 Mg wheeler’s load to increase the dry bulk density of soil from 1.21?±?0.05 to 1.45?±?0.1 g cm^?3. Soil samples were analysed by X-ray diffraction (XRD) and were treated by citrate bicarbonate (CB) and dithionite citrate bicarbonate (DCB) under controlled conditions. Major and minor elements (Fe, Al, Mg, Si and Mn) were analysed by ICP-AES in the CB and DCB extracts.

Results and discussion

It was found that X-ray diffraction is not an enough sensitive method to detect the quick mineralogical changes due to soil compaction. Results obtained from CB-DCB extractions showed that soil compaction resulted in larger CB and smaller DCB extractable elements as compared to non-compacted soil. Labile Fe was found 30 % of total Fe oxides in compacted soil against 10–14 % in non-compacted soils. Compaction thus resulted in Fe transfer from non-labile to labile oxides (s.l.). Results showed that soil compaction leads to the reduction of Fe³⁺ to Fe²⁺. The effects of hydromorphic conditions due to soil compaction were observed up to the depth of 35 cm in forest soil profile. Furthermore, a close association of Al with Fe oxides was observed in the soil samples, while Mn and Si were mainly released from other sources, Mg showing an intermediate behaviour.

Conclusions

Hydromorphic conditions owing to soil compaction affect the mobility and crystallisation process of iron mineral. CB-DCB selective extraction technique, in contrast to XRD technique, can be effectively used to examine the possible effects of soil compaction on iron minerals.

     

Some properties of a chrono-toposequence of soils from granite in New Zealand, 1. profile weights and general composition

A chrono-toposequence of five soils formed in strongly weathered granite in the South Island of New Zealand was described on the basis of soil profile morphology, landscape position, and the degree of weathering of the underlying granite. The sequence contains five progressively younger soils on increasing slopes from crest to backslope formed in a Parent Material of originally uniform mineralogical and chemical composition as Parent Rock under a uniform climate and vegetation. All profiles of the sequence are strongly weathered and leached with the final member of the sequence being considered to represent a terminal steady-state system.Changes in a number of chemical and physical soil properties were described with the increasing soil development (i.e. increasing weathering and leaching) represented in the sequence profiles. Profile soil weights (< 2mm) remained approximately constant, once weathering of stone material (> 2 mm) to fines was completed. An initial increase in profile silt and clay content was followed by a decline in both fractions with a corresponding increase in the sand fraction. Proportions of kaolin/metahalloysite initially increased and 2:1–2:2 Al intergrades decreased, but the final three sequence profiles had similar clay mineralogies with kaolin/metahalloysite being the main component. Profile weights of oxidisable carbon and total nitrogen followed an exponential decline after an initial increase between the first two sequence profiles. Profile cation-exchange capacity, total exchangeable bases, percent base saturation, and exchangeable cations showed no trends with increasing soil development. Depth-gradients of these parameters together with oxidisable carbon and total nitrogen became less pronounced, with the apparent tendency towards a steady-state system in the final sequence profile. Multiple regression analysis indicated that oxidisable carbon was the main contributing factor to cation-exchange capacity followed by clay content.Profile weights of total calcium and potassium were closely correlated and showed an approximately exponential decline with increasing soil development, whereas total magnesium showed a relatively linear decline. Total aluminium and iron both showed an initial increase and then declined. The relative rates of loss of the five total elements studied were in the order: Ca > Mg > K, Fe > Al.