A 10-year record of the weathering rates of surficial pebbles in Kärkevagge, Swedish Lapland

Between 1999 and 2004, nylon mesh bags containing  6.2 mm diameter fragments of crushed dolomite or granite were exposed to weathering on the surfaces of birch, Dryas heath, heath, meadow, solifluction meadow, and willow vegetation communities in Kärkevagge, a glaciated trough in Swedish Lapland. The material in the bags had previously been in the same locations from either 1994 or 1995 when it had been placed in the field as freshly crushed pebbles. Results of the mass losses resulting from chemical weathering during the first 4- or 5-year period were reported in Dixon et al. [Dixon, J.C., Thorn, C.E., Darmody, R.G., Schlyter, P. 2001. Weathering rates of fine pebbles at the soil surface in Kärkevagge, Swedish Lapland. Catena 45, 273–286.] in Catena. The spatial results of the second 5-year period confirm those determined during the first period. In general, wetter, more acidic sites promoted greater losses from the dolomite, but the losses from the granite were too small to reveal distinctive spatial patterns. Dolomite consistently weathered faster than granite in at-a-site comparisons with total mass losses between 1999 and 2004 ranging from 1.90% to 9.98% for dolomite and − 0.07% to 4.02% for granite. Spatial distinctions were blurred during the second period due to significant losses of bags at dry sites from atmospheric exposure. However, at-a-site comparisons between the two study periods revealed statistically significantly greater losses during the second period when compared to the first. In the absence of appropriate ground climate data, it is worth noting that air climates were both warmer and wetter during both study periods when compared to the 1961–1990 averages, and the second 5-year study period was also warmer and wetter than the first.     

Effect of Bacillus subtilis on granite weathering: A laboratory experiment

We performed a comparative experiment to investigate: (1) how the ubiquitous soil bacterium Bacillus subtilis weathers granite; and (2) which granite-forming minerals weather more rapidly via biological processes. Batch system experiments (granite specimen in a 500 ml solution including NaCl, glucose, yeast extract and bacteria B. subtilis at 27 °C) were carried out for 30 days. Granite surfaces were observed by SEM before and after the experiment. B. subtilis had a strong influence on granite weathering by forming pits. There were 2.4 times as many pits and micropores were 2.3 times wider in granite exposed to B. subtilis when compared with bacteria-free samples. B. subtilis appear to preferentially select an optimum place to adhere to the mineral and dissolve essential elements from the mineral to live. Plagioclase was more vulnerable to bacterial weathering than biotite among the granite composing minerals.     

不同风化年限的淮南矿区煤矸石理化性质变化规律

堆存于地表的煤矸石在遭受风化以后,其物理和化学性质可在短时间内发生较大变化,这些变化往往具有一定规律。该文选取淮南矿区潘北、潘一及新庄孜煤矿5个不同风化年限的煤矸石采样区进行分层采样。通过对135个样品的相关理化性质测试,对比分析了不同风化程度下煤矸石主要理化性质变化规律。结果表明,煤矸石在电导率、pH值和阳离子交换量等理化性质的变化具有一定规律:随着风化年限的增加,煤矸石电导率与pH值降低,阳离子交换量则不断增高。新鲜煤矸石的3项指标在2a内具有较快的降低速率,其中电导率在2 a内可降低30%,pH值下降接近10%,此后的降低变化速率则较缓慢。阳离子交换量在2 a内可增加17%,在后期的变化中则表现为缓慢上升趋势。在剖面变化特征方面,通过对30和30~60 cm之间的上下两层对比分析发现位于上层的电导率与pH值普遍略高于下层,阳离子交换量则为上层略低于下层。其中p H值的上、下两层的变化差距较小,仅在0.1~0.3之间。煤矸石的电导率、pH值,以及阳离子交换量等3项指标的时空变化均与风化作用的时间或风化程度密切相关。从植物生长条件角度出发,上述理化指标的变化均有利于煤矸石的复垦利用。     

Inverse models to analyze the spatiotemporal variations of chemical weathering fluxes in a granito-gneissic watershed: Mule Hole, South India

Water-rock reactions are driven by the influx of water, which are out of equilibrium with the mineral assemblage in the rock. Here a mass balance approach is adopted to quantify these reactions. Based on field experiments carried out in a granito-gneissic small experimental watershed (SEW), Mule Hole SEW (~ 4.5 km²), quartz, oligoclase, sericite, epidote and chlorite are identified as the basic primary minerals while kaolinite, goethite and smectite are identified as the secondary minerals. Observed groundwater chemistry is used to determine the weathering rates, in terms of ‘Mass Transfer Coefficients’ (MTCs), of both primary and secondary minerals.Weathering rates for primary and secondary minerals are quantified in two steps. In the first step, top red soil is analyzed considering precipitation chemistry as initial phase and water chemistry of seepage flow as final phase. In the second step, minerals present in the saprolite layer are analyzed considering groundwater chemistry as the output phase. Weathering rates thus obtained are converted into weathering fluxes (Q_weathering) using the recharge quantity.Spatial variability in the mineralogy observed among the thirteen wells of Mule Hole SEW is observed to be reflected in the MTC results and thus in the weathering fluxes. Weathering rates of the minerals in this silicate system varied from few 10 μmol/L (in case of biotite) to 1000 s of micromoles per liter (calcite). Similarly, fluxes of biotite are observed to be least (7 ± 5 mol/ha/yr) while those of calcite are highest (1265 ± 791 mol/ha/yr). Further, the fluxes determined annually for all the minerals are observed to be within the bandwidth of the standard deviation of these fluxes. Variations in these annual fluxes are indicating the variations in the precipitation. Hence, the standard deviation indicated the temporal variations in the fluxes, which might be due to the variations in the annual rainfall. Thus, the methodology adopted defines an inverse way of determining weathering fluxes, which mainly contribute to the groundwater concentration.     

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.     

Impacts caused by the addition of wine vinasse on some chemical and mineralogical properties of a Luvisol and a Vertisol in La Mancha (Central Spain)

Background, aim, and scope  Using wastewaters from wine production that are often discharged directly into soil, with previous treatment, we carried out an experiment to identify the impact over time, specifically to identify the benefits and risks of its application. Materials and methods  Experiments were carried out using approximately 200 g of samples of agricultural soils which were amended with increased amounts of vinasse: 1–5–10–20–40–70 ml. The doses used were not arbitrary but similar to the usual one amended. Soil electric conductivity was determined in distilled water with a glass electrode (soil to H₂O ratio 1:5). Clay identification of soil samples are by X-ray diffraction. Results  The pH decreased slightly and salinity increased, possibly leading to changes in crop productivity. The increase in salinity reflects the concentration of dissolved salts in the vinasse. Significant changes were observed in the clay minerals after amendment with the vinasse. Conclusions  Our study indicates that, under experimental incubation, the application of increasing doses of wine vinasse for increasingly long periods can affect certain chemical properties. The results show that application of winery wastewaters to soil results in significant increases of electrical conductivity, reaching levels that can be detrimental to crop growth. The decrease in pH values is somehow beneficial, and finally there is a small increase in the chemical weathering of clay minerals     

Silicon nutrition of rice is affected by soil pH,weathering and silicon fertilisation

Silicon (Si) is a beneficial element for tropical grasses such as rice (Oryza sativa) and responses to applications of Si are common on highly weathered soils. However, the importance of pH (and hence Si speciation), weathering and fertilisation on Si uptake is still poorly understood. The responses of rice to Si fertilisation were studied in two variably weathered basalt soils (Red Ferrosol, Grey Vertosol) adjusted at different pH values (5.5–9.5) with three levels of acidulated wollastonite. Soil Si was extracted using deionised water (H₂O), 0.01 M CaCl₂, or 0.5 M NH₄OAc. Significant increases in Si uptake and rice biomass were observed in the Red Ferrosol following fertilisation (p < 0.01). Greater biomass production was observed at lower pH, due to decreased Si sorption and higher solution Si concentrations. Silicon uptake by rice was greater at low pH, despite similar extractable Si concentrations; suggesting a relationship between Si speciation and uptake. In contrast, Si uptake and rice shoot dry matter in the less weathered Grey Vertosol were unaffected by Si fertilisation (p > 0.05) except at the highest rate and lowest pH (5.5). Solution Si concentrations were controlled by precipitation/polymerisation reactions in equilibrium with specific soil pH values rather than adsorption processes. Silicon speciation effects (monosilicic acid vs. silicate ions) were unable to be measured due to an induced phosphorus deficiency in both soils at pH values > 8.5. In conclusion, weathered soils are more responsive to Si fertilisation and Si uptake is increased at low pH.     

Effects of root exudates and humic substances on weathering kinetics

The effect of complex natural organic ligands on the weathering kinetics of aluminum oxide was investigated in laboratory experiments. A peat-derived humic substance and root exudates obtained from ectomycorrhizal (Picea abies — Hebeloma crustuliniforme) and non-mycorrhizal Norway Spruce trees; and γ-Al₂O₃ were used as a model system. The experimental weathering rates are in accordance with a surface-controlled dissolution mechanism. The effect of the humic material on dissolution rates appears to depend on the degree of protonation of the humic (macro)molecules: we observed dissolution-enhancement or -inhibition at pH 3 and 4, respectively. Ectomycorrhizal exudates proved to be effective weathering agents at pH 4, as opposed to humic material and non-mycorrhizal exudates. Our results suggest that (i) the role of humic materials in mineral weathering and podzolization is different from what is commonly thought, and (ii) mineral weathering rates in the rhizosphere may be higher than in the bulk soil.     

Artificial weathering of Spanish granites subjected to salt crystallization tests: Surface roughness quantification

For hundreds of years, two types of granite (Zarzalejo and Alpedrete) from the Madrid region, Spain, have been extensively used as building stones. Fresh specimens of both stone types have been sampled from their respective quarries and subjected to sodium sulphate salt crystallization test (SCT). The resulting physical and chemical weathering patterns have been characterized by polarized light optical and environmental scanning electron microscopy. Water absorption under vacuum conditions and mercury intrusion porosimetry techniques were used to determine the pre- and post-SCT porosity and pore size distribution. The following non-destructive techniques were performed to assess stone durability and decay: ultrasound velocity (US) and surface roughness determination (SR) of intra- and inter-granular quartz, feldspar and biotite minerals at the centre as well as at the corners and edges of specimen surfaces. Before the SCT, US values were lower and SR values higher in Zarzalejo (ZAR) than Alpedrete (ALP) granite. After SCT, the US values declined while SR rose in both types of granites, with greater average differences in ZAR than ALP for both parameters. Feldspar and biotite and their inter-granular contacts were found to be the weakest and therefore the most decay-prone areas of the stone.The initial SR parameters were generally higher and rose more steeply after SCT at the corners and around the edges of the specimens.While behaviour was found to be similar in the two types of granite, variations were greater in ZAR, the less durable and more decay-prone of the two. Surface roughness measurement of mineral grains in granite stones is a very useful, in situ, non-destructive technique for quantifying salt crystallization-mediated physical and chemical weathering. The resulting quantification of decay and of related durability provides insight into the future behaviour of this type of stone, commonly used in historic buildings.     

The Influence of Soil Properties on the Water Pollution (Nitrate,Phosphate and COD) Following a Single Application of Sewage Sludge to 70 Contrasting Agricultural Soils

The aim was to study the influence of soil properties on the leaching of nitrate, phosphate and organic matter (OM) following the application of sewage sludge to contrasting soils. Seventy agricultural soils from different parts of Spain were amended with sewage sludge (50 t dry weight ha⁻¹), and a controlled column study was developed. After 2, 4 and 6 months of incubation, distilled water, equivalent to an autumn rainfall event of 25 l m⁻² in Mediterranean environments, was applied and leachates collected and analysed: pH, electrical conductivity (EC), chemical oxygen demand (COD), phosphate and nitrate. The mean values of pH in the leachates after 2, 4 and 6 months were similar and close to the neutrality. The highest concentrations for the rest of the parameters analysed were found after 2 months of incubation and diminished for 4 and 6 months, especially COD. Soil pH and texture were the most relevant soil properties controlling the leaching of the analysed parameters. The OM mineralization seemed to be enhanced at high values of soil pH, thus increasing the nitrate and reducing the COD leaching. However, phosphate levels were reduced at high values of soil pH. In addition, leaching was promoted in sandy soils. Other soil properties influenced phosphate leaching being the equivalent calcium carbonate soil content as the most relevant. Soil organic carbon was negatively related to the EC and nitrate concentration in the leachates but resulting in a weak contribution compared with soil pH and texture. Concerns about nitrate pollution have been confirmed.     

The requirements of the rare moss, Hamatocaulis vernicosus (Calliergonaceae, Musci), in the Czech Republic in relation to vegetation, water chemistry and management

Hamatocaulis vernicosus, a rare moss, has been investigated in detail for its habitat preferences, ecology and population dynamics in the Czech Republic. At all its known sites plant species composition was described and relationships with environmental factors investigated (water table, pH, water conductivity). Experiments that included mowing and gap cutting were investigated at three sites over two years.Hamatocaulis vernicosus had the highest cover at neutral pH (6.7-7.2) and conductivity between 100 and 250 μS/cm, although most localities had lower values. It was influenced positively by mowing only at a site with a high vascular plants cover, and gap cutting was only beneficial at sites with a low water table. The growth and vitality of Hamatocaulis may, therefore, be supported by suitable management especially in drier habitats.     

Evaluation of elemental depletion weathering rate estimation methods on acid-sensitive soils of north-eastern Alberta, Canada

Immobile element-based weathering estimation methods assume that Zr (or Ti) is an immobile element, and that weathering rates of other elements can be estimated according to the enrichment of Zr in weathered horizons relative to an unweathered parent material. This approach was used to estimate base cation weathering rates for 33 soil profiles on acid-sensitive terrain in north-eastern Alberta. Zirconium generally showed enrichment within the rooting zone, but the deepest (subsoil) samples were not always associated with the lowest Zr concentrations. Weathering rates estimated with the Zr depletion and Pedological Mass Balance (PMB) methods were generally low (ranges: 0-51 and 0-58 mmol_c m^− 2 yr^− 1, respectively); however, low base cation oxide concentrations and heterogeneity within soil profiles complicated weathering rate calculations and net base cation gains were calculated for several (six) sites. Evaluation of the Zr depletion and PMB weathering estimates against those calculated with the process-oriented PROFILE model at a subset (n = 9) of the sites indicated the estimates were poorly related, with PROFILE rates typically being higher. The effects-based emissions management strategy for acid precursors in this region requires spatial coverage of soil properties (including weathering rates) across a large area, but the apparent limitations associated with the immobile element based methods in this region: identifying representative parent soils and deriving weathering rate estimates comparable to more robust methods are arguments against their candidacy for future use.     

Determining nitrogen fate by hydrological pathways and impact on carbonate weathering in an agricultural karst watershed

Identifying the nitrogen (N) fate is complicated and a great challenge in karst watersheds because of the co-existence of natural pools and anthropogenic sources. The objective of the study was to use stable isotopic composition of dual-isotope (δ¹⁵N_Nitrate and δ¹⁸O_Nitrate) and LOADEST model approaches to trace N sources, pathways in karst watershed. The study was conducted in the Houzhai watershed, which is a typical agricultural karst watershed from July 2016 to August 2018, to reveal the N fate and the coupled carbon(C)–N processes occurring in the riverine-watershed with agricultural activities. We found that the wet deposition of total nitrogen (TN) flux was 33.50 kg hm⁻²·a⁻¹ and dissolved nitrogen (DN) flux was 21.66 kg hm⁻²·a⁻¹. The DN runoff loss was 2.10 × 10⁵ kg·a⁻¹ and the loss of DN during the wet season accounted for 95.4% over a year. In the wet season, NO₃⁻-N daily efflux was 977.62 ± 516.66 kg ha⁻¹·day⁻¹and 248.77 ± 57.83 kg ha⁻¹·day⁻¹ in the dry season. The NH₄⁺-N efflux was 29.17 ± 10.50 kg ha⁻¹·day⁻¹ and 4.42 ± 3.07 kg ha⁻¹·day⁻¹ in the wet and dry seasons, respectively. The main form output load of N was NO₃⁻-N which was more than 30 times as much as NH₄⁺-N output loss. The NO₃⁻-N caused by rainfall contributed 11.82%–53.61% to the export load. Nitrate from soil contributed over 94% of the N to Houzhai river caused by N leaching. In addition, manure and farmland soil were the main sources of groundwater in the Houzhai watersheds, the contribution rates were 25.9% and 22.5%. The chemical N fertilizers affected carbonate weathering strongly, and the HCO₃⁻ flux caused by nitrification due to N fertilizers application in soil accounted for 23.5% of the entire watershed. This study suggested that carbonate weathering may be influenced by nitrogen nitrification in the karst watershed.     

Weathering and vegetation effects in early stages of soil formation

Bedrock surfaces in the Ouachita Mountains, Arkansas, exposed by spillway construction and which had not previously been subjected to surface weathering environments, developed 15–20 cm thick soil covers in less than three decades. All open bedrock joints showed evidence of weathering and biological activity. Rock surfaces and fragments also showed evidence of significant weathering alteration. The results suggest a soil production function whereby weathering and increases in thickness are initially rapid. The rapid initial rate (5 to 10 mm year^− 1) is facilitated by a weathering-favorable regional climate, local topography favoring moisture and sediment accumulation, and aggressive vegetation colonization. The ages of the trees on the bedrock benches suggests that a short period (< 10 years) of pedogenic site preparation is necessary before trees can become established. Initial chemical weathering within newly-exposed rock fractures in resistant sandstone strata and chemical weathering of weak shale layers, coupled with accumulation of organic and mineral debris in fractures and microtopographic depressions facilitates plant establishment, which accelerates local weathering rates.     

Rare earth and other trace and major elemental distribution in a pedogenic calcrete profile (Slimene,NE Tunisia)

A calcrete profile developed on the top of a calcareous consolidated dune located in a coastal area of NE Tunisia (semi-arid climate) was studied with the aim to investigate the behavior of the chemical elements (rare earth elements—REE—and other trace and major elements) during the processes associated with calcrete formation, particularly dissolution and precipitation of carbonates in the vadose zone. The profile shows a vertical sequence, with clear zonations from the surface downwards: a reddish soil at the land surface, a nodular horizon, a laminar-structured level, and the consolidated old dune. Chemical and mineralogical analyses of samples from all levels were performed by neutron activation analysis and X-ray diffraction (bulk samples and < 2 μm fraction). Detailed studies of iron speciation and iron minerals were done by Mössbauer spectroscopy. Microfauna, mineralogical and chemical variations in the Slimene weathering profile point to a long term aerial exposure of the old dune in a semi-arid carbonated environment leading to the development of a pedogenic calcrete profile. The mineralogical variations with depth indicate carbonate dissolution at the surface and downward leaching of calcium. Secondary carbonates overgrow primary ones and precipitate as coating or concretions below. Minerals found in the upper levels correspond to original materials of the old dune and also weathering phases and atmospheric inputs. Phyllosilicates decrease while calcite and K-feldspars increase with depth. Kaolinite was the only clay mineral found in the old dune. Illite and chlorite were also found in all samples with calcrete. Authigenic smectite formation occurs above the laminar-structured calcrete due to restricted drainage conditions. The most significant chemical variations associated with the calcrete formation are (i) enrichment of Co, U, Br, and REE in calcrete; (ii) depletion of middle REE in the upper levels, particularly Eu, and enrichment of middle REE and heavy REE in calcrete; and (iii) Hf, Zr, Cr, Th, Cs, Ta, Ga, Rb, and K appear to be retained in the upper levels, where calcite has been dissolved. Here iron is more reduced. Fe³⁺ occurs in iron oxides (goethite and hematite), and clay minerals. The ratio goethite/hematite appears to increase down the profile; and Fe²⁺ is mainly present in clay minerals.     

Mineral magnetic properties of a weathering sequence of soils derived from basalt in Eastern China

The magnetic properties and magnetic mineralogy of a weathering sequence of soils developed on basalt parent material from eastern China, were studied by rock magnetism, X-ray diffraction and soil chemical analyses to establish the connection between mineral magnetic properties and pedogenic development in a subtropical region. The magnetic susceptibility of soils formed on basalt varied greatly and did not increase with the degree of pedogenic development. The frequency-dependent susceptibility (χfd) values of soils ranged from 1.0 to 11.1% and increased with the pedogenic development. Highly significant linear relationship was found between the frequency-dependent susceptibility and the Fe_d content (R² = 0.683) and Fe_d/Fe_t ratio (R² = 0.780) in soils, indicating that pedogenic SP ferrimagnetic grains were associated with enrichment of the secondary iron oxide minerals in the weathering process of soil. Rock magnetism analysis showed that the major magnetic carriers in the weakly weathered soil profiles are magnetite and/or maghemite, and the highly developed soil profiles are generally enriched in magnetite/maghemite grains of pedogenic origin and the magnetically hard haematite, indicating that the magnetic component was transformed from a ferrimagnetic phase (magnetite) to antiferromagnetic phase (hematite) during pedogenic development. Results indicated that some of the magnetic parameters of soils, in this case χfd, can be useful for pedogenic comparisons and age correlations in the weathering sequence of soil. It is thus suggested that multiparameter rock magnetic investigations represent a more powerful approach for pedogenesis.     

Tritium radioactivity in the aquatic environment over Alexandria region,Egypt

Tritium activity is determined by standard techniques, in samples collected from fresh, marine and highly saline water around Alexandria. It has been found that, the average ³H activity is 6.32 ±1.17, 4.02±0.77 and 2.90±0.51 Bq.L^?1 for West Elnobaria Drain, Lake Marioute and Mediterranean coastal water, respectively. Tritium activity is found to be weakly correlated with water quality parameters; pH, dissolved O₂, Oxidation Reduction Potential (ORP), conductivity, salinity and chlorinity. Autocorrelation analysis indicates that the transport phenomena for ³H variations in fresh and saline water are not the same.     

南极Fildes半岛地衣在风化和成土过程中的作用

Lichens play an unparalleledly vital role in weathering and soil-forming processes in Antarctic region,In this study some related chemical components and micromorphological analyses have been carried out on the samples of the weathered rocks and the lichens grown on them from Files Peninsula,Antarctic,The results indicatied that the major chemical components in the bioweathering surface layer of the sampled rocks have been obviously altered and the weathering potential in this layer has greatly decreased by and average range around 4.66 percent in 4 samples,In the weathering surface layer ferruginiztion of some minerals in varying degress was seen by means of microscopic examination through the thin section of the weathered rocks,and its products proved to be dominated by hematitie,limonite,goethite and free iron oxides Meanwhile,the study suggested that the dissolution and absorption of lichens by their secretion accelerated the process of calcitization of minerals in the bio-weathering suface layer,Eventually,the results also show that different species of lichens play different roles in weathering and soil-forming proesses.     

Mineralogy and factors controlling charge development of three Oxisols developed from different parent materials

Three Oxisols, developed from serpentinite (Sungai Mas Series), basalt (Kuantan Series) and andesite (Segamat Series), selected to represent the most common Oxisols in Malaysia were sampled and studied. The objectives of this study were: (i) to determine mineralogical composition and factors responsible for changes in point of zero charge (pH₀) of the variable charge component of three Oxisols; (ii) to use pH₀ values to assess degree of chemical weathering; and (iii) to determine the magnitude of variable charge using corrected back-titration technique. The mineralogical composition was determined by X-ray diffraction analysis (XRD). The pH₀ was determined by potentiometric titration in different electrolyte strengths. The magnitude of variable charge generation as a function of soil pH was measured using corrected back-titration to allow elimination of charge overestimation caused by solid dissolution and hydrolysis reactions. The results showed that the mineralogical composition were similar (kaolinite, goethite, hematite and gibbsite) between profiles but different in proportion, except for gibbsite which was absent in the andesite-derived soil. The sequential removal of soil organic matter (SOM), iron oxides and SOM together with iron oxides resulted in the changes of pH₀ from 3.9–5.7 to 5.3–6.7, 2.6–3.7 and 3.3–4.5, respectively. These pH₀ changes indicate SOM and sesquioxides are masking mineral surfaces and are factors responsible for lowering and increasing pH₀ values, respectively. Regression correlation (R² = 0.87^??) showed that for every 1% organic C may decrease 1.0 unit of pH₀ value. The pH₀ values, after SOM removal, are in the order of Sungai Mas ～ Segamat > Kuantan Series. This suggests that the serpentinite and andesite-derived soils have achieved a relatively similar degree of chemical weathering and they are more weathered than the basalt-derived soil. The charge measured by corrected back-titration is 1.5–3.8 cmol_c kg^? 1 at pH 4.5 and increases to 4.2–10.8 cmol_c kg^? 1 at pH 6.5, indicating that the three Oxisols mainly bear variable charge. Charge overestimation resulted from dissolution and hydrolysis reactions during potentiometric titration ranges from 36 to 160%, depending on pH values (the lower the pH the higher is the overestimation). Hence, back-titration is a reliable technique to correct charge overestimation when using the traditional potentiometric titration for highly weathered tropical soils.     

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.