Soil Clay Minerals Research Articles

Three-Year Soil Carbon and Nitrogen Responses to Sugarcane Straw Management

Green harvest sugarcane management has increased soil organic C and N stocks over time. However, emerging sugarcane straw removal to meet increasing bioenergy demands has raised concerns about soil C and N depletions. Thus, we conducted a field study in southeast Brazil over nearly three years (1100 days) for assessing soil C and N responses to increasing sugarcane straw removal rates. In order to detect the C input as a function of the different amounts of straw over three years, a field simulation was performed, where the original soil layer (0–0.30 m) was replaced by another from an adjacent area with low total C and δ13C. The treatments tested were as follows: (i) 0 Mg ha−1 year−1 (i.e., 100% removal), (ii) 3.5 Mg ha−1 year−1 (i.e., 75% removal), (iii) 7.0 Mg ha−1 year−1 (i.e., 50% removal), (iv) 14.0 Mg ha−1 year−1 (i.e., no removal), and (v) 21.0 Mg ha−1 year−1 (i.e., no removal + extra 50% of the straw left on the field). The results showed that sugarcane straw removal affected the soil C and total N pools. In the first 45 days of straw decomposition, a small but important straw-derived C portion enters into the soil as dissolved organic carbon (DOC). The lower the straw removal rate, the higher was straw-derived DOC content found into the soil, down to 0.50 m depth. After 3 years of management, keeping sugarcane straw on soil surface significantly increased C and N stocks within surface soil layer (0–0.025 m). Our findings suggest that under no straw removal management (i.e., 14 Mg ha−1), approximately 364 kg ha−1 of C and 23 kg ha−1 of N are annually stored into this low-C soil. The contribution of the straw-derived C (C-C4) to the total soil C increases over time, which accounted for about 60% under no straw removal rate. The greatest contribution of the C storage preferentially occurs into the fraction of organic matter (< 0.53 μm) associated with soil clay minerals. We concluded that indiscriminate sugarcane straw removal to produce cellulosic ethanol or bioelectricity depletes soil C stocks and reduces N cycling in sugarcane fields, impairing environmental gains associated with bioenergy production. Therefore, this information, linked with other agronomic and environmental issues, should be taken into account towards a more sustainable straw removal management for bioenergy production in Brazil.

Game Changer in Soil Science. Functional role of clay minerals in soil

AbstractDefining soil clay minerals as all inorganic particles &lt; 2 µm would enhance their predictive power for soil functions. The future task is to unravel the prominent functional role of clay associations in the “ultimate particles” in soils–microaggregates. High energy techniques and decomposition of XRD patterns offer the prospect of understanding associations of silicates, oxides and organic matter within microaggregates.

Visible and Near-Infrared Reflectance Spectroscopy for Investigating Soil Mineralogy: A Review

Clay minerals are the most reactive and important inorganic components in soils, but soil mineralogy classifies as a minor topic in soil sciences. Revisiting soil mineralogy has been gradually required. Clay minerals in soils are more complex and less well crystallized than those in sedimentary rocks, and thus, they display more complicated X-ray diffraction (XRD) patterns. Traditional characterization methods such as XRD are usually expensive and time-consuming, and they are therefore inappropriate for large datasets, whereas visible and near-infrared reflectance spectroscopy (VNIR) is a quick, cost-efficient, and nondestructive technique for analyzing soil mineralogic properties of large datasets. The main objectives of this review are to bring readers up to date with information and understanding of VNIR as it relates to soil mineralogy and attracts more attention from a wide variety of readers to revisit soil mineralogy. We begin our review with a description of fundamentals of VNIR. We then review common methods to process soil VNIR spectra and summary spectral features of soil minerals with particular attention to those &lt;2 μm fractions. We further critically review applications of chemometric methods and related model building in spectroscopic soil mineral studies. We then compare spectral measurement with multivariate calibration methods, and we suggest that they both produce excellent results depending on the situation. Finally, we suggest a few avenues of future research, including the development of theoretical calibrations of VNIR more suitable for various soil samples worldwide, better elucidation of clay mineral-soil organic carbon (SOC) interactions, and building the concept of integrated soil mapping through combined information (e.g., mineral composition, soil organic matter-SOM, SOC, pH, and moisture).

Mutual Adsorption of Lead and Phosphorus onto Selected Soil Clay Minerals

Mutual adsorption of lead (Pb) and phosphorus (P) at pH 5 onto three soil clays materials (kaolinite, montmorillonite, and allophane) was studied to know interaction of the anion and the cation at surface of the clays. Adsorption of Pb was determined on montmorillonite, kaolinite and allophane with the following pretreatments; 1) untreated clay (control), 2) phosphate treated clay (P-clay) and 3) clay pre-treated with both P and Pb (P-Pb-clay). Adsorption of P was determined on montmorillonite, kaolinite and allophane with the following pretreatments; 1) control 2) Pb treated clay (Pb-clay) and 3) P-Pb-clay. The adsorption of Pb on the untreated clays was in the order: montmorillonite > allophane > kaolinite. On allophane and kaolinite Pb adsorption was in the order P-clay > P-Pb-clay > control. For montmorillonite, the trend was: P-Pb-clay = control > P-clay. Phosphorus adsorption was in the order Pb-clay = P-Pb-clay > control for montmorillonite and kaolinite, Pb-clay > control > P-Pb-clay for allophane. The findings suggested that pre-treatment with phosphate increases Pb adsorption on kaolinite and allophane, and decrease on montmorillonite, while pretreatment with Pb increases phosphate sorption on all clays, and both Pb and P increased adsorption on montmorillonite and kaolinite and decrease on allophane.

Distribution of Clay Minerals in Yellow Soils with Respect to Parent Rocks: An Example from the Bukcheon-myeon, Hadong-gun, Korea

Distribution of Clay Minerals in Yellow Soils with Respect to Parent Rocks: An Example from the Bukcheon-myeon, Hadong-gun, Korea

A novel multi-reaction model for kinetics of Zn release from soils: Roles of soil binding sites

The kinetics of Zn release from soil is a key process affecting the fate and bioavailability of Zn in soil, which may be complicated by multiple soil binding sites and reactions during the Zn release process. In this study, we developed a novel quantitative model for the dynamic Zn speciation in soil during the Zn release process, by integrating the mechanistic equilibrium model WHAM 7 and molecular level investigation of Zn speciation with X-ray absorption spectroscopy (XAS). The kinetics model specifically considered Zn reactions with soil organic matter (SOM), iron (hydr)oxides, and clay minerals, and dissolution of Zn precipitates. Batch and replenishment experiments were conducted to study kinetics of Zn release from laboratory-spiked soils and XAS was employed to identify soil Zn speciation. Overall, the model described the experimental data reasonably well with minimal model fitting parameters. We quantitatively assessed the dynamic changes of Zn speciation in different soil components and SOM binding sites at various time scales and reaction conditions. Our kinetics model provides a quantitative framework for predicting the dynamic behavior of Zn in soil environments.

Soil property trends and classification of alluvial floodplains, South Carolina Coastal Plain

Alluvial soils derived from sediments of Piedmont origin occur in the Atlantic Coastal Plain of the southeastern U.S. In South Carolina, high-order rivers receiving sediment are Savannah (SV), Congaree (CN), Wateree (WA), Santee (ST), Lynches (LY), and Great Pee Dee (PD). This study investigated distribution of soil properties on natural levee, valley flat, and backswamp landforms within and among the six river floodplains and compared results to the taxonomic classifications used historically during soil survey production. This study also investigated the contribution of sediment sources to soil characteristics. Data were compiled for 65 soil pedons sampled from 16 soil survey areas during a 41-year time period. Pedons were located in three Major Land Resource Areas (MLRAs), and the results should aid in development of both land resource area and ecological site concepts. Data analyses centered on clay and silt contents, mineralogy, base saturation (BS), and cation exchange activity (CEA) classes. Evaluation of data from the six river floodplains revealed common properties among soils on fluvial landscapes across multiple MLRAs. Four soil systems, each derived from a predominant rock type, composed the Piedmont drainage basin of each river. The absolute area of each soil system differed between rivers, but sediment supplied typically did not result in great soil texture variation per landform. The land area proportion of the Carolina Slate Belt soil system had a close relationship to silt contents of the alluvial soils. On the valley flat landform of five floodplains, particle-size control section (PSCS) median clay content ranged only between 302gkg−1 and 405gkg−1. Among all valley flats median PSCS silt content ranged between 361gkg−1 and 511gkg−1. Slight positive downriver trends for clay were observed on SV, WA, and PD and for silt on SV and WA. Although local BS variation could be high between pedons, slight positive downriver BS trends on valley flats were observed on SV, WA, and ST. Negative BS trends were seen on LY and PD. Kaolinite was the dominant clay mineral in soils of all landforms on the six floodplains. Weatherable primary minerals in sand and silt fractions resulted in higher BS except for the LY soils, where resistant minerals composed sand and silt fractions. The semiactive CEA class occurred predominantly on all three landforms. Taxonomic analysis supported soil assignment to the suborders Aquepts or Udepts on all floodplains. Great group level Eutrudepts in South Carolina are first reported here, and they occur extensively enough on SV, CN, WA, and PD to warrant recognition in soil map units. Because most soil properties showed little variability for soils occurring in floodplains for all six rivers, botanical assessments will be needed to determine the magnitude of the role the properties have toward defining ecological sites. Ultimately, the alluvial systems may be aggregated as a single land resource area within the Coastal Plain.

Behavior of ammonium adsorption by clay mineral halloysite

Ammonium pollution becomes severe during mining of ionic rare earth-ores in southern China. As one of the main clay minerals in soils of ionic rare earth mines, halloysite plays an important role in ammonium adsorption. In this study, the saturated adsorption capacity, factors affecting adsorption and adsorption kinetics of halloysite for ammonium were investigated. The results indicated that the ammonium adsorption of halloysite was saturated with 1.66 mg/g at 303 K, pH of 5.6 and initial ammonium concentration of 600 mg/L (about half of the actual initial in-situ leaching concentration). When the initial concentration of –N, pH values and temperatures (288 K to 313 K) increased, the ammonium adsorption capacity of halloysite increased. The ammonium isothermal adsorption of halloysite matched the Langmuir and Freundlich isotherms. The adsorption process of ionic rare earth mining soils for ammonium was favorable. And the adsorption process followed closely the pseudo-second kinetic equation.

Impacts of long-term nitrogen fertilization on acid buffering rates and mechanisms of a slightly calcareous clay soil

Acidification of cropland soils is a serious problem in China that may cause long term pH decline, which threatens the sustainability of soil fertility and crop yields. The objective of this research was to investigate those rates and mechanisms for a slightly calcareous soil. The field data were based on selected treatments from a long-term (1991–2015) N fertilization experiment at a rice-wheat rotation on a slightly calcareous (pH7.7), young and hardly weathered clay soil (Regosol), including two plots with urea and NH4Cl fertilization and a control plot (no N fertilization). Urea and NH4Cl were applied at annual rates of 300kg (1991–1996) and 285kg (1997–2015) N ha−1. Soil pH and exchangeable Ca2+ and Mg2+ declined in the first 12years for the urea treatments, but remained constant thereafter. For the NH4Cl and control treatment, the exchangeable base cations showed an oscillating decrease over the 25years. Soil pH and exchangeable Ca2+, and Mg2+ levels were significantly (P<0.05) lower in the NH4Cl treatment plot than the control. After 25years, soil pH in the plots receiving urea and NH4Cl were 0.9 and 2.0 units lower than the control, respectively, while the total exchangeable base cations were about 10% and 16% lower, respectively. Carbonate dissolution, base cation exchange and mineral weathering were the main H+ buffer pathways of the Regosol. Long-term N fertilization favored the formation of smectite and vermiculite and accelerated the weathering of mica, potassium feldspar, and illite. This was more evident in the NH4Cl treatment than in the urea treatment. Consequently, the soil acid-buffering capacity was slightly enhanced after year 13 in the order of urea>control>NH4Cl. The increased weathering of soil primary minerals and clay minerals was the main reason that soil pH and exchangeable base cations stayed constant after about 12years.

More aggressive sequential extraction procedure to access stable forms of Pb and As in clay minerals of soils

Lead (Pb) and arsenic (As) are potentially toxic in their most reactive forms, but can also occur as structural and specifically adsorbed (more stable forms) ions on clay minerals. Due to their lower mobility in soils, structural and specific adsorbed forms are usually neglected in environmental studies, but can be important, for example, in geochemistry researches. The aim of this study was to evaluate the occurrence of stable forms of Pb and As in the clay fraction of soils in a region rich in Pb ore (galena) in south Brazil using more aggressive sequential extractions (boiling 5molL−1 NaOH, successive citrate-bicarbonate-dithionite steps, and sodium bisulfate fusion). X-ray diffraction was used to follow possible mineralogical variations after each sequential extraction. We sampled soils from two areas rich in trace elements, with high occurrence of mineral deposits (PbS) in carbonate and phyllite/mica schist rocks. A reference soil formed on mineralized granite/gneiss was also sampled. In different mineral phases of the sequential extraction, preferential association of Pb with minerals extracted by 5molL−1 NaOH (kaolinite, low crystalline smectite and precipitated of Pb-S and Pb-P) was observed. In soils formed from parent materials at the phyllite/mica schist metamorphic stage, Pb contents solubilized by NaOH were extremely high (maximum >28,000mgkg−1). Lead exists in the mineral structure, such as Fe oxides, as result of the isomorphic substitution of Fe for Pb. Manganese oxides were also important host of Pb in the clay fraction. The largest As contents occurred at greater depth in the soil. The more aggressive sequential extraction procedure is more suitable to estimate the structural and specific adsorbed forms of Pb and As in clay minerals.

Adsorption of tungstate (WO4) on birnessite, ferrihydrite, gibbsite, goethite and montmorillonite as affected by pH and competitive phosphate (PO4) and molybdate (MoO4) oxyanions

The knowledge about adsorption characteristics of tungstate (WO4) on soil mineral components is critical to elucidate WO4 mobility in the soil and aqueous systems. The objectives of this study were to characterize WO4 adsorption on selected soil clay minerals (gibbsite, ferrihydrite, goethite, birnessite and montmorillonite), and to investigate the competition in adsorption of WO4 with PO4 and MoO4 on these clay minerals as affected by pH. Based on an adsorption isotherm study, adsorption affinity for WO4 follows the order of gibbsite=Fe (oxyhydr)oxides>birnessite>montmorillonite in acid to alkaline pH ranges. The adsorption maximum of WO4 was greater in the acid pH for all minerals with an exception of ferrihydrite whose adsorption characteristics were independent on solution pH. We found that WO4 has higher affinity for clay minerals than PO4 under acid pH, and than MoO4 under alkaline pH. This is evidenced by the result that, by adding WO4 and oxyanions with the same molar ratio, more WO4 than PO4 and MoO4 was usually adsorbed on soil minerals in acid and alkaline pH, respectively. In a case of gibbsite, for example, the adsorbed WO4/adsorbed PO4 molar ratio was 1.10 at pH4, and 0.49 at pH8, whereas the adsorbed WO4/adsorbed MoO4 molar ratio was 1.16 at pH4, and 8.19 at pH8. Unlike other clay minerals, birnessite and montmorillonite showed a unique adsorption behavior that the presence of PO4 and MoO4 in solution enhanced the WO4 adsorption along with the increase of pH. The information reported herein provides for better understanding of WO4 adsorption on major soil clay minerals in relation to the solution pH and competitive oxyanions.

A Critical Review on Occurrence of Fluoride and Its Removal through Adsorption with an Emphasis on Natural Minerals

Being the most reactive of all chemical elements and the lightest member of halogen group, fluorine is found in the environment as fluoride. Both natural and anthropogenic activities are responsible for fluoride contamination in groundwater. Fluoride has dual effect on human health. While lower concentration ( 1.5 mg/L) leads to skeletal fluorosis and even death. The present review paper is aimed at providing detailed occurrence of fluoride pollution around the globe and in India. Among the different defluoridation techniques to remove excess fluoride from contaminated drinking water at both community and domestic levels, adsorption is found to be very effective due to its technical feasibility, simple characteristics and comparative low-cost nature. Various adsorbents have been tested for their ability to treat fluoride contaminated water, viz., activated carbon, activated alumina, soil, clay and a variety of waste materials. Since fluoride pollution of drinking water is a major concern of poor people as they cannot afford to spend on purification of water, usage of low-cost natural mineral (natural mineral) as adsorbent for fluoride removal is one of the most essential issues in modern era. The present review bestows a detailed discussion on natural mineral as adsorbent used in defluoridation process with special emphasis on soil and low-cost clay minerals.

Radiocesium interception potential of agricultural soils in northeast Japan

ABSTRACTThe health risks associated with the long-term intake of radiocesium from food have been of great concern since the Fukushima Daiichi Nuclear Power Station accident in 2011. Therefore, evaluating the phytoavailability and mobility of radiocesium in soil and determining the ability of soil to fix radiocesium are important research topics. The capacity and selectivity for 137Cs sorption in soil is evaluated on the basis of the radiocesium interception potential (RIP). Here, we measured the RIP of 925 farmland soil samples collected from Fukushima Prefecture and surrounding regions and plotted the values on a soil classification map and a surface geological map. The RIP values ranged from 73 to 12700 mmol kg−1, and a wide range was also observed among samples with identical soil types and geological features; however, the RIP values were significantly low for Andosols (P ≤ 0.05). In addition, soils that did not contain vermiculite, illite and smectite had low RIP values. All measured RIP values were negatively correlated with the total C content and phosphate absorption capacity and positively correlated with the clay, silt, and exchangeable K and Ca content (P ≤ 0.01). However, the relationship between the RIP and the measured soil properties, except for the exchangeable K content, is strongly dependent on the soil classification and clay mineral composition. Therefore, the soil properties that most significantly influence the RIP values are probably differences in the soil parent materials, which influence the soil classification and clay mineral composition.

Bacillus subtilis biofilm development in the presence of soil clay minerals and iron oxides

Clay minerals and metal oxides, as important parts of the soil matrix, play crucial roles in the development of microbial communities. However, the mechanism underlying such a process, particularly on the formation of soil biofilm, remains poorly understood. Here, we investigated the effects of montmorillonite, kaolinite, and goethite on the biofilm formation of the representative soil bacteria Bacillus subtilis. The bacterial biofilm formation in goethite was found to be impaired in the initial 24 h but burst at 48 h in the liquid–air interface. Confocal laser scanning microscopy showed that the biofilm biomass in goethite was 3–16 times that of the control, montmorillonite, and kaolinite at 48 h. Live/Dead staining showed that cells had the highest death rate of 60% after 4 h of contact with goethite, followed by kaolinite and montmorillonite. Atomic force microscopy showed that the interaction between goethite and bacteria may injure bacterial cells by puncturing cell wall, leading to the swarming of bacteria toward the liquid–air interface. Additionally, the expressions of abrB and sinR, key players in regulating the biofilm formation, were upregulated at 24 h and downregulated at 48 h in goethite, indicating the initial adaptation of the cells to minerals. A model was proposed to describe the effects of goethite on the biofilm formation. Our findings may facilitate a better understanding of the roles of soil clays in biofilm development and the manipulation of bacterial compositions through controlling the biofilm in soils.

Soil variability description using Fourier transform mid-infrared photoacoustic spectroscopy coupling with RGB method

Fourier transform infrared photoacoustic spectroscopy (FTIR-PAS) has been successfully applied in recent years to predict soil properties due to its advantages as a rapid, non-destructive, and well-rounded characterization method. In this study, we investigated the potential of FTIR-PAS to characterize soil heterogeneity in an agricultural field in Lishui (China). On the basis that each FTIR-PAS spectrum represents a well-rounded soil characteristic from each sample, we aimed to explore a method to convert the spectral lines (multidimensional matrix) into data that can be used in soil mapping while retaining maximum soil information to display soil heterogeneity maps according to kriging interpolations. The Mahalanobis distance (MD) and red-green-blue tricolor (RGB) methods were employed to convert the soil information. A set of diverse soil samples (n=597) was scanned with a FTIR-PAS spectrometer in the mid-infrared range (4000–400cm−1). The peaks showing soil organic matter content (CO from carboxylic acids at 1755cm−1; COO, NH, and CN from carboxylates and amides at 1540cm−1), clay minerals (quartz overtones at 1953cm−1: carbonates, quartz, and iron oxides lower than 800cm−1), and water (O–H: 3700–3000cm−1 and 1700–1600cm−1, respectively). The first three PCs (85.05%) were extracted using principal component analysis (PCA). The results showed that both methods could convert the multidimensional matrix based primarily on absorbance bands associated with characteristic soil attributes. However, comparison of the maps suggested that the RGB method could translate information on spectral heterogeneity with higher accuracy than MD method which lost the direction of soil attributions. Therefore, FTIR-PAS spectra data combined RGB method might provide a supplemental method for use in agricultural modelling and soil management.

Impact of 60 Years of Intensive Rice Cropping on Clay Minerals in Soils Due to Si Exportation

Rice is cultivated as staple for over half of the World’s population. In Camargue (South of France) rice fields have been established on very young soils developed from historic fluvial deposits of the Rhône River. The comparison of clay mineralogy in a paddy field cultivated for 60 years and in a control shows a significant increase of the clay crystallinity in the paddy field soil, which implies a decrease of their solubility. In the paddy soils, phytoliths, poorly crystallized clays, such as smectite and to a lesser extent kaolinite, are progressively dissolved to supply Si for rice requirements. The sustainability of the crop system requires the clearing of silica exportations.

Soil Mineralogy of a Vernal Pool Catena in Southern California

Core Ideas Soil mineralogy, as with morphology, varies with slope position in the vernal pool watershed. Vernal pool soil mineralogy is determined by parent material, weathering, relative solubility of elements, and hydrology. Kaolin dominates on upper slope positions; smectite dominates on lower slope positions. Vernal pools are seasonal wetlands that are noted for their unique flora and fauna. Soils are critical to the hydrologic function of the vernal pools, but few studies have examined the mineralogy of vernal pool soils in detail. In this study, the soil mineralogy of a vernal pool catena on basalt in southern California was investigated to identify the mineral components, their origin, and their relationship to the soil hydrologic environment. Kaolin and smectite are found in the soil clay fraction throughout the catena. Kaolin, derived from plagioclase weathering, is the predominant clay mineral in the loamy upper slope soils. Excess silica and other soluble cations from the weathering of plagioclase and pyroxene are carried in throughflow to the lower slope positions where they precipitate primarily as smectite, producing clayey Vertisols. Minor amounts of quartz and biotite are present in the soils, apparently from eolian deposition. The biotite is interpreted to weather to vermiculite, and subsequently to a trioctahedral smectite, which is found in both upper and lower slope soils. Slope wash has transported soil material, including trioctahedral smectite and kaolin, from the upper slope to lower slope soils. Two dioctahedral smectite species are distinguished in the basin soils based on d(060) spacings and are interpreted to have neoformed and transformed from biotite or pyroxene. The mineralogy of the vernal pool catena is differentiated by pedogenic processes related to mineral weathering, relative solubility of released elements, slope wash, and soil hydrology.

Diversity of clay minerals in soils of solonetzic complexes in the southeast of Western Siberia

Data on the mineralogical composition of clay in soils of solonetzic complexes of the Priobskoe Plateau and the Kulunda and Baraba lowlands have been generalized. The parent materials predominating in these regions have loamy and clayey textures and are characterized by the association of clay minerals represented by dioctahedral and trioctahedral mica–hydromica, chlorite, kaolinite, and a number of irregular interstratifications. They differ in the proportions between the major mineral phases and in the qualitative composition of the minerals. Mica–hydromica and chlorites with a small amount of smectitic phase predominate on the Priobskoe Plateau and in the Kulunda Lowland; in the Baraba Lowland, the portion of mica–smectite interstratifications is higher. An eluvial–illuvial distribution of clay fraction in solonetzes is accompanied by the acid–alkaline destruction and lessivage of clay minerals, including the smectitic phase in the superdispersed state. This results in the strong transformation of the mineralogical composition of the upper (suprasolonetzic) horizons and in the enrichment of the solonetzic horizons with the products of mineral destruction; superdispersed smectite; and undestroyed particles of hydromica, kaolinite, and chlorite from the suprasolonetzic horizons. A significant decrease in the content of smectitic phase in the surface solodic horizons of solonetzic complexes has different consequences in the studied regions. In the soils of the Priobskoe Plateau and Kulunda Lowland with a relatively low content (10–30%) of smectitic phase represented by chlorite–smectite interstratifications, this phase virtually disappears from the soils (there are only rare cases of its preservation). In the soils of the Baraba Lowland developed from the parent materials with the high content (30–50%) of smectitic phase represented by mica–smectite interstratifications, the similar decrease (by 10–20%) in the content of smectitic phase does not result in its complete disappearance. However, the smectitic phase acquires the superdispersed state and the capacity for migration.

The 3D distribution of phyllosilicate clay minerals in western Victoria

The mineralogy of the clay fraction of soils is a major determinant of the behavior of soil. Conventionally these clay minerals have been determined using techniques such as X-ray Diffraction (XRD), but new complementary methods such as infrared spectroscopy can be used to rapidly and economically predict these minerals. This paper presents a methodology to predict these clay minerals at high-resolution that adhere to GlobalSoilMap (GSM) standards. Mid-infrared (MIR) spectroscopic models were formulated for clay minerals kaolinite, illite and smectite using partial least squares regression (PLSR) and legacy quantitative XRD determinations. Very strong models were achieved for kaolinite, illite and smectite and the root mean square error of cross validation (RMSECV) were all <12wt.%. Spectroscopic models were applied to 11,500 samples from western Victoria and harmonized to the GSM specified depth intervals using equal area splines. Clay minerals were then mapped using data mining model trees with a 10-fold cross validation to derive a mean prediction estimate and 90% prediction interval. Spatial models accounted for 26 to 77% of the total variation with kaolinite predictions for all 6 GSM depths≥65%. Kaolinite is the dominant soil clay mineral of western Victoria for uplands and weathered volcanic terrains. Illite concentrations are higher where associated with weathered granitic plutons and in aeolian deposits of semi-arid environments. Smectite tends to occur associated with depressions of plains (volcanic and sedimentary). Further supplementation of additional sites and samples for landscapes with relatively sparse observations should contribute to refined and improved maps of these clay minerals.

Adsorption-desorption behavior of dissolved organic carbon by soil clay fractions of varying mineralogy

Soil clay minerals significantly influence the accumulation and stabilization of organic carbon (OC). However, the effect of interactions among phyllosilicate clay minerals, native OC and sesquioxides (Fe/Al oxides) on the adsorption-desorption of dissolved organic carbon (DOC) under different background electrolyte types and concentration is poorly understood. A set of batch adsorption-desorption experiments were conducted using pedogenic clays extracted from soils dominated by kaolinite-illite (Kaol-Ill), smectite (Smec) and allophane (Allo). The clay samples were sequentially treated to remove native OC and sesquioxides, and tested for adsorption-desorption of DOC under various solution conditions. All the experiments were conducted at pH7 using water extractable fraction of OC from wheat residues. DOC adsorption increased with increasing background electrolyte concentration, and the presence of Ca2+ significantly enhanced the uptake in comparison to Na+ due to a possible cationic bridging effect. Under all electrolyte conditions, the maximum DOC adsorption capacity (Qmax) (mgg−1) of the soil clay fractions (SCF) maintained the order: Allo>Smec>Kaol-Ill. A similar order was also observed when the adsorption capacities were normalized to the specific surface area (SSA) of the SCFs (mgm−2). DOC adsorption showed a positive relationship with SSA, and sesquioxides and allophanic minerals provided the largest contributions to the SSA in the SCF. Removal of sesquioxides from the SCF resulted in a decrease in SSA and thus DOC adsorption, whereas removal of native OC increased the SSA and subsequent DOC adsorption. Because this study used pedogenic SCFs which represented soils formed in different environments instead of processed clays from geological deposits, it provided realistic information about the interaction of DOC with SCF in relation to their native OC and sesquioxide contents. It also revealed the importance of Ca2+ in enhancing the carbon adsorption capacities of these SCFs.