Hydroxy-interlayered Vermiculite Research Articles

Changes in soil mineralogy due to nitrogen fertilization in an agroecosystem

Additions of nitrogen (N) fertilizer to surface soil can trigger a wide array of complex effects, including changes in soil chemical, physical, and microbial properties which impact both the nitrogen and carbon cycles. While abundant literature exists regarding the influence of N amendments on processes and properties influencing the soil carbon cycle, there is little recognition given to potential changes in soil mineralogy. We collected surface soil (Maury silt loam, Typic Paleudalf) from a long-term agroecosystem under no-tillage management which has received annual inorganic N additions (0, 168, 336kgNha−1) for 34years and characterized the soil mineralogy and other relevant properties. With recent liming, N fertilizer addition was associated with only slight changes in soil pH and the suite of exchangeable cations. Silt mineralogy was not influenced by N, but x-ray diffraction found changes in the clay fraction. The clay mineral assemblage in the control (0kgNha−1) is complex, comprised of layers of pedogenic chlorite, vermiculite, hydroxy-interlayered vermiculite, mixed-layer chlorite–vermiculite, illite, kaolinite, and quartz. Fertilizer N additions resulted in a 3- and 3.3-fold decrease in the 1.4nm/1.0nm peak area ratios for Mg-clay slides in the 168 and 336kgNha−1 treatments. This decrease is likely due in part to fixation of NH4+ in vermiculite layers, corroborated by increases in the intensity of the infrared (IR) band at 1430cm−1 (assigned to the NH bending mode of NH4+) with added N. The role of biota emerged as shown by a negative relationship between water-soluble oxalate concentrations and the 1.4nm/1.0nm peak area ratios (r=0.56, P=0.06). Past acidification due to nitrification of added NH4+ which occurred prior to liming might have played a role in weathering of pH-sensitive minerals such as chlorite. Our findings raise questions about the capacity of reactive 2:1 mineral layers to fix NH4+ and how this is moderated by biota in no-till agroecosystems.

Biotite controlled chemical alteration of granodioritic–tonalitic saprock: Exploring the use of sieving to enhance compositional linear trends

The Peninsular Ranges, southern California, USA, commonly contains eroded sections of regolith composed of (1) saprock enclosing corestone or (2) alluvial deposits overlying plutonic/metamorphic basement. In this paper, we present a study focused on evaluating the chemical weathering patterns exemplified by type 1 regolith. In an attempt to enhance the coverage of altered samples about calculated compositional linear trends, the <45μm fractions were separated from saprock samples and were chemically analyzed.Though Alfisol is common within the study area, due to erosional exhumation, such soil is absent at the specific site sampled during this investigation, and colluvium directly overlies saprock and corestones. Mean annual precipitation ranges from 25cm–51cm with a mean annual air temperature of 13°C–18°C.In centered p(A), p(CN), and p(K) ternary space, A, CN, and K are compositions representing molar percentages of Al2O3, CaO*+Na2O, and K2O, respectively, in the sum of A, CN, and K. CaO* is the molar percent of CaO following correction for CaO in apatite. Centering preserves the geometrical relationships between plotted samples, but moves them to the center of the ternary diagram.Saprock samples plot about a linear compositional trend directed away from the p(K) apex. Using orthogonal projection, the average weathering intensity value, t, of the bulk saprock samples is 0.09 (±0.04). When the sieved <45μm fractions are considered along with the bulk saprock samples, a similar linear compositional trend is calculated, but sieved fractions plot closer to the A–CN join, and have an average t-value of 0.89 (±0.04). Hence, inclusion of the sieved fraction extended the coverage of altered samples about the weathering trend.Using Na as a reference frame, a statistically significant loss of 10% (+9/−8%) and 14% (+6/−5%) K and P mass, respectively, occurred during the development of saprock. The loss of K mass is attributed to the alteration of biotite to hydroxy-interlayered vermiculite and mixed-layer biotite/vermiculite, while the loss of P mass is due to the dissolution of apatite. In addition, statistically significant increases in Si (6%±4%) and LOI (56% (+37/−30%)) mass occurred. The increase in Si mass reflects either, or both, illuviation of small amounts of kaolinite into the prominent crack system observed at the study site, or the addition of pedogenic silica. Such increases are apparently the result of processes that were operative in the now erosionally removed section of Alfisol. The results of the above elemental mass changes produced a statistically significant increase in bulk mass of 5%±3%.The results of our work suggests that the inclusion of <45μm fraction chemical data can significantly enhance the coverage of calculated linear compositional trends, and could be a useful investigative tool for future studies.

Enrichment of cesium and rubidium in weathered micaceous materials at the Savannah River Site, South Carolina.

The enrichment of Cs and Rb relative to Ba, Sr, and K in three soils representing a range of soil maturities was determined to investigate the long-term sorption behavior of these elements in upland soils of the Savannah River Site (SRS). Elemental mass fractions normalized to upper continental crust (UCC) decreased in the order Cs > Rb > Ba > K > Sr in the soil fine fractions. Only the UCC-normalized amount of Cs was greater than unity. The UCC-normalized amounts in strong-acid extracts decreased as Cs > Rb > Ba > K ≈ Sr. In all three soil cores, the trends of the UCC-normalized amounts of acid-extractable metals were similar to trends of cation-exchange capacity (CEC) calculated from synchrotron-X-ray diffractometry measurements of soil mineralogy. Consequently, the relative enrichment of Cs and Rb is largely controlled by selective sorption to micaceous minerals, including hydroxy-interlayered vermiculite, that dominate the CEC. Where high clay content had caused retention of soil solution, amounts of acid extractable K, Sr, and Ba were enhanced. The retention of natural Cs by these three soils, which developed over many thousands of years, is a strong indicator that radiocesium will likewise be retained in SRS soils.

Experimental aluminization of vermiculite interlayers: An X-ray diffraction perspective on crystal chemistry and structural mechanisms

Natural aluminization of swelling clay minerals is ubiquitous in acidic soils leading to the formation of hydroxy-interlayered (HI) minerals. This process has drawn special attention over the last 4–5 decades owing to the negative impact on soil fertility of the induced reduction of cation exchange capacity. Combination of chemical analyses and of X-ray diffraction profile modeling on a series of experimentally self-aluminized samples aimed at an improved description of HI minerals and more especially of their interlayer crystal chemistry. Both exchangeable alkali cations and hydroxy (Al, Fe) cations coexist within HI vermiculite interlayers. The latter form discontinuous gibbsite-like interlayer sheets with ~15% completeness, despite layer-to-layer distances similar to that of chlorite. From charge compensation considerations, the isolated hydroxy (Al, Fe) cation clusters bear ~1.5 positive charges per cation. Analysis of X-ray diffraction data indicates that aluminization of initially swelling interlayers is a layer-by-layer process leading to mixed layers composed of randomly interstratified swelling and aluminized layers, all aluminized layers likely hosting a similar number of hydroxy (Al, Fe) cations along the aluminization process. This model contradicts the widely accepted description of HI minerals as a solid solution between expandable 2:1 clay and aluminous chlorite end-members. As a consequence, the proportion of HI layers in the mixed layer is a robust estimate of aluminization progress and both the amount of extractible (Al, Fe) and the extent of CEC decrease are positively correlated to this essential parameter.

Investigation of clay mineralogy in a temperate acidic soil of a forest using X-ray diffraction profile modeling: Beyond the HIS and HIV description

The crystal structures as well as the amount of hydroxy-interlayered clay minerals resulting from the Al-hydroxylation process in temperate acidic soil developed in a forest were studied by modeling experimental X-ray diffraction (XRD) patterns. The <2μm clay fraction of this soil was separated into four sub-fractions of different particle sizes (<0.05, 0.05–0.1, 0.1–0.2, and 0.2–2μm), and XRD patterns were obtained after different treatments of the samples (i.e., calcium saturation and air-drying, ethylene-glycol solvation, or potassium saturation and heating at 110°C). The qualitative analysis of the experimental XRD patterns revealed the presence of hydroxy-interlayered-smectite (HIS) and hydroxy-interlayered-vermiculite (HIV) phases commonly considered in the description of clay mineralogy in acidic soil environments. Quantitative information derived from the fitting procedure for the different sub-fractions allowed for the determination of the complex mineralogy of the <2μm clay fraction through the identification and quantification of ten clay phases. The results revealed that the HIS and HIV clay phases considered so far in the literature actually consist of three-component randomly interstratified mixed-layer minerals (MLMs) with contrasting relative proportions of smectite, hydroxy-interlayers (HI) and illite layers. Moreover, the quantitative results derived from this study showed that these two MLMs account for less than 50% of the total HI layer abundance. In that regard, most HI layers are present in illite-rich MLMs with poorly resolved XRD reflections. The contrasting behaviors of smectite and HI layers in the coarsest 0.2–2μm sub-fraction compared to those in the finer sub-fraction indicated the possible increase of the layer charge of the expandable layers in the coarsest sub-fraction. Finally, it was shown that the detailed structural analysis derived from the methodology used in the present study is conducive for estimating the degree of expandable layer transformation in natural soil samples while reconciling the specific behaviors of expandable and HI layers from natural soil with those described by laboratory experiments performed on monomineralic samples.

Vertical migration of radiocesium and clay mineral composition in five forest soils contaminated by the Fukushima nuclear accident

In forest soils contaminated by radiocesium (134Cs and 137Cs), deposition from the Fukushima nuclear accident, clay minerals might play important roles in long-term cesium (Cs) dynamics through sorption. To determine whether radiocesium can be retained within the organic layer and the upper mineral soil layers in the Fukushima region, we investigated the vertical distribution of 134Cs and 137Cs and the clay mineral composition in five soil profiles of varying radiocesium deposition levels and vegetation types. X-ray diffraction analyses and oxalate extraction suggested that hydroxy-interlayered vermiculites and short-range-ordered aluminum (Al) and iron (Fe) compounds (i.e, allophane and ferrihydrite) were major clay mineral species of the upper soil layers. The vertical soil distribution of 134Cs and 137Cs suggested that most of them were retained in the organic layer and upper mineral soil layer under different levels of deposition. Within 1.5 years after the accident, both 134Cs and 137Cs were leached from the organic layer, and most of these (59–73%) were accumulated in the upper soil layer (0–5 cm). The proportion of 137Cs (or 134Cs) leaching from the organic layer was greater at sites receiving greater amounts of precipitation. The substantial accumulation of 137Cs in the upper soil layer, irrespective of the 137Cs deposition level or clay mineral composition, suggests that sorption capacities of clays and organic matter are sufficiently high to retain 137Cs in the surface soil during at least the initial stage of contamination.

Soft X-ray spectromicroscopy study of mineral-organic matter associations in pasture soil clay fractions.

There is a growing acceptance that associations with soil minerals may be the most important overarching stabilization mechanism for soil organic matter. However, direct investigation of organo-mineral associations has been hampered by a lack of methods that can simultaneously characterize organic matter (OM) and soil minerals. In this study, STXM-NEXAFS spectroscopy at the C 1s, Ca 2p, Fe 2p, Al 1s, and Si 1s edges was used to investigate C associations with Ca, Fe, Al, and Si species in soil clay fractions from an upland pasture hillslope. Bulk techniques including C and N NEXAFS, Fe K-edge EXAFS spectroscopy, and XRD were applied to provide additional information. Results demonstrated that C was associated with Ca, Fe, Al, and Si with no separate phase in soil clay particles. In soil clay particles, the pervasive C forms were aromatic C, carboxyl C, and polysaccharides with the relative abundance of carboxyl C and polysaccharides varying spatially at the submicrometer scale. Only limited regions in the soil clay particles had aliphatic C. Good C-Ca spatial correlations were found for soil clay particles with no CaCO3, suggesting a strong role of Ca in organo-mineral assemblage formation. Fe EXAFS showed that about 50% of the total Fe in soils was contained in Fe oxides, whereas Fe-bearing aluminosilicates (vermiculite and Illite) accounted for another 50%. Fe oxides in the soil were mainly crystalline goethite and hematite, with lesser amounts of poorly crystalline ferrihydrite. XRD revealed that soil clay aluminosilicates were hydroxy-interlayered vermiculite, Illite, and kaolinite. C showed similar correlation with Fe to Al and Si, implying a similar association of Fe oxides and aluminosilicates with organic matter in organo-mineral associations. These direct microscopic determinations can help improve understanding of organo-mineral interactions in soils.

Interactions of Radioactive and Stable Cesium with Hydroxy-Interlayered Vermiculite Grains in Soils of the Savannah River Site, South Carolina, USA

AbstractSorption and fixation of Cs by the upland soils of the US Department of Energy’s Savannah River Site (SRS) have been attributed to micaceous grains consisting mostly of hydroxy-interlayered vermiculite (HIV). Results of experiments to characterize SRS soil samples, to examine aspects of their radiocesium sorption, and to determine how much of their natural Cs is accessible for chemical extraction and isotope dilution are presented in support of mechanistic hypotheses to explain Cs sorption and fixation in HIV grains. The HIV is responsible for most of the soil cation exchange capacity, and concentrations of naturally occurring Cs, Rb, and K in soil samples are closely related to the concentration of HIV. Experiments with 137Cs to examine (1) sorption kinetics, (2) blocking of exchange sites with silver thiourea, and (3) susceptibility of sorbed 137Cs to chemical extraction, support the idea that added Cs is sorbed at different kinds of cation exchange sites in HIV grains. Sites highly selective for Cs but relatively few in number are inferred to exist in interlayer wedge zones within such grains. Little of the naturally occurring Cs in the soil samples was extractable by chemical agents that would remove Cs from ordinary cation-exchange sites and from within non-silicate soil components. Furthermore, most of the natural Cs was inaccessible for isotope dilution under slightly acidic conditions approximating the natural soil environment. These observations support the idea that most of the Cs in these soils has become effectively fixed in the narrower parts of interlayer wedge zones. Control of Cs uptake and fixation by highly Csselective interlayer wedge sites would account for the large distribution coefficients found for 137Cs at the low aqueous Cs concentrations typical of environmental systems and also for the relatively large concentrations of stable Cs in the SRS soils.

Pedogenic alteration of illite in subtropical China

AbstractPedogenic alteration of illite from red earth sediments in Jiujiang in subtropical China was investigated using X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). Illite, hydroxy-interlayered vermiculite (HIV), kaolinite and mixed-layer illite-HIV (I-HIV) are present in the soils. The characteristic reflections of the clay phases were 14 Å, 10–14 Å, 10 Å, and 7 Å, respectively. After Mg-glycerol saturations, the 14 Å peak of the samples did not expand, and after heating at 350°C and 550°C it shifted to 13.8 Å and 12 Å respectively, with no residual 14 Å reflection, suggesting the occurrence of hydroxy-interlayered vermiculite. The randomly interstratified I-HIV clays were characterized by a broad peak at 10–14 Å, which did not change its position after Mg-glycerol saturation, but collapsed to 10 Å after heating at 350°C and 550°C. HRTEM analysis showed different lattice fringes of 12 Å, 10 Å and 7 Å . Mixed-layer I-HIV, HIV-K and illite-kaolinite (I-K) were observed in the HRTEM images which represented the intermediate phases during illite alteration. The merging of two 10 Å illite layers into a 12 Å HIV layer, lateral transformation of one HIV layer into one kaolinite layer and alteration of one illite layer into two kaolinite layers illustrated the mechanisms of illite-to-HIV, HIV-to-kaolinite and illite-tokaolinite transformation, respectively. The proposed pedogenic alteration of illite and the weathering sequence of the clay minerals in Jiujiang is illite → I-HIV → HIV → HIV-K → kaolinite. In addition, illite may transform directly to kaolinite.

Formation of hydroxy-interlayer vermiculite (HIV) in rhizosphere soils under German camomile cultivation and manure application

AbstractThis study examines the effect of German camomile (Matricaria camomilla L.) cultivation on the weathering of minerals present in the clay fraction of five different soil series modified or not unmodified (control) with cattle manure. A factorial experiment was performed in a randomized complete block design (RCBD) with three replications. At harvest time the rhizosphere soil was separated from bulk soil and the mineralogy was examined by X-ray diffraction (XRD). Trioctahedral chlorite transformed to hydroxy-interlayer vermiculite (HIV), while kaolinite disappeared. The presence of HIV was identified by an increase in the intensity ratio of the 1.0 and 1.4 nm peaks after Na-citrate pretreatment and K-saturation. It is suggested that Al3+ is released during dissolution of kaolinite and subsequently enters into the interlayer sheet of chlorite, and consequently the brucite layer disappears. This was verified by a significant Mg2+ increase in the soil solution in three out of five experiments at harvest time compared to pre-planting. The pots amended with manure showed the same changes as the pots without manure. The results of this study may explain the presence of unexpected clay minerals in regions with unsuitable conditions for their formation.

Characterisation of the hydroxy-interlayered vermiculite from the weathering of illite in Jiujiang red earth sediments

The clay mineralogy and formation of hydroxy-interlayered vermiculite (HIV) in the Jiujiang red earth sediments were investigated using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC) and inductively coupled plasma-atomic emission spectrometer (ICP-AES) analyses. The 1.4-nm peak of HIV did not change after Mg2+ saturation and glycol solvation, but it exhibited partial collapse to 1.0 nm after K+ saturation followed by heat treatment at successively higher temperatures. HIV was also characterised by FTIR adsorption bands at ~3485 cm–1 and ~3415 cm–1, which did not change with increasing temperature. DSC analysis revealed that the dehydroxylation of hydroxides in the interlayer of HIV began at ~400°C, and a further dehydroxylation was confirmed by the XRD of the sample heated to ~600°C. The ICP-AES analysis of sodium citrate extracts showed that the Al concentration was higher than that of Fe, indicating that the Al was probably present as hydroxy-Al in the interlayer of HIV. The presence of hydroxy-Al polymers in the interlayer influenced both expandability and thermal properties of HIV clays from Jiujiang red earth sediments.

Hydrophysical properties of Humic Latosols from Brazil

A b s t r a c t. The hydrophysical properties of the prevalent Humic Latosols (organic matter rich and charcoal stained soils) were related to structural sustainability under loading. Intact cores collected at the Ap, AB, Bw horizons were used for hydrophysical characterization. Precompression stresses at 10 suctions were obtained to estimate the load bearing capacities. We observed the dominance of kaolinite with some occurrences of gibbsite and hydroxy-interlayered vermiculite in the clay mineralogy. The high organic matter content in the Ap horizon favours crumb structure with the structural unit presenting high porosity and water retention. The structure of the AB and Bw horizons was, however, granular with structural units having low porosity. Possible influence of earlier incidences of fire enhanced the organic mat ter and carbon content in the soil reducing down the profile from 42.5 g kg -1 at the Ap to 16.4 g kg

Surface charge properties of kaolinite from Thai soils

Kaolinite is the major clay mineral in many Thai soils and the charge characteristics of soil kaolinites have not been properly evaluated. Soil samples from sub-soil horizons of fourteen Oxisols and Ultisols from Thailand were taken for this study. Clay fractions were separated from the soils, and free iron oxides and organic matter were removed to obtain soil kaolinites. Surface charge properties of soil kaolinites and a reference kaolinite (KGa-2) were evaluated by various procedures. Permanent negative charge (−σp) was determined by cesium adsorption method. Point of zero charge (PZC) was measured by ion adsorption, potentiometric titration and electrophoretic mobility methods. X-ray diffraction analysis showed that kaolinites isolated from the Thai soils contain traces to small amounts of quartz, anatase, gibbsite, hydroxy-Al interlayered vermiculite (HIV) and illite. The value of −σp of soil kaolinites varied between 34.9 and 145.3mmolc kg−1; excluding a few anomalous values for the kaolinites with impurities of 2:1 clay minerals, soil kaolinites have an average −σp of 52mmolc kg−1. The −σp on soil kaolinites was positively correlated with the specific surface area (SSA) of the samples. All kaolinites exhibited increasing negative charge and decreasing positive charge with increasing equilibrium solution pH confirming the presence of variable charge on the mineral surface. The PZNC values of soil kaolinites varies from 2.1–3.1, except for two samples where it could not determined as the CEC values were greater than the AEC values at all pH values. Potentiometric titration curves for all kaolinites exhibited variable net proton charge (ΔH−ΔOH or σH) with changing equilibrium solution pH from 2 to 10. The PZSE values were slightly higher than the PZNC values of the soil kaolinites, and varied between 2.3 and 2.9. The zeta potential of all kaolinites became more negative with increasing solution pH (2 to 6) but not low enough to reach a positive value for all but one soil kaolinites; hence the IEP of soil kaolinites could not be obtained. The PZC of kaolinite from Thai soils are at lower pH than the natural pH of these soils, thus soil kaolinites carry a net negative charge under natural conditions and they have some limited ability to adsorb cations. Changes in soil pH from soil acidification, liming or addition of any other amendments or practices can substantially change the cation- or anion-exchange capacity of soil kaolinites, which may influence the availability of nutrient elements in the soils.

Hydroxy-interlayered vermiculite genesis in Jiujiang late-Pleistocene red earth sediments and significance to climate

To understand the paleoclimatic significance of the red earth sediments in Jiujiang, southern China, hydroxy-interlayered vermiculite (HIV) in the deposits was investigated using X-ray diffraction (XRD), chemical extraction, and high resolution transmission electron microscopy (HRTEM). The XRD results indicated that the clay mineral assemblages of the sediments are mainly illite, kaolinite, HIV, vermiculite, with minor illite-HIV mixed-layer clays. The 1.41-nm reflection did not expand after Mg saturation and glycerol solvation, but it shifted to 1.38nm and 1.20nm when the samples were heated to 350°C and 550°C, respectively. The 1.41-nm spacing showed partial collapse after K+ saturation at 80°C in association with the reinforcement of the 1.0nm reflection. Collapse of the HIV (001) spacing was related to the release of Al, Mg, and Ca from the interlayer after KCl extraction. The Al was probably present as hydroxy-Al in HIV interlayers before being released. The HRTEM image revealed 1.4-nm lattice fringes interstratified with illite 1.0nm fringes and two illite lattice fringes merged into one HIV fringe. This suggests that the HIV clays in Jiujiang soils resulted from illite weathering. The shift of a small X-ray diffraction reflection between 1.0 and 1.4nm for an air-dried and glycerol-solvated sample to 0.997nm after heating at 550°C indicated the presence of an irregularly interstratified illite–HIV mineral. The common occurrence of HIV minerals in the Jiujiang red earth sediments suggests a climate with frequent wet and dry cycles prevailed during late-Pleistocene time.

K-Ar Age Constraints on the Origin Of Micaceous Minerals in Savannah River Site Soils, South Carolina, USA

AbstractK-Ar measurements were used in this study of upland Savannah River Site soils to distinguish between sorbed K and the K remaining in remnants of primary minerals. Study of sorbed K contributes to understanding further the interaction of alkali metals (Cs in particular) with the soils. Primary mineral K and the associated radiogenic Ar were studied to characterize soil mica with respect to its provenance and its relationship to hydroxy-interlayered vermiculite. K-Ar age values of Na-saturated clay fractions from five samples of these soils range in age from 270 to 370 Ma. After a moderate acid treatment (6% HNO3 v/v, ~1 mol dm-3, 3 h, 80°C) of the clay fractions, K-Ar age values (270-325 Ma) were little changed on the whole, but they were more closely grouped near 300 Ma. Earlier work had shown that most of the K in these soils is found in material resistant to moderate acid extraction. The K-Ar age values show that this acid-resistant material is much older than any pedogenic minerals could be, even much older than the sedimentary parent rocks from which the soils were derived. These observations support earlier inferences by others that the K in these well leached soils is largely in remnants of primary muscovite from the parent sediments. Age values near 300 Ma suggest that the muscovite is largely from proximal Piedmont terranes of the Appalachian orogen, where the K-Ar relationship in most micas was set by Alleghanian tectonic processes late in the Paleozoic Era. The structural location of the K within mica, shown by the retention of the associated radiogenic Ar, is in contrast to the sorption-dominated behavior of the Cs and most of the Rb in these soils during pedogenesis. Stronger acid treatment (~6 mol dm-3 HNO3, 3 h, 100°C) extracted substantial fractions of both the K and the radiogenic Ar from bulk-soil portions, indicating destruction of some of the primary mica. K-Ar age values for the sand-rich bulk soils were not useful for this study because the sand contains excess radiogenic Ar, probably in sand-sized vein quartz.

Estimation of −1500 kPa Water Retention in Soil Survey

Water retention at −1500 kPa is a crucial parameter in soil survey interpretations and in many hydrologic, agronomic, and ecological applications. Because it is impractical to measure −1500 kPa water contents everywhere in a soil survey area, it is often estimated using a model that is considered rule of thumb. The objectives are to validate this model and determine if a superior model can be developed using general linear models. Validation results indicate a consistent bias between the measured and predicted values. The 1500 kPa model was found to overestimate the contribution of organic matter, and the 1500 kPa water-to-clay ratio (WCR) of 0.4 was shown to be in agreement with measured ratios from a wide range of soils. For soils dominated by specific clay minerals, the 1500 WCR ratios varied from 0.25 for soils containing gypsum to 0.50 for soil clays containing hydroxy-interlayered vermiculite. The accuracy of the model could be improved if the clay mineralogy is known and the corresponding 1500 WCR is used instead of 0.4. The development of a new model resulted in total clay, organic carbon, and cation exchange capacity explaining 91% of the variation in water contents retained at −1500 kPa. The new model had the lowest root mean square error of 1.967, indicating a more accurate model than just using total clay or total clay and organic carbon as predictor variables. On a wide range of soils, validation of the newly developed model indicated no bias and was superior to the rule of thumb model in accuracy. The new model will be useful in soil survey as an approximation when data are lacking and is not intended to replace measured values.

Long-Term Selective Retention of Natural Cs and Rb by Highly Weathered Coastal Plain Soils

Naturally occurring Cs and Rb are distinctly more abundant relative to K in the highly weathered upland soils of the Savannah River Site, South Carolina, than in average rock of Earth's upper continental crust (UCC), by factors of 10 and 4, respectively. Naturally occurring Cs has been selectively retained during soil evolution, and Rb to a lesser extent, while K has been leached away. In acid extracts of the soils, the Cs/K ratio is about 50 times and the Rb/K ratio about 15 times the corresponding ratios for the UCC, indicating that relatively large amounts of natural Cs and Rb have been sequestered in soil microenvironments that are highly selective for these elements relative to K. Cation exchange favoring Cs and Rb ions, and subsequent fixation of the ions, at sites in interlayer wedge zones within hydroxy-interlayered vermiculite particles may account for the observations. The amounts of stable Cs retained and the inferred duration of the soil evolution, many thousands of years, provide new insights regarding long-term stewardship of radiocesium in waste repositories and contaminated environments. Study of natural Cs in soil adds a long-term perspective on Cs transport in soils not available from studies of radiocesium.

The effect of clear cutting on podzolisation and soil carbon dynamics in boreal forests (Middle Taiga zone, Russia)

Boreal forests are one of the most important terrestrial carbon sink, and a large portion of C is allocated in soil for long-term storage. However forest harvesting may quickly affect soil carbon stocks and dynamics, especially where organic substances drive the soil-forming processes, such as in Podzols. To evaluate the effects of clear cutting on carbon dynamics and podzolisation process over a short time period, a pristine boreal forest (Komi Republic, Russian Federation) and a recently clear cut site (5year-old) were selected. Soils are polygenic: podzolisation occurs within the clay-depleted eluvial horizon, formed by a previous lessivage process. Because podzolisation can start only after the eluvial horizon has reached a sort of threshold, bisequal soils allow to individuate comparable pedogenic conditions prior to anthropogenic disturbances.After harvesting, C storage tended to increase in the upper part of the soil profile (organic layer and podzolic sequum) from 2.2 to 5.0kgm−2. The abundance of woody materials on the forest floor together with an increase in soil water saturation, discernible by the vegetation survey and iron fractionation, prevented litter degradation and allowed organic matter accumulation at the soil surface. Fulvic acids (FA) in the organic layer of the pristine site showed a low incorporation of polysaccharide and proteinaceous moieties, confirming a higher degradation of the humified fraction than at the clear cut site. The lack of disturbances allowed a selection of FA with the more oxidised and mobile fractions accumulating in the deeper horizons, as currently observed in Podzols. Almost no differences were instead found in the chemical composition of FA along the profile from the clear cut site. A larger portion of FA showed the tendency to migrate through the profile after clear cutting even below the Bhs horizon (C-fulvic acid/C-humic acid >1) with a marked increase in the FA-carbon stocks with respect to the pristine forest soil (0.66 and 0.30kgm−2 down to 30cm, respectively).Clear cutting also affected Al and Fe dynamics. The reducing conditions acted upon soil mineral surfaces and enhanced Fe mobilisation probably both in the ionic form and complexed with organic matter. The Al dynamics was instead more related to short term transformations of the layer silicate phases. Traces of a poorly crystalline chlorite were detectable in the Bhs in the pristine forest, but at the clear site only hydroxy-interlayered vermiculite was present. The high amounts of organic acids that migrated through the Bhs after clear cutting may have partially complexed the Al from pedogenic chlorite, giving rise to hydroxy-interlayered behaviour, as normally occurs in Podzol eluvial horizons from where the organic Al-complexes migrate.Our findings suggested that if this trend proceeds further the whole podzolic sequum may migrate downwards. This may have important implication on C budget, as organic carbon will be transferred deeper in the soil profile limiting its losses at least over a short time period.

Unraveling complex &lt;2 m clay mineralogy from soils using X-ray diffraction profile modeling on particle-size sub-fractions: Implications for soil pedogenesis and reactivity

A specific methodology was developed to refine the complex clay mineralogy commonly encountered in soil environments. The soil examined was a Cambisol developed into a ferralitic paleosol. The sample was split into four sub-fractions of different particle sizes (&lt;0.05, 0.05-0.1, 0.1-0.2, and 0.2-2 μm), and their respective mass contributions to the overall &lt;2 μm clay fraction were determined. For each sub-fraction, X-ray diffraction (XRD) patterns were modeled using a trial-and-error approach based on the direct comparison of experimental and calculated profiles. Quantitative information derived from the fitting procedure for the different sub-fractions allowed for the determination of the complex mineralogy of the &lt;2 μm clay fraction through the identification and quantification of eight clay phases. The results show that the finest and most reactive clay fraction (&lt;0.05 μm) was totally hidden in the XRD pattern of the &lt;2 μm fraction, the fraction commonly considered in soil mineralogical analyses. Similarly, this procedure revealed the presence of illite-smectite-chlorite and kaolinite-illite mixed-layer minerals seldom described in soil literature using classical methods. The use of this methodology improved our understanding of the pedogenesis of this soil through the identification and quantification of clay phases structural properties. The analysis of the evolution of structural parameters with particle size allowed for the detection of local modifications in the interlayer composition of expandable and hydroxy-interlayered vermiculite layers. Following this approach, key information can be derived to determine subtle changes in clay mineralogical composition that are related to microorganism and/or plant activity.

Effects of clay mineralogy and physico-chemical properties on potassium availability under soil aquic conditions

Aquic conditions performed for paddy cultivation may cause many nutritional problems. Clay mineralogical composition plays an important role in this relation by affecting many physico-chemical and biological characteristics of soils. Three forms of K including unavailable, slowly available and readily available K, ex- ist in equilibrium in the soil system. Relationships between soil potassium forms with the clay mineralogical suite and physico-chemical properties were determined in 33 soil samples collected from five from paddy soils, two from kiwi fruit and one from citrus land pedons in the North of Iran. Mineralogical analyses showed that smectite, illite and vermiculite were the most abundant clay minerals in the studied soils. Mean soluble and exchangeable potassium in paddy soils were rather low compared to non-paddy soils due to potassium fertilization in non-paddy soils. Non-exchangeable potassium in the samples containing smectite as dominant clay mineral was lower than the samples dominated by vermiculite, hydroxy-interlayer vermiculite (HIV) and illite. Inability of the studied soils for potassium fixation was related to prevailed montmorillonitic type of smectite, which its layer charge origi- nates from octahedral sheet. Significant correlations (0.56 ** , 0.54 ** , 0.56 ** ) were ob- tained between exchangeable potassium and clay content, organic carbon and cation exchange capacity respectively. Therefore, taking into account both exchangeable and non-exchangeable K beside clay mineralogy and physico-chemical properties gives better indications for K potential and soil-quality management, compared to exchangeable form of K alone.