Pedogenic Iron Oxides Research Articles

Edaphic Factors Modulating Phosphorus Availability in Lowland Rice Systems, Nigeria

ABSTRACT The low rice production level in Africa is attributed to poor soil fertility despite the potential for rice cultivation in the inland valleys. This study was conducted to evaluate soil characteristics in some lowlands that have been continuously and intensively used for rice production for over two decades in southwestern Nigeria. Soil physical, chemical, and mineralogical properties, as well as phosphorus sorption characteristics, were assessed in soil samples collected from diagnostic horizons of profiles in six benchmark soils. The results show kaolinite, quartz, potassium(K)-feldspars, and mixed-layered smectite as the predominant minerals. The iron oxide fractions indicate that the soils are at varying degrees of development where the soils at the advanced stage of development are predominantly rich in kaolinite and quartz with a high concentration of total pedogenic iron oxide and generally low in soil fertility. Soil phosphorus buffering capacity is low and varies inversely with pH, confirming the deficiency of phosphorus at low pH. Short-range-order (SRO) iron (Fe) oxides show a strong affinity for phosphorus and this is stronger at lower soil horizons and mediated by some redoxomorphic reactions. The SRO iron oxide concentration has more influence on phosphorus retention in soils rather than the absolute concentration of iron oxides. The study concludes that SRO Fe oxides and soil pH are the major edaphic factors playing a prominent role in phosphorus retention and availability in the wetland soils rather than the clay minerals.

Thermomagnetic analysis applied for identification of lithogenic and pedogenic iron oxides in topsoils from Bulgaria

Identification of magnetic minerals, which determine the magnetic properties of natural rocks, sediments and soils, is of crucial importance for any further interpretation of their magnetic signature in environmental or paleogeographical context. One of the major and widely used methods for determination of kind of magnetic phases in natural materials is by obtaining the Curie/Neel temperature of the respective magnetic mineral. In this contribution, we report a set of thermomagnetic measurements of high-temperature behavior of magnetic susceptibility for topsoils from the territory of Bulgaria, aiming to reveal the pedogenic and lithogenic signature. The data are considered and reported in respect to the parent rock type. Our results suggest that pedogenic magnetic minerals are represented mainly by fine-grained maghemite and/or hematite with possible Al-substitutions in the crystal lattice. This phase is expressed on the heating run of the thermomagnetic curve as a “hump” with a maximum at 250–300 °C. Coarse-grained magnetite is identified as a dominant lithogenic magnetic mineral. Hematite’s presence in a sub-set of red-colored soils is confirmed by additional analyses of isothermal remanence acquisition curves. Two hematite coercivity components were identified, related to pedogenic and lithogenic origin, respectively.

Pedogenesis at the coastal arid-hyperarid transition deduced from a Late Quaternary chronosequence at Paposo, Atacama Desert

Under hyperarid climate conditions, pedogenesis is driven by the atmospheric deposition and subsequent accumulation of easily to moderately soluble salts forming extreme types of Aridisols. In contrast, the processes and timescales of soil formation in coastal desert environments are as yet not well understood. Therefore, a soil chronosequence at the arid-hyperarid transition in the south-central coastal Atacama Desert is investigated. We measured physicochemical soil parameters of shallow soil profiles on four generations of a multi-stage alluvial fan system, assisted by micromorphological analysis, allowing to deduce the soil-forming processes. We further established a geochronological framework by applying 10Be cosmogenic nuclide exposure dating on the abandoned surface generations, which allowed to infer the timescales for soil formation. The results indicate initial yet clearly identifiable pedogenesis occurring since MIS 5e–c. Along the Late Quaternary chronosequence, physicochemical soil properties show decreasing trends with growing age for the median grain size, salinity, long-term pH development, and CaCO3 contents. Contrarily, organic carbon, the ratio of organic to total phosphorus, the redness, and the contents of total and poorly crystalline pedogenic iron oxides tend to increase with age. This provides evidence for soil formation under an arid to hyperarid coastal climate characterized by (i) redistribution of salts by leaching causing topsoil desalinization; (ii) decalcification coupled with (iii) dealkalinization in totally decalcified horizons; (iv) accumulation of soil organic matter; (v) initial rubification through formation of pedogenic iron oxides (i.e. hematite); and (vi) initial loamification that, in turn, might have impeding effects on the translocation of soil constituents on timescales larger than 105 a. Furthermore, in situ pedogenesis might be superimposed considerably by aeolian dust influx. At Atacama’s south-central coast, the state of pedogenic alteration scales with the cumulative precipitation received throughout the entire Late Quaternary, yet evolved strongest between MIS5 and the MIS4/3-transition.

Sources and formation of iron minerals in eastern Mediterranean coastal sandy soils – A HRTEM and clay mineral study

Pedogenic iron minerals and their magnetic and color properties have been studied mainly as reflectors of environmental conditions, especially in Chinese and European loess/palaeosol sequences. The suggested sources of iron for the formation of pedogenic iron oxide minerals were inherited iron oxide minerals, primary iron-bearing minerals and phyllosilicates. Reddening of soils is caused by the addition of neoformed pedogenic hematite from the source minerals under favorable conditions. The sources and pedogenic processes of reddening in red Mediterranean soils have been sporadically studied, but not in the abundant sandy coastal plain soils. The current study attempts to fill two gaps: 1. exploring reddening of coastal sandy soils in relation to iron sources, accumulation of dust and soil evolution. The sandy soils were compared to modern dust and to mountainous soils developed on carbonates under the same climatic regime; 2. revealing the role of clay transformation in the release and secretion of iron. For this purpose, mineralogical compositions of bulk samples and clay fraction, as well as bulk chemical composition, were determined. High-resolution transmission electronic microscopy (HRTEM), coupled with energy dispersive spectrometry (EDS) and selected area electron diffraction (SAED), was used for visual observation, point chemical analysis, and crystal structure designation of nanoscale soil particles. The study documents hematite and goethite as the dominant iron minerals in dust, beach sand and coastal sandy soils, as well as in mountainous soils in Israel. The hematite content is higher than that of goethite in all types of samples, except in Calcaric Cambisols, where goethite is dominant. Iron-bearing minerals in dust are transformed in the soil environment to form pedogenic hematite, goethite and ferrihydrite; the scarcity of the latter indicates that the transformation processes are rather rapid. Ilmenite is found, apparently for the first time, as an important source for iron. Dust clays start transformation in the sandy soil environment immediately after deposition, during which adsorbed and structural iron is released and used to form pedogenic iron minerals. The pedogenic processes of iron mineral formation are similar in mountainous and coastal plain soils but are more intense in the latter due to the higher permeability of the sandy substrate.

Topography-dependent formation and transformation of lithogenic and pedogenic iron oxides on a volcano under a tropical monsoon climate

Volcanoes are widely distributed on the surface of the Earth and on other planets. Volcanic activities play a key role in the long-term evolution of the planetary atmosphere and geomorphology. Iron oxides on volcanoes often exist as the products of rapid magmatic oxidation driven by earlier lithogenesis or the weathering of primary iron-bearing minerals driven by later pedogenesis. The antiferromagnetic (AFM) and ferrimagnetic (FM) particles associated with the formation and transformation of iron oxides dominate the color and magnetism of soil and can potentially serve as an indicator to trace these processes. In this study, we systematically investigated the subsoils derived from a Pliocene volcano under a tropical monsoon climate. We observed that the crystalline iron oxides are differentiated along the leeward and windward slopes, while amorphous iron oxides are mainly differentiated along the shady and sunny slopes. The AFM hematite is more concentrated on the purplish top and the reddish leeward slopes, while the AFM goethite is more concentrated on the yellowish windward slopes. Coarser FM particles accompanied by lithogenic hematite form via early lithogenesis at the top, while more fine FM particles are concentrated on the leeward slopes with the neoformation of pedogenic hematite. Fewer but finer FM particles form with goethite on the windward slopes. These results suggest that FM particles could coexist with AFM hematite via lithogenesis or pedogenesis and that goethite would be a more reasonable pedogenic indicator in volcanic regions.

Rare earth elements associated with pedogenic iron oxides in humid and tropical soils from different parent materials

Rare earth elements (REEs) from parent materials are easily trapped by secondary minerals in highly weathered soils that contain high pedogenic iron (Fe) oxide levels. However, few studies have investigated REEs fractionation by Fe oxides during pedogenesis. Therefore, this study examined REEs partitioned in the pedogenic Fe oxides of four pedons developed from schist, andesite, shale, and mafic rocks in Eastern Taiwan; this was achieved by combining the dithionite-citrate-bicarbonate (DCB) extraction for bulk soil samples and microspectroscopic approaches for thin sections. The DCB extraction was applied to pedogenic Fe oxides (Fed). Furthermore, REE concentrations of the Fe extraction were measured to assess the potential mobility of REEs through the dissolution of pedogenic Fe oxides. The spatial distribution of REEs in Fe nodules and surrounding soil matrix was achieved by using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and electron probe microanalysis (EPMA). The results revealed that total REE content varied with soil depth and among pedons. Total REEs were fractionated during pedogenesis; thus, the ratio of light REEs (LREEs) to heavy REEs (HREEs) varied substantially among all pedons. However, the DCB-extractable REE contents significantly (p < 0.05) increased when the Fed contents increased, indicating the high affinity of pedogenic Fe oxides for REEs. Moreover, the pedogenic Fe oxides exhibited an association preference for HREEs over LREEs even though the DCB-extractable concentrations of LREEs were higher than those of HREEs in the soils. Additionally, the association of REEs with Fe oxides led to HREEs condensation in the Fe nodules identified through LA-ICP-MS and EPMA. Our results elucidated pedogenic Fe oxides as the major carrier of REEs while clarifying the preferential trapping of HREEs by pedogenic Fe oxides.

Soil geochemistry as a driver of soil organic matter composition: insights from a soil chronosequence

Abstract. A central question in carbon research is how stabilization mechanisms in soil change over time with soil development and how this is reflected in qualitative changes in soil organic matter (SOM). To address this matter, we assessed the influence of soil geochemistry on bulk SOM composition along a soil chronosequence in California, USA, spanning 3 million years. This was done by combining data on soil mineralogy and texture from previous studies with additional measurements on total carbon (C), stable isotope values (δ13C and δ15N), and spectral information derived from diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). To assess qualitative shifts in bulk SOM, we analysed the peak areas of simple plant-derived (S-POM), complex plant-derived (C-POM), and predominantly microbial-derived organic matter (OM; MOM) and their changes in abundance across soils with several millennia to millions of years of weathering and soil development. We observed that SOM became increasingly stabilized and microbial-derived (lower C : N ratio, increasing δ13C and δ15N) as soil weathering progressed. Peak areas of S-POM (i.e. aliphatic root exudates) did not change over time, while peak areas of C-POM (lignin) and MOM (components of microbial cell walls (amides, quinones, and ketones)) increased over time and depth and were closely related to clay content and pedogenic iron oxides. Hence, our study suggests that with progressing soil development, SOM composition co-varied with changes in the mineral matrix. Our study indicates that structurally more complex OM compounds (C-POM, MOM) play an increasingly important role in soil carbon stabilization mechanisms as the mineral soil matrix becomes increasingly weathered.

DCB dissolution of iron oxides in aeolian dust deposits controlled by particle size rather than mineral species

Dithionite-citrate-bicarbonate (DCB) treatment is a classical method for removing iron oxides from soil. The DCB-induced dissolution effects on iron oxides are controversial. In this paper, samples from a typical loess-paleosol sequence in the Chinese Loess Plateau (CLP) and from other aeolian dust deposits in southern China were collected, and changes in the grain size composition and magnetic properties of the samples after DCB treatment were analyzed. The results show that the dissolution of iron oxides in loess-paleosol samples from the CLP is highly grain size dependent. In addition to completely dissolving nanometer-sized pedogenic iron oxides (< 0.2 μm), the standard DCB procedure can also dissolve submicron- and micron-sized aeolian iron oxides (0.2–6 μm). For these aeolian iron oxides, the submicron-sized (0.2–1 μm) iron oxides are sufficiently dissolved, and the solubility of the micron-sized (1–6 μm) iron oxides decreases with increasing particle size. The dissolution of > 6 μm aeolian iron oxides is negligible. DCB can neither separate pedogenic iron oxides from aeolian iron oxides nor selectively dissolve magnetite or maghemite. Although the total amount of dissolved iron oxides in the profiles from southern China is higher than that in the LC profile from northern China, the submicron- and micron-sized aeolian iron oxides in the latter are more easily dissolved.

Changes in profile distribution and chemical properties of natural nanoparticles in paddy soils as affected by long-term rice cultivation

Systematic studies on the genesis, properties, and distribution of natural nanoparticles (NNPs) in soil remain scarce. This study examined a soil chronosequence of continuous paddy field land use for periods ranging from 0 to 1 000 years to determine how NNPs in soil changed at the early stages of soil genesis in eastern China. Soil samples were collected from coastal reclaimed paddy fields that were cultivated for 0, 50, 100, 300, 700, and 1 000 years. Natural nanoparticles were isolated and characterized along with bulk soil samples (< 2-mm fraction) for selected physical and chemical properties. The NNP content increased with increasing soil cultivation age at 60 g m−2 year−1, which was related to decreasing soil electrical conductivity (172–1 297 µS cm−1) and NNP zeta potentials (from –22 to –36 mV) with increasing soil cultivation age. Changes in several NNP properties, such as pedogenic iron oxide and total organic carbon contents, were consistent with those of the bulk soils across the soil chronosequence. Notably, changes in NNP iron oxide content were obvious and illustrated active chemical weathering, pedogenesis, and potential impacts on the microbial community. Redundancy analysis demonstrated that the soil cultivation age was the most important factor affecting NNP properties, contributing 60.7% of the total variation. Cluster and principal component analysis (PCA) revealed splitting of NNP samples into age groups of 50–300 and 700–1 000 years, indicating rapid evolution of NNP properties, after an initial period of desalinization (approximately 50 years). Overall, this study provides new insights into NNP evolution in soil during pedogenesis and predicting their influences on agriculture and ecological risks over millennial-scale rice cultivation.

Understanding the nature of Fenton processes in soil matrices: The role of iron forms and organic matter

Understanding the processes of pollutants removal in soil remediation practices is crucial to apply the appropriate treatment method. Although widely employed in soil contamination events, the mechanisms of the Fenton reaction are still debatable. To investigate the catalytic performance of soils towards the degradation of p-xylene in Fenton reactions, we performed a series of experiments employing two soil samples with different physical-chemical properties, Oxisol and Alfisol. These soils were subjected to extraction procedures that separated the different types of pedogenic iron oxides (amorphous and crystalline) and produced soil fractions with different organic matter contents. We observed that Oxisol, which contains high amounts of amorphous pedogenic iron oxides, performed better in hydrogen peroxide decomposition and radical generation but worse in p-xylene degradation. These results originated from the presence of hematite in Oxisol, which has a lower catalytic activity than goethite, the pedogenic oxide present in Alfisol. Samples containing high concentrations of organic matter performed better in decomposing hydrogen peroxide but worse in degrading p-xylene due to the scavenging of active species by labile organic matter compounds.

Forms and distribution pattern of soil Fe (Iron) and Mn (Manganese) oxides due to long-term rice cultivation in fars Province Southern Iran

ABSTRACT To assess the effect of long-term rice cultivation and aquic conditions on the physicochemical properties and different forms of iron and manganese and also to make a comparison with non-paddy and adjoining uncultivated soils in calcareous soils of southern Iran. The soil samples were taken from the paddy and non-paddy soils with similar parent materials and landforms. The results showed that in the paddy fields, the amounts of available (extracted by DTPA), total (extracted by nitric acid), and poorly crystalline iron and manganese oxides (extracted by ammonium oxalate) were higher than those in the non-paddy soils and show the highest amount in the surface horizons of paddy fields. In general, the paddy land use increases the iron and manganese oxides with poorly crystalline form, Feo/Fed and Mno/Mnd ratios by 1.84, 1.57, 2 and 3.2, respectively. It also reduces the amount of pedogenic iron and manganese oxides and their crystalline forms relative to the adjacent uncultivated soils. Finally, a significant positive relationship was found between the amounts of poorly crystalline iron and manganese oxides and total form and the amount of organic matter and clay in the soils under study. Also, by changing the land use from the non-paddy to paddy field, the amount of smectite increased and, in contrast, the amounts of illite and chlorite decreased. Therefore, it seems that the significant amount of organic matter introduced and their burial caused by the plowing operation and, on the other hand, their slow decomposition due to the inactivation of aerobic activities in the reduction conditions is among the factors affecting the variations of chemical properties in paddy soils and, generally, wet land soils.

How strong was pedogenesis in Schirmacher Oasis during the Late Quaternary?

We compare the sediment magnetic properties of two lakes in Schirmacher Oasis, East Antarctica and discuss the intensity of pedogenesis during the Late Quaternary. The magnetic minerals are land-derived with no obliteration of the signal due to bacterial or anthropogenic sources. The magnetic grains of Lake L-49 are much smaller in size compared to Lake Sandy's (low ratio values of IRM20mT/ARM and S20 ). The iron oxide minerals in Lake Sandy's sediments are coarse stable single domain (SSD) and multi-domain (MD) grains with the SSD concentration increasing during the Holocene. Percentage frequency-dependent susceptibility, a marker for pedogenic iron oxide minerals shows a similar trend for Lake Sandy, with the values remaining low during the Last Glacial Stage (LGS) and increasing towards the Holocene. Such a pattern is not observed in the sediments of Lake L-49. Principal Component Analysis reveals a relatively stronger intensity of pedogenesis in the Lake Sandy catchment during the deglaciation. From the data, we observe that even though pedogenesis is active, the intensity is not strong enough to form superparamagnetic (SP) grains. The peaks in χfd for both the lakes show a direct relationship with Antarctic Isotope Maximum events recorded in an ice-core data suggesting that χfd can be used as a proxy for paleo-pedogenesis in Antarctica.

Magnetic susceptibility in the prediction of soil attributes in southern Brazil

AbstractGlobal demand for soil information has led to investigations that have adopted ways to estimate soil attributes quickly and effectively. In this context, magnetic susceptibility (χ) has gained prominence because it is a technique capable of estimating other attributes that are more difficult to acquire. This study aimed to (a) evaluate the performance of χ for the prediction of sand, silt, clay, hue, hematite/(hematite + goethite) ratio, Fe content of pedogenic iron oxides, and remaining phosphorus and (b) develop maps of χ, soil attributes and attributes predicted by χ in the state of Rio Grande do Sul (RS), Brazil. Here, 198 soil samples under forest and native pasture were used for testing the potential of χ as a predictive technique, separating the data into calibration (nc = 149) and validation sets (nv = 49). Linear regression was used to obtain the pedotransfer equations according to soil classes and lithology. To visualize the distribution of the values of χ and other soil attributes in RS, maps were made with the real values of χ and the real and estimated values of soil attributes. The great range of the χ values and related attributes was associated with the lithological and pedological influence, allowing the construction of predictive models that encompass a large gradient of χ. In the predictions made in groups, the attributes of Oxisols and Ultisols were best estimated by χ; however, among the lithology groups, the extrusive igneous rocks stood out.

Magnetic properties of surface sediments in Schirmacher Oasis, East Antarctica: spatial distribution and controlling factors

PurposeWe investigated the magnetic properties (abundance, grain size, and mineralogy) of iron oxides present in Lake L-55 sediments, Schirmacher Oasis, East Antarctica, with an aim to understand their spatial distribution and the underlying mechanisms that control their formation and distribution.MethodsTwenty-five surficial sediments retrieved from different parts of Lake L-55 were subjected to the entire range of environmental magnetic (magnetic susceptibility, anhysteretic remanent magnetization (ARM), isothermal remanent magnetization (IRM)) measurements (at different field strengths). Inter-parametric ratios (χARM/SIRM, χARM/χlf, χARM/χfd, IRM20 mT/SIRM, IRM20 mT/ARM, S-ratio, L-ratio) provided insights into the magnetic properties (abundance, grain size, and mineralogy of iron oxides). Scanning electron microscopic-energy dispersive X-ray spectroscopic (SEM-EDS) analysis was performed on magnetic extracts from a few sediments. Besides, organic matter (%) was also calculated for the sediment samples. Principal component analysis was performed to gain information on the presence of different components and their relative dominance.ResultsThe iron oxides are strongly magnetic (high values of concentration-dependent parameters). The principal iron oxide is magnetite (S-ratio > 0.90) which is coarse-grained (multi-domain (MD) and stable single-domain (SSD) grains), and there is no influence of authigenic greigite, bacterial magnetite, and anthropogenic magnetite. The mineralogy is confirmed by SEM-EDS data. The iron oxides are of different grain sizes, and their contribution is in the order of MD > SSD > SP as shown by the principal component analysis. Pedogenic iron oxide minerals seem to be present in the samples whose formation is due to the oxidation of magnetite into hematite. However, they are of SSD size and not SP, suggesting that the intensity of pedogenesis is not sufficient to form SP grains.ConclusionThe iron oxide minerals are mainly terrigenous, and the biogenic activity within the lake is not sufficient to modify the ferrimagnetic minerals. Spatial distribution patterns suggest the non-uniform distribution of magnetite/titanomagnetite of varying sizes in the lake basin which is transported by both melt water streams and winds.

Iron oxide characteristics of the Chinese loess-red clay sequences and their implications for the evolution of the East Asian summer monsoon since the Late Oligocene

The most significant climatic change in East Asia involved the formation and evolution of the coupled monsoon-arid environmental system during the Cenozoic, which was dynamically linked to global cooling and the growth of the Qinghai-Tibet Plateau. However, our understanding of monsoon evolution since the Late Oligocene remains controversial due to differences in the interpretation of various environmental proxies from loess archives. In this study, the composition and relative abundance of pedogenic iron oxides of two eolian sequences from the Lingtai section and the Zhuanglang drill core were investigated using rock magnetic, optical spectroscopy, and geochemical analyses. The results indicate that while the composition of iron oxides in the Mio-Pliocene red clay and Quaternary loess-paleosol sequences is similar, the concentrations of magnetite, maghemite, hematite and goethite are quite different between loess, paleosol and red clay. The combination of magnetic proxies (magnetic susceptibility, Fed/Fet, and color a* values) reveals that precipitation and temperature played varying roles in driving pedogenesis and chemical weathering since the Late Oligocene. During the Late Oligocene to Early Miocene, the occurrence of relatively strong pedogenesis and chemical weathering was likely the result of high temperatures rather than high precipitation. The East Asian summer monsoon intensified during the Middle Miocene Climate Optimum (16.5–14.5 Ma), in response to global climatic warming; subsequently, during the Middle to Late Miocene, the monsoon intensity gradually weakened due to global cooling. After ~8 Ma, the summer monsoon strengthened substantially and reached a maximum during 4.5–2.6 Ma, possibly as a result of the uplift of the northeastern margin of the Qinghai-Tibetan Plateau during the Late Miocene. The monsoon intensity then commenced a decreasing trend in the Early Pleistocene, but then strengthened again after ~1.0 Ma.

Modeling the formation of soil microaggregates

The functions of soils are intimately linked to its aggregated structure. Microaggregates formed during pedogenesis from a vast variety of mineral, organic, and biotic materials are the smallest conceivable compounds at the basis of soil structural hierarchy. Qualitative hypotheses and concepts on how aggregates form are quite elaborate, but the consistent quantitative application within a mechanistic and physically rigorous framework is still missing. This is partly caused by the fact that such an endeavor requires the combination of synergistic concepts on the transport of particles and their interactions affected by shape, size, and surface properties in aqueous suspension. Here we present a novel quantitative approach for the formation of soil microaggregates in silico that integrates diffusion-limited cluster-cluster aggregation with a probabilistic attachment following the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of particle interactions. To represent universal mineral shapes found in soils, we implemented spherical, plate-like as well as rod-like particle morphologies inspired by weathered silicates, secondary clay minerals and pedogenic iron oxides. In this way, we developed a model that produces an explicit three-dimensional structure of aggregates emerging from hetero- as well as homo-aggregation in response to the chemical milieu. We then exploited the 3D morphology to assess functional properties of aggregates exemplified with the water retention curve and pore size distribution.

The occurrence, soil parameters and genesis of rubified soils (‘Fuchserden’) of northeastern Germany

The local occurrence of red subsoils on the Late Pleistocene sandy deposits of the Northern European lowlands invites investigation into the formation and paleoenvironmental significance of these soils. According to the World Reference Base for Soil Resources (WRB), these soils are classified as Rhodic Brunic Arenosols or Chromic Brunic Arenosols; however, because of their unclear origins, these soils are still not considered in the German Guidelines for Soil Mapping. We aim to characterize the genesis of these rubified soils by combining pedological findings with chemical analyses and micromorphological studies at four sites in southeastern Brandenburg, Germany. Our novel analytical approach differentiates pedogenic iron (hydr-)oxides using a combination of Fourier transform infrared-multiple internal reflection (FTIR-MIR) spectroscopy and sequential sample heating (from 21 °C to 100 °C, 250 °C, 500 °C, and 950 °C). Together with a comparison of synthetic iron (hydr-)oxides, the results of this analytical approach demonstrate that the mineralogy of these Rhodic Brunic Arenosols and Chromic Brunic Arenosols are characterized mainly by hematite; some goethite and minor amounts of maghemite are also present. Our results confirm recent findings from Jankowski (2013) that rubification is not primarily a relict process in Central European soils; consequently, the German Guidelines for Soil Mapping require revision. The high total iron contents in the rubified soils suggest strongly that the iron is allochthonous in origin, emphasizing the importance of lateral inputs of iron compounds in the genesis of these soils. This finding is highly consistent with the ‘translocation catena’ concept (Sommer and Schlichting, 1997); i.e., the Rhodic Brunic Arenosols and Chromic Brunic Arenosols studied here formed due to inputs of allochthonous compounds. These rubified soils examined in this study share some genetic features with other soils described from Central Europe, such as the ‘Lockerbraunerden’ or the ‘Ockererden’. Further research should focus on these commonalities and compare the genetic pathways of such soils to support the argument that these soils should be considered in a separate soil class in the German Guidelines for Soil Mapping.

Pedogenic iron oxides in soils of the Acre State, Brazil

ABSTRACT: Iron oxides are environmental indicators and influence on physical and chemical behavior of soils. This research aimed to identify and characterize pedogenic iron oxides in soils of the Acre state, Brazil. The soils developed from sedimentary rocks of the Solimões Formation. Twenty one samples of subsurface horizons were collected from ten soil profiles. Soil samples were analyzed by x-ray diffractometry associated to selective chemical dissolutions. Fe contents in the clay minerals (Fes), in the pedogenic iron oxides (Fed) and in the poorly crystalline oxides (Feo) were determined. The Al3+-substitution and the mean crystal diameter of goethite and hematite were estimated. Soils presented low contents of Fes and Fed. The Fed/Fes ratio indicated soils in the intermediate stage of weathering, with dominance of goethite, hematite and maghemite. This condition of weathering was confirmed by the higher frequency of goethites and hematite with intermediate Al3+-substitution. Goethites have isodimensional forms and hematite have flat plaques forms.

Drivers of organic carbon allocation in a temperate slope-floodplain catena under agricultural use

The distribution of soil organic carbon (OC) in the landscape shows high variability between landform positions. Soils in depositional sites are characterized by a highly diverse vertical structure and can contain significant amounts of OC over the whole soil profile. However, anthropogenic modifications of the landscape such as land use and artificial drainage can significantly alter the OC balance in a catchment. Here, we analyse the spatial distribution of OC stocks along an agriculturally used catena in a catchment, tributary to the Danube River in the Bavarian Forest in Southern Germany. The combination of geomorphic aspects with highly detailed soil data based on 16 soil profiles along a catena allowed us to closely examine the relationship between soil OC and the factors of landform position, soil inherent properties and grassland vs. cropland land use. We determined bulk density, total carbon (TC) and inorganic carbon (IC) for each soil horizon in order to quantify OC stocks and picture the vertical OC distribution in the soils. Pedogenic oxides were analysed as an indicator for the redox conditions as well as the soil development. Furthermore, we analysed the clay fraction as well as poorly crystalline iron oxides (FeOX) as potential binding agents for OC. The persistence of OC in the landform positions could be designated by the proportion of 14C content of the bulk soil OC. Our data revealed that the amount of OC stored in the topsoil is mostly determined by land use, but subsoil OC stocks highly depend on landform position. Most OC of this catena is stored in the floodplain featuring particularly high amounts of OC in their subsoils. Radiocarbon signatures of the floodplain OC indicated a young 14C age compared to the footslope subsoils. We assume that these high OC stocks were due to 1.) formerly high input of OC-rich sediments, 2.) preservation of this OC from decomposition due to high water saturation and 3.) current land use (grassland). Dropping of the ground water table due to artificial drainage measures left detectable footprints within a few decades in the depth distribution of pedogenic iron oxides and indicated currently dominant oxic conditions within the uppermost 100 cm of the floodplain soils. We suspect that under these conditions the OC stocks are now vulnerable to OC losses. Even though we found no correlation between FeOX and OC, it can be assumed that the interplay between FeOX and OC will play a key role for the stabilization of the floodplain OC in the future. Today most floodplains in Central Europe are not in their natural state, but drained and under agricultural land use. Therefore this work points to the necessity of monitoring the OC pools in these landform positions and to consider quick responses when it comes to changes in the hydrologic regime and/or land-use system.

Soil organic carbon and mineral interactions on climatically different hillslopes

Climate and topography have been widely recognized but studied separately as important factors controlling soil organic carbon (SOC) dynamics. Subsequently, the significance of their interplay in determining SOC storages and their pools is not well understood. Here we examined SOC storages and SOC-mineral interactions along two hillslope transects in differing climate zones (MAP = 549 mm in semi-arid eucalyptus savannah vs. 816 mm in temperate eucalyptus forest) in southeastern Australia. On eroding slopes, SOC inventories were twice as large at the wetter site (4.5 ± 0.6 vs. 2.3 ± 0.9 kg m−2), whereas depositional soils had similar SOC inventories at both locations (7.5 ± 2.0 vs.7.0 ± 2.2 kg m−2). On eroding slopes, carbon concentrations of the mineral-associated SOC fraction (<250 μm and >2.0 g cm−3) increased by ~50% with increasing rainfall, which was also positively correlated to abundances of clay minerals and pedogenic iron oxides. Within individual hillslopes, carbon concentrations of the mineral-associated SOC fraction doubled from eroding to depositional soils at the drier site, but no topographic trend was observed at the wetter site. The effects of topography on SOC inventories and mineral-associated SOC were more strongly expressed under the drier climate, where vegetation was sparser and soil erosion involved mineral grain size sorting. Our results demonstrate that SOC pools and their interactions with minerals are dependent on topographic locations, emphasizing the need to include geomorphic data when assessing climatic controls of SOC.