Dithionite-citrate-bicarbonate Research Articles

Chemical and mineralogical constitution of redoximorphic features and mechanism of formation of Plinthosols from the Araguaia River plain, Brazil

ABSTRACT Currently in Brazil, large grain cultivation projects on Plinthosols are a reality, however, there is little or no knowledge of the real mechanism of formation of the plinthite feature, in addition to what is reported in the literature as being a product of oxidation-reduction processes of iron element. This study evaluates iron redoximorphic features and investigates their chemical and mineralogical composition in two profiles of Plinthosols from the Araguaia River plain (P1 and P2). The study strengthens the understanding of the pedogenetic processes involved in the formation of mottles and plinthite. In this sense, it assesses whether the formation mechanisms corroborate the literature. Soil features were sampled in the upper right and left position at the initial plinthic horizon, upper right and left position at the main plinthic horizon, and lower right position at the base horizon of the plinthite zone in the profile. Separated samples comprising the soil matrix, mottles, and plinthite under natural moisture conditions were ground into powder form for chemical determinations by X-ray fluorescence (XRF), sulfuric acid attack (H 2 SO 4 ), sodium dithionite-citrate-bicarbonate (DCB), and ammonium acid oxalate; and mineralogical determinations by X-ray diffraction. Iron contents in all determined forms were always higher in the plinthite feature, intermediate in the mottle feature, and lower in the soil matrix feature. Most of the Fe in all redoximorphic features is included in the structure of primary minerals and their derivatives (vermiculite, illite, and VHEs). Only part of the iron present (about 35.40 % in P1 and 41.98 % in P2) is detected in the form of oxides such as goethite and hematite, which could be formed in redox processes. The mottle and plinthite features under study are not the product of the classic process of segregation, mobilization, and accumulation of iron as a consequence of redox processes. These features were formed or emerged as a result of a relatively slow and constant weathering process of their source material, which is gradually decomposed in an aqueous medium, releasing most of its components. These components include iron and more mobile elements such as bases and silicon, which leave the system through drainage water, and of which a small part may eventually recombine to form new less complex minerals such as kaolinite and oxides.

Variations in Root Characteristics and Cadmium Accumulation of Different Rice Varieties under Dry Cultivation Conditions.

Variations in the cadmium (Cd) accumulation and root characteristics of different genotypes of rice during three developmental periods of dry cultivation were investigated in pot experiments in which two levels of Cd were added to the soil (0 and 10 mg kg-1). The results show that the Cd concentration in each organ of the different rice genotypes decreased in both the order of roots > shoots > grains and during the three developmental periods in the order of the maturity stage > booting stage > tillering stage. The lowest bioaccumulation factor (BCF) and translocation factor (TF) were found in Yunjing37 (YJ37) under Cd stress. At maturity, Cd stress inhibited the root length of Dianheyou34 (DHY34) the most and that of Dianheyou 918 (DHY918) the least, also affecting the root volume of DHY34 and Dianheyou615 (DHY615) the most and that of YJ37 and Yiyou 673 (YY673) the least; the inhibition rates were 41.80, 5.09, 40.95, and 10.51%, respectively. The exodermis showed the greatest thickening in YY673 and the lowest thickening in DHY615, while the endodermis showed the opposite result. The rates of change were 16.48, 2.45, 5.10, and 8.49%, respectively. The stele diameter of DHY615 decreased the most, and that of YY673 decreased the least, while the secondary xylem area showed the opposite result; the rates of change were -21.50, -14.29, -5.86, and -26.35%, respectively. Under Cd stress treatment at maturity, iron plaque was extracted using the dithionite-citrate-bicarbonate (DCB) method. The concentration of iron (DCB-Fe) was highest in YJ37, and the concentration of cadmium (DCB-Cd) was lowest in DHY34. YJ37 was screened as a low Cd-accumulating variety. The concentration of available Cd in the rhizosphere soil, iron plaque, root morphology, and anatomy affect Cd accumulation in rice with genotypic differences. Our screening of Cd-accumulating rice varieties provides a basis for the dry cultivation of rice in areas with high background values of Cd in order to avoid the health risks of Cd intake.

Using the pedogenic oxide ratio in distinguishing volcanic soil weathering intensity in subtropical region

Pedogenic oxide ratio (POR) helps identify and understand pedogenic processes. However, POR is seldom used to study and distinguish the weathering intensity of volcanic soils that are moderately weathered in the subtropical region. The research was conducted by collecting thirty-three subtropical volcanic soils from the northeastern Tatung Volcanic Group (TVG) region in Yangmingshan National Park, near Taipei, located in northern Taiwan. These volcanic soils could be grouped into two groups with different weathering intensities based on three weathering indices, pedogenic Fe-oxides ratios (PORs) - Feo/Fed, ((Fed-Feo)/Fet), and total reserve in bases (TRB): the highly weathered (HW) group and the lower weathered (LW) group. The two PORs show significant differences between the two groups, but not for TRB, which indicates the PORs are a good indicator for providing quantitative measurements of weathering intensity. Except for the soil bulk density (BD) meeting andic soil properties, the soils of the HW group have a lower content of Al plus 1/2 Fe content (by ammonium oxalate) (Alo + 1/2Feo) and phosphorus (P) retention, and soils of the LW group only have a lower P retention. Within those soil physicochemical properties and selective dissolution analysis results and index values, Alo + 1/2Feo, pyrophosphate extractable C (Cp), P retention, and dithionite-citrate-bicarbonate (DCB) extractable aluminum (Ald) are four critical factors for the discrimination of the soils. The soils in the study area still have relict volcanic soil properties after moderately weathered. Our research results indicate that the PO and POR are good indicators of moderately weathered volcanic soils in the subtropical region, allowing for a more precise assessment of the soil weathering process, in addition to quantitative measurements of weathering intensity.

Texture and clay mineralogy as main drivers of the priming effect in temperate forest soils

Abstract Background and aims This work aimed to determine how the soil parameters affect the magnitude and direction of priming effect (accelerated or decreased decomposition of native SOM under addition of new organic substrates, PE) in temperate acidic forest soils. Methods Thirteen topsoil samples were incubated for 163 days with the addition of maize residues. Soil respiration was measured and natural isotope labelling was used in order to separate the respiration sources (SOM, maize and PE). The effect of soil parameters on PE was studied using linear regression and structural equation modelling (SEM). Results Soils with high C/N ratio showed the lowest magnitude of cumulative PE (R2 = 0.321, p < 0.05) and the longest negative PE period. A positive relationship was found between PE and the pH (R2 = 0.511, p < 0.05). SEM analysis showed that pH and C/N ratio has direct (β = 0.50) and indirect (β = 0.20, via modifying soil texture and mineralogy) effect on PE. Soil texture and mineralogy had a significant effect on PE: texture affects the proportions of soil respiration sources and PE was reduced by the dithionite–citrate–bicarbonate–extractable Al (AlDCB, R2 = 0.454, p < 0.05), silt + clay (R2 = 0.421, p < 0.05), non-swelling clay mineral (R2 = 0.575, p < 0.05) and illite (R2 = 0.522, p < 0.05) contents. SEM analysis also highlighted that the AlDCB, illite and silt + clay contents has a great effect (β=−0.59) on the PE. Conclusion The silt + clay content and mineral composition of the soil, including the Al oxide and illite contents may thus significantly inhibit the magnitude of PE, and consequently the decomposition of SOM under acidic conditions.

A modified dithionite reduction method for the quantification of iron-bound organic carbon in marine sediments

Iron-bound organic carbon (Fe-OC) plays a significant role in the global marine carbon cycle, facilitating the long-term accumulation and storage of organic carbon. However, the conventional method for quantifying Fe-OC, known as the citrate-bicarbonate-dithionite (CBD) method, has limitations in terms of its efficiency in extracting both Fe-OC and iron (Fe). Another CBD method, commonly used for Fe speciation analysis in paleoenvironmental reconstructions, has shown efficient and selective extraction of Fe oxides; however, its effectiveness in extracting Fe-OC remains uncertain. To evaluate the efficiency of the two CBD methods, synthetic samples spiked with organic acid/environmental organic matter‑iron (hydr)oxide coprecipitates and a marine sediment sample were subjected to extraction. The CBD approach for Fe speciation analysis at pH 4.8 on spiked samples demonstrated higher efficiency compared to the conventional CBD method, but not for samples of marine sediment. This result suggests that the synthetic Fe-OC in the spiked samples did not accurately represent the realistic state of Fe-OC in marine sediments. We found that the pH of the leaching solution during the CBD extraction significantly influenced the efficiency of the Fe-OC and Fe extraction. Our tests at pH 4.8, 5.5, 6.0, 6.5, and 7.0 revealed that lower pH enhanced the extraction of Fe, but hindered the release of Fe-OC. Our study revealed that the new CBD (pH 6.5) method resulted in approximately 30% higher yields of Fe-OC and Fe compared to the conventional CBD method, despite of significant variability in the Fe-OC data. The modified CBD (pH 6.5) method uses lower concentration of extraction reagent and reaction temperature, both of which is considered beneficial. Therefore, we suggest that the new CBD method (pH 6.5) is at least of similar efficiency as the conventional method, and can be employed alongside the established method to facilitate the evaluation of the role of Fe-OC in the marine carbon cycle.

Boron deficiency decreased the root activity of Ga-exposed rice seedlings by reducing iron accumulation and increasing Ga in iron plaque.

Gallium (Ga) is an emerging chemical pollutant chiefly associated with high-tech industries. Boron (B) alleviates the negative effects of toxic elements on plant growth. Thereby, the effects of B fertilization on Ga toxicity in rice seedlings was studied to clarify the role of iron plaque in the distribution of Ga, Fe, and B in Ga-treated rice seedlings in the presence or absence of B. Gallium exposure significantly reduced the biomass of rice seedlings. Boron deficiency induced a significant change in the distribution of B in Ga-treated rice seedlings compared with "Ga+B" treatments. Accumulation of Ga in roots, dithionite-citrate-bicarbonate (DCB) extracts, and shoots showed a dose-dependent manner from both +B and -B rice seedlings. Boron nutrition levels affect the distribution of Fe in roots, DCB extracts, and shoots, in which DCB-extractable Fe was significantly decreased from "Ga-B" treatments compared with "Ga+B" treatments. Root activity was significantly decreased in both Ga-exposed rice seedlings; however, B-deficient seedlings showed a severe reduction than +B rice seedlings. These results reveal that Fe plaque might be a temporary sink for B accumulation when plants are grown with proper B, wherein the re-utilization of DCB-extractable B stored in Fe plaque is mandatory for plant growth under B deficiency. Correlation analysis revealed that B deficiency decreased the root activity of Ga-exposed rice seedlings by reducing DCB-extractable Fe and increasing DCB-extractable Ga in Fe plaque. This study enhances our understanding of how B nutritional levels affect Ga toxicity in rice plants.

The effects of different iron and phosphorus treatments on the formation and morphology of iron plaque in rice roots (Oryza sativa L).

Rice (Oryza sativa L.) is a pivotal cereal crop worldwide. It relies heavily on the presence of iron plaque on its root surfaces for optimal growth and enhanced stress resistance across diverse environmental conditions. To study the crystallographic aspects of iron plaque formation on rice roots, the concentrations of Fe2+ and PO4 3- were controlled in this study. The effects of these treatments were assessed through comprehensive analyzes encompassing root growth status, root surface iron concentration, root vitality, enzyme activities, and microstructural characteristics using advanced techniques such as root analysis, scanning electron microscopy (SEM), and ultrathin section transmission electron microscopy (TEM). The results demonstrated that an increase in the Fe2+ concentration or a decrease in the PO4 3- concentration in the nutrient solution led to improvements in various root growth indicators. There was an elevation in the DCB (dithionite-citrate-bicarbonate) iron content within the roots, enhanced root vitality, and a significant increase in the activities of the superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) enzymes. Moreover, as the Fe2+ concentration increased, amorphous iron oxide minerals on the root surface were gradually transformed into ferrihydrite particles with sizes of approximately 200 nm and goethite particles with sizes of approximately 5 μm. This study showed that an increase in the Fe2+ concentration and a decrease in the PO4 3- concentration led to the formation of substantial iron plaque on the root surfaces. It is noteworthy that there was a distinct gap ranging from 0.5 to 3 μm between the iron plaque formed through PO4 3- treatment and the cellular layer of the root surface. This study elucidated the impacts of Fe2+ and PO4 3- treatments on the formation, structure, and morphology of the iron plaque while discerning variations in the spatial proximity between the iron plaque and root surface under different treatment conditions.

Effect of soaking time of dithionite–citrate–bicarbonate solution on strength and deformation characteristics of lateritic soil

Effect of soaking time of dithionite–citrate–bicarbonate solution on strength and deformation characteristics of lateritic soil

Oxidation State and Structure of Fe in Nontronite: From Oxidizing to Reducing Conditions.

The redox reaction between natural Fe-containing clay minerals and its sorbates is a fundamental process controlling the cycles of many elements such as carbon, nutrients, redox-sensitive metals, and metalloids (e.g., Co, Mn, As, Se), and inorganic as well as organic pollutants in Earth's critical zone. While the structure of natural clay minerals under oxic conditions is well-known, less is known about their behavior under anoxic and reducing conditions, thereby impeding a full understanding of the mechanisms of clay-driven reduction and oxidation (redox) reactions especially under reducing conditions. Here we investigate the structure of a ferruginous natural clay smectite, nontronite, under different redox conditions, and compare several methods for the determination of iron redox states. Iron in nontronite was gradually reduced chemically with the citrate-bicarbonate-dithionite (CBD) method. 57Fe Mössbauer spectrometry, X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure (XANES) spectroscopy including its pre-edge, extended X-ray absorption fine structure (EXAFS) spectroscopy, and mediated electrochemical oxidation and reduction (MEO/MER) provided consistent Fe(II)/Fe(III) ratios. By combining X-ray diffraction (XRD) and transmission electron microscopy (TEM), we show that the long-range structure of nontronite at the highest obtained reduction degree of 44% Fe(II) is not different from that of fully oxidized nontronite except for a slight basal plane dissolution on the external surfaces. The short-range order probed by EXAFS spectroscopy suggests, however, an increasing structural disorder and Fe clustering with increasing reduction of structural Fe.

Zinc and iron enrichment of vermicompost can reduce the arsenic load in rice grain: An investigation through pot and field experiments

The heavy metalloid arsenic (As), occurring in both trivalent and pentavalent is extremely toxic and has detrimental effect on humans through water-soil-crop transfer. Previously organic and inorganic amendments have been used separately for mitigation of As in rice but there exists a research gap regarding use of them simultaneously. In this study, both pot and field scale investigations were undertaken for four consecutive years in As-contaminated locations to assess the efficacy of zinc (Zn) and iron (Fe)-enriched vermicompost in reducing the As uptake in rice grain. Altogether seven types of vermicompost and three application rates were evaluated. The treatment combination V4D1 (enriched vermicompost V4 applied to soil at 3 t/ha) recorded the lowest soil available As (2.525 mg kg−1) and the highest soil available As (2.982 mg kg−1) was observed with V5D3 (enriched vermicompost V5 applied to soil at 1.5 t/ha). Application of enriched and non-enriched vermicompost reduced As in grain by 58.14 and 31.40% respectively over no vermicompost (control). The partial dependence plot from stepwise regression modelling of different fractions of As revealed that an increase in organically bound As resulted in a decrease in the availability of As and hence uptake by rice. Further, Zn and Fe-enriched vermicompost resulted in increase of iron plaque formation on the root. A significant positive relationship (r = 0.462) was observed between dithionite-citrate-bicarbonate (DCB) extractable -Fe andAs. A significant negative correlation (r = ˗0.410) between DCB-Fe and grain As, advocates better root plaque formation resulting a higher capacity to sequester As onto the root surface and reducing its's entry into the rice system. The carcinogenic risk somewhat was benign (TCR of 2.69 × 10−3 and SAMOE of 0.101) against no vermicompost (TCR of 6.64 × 10−3 and SAMOE of 0.04). Therefore enriching vermicompost with ZnSO4 and FeSO4 at 10% dry weight basis (V4) of the composting substrate can lower arsenic build-up in rice grains without affecting yield.

Composition of organic matter‑iron‑phosphorus associations in sediments of algae- and macrophyte-dominated zones in Lake Taihu

The biogeochemical cycles of phosphorus (P) and organic matter (OM) are significantly influenced by iron (Fe) through forming OM-Fe-P associations. The sources and compositions of organic matter in the two typical states (algae-dominated and macrophyte-dominated) of shallow lakes are different, which could modulate the formation of OM-Fe-P associations and influence the internal loadings of P in lakes. In this study, OM and P bound with Fe were extracted from the sediments of algae-dominated zone (A-zone) and macrophyte-dominated zone (M-zone) in Lake Taihu via the citrate-bicarbonate-dithionite (CBD) reduction method, and were analyzed to elucidate the differences in OM-Fe-P associations between the two zones. The results showed that OM-Fe-P associations in the sediments of M-zone had higher a molar C/Fe ratio but a lower molar P/Fe ratio. Four components identified by excitation-emission matrix fluorescence coupled with parallel factor analysis (EEM-PARAFAC) in OM-Fe-P associations in the two zones were all humic-like substances and the relative abundance of the corresponding PARAFAC components had no significant difference between the two zones. However, the total fluorescence intensity of the humic-like components was higher in M-zone than those in A-zone. This could be attributed to the higher aromaticity of OM produced from macrophyte than that from algae, as evidenced by the difference in the easily-desorbed OM between two zones. In M-zone, high aromatic compounds which are preferentially associated with Fe, could be more produced from macrophyte plants than from algae and resulted in the higher C/Fe molar ratio. The higher C/Fe molar ratio (2.1 ± 0.9) of OM-Fe-P associations in M-zone reflected that more OM originated from macrophyte could be involved in the coprecipitation with Fe (III) and OM-Fe-P associations in M-zone were probably more resistant to the microbial and chemical reduction. Our results from the real situation confirmed the mechanism related to the role of OM composition in the reduction of OM-Fe-P associations to explain the lower internal loading of P in M-zone than that in A-zone.

FERTILIZER-INDUCED GEOCHEMICAL FRACTIONS OF PHOSPHORUS IN FRUIT ORCHARDSS

Phosphorus (P) is essential to all known life forms because it is a key element in many physiological and biochemical processes. In this study, different physiographic units of Atabey Plain have developed soils with varying characteristics. The variability in land use and fertilizer input management causes changes in the availability and geochemical fractions of phosphorus. In this study, 29 surface soil samples (0-20 cm) were taken from various fruit orchards to investigate the plant cover-related inter-fraction mobility and availability of P as a function of soil characteristics. Sequentially extracted geochemical P fractions were: sodium bicarbonate [NaHCO3-P (organic Po, inorganic Pi, and Total Pt)], sodium hydroxide [NaOH-P (Po, Pi, and Pt)], citrate bicarbonate dithionite (CBD-P), hydrochloric acid (Ca-P), and residual (Res-P) phosphorus. In order to study the relationship of phosphorus fractions with soil properties, chemometric analyses were performed. According to the results, the average concentrations of the fractions in descending order were: Ca-P (200-1029 mg kg-1) > Res-P (55.3-449 mg kg-1 ) > CBD-P (56.5-204 mg kg-1) > NaOH-Pi (26.0-206 mg kg-1) > NaHCO3-Pi (4.38-93.4 mg kg-1) > NaOH-Po (0.55-30.0 mg kg-1) > NaHCO3-Po (0.26-7.56 mg kg-1). The added fertilizer P was likely to be converted to NaOH-P in the short term and Ca-P and/or CBD-P for a relatively longer time. It was determined that the fractal distribution of these fractions varied according to the land use pattern and soil properties such as texture, organic matter, carbonates, and iron and manganese oxide minerals.

Probable causes of hardening of redoximorphic features in Plinthosols of the Araguaia River floodplain, Central region of Brazil

This study aimed to investigate the possible causes or conditions for the greater hardening or more hardened consistency of the plinthic feature, particularly in relation to forms and quantities of iron both in wet and dry conditions, taking into account that variations in the degree of consistency of these features are a phenomenon of great importance in the behavior of the soils of the Araguaia River floodplain, Brazil, due to its wide agricultural use. Soil features were collected at five positions in the profiles, where initial, central, and base plinthic horizons were detected. The separated samples of matrix, mottle, and plinthite were broken by the Instron apparatus and ground into powder. Then, chemical determinations were carried out by X-ray fluorescence (XRF), sulfuric acid attack (H2SO4), citrate-bicarbonate-dithionite (CBD), acid ammonium oxalate, and X-ray diffraction. The greater hardness of the plinthic features of the soils is directly related to the iron content in the oxidized form contained in its fine fraction and iron oxides are the elements responsible for promoting and maintaining the hardening of the features. The presence of the plinthic feature with the adequate hardness requirements was observed in the cases that presented a combination of the following factors: seasonality of the water regime; absolute Fed contents of at least 40.0 g kg−1 associated with a Fed/Fes ratio of at least 0.35; and Fes values equal to or >100 g kg−1 associated with a Fes/Fet ratio of at least 0.70. High Feo/Fed ratios, above 0.07, are indicative of more unstable features and, at the same time, with greater potential for hardening after drying.

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.

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.

Chemical and mineralogical properties of Guanajibo clay (Plinthic Kandiudults)

A profile of Guanajibo clay (Plinthic Kandiudults) was evaluated to determine the ironstone content in the horizons and its chemical, physical and mineralogical properties. The mineralogy was characterized by X-ray diffraction (XRD) and the iron and aluminum oxide content determined by selective dissolution, using citrate-bicarbonate dithionite (CBD) and ammonium oxalate (OX) extractions. Soil color and the color of ground ironstone were determined with Munsell color charts. Soil texture, pH, and effective cation exchange capacity (ECEC) were also evaluated. The highest content of ironstone per unit mass was found in the Ap horizon (4.6%), decreasing in the Bt (0.9%) and Btv1 (1.0%) horizons. An increase in ironstone content was observed in the Btv2 horizon (3.8%), which suggests that this horizon was formed from alluvial deposits containing ironstone. No ironstone was found in the Btv3 horizon. The texture of the Ap horizon was a sandy clay, and the other horizons had a clay texture. Clay content reached a maximum value of 92.8% in the Btv3. The ECEC was low for the whole profile, ranging from 2.37 cmolc/kg in the Ap horizon to 4.37 cmolc/kg in the Btv3 horizon. The low ECEC is indicative of mineralogy dominated by highly weathered clay minerals like kaolinite and iron and aluminum oxides. The XRD analysis confirmed the presence of kaolinite, goethite, quartz, hematite and gibbsite in the clay fraction (&lt; 2 mm) of the soil. A small peak corresponding to 2:1 clay minerals, probably Al-hydroxy interlayer vermiculite or montmorillonite, was observed in clay samples from the Ap, Bt and Btv1 horizons. The major mineral components of the ironstone were goethite, kaolinite and quartz. The fine earth fraction (150 µm) of the Btv1 horizon had a 2.8% CBD extractable iron content and a ratio of OX/CBD less than 0.10, meeting the two requirements established by Soil Taxonomy to qualify as plinthic. The other horizons met the requirement of OX/CBD ratios of less than 0.10, but had less than 2.5% iron extractable by the CBD method. Soil management practices that prevent soil erosion must be implemented in this soil to ward off exposure of plinthite to the surface and its irreversible hardening to ironstone.

A comparison of machine learning algorithms for mapping soil iron parameters indicative of pedogenic processes by hyperspectral imaging of intact soil profiles

AbstractSoil iron (Fe) performs vital functions in the biogeochemical cycles of soil environments. The amount and profile allocation of various Fe parameters can be used as sensitive indicators of soil development and pedogenic processes. This study aimed to evaluate the potential of ground‐based hyperspectral imaging (HSI: 400–1010 nm) spectroscopy to predict and map six Fe parameters indicative of pedogenic processes: total Fe (Fet), dithionite‐citrate‐bicarbonate (DCB)‐extracted Fe (Fed), oxalate‐extracted Fe (Feo), weathering index (FeW), active ratio (FeA) and crystallinity ratio (FeC). In total, 17 intact soil profiles at a depth of 100 ± 5 cm were collected to acquire HSI images. Four non‐linear machine learning techniques, namely, random forest (RF), XGBoost, CatBoost and support vector machine regression (SVMR), were implemented and compared with linear partial least squares (PLS) to identify the models with the best performance for different soil Fe parameters. Our results indicate that the four non‐linear machine learning models outperformed PLS for most soil Fe parameters, with low root mean square error (RMSE) and high Lin's concordance correlation coefficient (LCCC) values. Based on the testing set, SVMR showed better performance over the other tested models for Fet (RMSEP = 2.645 g kg−1, LCCCP = 0.89), Fed (RMSEP = 0.972 g kg−1, LCCCP = 0.97), Feo (RMSEP = 0.273 g kg−1, LCCCP = 0.97), FeW (RMSEP = 0.035, LCCCP = 0.97), FeA (RMSEP = 0.033, LCCCP = 0.97) and FeC (RMSEP = 0.031, LCCCP = 0.97). According to the LCCCP values, soil Fet was predicted to be in substantial agreement by SVMR, and the other soil Fe predictions were considered to be in near perfect agreement. Moreover, SVMR required lower computational costs. Given these results, the combination of HSI spectroscopy and SVMR is recommended due to its more reliable estimation and profile mapping of the selected soil Fe parameters than that of PLS, RF, XGBoost and CatBoost.Highlights Hyperspectral imaging was used to map six soil Fe parameters of intact profiles. Nonlinear machine learning models were compared to select the most suitable model. Nonlinear techniques outperformed the linear PLS model in most cases. SVMR showed higher comprehensive performance than other models. Hyperspectral imaging combined with SVMR is recommended to map soil Fe in profiles.

Combined amendment improves soil health and Brown rice quality in paddy soils moderately and highly Co-contaminated with Cd and As

In situ remediation technology applied aims to not only decrease cadmium (Cd) and arsenic (As) uptake by rice but also improve soil health and rice quality in contaminated paddy soils. Here the effects of a combined amendment, consisting of limestone, iron powder, silicon fertilizer, and calcium-magnesium-phosphate fertilizer, with three application rates (0, 450, and 900 g m−2) on soil health, rice root system, and brown rice quality were compared in moderately versus highly Cd and As co-contaminated paddy fields. After the amendment application, soil pH, cation exchange capacity, four kinds of soil enzyme activities increased (sucrase, urease, acid phosphatase, and catalase), and concentrations of leached Cd/As decreased, as measured by the DTPA (diethylene triamine pentaacetic acid) and TCLP (toxicity characteristic leaching procedure). Changes in the above soil indicators promoted soil health. In both fields, the dithionite-citrate-bicarbonate (DCB)-Fe and DCB-Mn concentration in iron plaque increased and root length became longer. Changes in the above root system indicators reduced the root system's absorption of Cd and As but increased that of nutrients. Under 900 g m−2 treatment, the Cd concentration in brown rice of two sites decreased by 55.8% and 28.9%, likewise inorganic As (iAs) decreased by 50.0% and 21.1%, whereas essential amino acids increased by 20.4% and 20.0%, respectively. Furthermore, the Cd and iAs concentrations in brown rice were <0.2 mg kg−1 (maximum contaminant level of Cd and iAs in the Chinese National Food Safety Standards GB2762-2017 for brown rice) under the 900 g m−2 in the moderately contaminated field. These results suggest the combined amendment can improve soil health and brown rice quality in the moderately and highly Cd- and As-co-contaminated paddy soils, offering potential eco-friendly and efficient remediation material for applications in such polluted paddy soils.

Biochar promotes arsenic sequestration on iron plaques and cell walls in rice roots

The iron (Fe) cycle in the rice-soil system affects arsenic (As) uptake by rice. The effect of Fe on As uptake can be influenced by the addition of biochar, but has not been thoroughly investigated. In this study, the effects of maize straw-derived biochar (MB) on Fe and As translocation were determined by analysing the Fe and As concentrations in pore water, dithionite-citrate-bicarbonate (DCB) extracts, and rice plants. As-contaminated soils were supplemented with 0 or 1% biochar and 0 or 90 mg kg−1 P, and rice plants were grown for 70 d. Results indicated that biochar addition increased the concentrations of Fe and As in pore water, while P did not affect them. Additionally, biochar promoted the accumulation of Fe and As in roots. However, the rice biomass increased by 28% upon biochar addition, indicating that the rice plants became more tolerant to As toxicity with biochar. Specifically, biochar increased the root triphenyl tetrazolium chloride (TTC) reductive intensity, reduced the root H2O2 concentration, and promoted iron plaque (IP) formation. Moreover, the positive correlation between IP/DCB-extractable As and crystalline Fe on the rice root surface indicated that crystalline Fe appeared to be the determinant species of IP and played a central role in As segregation. In addition, biochar increased both crystalline Fe formation on the root surface and the Fe content in the cell wall, which enhanced As sequestration. Overall, rice could effectively tolerate As stress under biochar treatment since As could be retained on the root surface and root cell wall with MB.

Varietal variation and formation of iron plaques on cadmium accumulation in rice seedling

This study examined the impact of iron (Fe) plaque deposition and varietal variation on cadmium (Cd) accumulation in the rice plants (Oryza sativa L.) in a hydroponic experiment under controlled conditions. Fe was applied at the rate of 0, 50 and 100 mg L−1 to the nutrient solution to generate varying amounts of Fe plaque deposition around the root of the rice seedlings. The seedlings were then treated with Cd at the rate of 0, 0.5 and 1.0 mg L−1 in the nutrient solution. Reddish-brown colored Fe plaque is induced gradually on the roots of rice seedlings after Fe supplementation in the nutrient solution. Results showed that the biomass production differed markedly among the rice varieties due to the application of Fe with or without Cd stress. The Quest variety demonstrated the highest capacity of Fe plaque formation compared to the other varieties. The application of Fe and Cd significantly affected the Cd concentration in the citrate-bicarbonate-dithionite (CBD) extracts of roots and in the rice seedlings. The exogenous application of Cd significantly increased the root Cd content, which was greater than the shoot Cd content. The Fe plaque deposition capacity markedly varied among the examined varieties. The Cd concentrations in shoots declined by adding Fe. This study results demonstrated that boosted Fe plaque formation can minimize detrimental effects of Cd on rice shoot growth to some extent, but the root tissues are the main barrier to Cd accumulation and movements in the interior of the rice plants.