Iron Oxides In Soils Research Articles

Characteristics of phosphorus fractions and their driving factors in forest soils with different parent materials in the mid-subtropics, China

With the aim to understand the influence degree and mechanism of parent material and forest type on soil phosphorus component, we analyzed soil P fractions, iron and aluminum oxides, microbial biomass, and phosphatase activity in Castanopsis carlesii and Cunninghamia lanceolata forest soils developed from two types of parent materials, sandstone and granite, in Sanming, Fujian Province. The results showed that both parent material and forest type significantly affected the contents of different P fractions. The contents of total P, labile inorganic/organic P, moderately labile inorganic/organic P, and non-labile P from sandstone-developed soils were significantly higher than those from granite-developed soils. Moreover, soil labile organic P, moderately labile inorganic/organic P and non-labile P fraction in sandstone-developed soils of C. carlesii forest were significantly higher than those of C. lanceolata forest, while the corresponding soil P fractions in granite-deve-loped soils had no significant difference between the two types of forests. The activity of acid phosphatase (ACP) in granite-developed soils was significantly higher than that in sandstone-developed soils under two types of forest, while soil microbial biomass carbon (MBC) and P (MBP) in sandstone-developed soils were significantly higher under C. carlesii forest than under C. lanceolata fo-rest. The content of soil P fractions was significantly positively correlated with the content of different forms of iron and aluminum oxides, MBP, and MBC, but negatively correlated with soil pH and acid phosphatase activity (ACP). Our results indicated that parent material and forest types might affect soil P fractions and characteristics mainly through altering soil iron and aluminum oxides, ACP, MBP in mid-subtropical forest soils.

Adsorption of double-stranded ribonucleic acids (dsRNA) to iron (oxyhydr-)oxide surfaces: comparative analysis of model dsRNA molecules and deoxyribonucleic acids (DNA).

Double-stranded ribonucleic acid (dsRNA) molecules are novel plant-incorporated protectants expressed in genetically modified RNA interference (RNAi) crops. Ecological risk assessment (ERA) of RNAi crops requires a heretofore-missing detailed understanding of dsRNA adsorption in soils, a key fate process. Herein, we systematically study the adsorption of a model dsRNA molecule and of two double-stranded deoxyribonucleic acid (DNA) molecules of varying lengths to three soil iron (oxyhydr-)oxides - goethite, lepidocrocite, and hematite - over a range of solution pH (4.5-10), ionic strength (I = 10-100 mM NaCl) and composition (0.5, 1, and 3 mM MgCl2) and in the absence and presence of phosphate (0.05-5 mM) as co-adsorbate. We hypothesized comparable adsorption characteristics of dsRNA and DNA based on their structural similarities. Consistently, the three nucleic acids (NAs) showed high adsorption affinities to the iron (oxyhydr-)oxides with decreasing adsorption in the order goethite, lepidocrocite, and hematite, likely reflecting a decrease in the hydroxyl group density and positive charges of the oxide surfaces in the same order. NA adsorption also decreased with increasing solution pH, consistent with weakening of NA electrostatic attraction to and inner-sphere complex formation with the iron (oxyhydr-)oxides surfaces as pH increased. Adsorbed NA concentrations increased with increasing I and in the presence of Mg2+, consistent with adsorbed NA molecules adopting more compact conformations. Strong NA-phosphate adsorption competition demonstrates that co-adsorbates need consideration in assessing dsRNA fate in soils. Comparable adsorption characteristics of dsRNA and DNA molecules to iron (oxyhydr-)oxides imply that information on DNA adsorption to soil particle surfaces can inform dsRNA ERA.

Effects of Heavy Metal-immobilizing Bacteria on Reducing Wheat Uptake Cd by Regulation Soil Iron Oxides

Iron oxides are widely distributed in soils and have large specific surface areas and strong affinities with heavy metals, and thus play a significant role in the transformation of heavy metals in soils. Pot experiments were conducted to study the effects of heavy metal-immobilizing bacteria Bacillus megaterium N3, Serratia liquefaciens H12, and strain N3 combination with H12 on the absorption of Cd by ZM-3 (Triticum aestivum L.). The effects of different treatments on soil pH, iron oxide content, and relative abundance of iron oxide bacteria, clay particle mineral composition, and aggregates in wheat rhizosphere were also measured. The results showed that strains N3, H12, and N3+H12 significantly (P<0.05) increased the dry weight (36.5%-75.3%) of wheat grains, and significantly (P<0.05) decreased the content of Cd (30.2%-84.9%) in wheat grains. Furthermore, compared with the single application of strain N3 or H12, the application of strain N3 combined with H12 had a stronger ability to reduce the absorption of Cd by ZM-3. The results suggested that strain N3, combined with H12, decreased the uptake of Cd in wheat by:①increasing the pH (from 6.74 to 7.08) of wheat rhizosphere soil; ② increasing the Fe oxide (67.9%) and goethite contents of wheat rhizosphere soil; ③ increasing the relative abundance of iron-oxidizing bacteria (Leptothrix spp. and Gallionella spp.), and ④ increasing the content of small particle size (<0.25 mm) aggregates. The results provide a theoretical basis and technical support for the use of heavy metal-immobilizing bacteria to repair farmland contaminated by heavy metals.

New insight into adsorption and reduction of hexavalent chromium by magnetite: Multi-step reaction mechanism and kinetic model developing

Magnetite is a common mixed ferrous–ferric iron oxide in soils and sediments, which has both adsorption and reduction capacities for Cr(VI). Previous studies mainly focused on the application of magnetite in Cr(VI) removal from aqueous phase by adsorption, while few efforts has been devoted to the reduction process of adsorbed Cr(VI) on the solid phase. In this study, the kinetic and thermodynamic experiments were simultaneously conducted to verify the relationship between Cr(VI) adsorption and reduction, and SEM-EDS, XPS and Raman spectroscopies were utilized to reveal the mechanisms of Cr(VI) reduction and magnetite transformation. According to the results, Cr(VI) was found to be quickly adsorbed onto magnetite surface with a very high adsorption rate constant of 11.369 d−1, and then the adsorbed Cr(VI) as an intermediate was directly reduced to Cr(III) by the active Fe(II) on magnetite surface with a fast reaction rate constant of 1.606 d−1. Notably, the reduction rate constant gradually decreased as low as 0.032 d−1 with time, which might be induced by the maghemite passive layer formed on the outer-sphere of magnetite, where the adsorbed Cr(VI) could only be reduced indirectly by the Fe(II) located at the inner-sphere of magnetite, and thus the electron transfer rate from internal Fe(II) to adsorbed Cr(VI) became the rate-limiting step of Cr(VI) reduction. Accordingly, a coupling mechanism of “fast adsorption-direct reduction-indirect reduction” was proposed, and a multi-step kinetic model was developed based on that, which had a much better fitting effect (0.936 <R2 < 0.996) than the classical first-order or second-order kinetic models (0.885 <R2 < 0.956). This work highlighted the multi-step reaction mechanism of Cr(VI) with magnetite, especially the distinguished Cr(VI) reduction schemes by magnetite with different passivation extents, and these findings may favor the understanding of the interfacial processes of Cr(VI) on Fe(II) containing minerals in subsurface environment, where the minerals may exist in various passivation degrees due to natural oxidation.

Reduction and stabilization of Cr(VI) in soil by using calcium polysulfide: Catalysis of natural iron oxides

Cr(VI)-contaminated soils could be remediated by using calcium polysulfide (CPS), while natural iron oxides as a main composition of soil would influence the pathways of the remediation. Through kinetic batch tests, the kinetics of Cr(VI) removal from soil, the effects of the contents of natural iron oxides, soil environmental conditions and mechanisms of Cr(VI) removal by using CPS with the presence of natural iron oxides were investigated. The results show that the removal of Cr(VI) by using CPS in soil fitted the pseudo-second-order model best, and the appearance of goethite increased the apparent rate constant from 0.0002 kg mg−1 h−1 to 0.0005 kg mg−1 h−1. The presence of iron oxides enhanced the removal of Cr(VI) by using CPS, and an extended reductive atmosphere of soil was created. The enhancement of Cr(VI) removal increased with the contents of iron oxides from 0 to 9 g kg−1, and declined from 9 to 12 g kg−1. Acidic environment favored the removal of Cr(VI) from soil by using CPS with or without the iron oxides compared to neutral soil and increased it from 87% to 100% because of proton-consuming reactions and electrostatic attraction. Twenty-nine percent of exchangeable and bound-to-carbonates species of chromium declined after the remediation, while 24% bound-to-iron-and-manganese-oxide species increased simultaneously. The findings of the study indicate that natural iron oxides in soils catalyze the reduction of Cr(VI) in soil and facilitate significantly the remediation of Cr(VI)-contaminated soil by using CPS.

Arsenate Adsorption on Different Fractions of Iron Oxides in the Paddy Soil from the Karst Region of China.

Adsorption of arsenate, a common soil contaminant, on natural occurring iron oxides controls arsenate adsorption on soils, especially on iron rich soils. However, soils contain multiple iron minerals and their contributions for arsenate adsorption on the bulk soil are unclear. In this study, Wenzel's sequential extraction procedure (SEP) was used to characterize different fractions of iron oxides in the paddy soils from Guangxi, China. Soils treated with different extractants in the SEP (i.e. soils contained different iron fractions) were used to study arsenate adsorption on different iron fractions in the soil. Langmuir model was used to assess arsenate adsorption capacities of those iron fractions. The results showed that though amorphous iron made up only 16% of the total iron in the soil, it accounted for 82% of the soil adsorption capacity. Besides, arsenate adsorption capacity on the soils could be estimated by linear addition of arsenate adsorption capacity on the amorphous iron oxides fraction and that on the other iron oxides fraction. This study implied a possibility to use iron fractions characterized by Wenzel's SEP to evaluate arsenate adsorption on the bulk soils.

Effect of Organic Matter Content on the Spectral Signature of Iron Oxides across the VIS–NIR Spectral Region in Artificial Mixtures: An Example from a Red Soil from Israel

The investigation of iron oxides in soil using spectral reflectance is very common. Their spectral signal is significant across the visible–near infrared (VIS–NIR) spectral range (400–1000 nm). However, this range overlaps with other soil chromophores, such as those for water and soil organic matter (SOM). This study aimed to investigate the effect of different SOM species on red soil from Israel, which is rich in hematite iron oxide, under air-dried conditions. We constructed datasets of artificially mixed soil and organic matter (OM) with different percentages of added compost from two sources (referred to as A2 and A5). Eighty subsamples of mixed soil–OM were prepared for each of the OM (compost) types. To investigate the effect of OM on the strong iron-oxide absorbance at 880 nm, we generated two indices: CRDC, the absorbance spectral depth change at 880 nm after continuous removal, and NRIR, the normalized red index ratio using 880 and 780 nm wavelengths. The different OM types influenced the soil reflectance differently. At low %SOM, up to 1.5%, the OM types behaved more similarly, but as the OM content increased, their effect on the iron-oxide signal was greater, enhancing the significant differences between the two OM sources. Moreover, as the SOM content increased, the iron-oxide signal decreased until it was completely masked out from the reflectance spectrum. The masking point was observed at different SOM contents: 4% for A5 and 8% for A2. A mechanism that explains the indirect chromophore activity of SOM in the visible region, which is related to the iron-oxide spectral features, was provided. We also compared the use of synthetic linear-mixing practices (soil–OM) to the authentic mixed samples. The synthetic mixture could not imitate the authentic soil reflectance status, especially across the overlapping spectral position of the iron oxides and OM, and hence may hinder real conditions.

Response of soil microbial communities to additions of straw biochar, iron oxide, and iron oxide-modified straw biochar in an arsenic-contaminated soil.

Anthropogenic activities have caused extensive arsenic (As) contamination in soils. The role of biochar in the remediation of As-contaminated soils has been attracting attention lately. In this study, effects of straw biochar, iron oxide, and iron oxide-modified biochar on soil microbial community composition and soil chemical properties were tested in an As-contaminated soil. After 9months of incubation, soil chemical properties and microbial communities were analyzed. Our results showed that biochar addition significantly increased soil pH value, soil organic carbon (SOC) concentration, and the ratio of soil carbon to nitrogen (soil C:N ratio) but decreased soil dissolved organic C. Adding iron oxide also increased soil pH value, while iron oxide-modified biochar decreased it. Interestingly, compared with the control, all treatments significantly decreased soil total microbial biomass and biomasses of soil bacteria, fungi, Actinomyces, and protozoa. In addition, significantly positive correlations were found between soil pH and soil total microbial biomass as well as bacterial, Actinomyces, and arbuscular mycorrhizal fungal biomass. There were negative relationships between SOC, soil C:N ratio, and all soil microbial biomass indicators in all treatments. These results indicated that biochar and iron oxide-modified biochar affected soil microbial community composition by altering the soil C:N ratio, but iron oxide affected it via adjusting soil pH. Furthermore, the iron oxide-modified biochar effects on soil microbial community and soil chemical properties are not the same as the additive effects of biochar and iron oxide alone, and its effect on soil microbial community is regulated by the soil C:N ratio. These findings will help guide the development of remediation practices for As-contaminated soil using biochar.

Natural organic matter decreases uptake of W(VI), and reduces W(VI) to W(V), during adsorption to ferrihydrite

Tungsten is both naturally occurring and an anthropogenically released contaminant metal in soils, sediments and water systems that typically exits as the soluble tungstate oxyanions, W(VI)O42−. Tungsten mobility and fate are strongly dependent on the adsorption of tungstate to mineral surfaces. However, environmental mineral surfaces are commonly coated with natural organic matter (NOM), and the role of this coating in the tungsten adsorption process, and thus in controlling tungsten reactivity and transport, is unclear. This study investigates W(VI) adsorption to ferrihydrite (Fh), a ubiquitous iron (hydr)oxide in soils and sediments, both in the absence and presence of humic acid (HA), a widely occurring type of NOM, using batch experiments coupled with spectroscopic and thermodynamic techniques. Kinetic results indicate that access to the adsorption sites for W(VI) on the organomineral surfaces is limited when Fh is coprecipitated with HA. Commensurate with this observation, batch experiments show that HA decreases W(VI) adsorption to Fh over a wide pH range (4–11), and this inhibitory effect is more pronounced at higher HA concentration. X-ray photoelectron spectroscopy (XPS) measurements demonstrate the formation of inner-sphere type W complexes on both the Fh and HA fraction of the Fh-HA binary composite. In particular, ~40% of the adsorbed W(VI) species is reduced to W(V) in the presence of HA. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) results show the presence of poly tungstate species on Fh, particularly at lower pH and in the presence of HA. Isothermal titration calorimetry shows that W(VI) adsorption to Fh is an exothermic process both in the presence and absence of HA, and that process is accompanied by a positive entropy. The findings of this work suggest that NOM not only mobilizes tungstate but also reduces tungstate from W(VI) to W(V) at environmental iron (hydr)oxide-water interfaces, which is of significance for evaluating the migration and bioavailability of tungsten in both natural and contaminated environments.

Paddy periphyton reduced cadmium accumulation in rice (Oryza sativa) by removing and immobilizing cadmium from the water–soil interface

Periphyton plays a significant role in heavy metal transfer in wetlands, but its contribution to cadmium (Cd) bioavailability in paddy fields remains largely unexplored. The main aim of this study was to investigate the effect of periphyton on Cd behavior in paddy fields. Periphyton significantly decreased Cd concentrations in paddy waters. Non-invasive micro-test technology analyses indicated that periphyton can absorb Cd from water with a maximum Cd2+ influx rate of 394 pmol cm−2 s−1 and periphyton intrusion significantly increased soil Cd concentrations. However, soil Cd bioavailability declined significantly due to soil pH increase and soil redox potential (Eh) decrease induced by periphyton. With periphyton, more Cd was adsorbed and immobilized on organic matter, carbonates, and iron and manganese oxides in soil. Consequently, Cd content in rice decreased significantly. These findings give insights into Cd biogeochemistry in paddy fields with periphyton, and may provide a novel strategy for reducing Cd accumulation in rice.

Organo-mineral complexes protect condensed organic matter as revealed by benzene-polycarboxylic acids

Condensed organic matters (COM) with black carbon-like structures are considered as long-term carbon sinks because of their high stability. It is difficult to distinguish COM from general organic matter by conventional chemical analysis, thus the contribution by and interaction mechanisms of organo-mineral complexes in COM stabilization are unclear and generally neglected. Molecular markers related to black carbon-like structures, such as benzene polycarboxylic acids (BPCAs), are promising tools for the qualitative and quantitative analysis of COM. In this study, one natural soil and two cultivated soils with 25 y- or 55 y-tillage activities were collected and the distribution characteristics of BPCAs were detected. All the investigated soils showed similar BPCA distribution pattern, and over 60% of BPCAs were detected in clay fraction. The extractable BPCA contents were substantially increased after mineral removal. The ratios of BPCA contents before and after mineral removal indicate the extent of COM-mineral particle interactions, and our results suggested that up to 73% COM were protected by mineral particles, and more stronger interactions were noted on clay than on silt. The initial cultivation dramatically decreased COM-clay interactions, and this interaction was recovered only slowly after 55-y cultivation. Kaolinite and muscovite are important for COM protection. But a possible negative correlation between BPCAs and reactive iron oxides of the cultivated soils suggested that iron may promote COM degradation when disturbed by tillage activities. This study provided a new angle to study the stabilization of COM and emphasized the importance of organo-mineral complexes for COM stabilization.

水稻土中铁氧化物对产甲烷古菌群落结构的影响

水稻土中铁氧化物对产甲烷古菌群落结构的影响

Spectroscopic diagnosis of zinc contaminated soils based on competitive adaptive reweighted sampling algorithm and an improved support vector machine

Soil contamination by heavy metals is of great concern due to the increasing intensity of anthropogenic activities and requires a reliable approach to rapidly diagnose heavy metal pollution in soils. In this study conducted in northern Shandong Province, China, a total of 110 soil samples were collected to determine the zinc concentration and their visible and near-infrared reflectance spectroscopy was applied to diagnose zinc contaminated soils. Pearson’s correlation analysis and competitive adaptive reweighted sampling algorithm were compared to select active bands as independent variables. Partial least squares regression and particle swarm optimization support vector machine method were applied to estimate zinc concentration and a leave-one-out cross-validation was used to validate data sets. Results show that the obvious accumulations of zinc were in topsoil at the study area. Partial least squares regression estimates of zinc concentrations were improved with competitive adaptive reweighted sampling algorithm. The optimized support vector machine model, based on the first derivative of spectra, achieved the best accuracy for zinc concentration estimation with ratio of performance to deviation of 2.74. The method of active band extracting by competitive adaptive reweighted sampling algorithm was responsible for the improved model performance because competitive adaptive reweighted sampling algorithms determined active bands which can be useful for interpreting complex relations with soil compounds, such as soil organic matter, iron oxide, and clay minerals. In addition, the climate in specific research areas plays an important role in active band selection as a result of soil constituent changes. This study indicated that a combination model of competitive adaptive reweighted sampling algorithm and optimized support vector machine (R2 = 0.88; RMSE = 6.74, RPD = 2.74) can be recommended for estimating zinc concentrations in polluted soils and can provide improved understanding of the estimation mechanism and an alternative approach for diagnosing soil contamination.

Hematite enhances the immobilization of copper, cadmium and phosphorus in soil amended with hydroxyapatite under flooded conditions

Hydroxyapatite (HA) is often applied as chemical amendment in soils contaminated with trace metals such as copper (Cu) and cadmium (Cd). Large amounts of iron oxides in red soil may interacts with phosphate released from HA and influence trace metal immobilization of HA. Here we simulated a red paddy soil with 1–5% wt iron oxides by adding hematite and evaluated the Cu and Cd availability in soil amended with HA under flooded conditions. Changes in phosphorus and iron oxide fractions were also evaluated after a 42-day flooding incubation experiment. Results showed that the addition of HA-only and hematite-only decreased soil redox potential and increased pore water pH compared to the control. HA combined with hematite could effectively decrease phosphate, Cu and Cd in soil pore water compared to HA-only. Additionally, HA combined with hematite could also increase soil pH and decrease soil CaCl2-extractable Cu and Cd. In particular, HA combined with 5% hematite was most effective in reducing soil exchangeable fractions of Cu and Cd by 53.7% and 65.6% compared to the control, respectively. The addition of HA-only increased water-soluble phosphorus, NaHCO3-extractable inorganic phosphorus, NaOH-extractable inorganic phosphorus, and HCl-extractable phosphorus. Conversely, HA combined with hematite treatments decreased NaHCO3-extractable inorganic phosphorus by 11.3–43.0% compared to HA-only. Vivianite and metal–phosphate precipitates were not observed using the Visual MINTEQ model, X-ray diffraction, and chemical analysis. The addition of hematite with or without HA increased free and crystal iron oxide fractions, while it substantially enhanced amorphous iron oxides in the soil. Thus, this study indicates that soil with high hematite content could enhance Cu and Cd immobilization while decreasing phosphorus availability in the red paddy soil amended with HA under the flooded conditions.

Application of chemical and biological tests for estimation of current state of a tailing dump and surrounding soil from the region of Tarniţa, Suceava, Romania.

This paper presents the results from a study on the current state of tailing dump, surrounding soil and water in the region of Tarniţa-Suceava, Romania. A number of chemical analyses and germination tests were applied in an attempt to estimate the ability of soil to maintain the plants growing, the bioavailability, and heavy metals uptake. Total heavy metals, exchangeable metals, acidity, and carbon and nitrogen content were determined. A modified sequential extraction method was used to determine geochemical phase distribution of heavy metals. The most abundant heavy metals in the studied samples were Cu, Zn, and Pb. Elevated concentrations of As were also found. The results from sequential extraction revealed that up to 51% of copper was retained by amorphous and crystalline iron oxides in soil. Higher content of lead was noticed in amorphous iron oxide fraction. The heavy metal concentration in river water during dry season varied from 0.13 mg/L (Fe) to 4.2 mg/L (Zn) and was below the maximum contamination level for drinking water. The soil toxicity and heavy metal bioavailability of tailing dump material and surrounding soils were studied by germination tests. The germinated plantlets on the studied soils were found to accumulate elevated concentrations of heavy metals thus indicating the bioavailability of soil contaminants. Soil decontamination by distilled water or magnesium nitrate solution was found to be efficient enough to improve the capability of the studied soils to support the germination process.

Soil Organic Matter and Phosphate Sorption on Natural and Synthetic Fe Oxides under in Situ Conditions

Iron (Fe) oxides in soils are strong sorbents for environmentally important compounds like soil organic matter (SOM) or phosphate, while sorption under field conditions is still poorly understood. We installed polyvinyl chloride plastic bars which have been coated either with synthetic Fe or manganese (Mn) oxides for 30 days in a redoximorphic soil. A previous study revealed the formation of newly formed ("natural") Fe oxides along the Mn oxide coatings. This enables us to differentiate between sorption occurring onto the surfaces of synthetic versus natural Fe oxides. After removal of the bars, they were analyzed by nanoscale secondary ion mass spectrometry (NanoSIMS) to study the distribution of Fe (56Fe16O-), SOM (12C14N-), and phosphorus (31P16O2-) at the microscale. Image analysis of individual Fe oxide particles revealed a close association of Fe, SOM, and P resulting in coverage values up to 71%. Furthermore, ion ratios between sorbent (56Fe16O-) and sorbate (12C14N- and 31P16O2-) were smaller along the natural oxides when compared with those for synthetic Fe oxides. We conclude that both natural and synthetic Fe oxides rapidly sequester SOM and P (i.e., within 30 days) but that newly, natural formed Fe oxides sorbe more SOM and P than synthetic Fe oxides.

Spatial variability of iron oxides in soils from Brazilian sandstone and basalt

Iron oxides as goethite (Gt) and hematite (Hm) are key minerals to better understand the soil–landscape relationships. Soil samples were collected at three stages of landscape dissection from the geological formations of Vale do Rio do Peixe (sandstone) and Serra Geral (basalt) in the Western Paulista Plateau (WPP), Brazil. Both iron oxides were quantified by X-ray diffraction (XRD) and diffuse reflectance spectroscopy (DRS), and the results were subjected to geostatistical analysis in order to assess the usefulness of DRS for characterizing the spatial variability in Gt and Hm. The prevalence and spatial variability of Hm and Gt in the soils were governed by the sandstone/basalt lithological contrast and landscape dissection. Iron oxides in the clay fraction exhibited high spatial variability over a large area and can be robust indicators of geological diversity and landscape dissection in pedoenvironments with low or high contents of iron oxides. Goethite had the highest spatial variability. Based on the spatial pattern of the differences between DRS and XRD estimates, the saturated red color in soil made DRS less useful for quantifying Hm in environments with high iron oxide contents. The maps indicate the sensitivity of XRD and DRS techniques to represent Hm and Gt spatial variability patterns. Gt was more sensitive to landscape dissection while Hm sensitive to lithology. Thus, the DRS technique is efficient in characterizing the spatial variability of these soil oxides in large areas, even considering the complex relations between soil and landscape.

Study on Hyperspectral Characteristics and Content Estimation of Soil Iron Oxide

The iron oxide in the soil comes from the re-deposition of the weathering products of the parent material. It can be divided into free iron oxide (Fed) and amorphous iron oxide (FeO) according to chemical methods. The content of iron oxide in soil can be used as the soil type in soil system classification. Diagnostic indicators are considered to be a function of soil development and development. Although the traditional method for measuring iron oxide in soil has high precision, it is time-consuming and laborious, and it is impossible to obtain real-time monitoring data of soil physical and chemical properties in time. Hyperspectral remote sensing technology can quickly obtain the spectral information of soil by using hyperspectral instrument, combine the chemical data measured indoors with the physical model to establish the relationship between soil properties, realize the quantitative estimation of soil information, and have the tradition of rapid and current situation. The unparalleled advantages of soil iron oxide measurement methods provide broad application prospects for understanding soil iron oxide spectral information and its content prediction.

Iron oxides selectively stabilize plant‐derived polysaccharides and aliphatic compounds in agricultural soils

AbstractA strong link exists between iron oxides and soil organic carbon (SOC). However, the role of iron oxides in the preservation of SOC in agricultural soil remains poorly understood. In this study, we comprehensively examined the concentration, molecular composition and biological sources of iron oxide‐bound organic carbon (Fe‐bound OC) in arable soils collected from 12 sites in central and east China. The results indicated that 6.2–31.2% of the SOC was bound to iron oxides in agricultural soil and that the binding mechanisms varied from adsorption in most soils to coprecipitation in those with a large content of organic carbon. The distribution of Fe‐bound OC showed no clear variation in relation to site, but Fe‐bound OC reached a peak in soils with an annual mean temperature of 16.4°C. Correlation analysis demonstrated that TOC might be the main determinant for the amount of Fe‐bound OC and that the binding mechanism is influenced by both TOC and the active Fe ratio. Comprehensive studies of C/N, 13C isotope and synchrotron radiation‐based Fourier transform infrared (SR‐FTIR) spectroscopy showed that iron oxides selectively protected plant‐derived aliphatic compounds and polysaccharides in agricultural soil. This study revealed the quantitative characterization, biological sources and molecular composition of Fe‐bound OC in arable soils, which provides useful information for evaluating and managing the global C cycle under the framework of climate change.

The cation exchange behavior of tylosin in loess-derived soil

Tylosin (Tyl) is a veterinary antibiotic commonly used in swine and poultry production. Due to metabolic inefficiencies, it enters the environment through manure applications. Ion exchange is an important retention mechanism for Tyl, particularly for smectite clay. The objectives of this study are to characterize the exchange interactions of Tyl with common soil cations in subsoil horizons that contain smectite and to investigate the interactions using in situ Fourier transform infrared (FTIR) spectroscopy. Adsorbed Tyl in pH neutral, smectitic subsoil horizons is divided into exchangeable and nonexchangeable forms. The percentage of adsorbed Tyl that is exchangeable varies from 36% to 43% when Na+ is the competing cation, and from 57% to 66% when Ca2+ competes. In NaX-TylX binary exchange systems, neither Na+ nor Tyl+ is preferred by the clay exchange phase, and the Vanselow selectivity coefficients (KV) for the NaX→TylX exchange reaction range between 0.79 and 1.41. In the CaX2-TylX systems, Tyl+ is preferred by the clay exchange phase when the equivalent fraction of TylX (ETylX) is less than 0.4. The KV values for the CaX2→TylX exchange reaction are at a maximum at the lowest ETylX values, with 17.6 <KV < 58.1, then decrease with increasing ETylX to 1.34 <KV < 6.28. Adsorbed Tyl masks the CEC of the soil clays; the effect is greatest in systems that are initially Tyl-saturated, and is attributed to the steric effects of the large Tyl molecule. In situ FTIR indicates that Tyl interacts with soil iron oxides through the dimethylamine moiety.