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1.
As repeatedly reported, soil flooding improves the availability of P to rice. This is in contrast with an increased P sorption in paddy soils. The effects of soil flooding on the transformation of Fe oxides and the adsorption/desorption of P of two paddy soils of Zhejiang Province in Southeast‐China were studied in anaerobic incubation experiments (submerging with water in N2 atmosphere). Soil flooding significantly increased oxalate‐extractable Fe (Feox), mainly at the expense of dithionite‐soluble Fe (FeDCB), as well as oxalate‐extractable P (Pox), but decreased the ratio of Pox/Feox. Flooding largely increased both, P adsorption and the maximum P adsorption capacity. The majority of newly sorbed P in the soils was Pox, but also more newly retained P was found to be not extractable by oxalate. Flooding also changed the characteristics of P desorption in the soils. Due to a decrease of the saturation index of the P sorption capacity, P adsorbed by flooded soils was much less desorbable than that from non‐flooded soils. There are obviously significant differences in the nature of both, the Feox and Pox fractions under non‐flooded and flooded conditions. The degree of the changes in Feox, Pox, P adsorption and P desorption by flooding depended on the contents of amorphous and total Fe oxides in non‐flooded soils. Our results confirm that the adsorption and desorption behavior of P in paddy soils is largely controlled by the transformation of the Fe oxides. The reasons of the often‐reported improved P availability to rice induced by flooding, in spite of the unfavorable effect on P desorbability, are discussed.  相似文献   

2.
Minerals with large specific surface areas promote the stabilization of soil organic matter (SOM). We analysed three acidic soils (dystric, skeletic Leptic Cambisol; dystric, laxic Leptic Cambisol; skeletic Leptic Entic Podzol) under Norway spruce (Picea abies) forest with different mineral compositions to determine the effects of soil type on carbon (C) stabilization in soil. The relationship between the amount and chemical composition of soil organic matter (SOM), clay content, oxalate‐extractable Fe and Al (Feo; Alo), and dithionite‐extractable Fe (Fed) before and after treatment with 10% hydrofluoric acid (HF) in topsoil and subsoil horizons was analysed. Radiocarbon age, 13C CPMAS NMR spectra, lignin phenol content and neutral sugar content in the soils before and after HF‐treatment were determined and compared for bulk soil samples and particle size separates. Changes in the chemical composition of SOM after HF‐treatment were small for the A‐horizons. In contrast, for B‐horizons, HF‐soluble (mineral‐associated) and HF‐resistant (non‐mineral‐associated) SOM showed systematic differences in functional C groups. The non‐mineral associated SOM in the B‐horizons was significantly depleted in microbially‐derived sugars, and the contribution of O/N‐alkyl C to total organic C was less after HF‐treatment. The radiocarbon age of the mineral‐associated SOM was younger than that of the HF‐resistant SOM in subsoil horizons with small amounts of oxalate‐extractable Al and Fe. However, in horizons with large amounts of oxalate‐extractable Al and Fe the HF‐soluble SOM was considerably older than the HF‐resistant SOM. In acid subsoils a specific fraction of the organic C pool (O/N‐alkyl C; microbially‐derived sugars) is preferentially stabilized by association with Fe and Al minerals. Stabilization of SOM with the mineral matrix in soils with large amounts of oxalate‐extractable Alo and Feo results in a particularly stable and relatively old C pool, which is potentially stable for thousands of years.  相似文献   

3.
Differentiation of oxalate soluble iron oxides According to their mode of formation the oxalate soluble Fe(III)-oxides show different rates of dissolution especially within the first 30 min of oxalate extraction. Precipitates formed by bacterial or oxidative decomposition of ferric citrate have a very high dissolution rate with more than 90 percent being dissolved within 30 min. Ferrihydrites prepared by alkaline hydrolysis of Fe(III) salts, after slow drying, dissolve to about 60 to 80 percent within this period, whereas similar products quenched with liquid N2 dissolve more rapidly. A plot of (Fe30/Feo) vs. (Feo/Fed) allows to draw a boundary line between the field of podzol values and the field of the other pedogenic iron oxides, as from brown earths and black earths (chernozems).  相似文献   

4.
Soils that are forming on volcanic parent materials have unique physical and chemical properties and in most cases, on wet and humid climates, are classified as Andisols. The main purpose of this study is to examine if the soils that are forming on volcanic materials under a dry Mediterranean climate, in Nisyros Island (Greece), meet the requirements to be classified as Andisols. Soils from seven sites were sampled and examined for their main physico-chemical properties and selective dissolution analysis. Dithionite–citrate–bicarbonate (DCB) extractable Al and Fe (Áld, Fed), acid ammonium oxalate extractable Al, Fe, and Si (Álo, Feo and Sio), and sodium pyrophosphate extractable Al and Fe (Alp, Fep) were measured. In addition, Al and Si were determined after reaction with hot 0.5 M NaOH, (AlNaOH and SiNaOH) and with Tiron-(C6H4Na2O8S2), (AlT and SiT). P-retention was also measured. The soils are characterised by coarse texture, low organic matter content, low values of cation exchange capacity (CEC), and high pH values. Values of Sio, Alo and Feo are less than 0.022%, 0.09% and 0.35% respectively, highlighting the lack of noncrystalline components. The ratio (Fed–Feo)100/Fed is quite high expressing the degree of crystallisation of free iron oxides. For all samples tested, values of the Alo + 1/2Feo index are extremely low (< 0.24%). High SiNaOH and SiT (arising 2.76% and 2.18% respectively) indicate the presence of silica in amorphous forms. P-retention values are very low (< 12.6%). The results indicated the absence of noncrystalline minerals except for amorphous silica, and do not exhibit andic or vitric soil characteristics to be classified as Andisols.  相似文献   

5.
Losses of phosphorus (P) to water that follow manure applications can be high while water treatment residuals (WTR) have an appreciable capacity to sorb soluble P which is an important risk factor in determining the susceptibility of manure P to run‐off losses. The objective of this study was to assess whether co‐blending WTR with dairy cow manure prior to surface application would reduce P concentrations in run‐off from grassland. An alum‐derived WTR was collected from a water treatment works (WTW), dried and characterized for its phosphorus sorption capacity (PSC) based on oxalate‐extractable Al and Fe. Multipoint P sorption isotherms were used to calculate the Langmuir P sorption maximum (Pmax) and equilibrium P concentration (EPC0). The WTR contained 170 g Alox/kg and 2.2 g Feox/kg with a nominal long‐term PSC of 118 g/kg. Following a 6 day incubation of WTR, the Langmuir Pmax was 82.6 g/kg and the EPC0 of 0.13 mg P/L. Laboratory incubations of manure co‐blended with WTR indicated that 144 g WTR/kg dry matter (DM) manure significantly lowered (P < 0.001) manure WSP by 71.5 ± 16.6% after 108 h, but lower WTR mixing rates of 72 and 36 g WTR/kg had no statistical effect on manure WSP. Results from a field experiment using simulated rain on 0.5‐m2 grassland plots showed no significant effect on run‐off P 2 days after applying 50 m3/ha of 6% DM manure co‐blended WTR at rates of 150 and 250 g WTR/kg.  相似文献   

6.

Purpose

We review 2,4-dichlorophenoxyacetic acid (2,4-D) and other phenoxy herbicide sorption experiments.

Methods

A database with 469 soil–water distribution coefficients K d (in liters per kilogram) was compiled: 271 coefficients are for the phenoxy herbicide 2,4-D, 9 for 4-(2,4-dichlorophenoxy)butyric acid, 18 for 2-(2,4-dichlorophenoxy)propanoic acid, 109 for 2-methyl-4-chlorophenoxyacetic acid, 5 for 4-(4-chloro-2-methylphenoxy)butanoic acid, and 57 for 2-(4-chloro-2-methylphenoxy)propanoic acid. The following parameters characterizing the soils, solutions, or experimental procedures used in the studies were also compiled if available: solution CaCl2 concentration, pH, pre-equilibration time, temperature, soil organic carbon content (f oc), percent sand, silt and clay, oxalate extractable aluminum, oxalate extractable iron (Oxalate Fe), dithionite–citrate–bicarbonate extractable aluminum, dithionite–citrate–bicarbonate extractable iron (DCB Fe), point of zero negative charge, anion exchange capacity, cation exchange capacity, soil type, soil horizon or depth of sampling, and geographic location. K d data were also compiled characterizing phenoxy herbicide sorption to the following well-defined sorbent materials: quartz, calcite, α-alumina, kaolinite, ferrihydrite, goethite, lepidocrocite, soil humic acid, Fluka humic acid, and Pahokee peat.

Results

The data review suggests that sorption of 2,4-D can be rationalized based on the soil parameters pH, f oc, Oxalate Fe, and DCB Fe in combination with sorption coefficients measured independently for humic acids and ferrihydrite, and goethite.

Conclusions

Soil organic matter and iron oxides appear to be the most relevant sorbents for phenoxy herbicides. Unfortunately, few authors report Oxalate Fe and DCB Fe data.  相似文献   

7.
Abstract

The importance of various soil components on copper (Cu) retention by Spodosois was investigated. Copper sorption and extraction were conducted on samples from the B horizon from six Danish Spodosois. The investigation was conducted on untreated samples, on hydrogen peroxide‐treated samples (to remove organic matter), on oxalate‐treated samples [to remove amorphous to poorly crystalline aluminum (Al) and iron (Fe) oxides], on hydroxylamine‐treated samples [to remove manganese (Mn) oxides]. Subfractions treated with hydrogen peroxide (H2O2) were further treated with oxalate and citrate‐bicarbonate‐dithionite (CBD). Sorption of Cu from an initial 10‐6 M solution after 48 hours was determined in the pH range 3 to 7 using 0.1M sodium nitrate (NaNO3) as the background electrolyte. The pH‐dependent sorption curve (sorption edge) was shifted to a higher pH with decreasing Al oxide content in the soils, and for the treated sample after removal of organic matter and Al and Fe oxides. A negligible effect was seen after removal of the Mn oxides because of their low abundance. Extraction of sorbed Cu at pH 4 to 6 with 0.1M nitric acid (HNO3) for 24 hours confirmed the sorption results, in inasmuch as removal of the Al (and Fe) oxides increased Cu extractability. Therefore, it was concluded that in the soils investigated, Cu retention is mainly determined by the oxalate‐extractable Al fraction with a minor contribution due to crystalline Fe oxides.  相似文献   

8.
The prediction of the mobility of arsenic (As) is crucial for predicting risks in soils contaminated with As. The objective of this study is to predict the distribution of As between solid and solution in soils based on soil properties and the fraction of As in soil that is reversibly adsorbed. We studied adsorption of As(V) in suspensions at radiotrace concentrations for 30 uncontaminated soils (pH 4.4–6.6). The solid–liquid distribution coefficient of As (Kd) varied from 14 to 4430 l kg?1. The logarithm of the concentration of oxalate‐extractable Fe explained 63% of the variation in log Kd; by introducing the logarithm of the concentration of oxalate‐extractable P in the regression model, 85% of the variation in log Kd is explained. Double labelling experiments with 73As(V) and 32P(V) showed that the As to P adsorption selectivity coefficient decreased from 3.1 to 0.2 with increasing degree of P saturation of the amorphous oxides. The addition of As(V) (0–6 mmol kg?1) reduced the Kd of 73As up to 17‐fold, whereas corresponding additions of P(V) had smaller effects. These studies suggest that As(V) is adsorbed to amorphous oxides in soils and that sites of adsorption vary in their selectivity in respect of As and P. The concentration of isotopically exchangeable As in 27 contaminated soils (total As 13–1080 mg kg?1) was between 1.2 and 19% (mean 8.2%) of its total concentration, illustrating that a major fraction of As is fixed. We propose a two‐site model of competitive As(V)–P(V) sorption in which amorphous Fe and Al oxides represent the site capacity and the isotopically exchangeable As represents the adsorbed phase. This model is fitted to 73As adsorption data of uncontaminated soils and explains 69% of the variation of log Kd in these soils. The log Kd in contaminated soils predicted using this two‐site model correlated well with the observed log Kd (r = 0.75). We conclude that solubility of As is related to the available binding sites on amorphous oxides and to the fraction of As that is fixed.  相似文献   

9.
Abstract

Highly calcareous soils are abundant in Iran. The calcium carbonate equivalent (CCE) of these soils reach up to 650 g kg?1. Although phosphorus (P) fertilizer is being widely used in these soils, little information, if any, is available about P status in such soils. The objectives of this study were to 1) determine inorganic P forms in 18 surface soils of southern Iran, 2) study P readsorption during different stages of fractionation schemes, 3) assess the ability of NaOH to extract aluminum (Al)‐P, and 4) evaluate the relationships between P availability indices and inorganic P forms. Eighteen soil samples with a wide range of physicochemical properties were selected for this study. Inorganic P forms was determined by sequential extraction with NaHCO3, NH4OAc, NH4F, NaOH, citrate dithionite (CD), and H2SO4, which are referred to as Ca2‐P, Ca8‐P, Al‐P, Fe‐P, occluded P (O‐P), and Ca10‐P. Phosphorus readsorption in different stages was determined by 1 M MgCl2. Furthermore, a fractionation scheme without an NH4F step was used to evaluate the ability of NaOH to extract Al‐P. NaHCO3 (Olsen‐P) and MgCl2‐extractable P (Exch‐P) were regarded as P-availability indices. The abundance of different P forms was in the order Ca2‐P<Fe‐P<Al‐P<O‐P<Ca8‐P<Ca10‐P. Ca2‐P was highly correlated with Olsen‐P and Exch‐P. Ca2‐P, Olsen‐P, and Exch‐P showed a relationship with CCE, citrate–bicarbonate–dithionite extractable Fe (Fed), and Al (Ald). Phosphorus readsorption appeared to be important only in the Ca8‐P step, and the content of readsorbed P was related to Ca8‐P, CCE, and clay content of the soils. In the present study, Al‐P and Fe‐P accounted for 10 and 5% of the sum of the inorganic P fractions, respectively, and Fe‐P showed a strong relationship with Feo, whereas Al‐P showed a significant relationship with oxalate‐extractable Al (Alo) and Ald. It was found that one extraction with NaOH is not a good indicator for Fe‐ and Al‐P, and the ability of NaOH to extract Al‐P was reduced with increase in Al‐P content.  相似文献   

10.
Abstract

The content of various forms of iron (Fe) (free, reducible, and organic) were determined by selective extraction methods in three wetland profiles between 1993 and 1995 seasons. The result showed that Fe distribution was in the order: dithionite (Fed) > hydroxylamine (FeH) > pyrophosphate (Fep) iron in the three pedons. The hydroxylamine‐Fe constituted between 10–42% (1993), 20–47% (1994), and 10–12% (1995) of the total free Fe oxides. The pyrophosphate‐Fe, on the other hand, constituted between 0.2–1.0% (1993), 19–52% (1994), and 3–9% (1995) of the total free Fe oxides. Dithionite‐Fe (total free iron oxides) content increases with the increasing depth, while hydroxylamine‐Fe decreases, suggesting that larger proportions of Fe oxides are present as crystalline forms in the lower horizons. The active Fe ratios were generally high in the top soils and low in the subsoil. It ranged between 0.03 and 0.69 (1993), 0.05 and 68 (1994), 0.05 and 0.53 (1995) in all pedons. This suggests that poor drainage slowed down soil development. Highly significant correlations (0.1%) were evident between phosphorus (P) and organic carbon; ECEC and base saturation; FeH and active Fe ratio. Significant correlations (1%) were also evident between Fe2+ and organic carbon; P and FeH; ECEC and clay. Furthermore, significant correlations (5%) were also obtained between clay and Fed; pH and Fed; active Fe ratio and P; FeH and clay; active Fe ratio and Fed.  相似文献   

11.
The various iron fractions were quantified by selective dissolution (Fed, Feo, Fet) in four Red Mediterranean soils, developed on metarhyolite and metadolerite. They were similar in all profiles. A strong trend of iron removal from the surface horizon and of its subsequent illuvial translocation to the argillic horizons was observed. In all profiles, Feo was not related to the organic matter content indicating the Mediterranean xeric soil environment. The Feo/Fed ratio and the percentage of crystalline iron oxides (Fed-Feo) suggested that the pedoenvironment in which the profiles P1, P2 were formed, allowed the high crystallization of iron oxides. As indicated by the Fed/Fet values, the weathering process was more intense in the metarhyolite-developed soils. In contrast, the metadolerite-developed soils present conditions of poorly crystallized iron oxides and a lower degree of development.  相似文献   

12.
Iron forms which characterize the clay fraction of four Cambisols from granite and the distribution of iron oxides in the soil profiles are studied by Mössbauer spectroscopy, X-ray diffractometry and selected chemical methods. The results obtained show that about 50 % of the Fet(6–10 % Fe) is not extracted by dithionite. The Feo/Fed ranges between 0.15 and 0.35. Oxalate dissolves goethite in different amounts, probably depending on the particle size and the Al-substitution level. The highest amount of hematite is found in the samples characterized by the lowest internal magnetic fields, suggesting that the soil distribution of hematite and goethite is related to environmental factors favouring or disfavouring Al-for-Fe substitution and crystallinity of the oxides.  相似文献   

13.
A soil toposequence in NE Italy was studied, which consists of a terra rossa on Cretaceous limestone on the upper slope grading downwards into a colluvial fan with terra rossa material and finally into alluvial river sediments. It is postulated that the red colluviated terra rossa material has come under a moister hydroregime which provided reducing conditions. Because hematite of the terra rossa dissolved preferentially over goethite, as shown by quantitative Fe oxide mineralogy, soil color changed from 2.5YR to 7.5YR. The soils contained two types of concretions, red ones with a low Feo/Fed ratio and a high content of hematite and low content of Mn-oxides and black ones with a high Feo/Fed ratio, a small amount of hematite and abundant Mn oxides. The red concretions are therefore considered as inherited from an earlier period of pedogenesis whereas the black ones are neoformed in the present pedoenvironment. This is further supported by the lower Al-for-Fe substitution of goethite in the black concretions as compared to a higher Al substitution in the goethite inherited from the terra rossa.  相似文献   

14.
Inositol phosphates are abundant organic phosphates found widely in the environment. The sorption and desorption of organic phosphate (Po) are important processes in controlling the mobility, bioavailability and fate of phosphorus (P) in soil and sediment. The desorption characteristics of myo‐inositol hexakisphosphate (IHP) and inorganic phosphate (Pi) from goethite were studied by pre‐sorption of IHP or Pi followed by desorption by KCl, H2O, and citrate. Batch experiments and in situ attenuated total reflectance Fourier transform infrared (ATR‐FTIR) spectroscopy were used to investigate the desorption of IHP/Pi. The desorption percentage of IHP/Pi by citrate was much higher than that by H2O/KCl. The desorption of P by citrate was mainly achieved through ligand exchange, and the desorption increased with decreasing pH. Desorption by H2O was slightly greater than that by 0.02 M KCl because the electrostatic repulsion between the P molecules is larger in H2O. Due to the higher affinity of IHP for goethite than that of Pi, the maximum desorption of IHP was lower than that of Pi. Desorption curves (desorption concentration in solution vs. sorption density) of IHP or Pi on goethite by KCl or H2O was well fitted by an exponential equation, while those by citrate were well fitted by a linear equation. The desorption amounts of P in the first cycle account for more than 58% of the total desorption followed by substantial decreases in the second and third cycles. There was a re‐sorption of Pi from solution in the late stage of desorption by KCl and H2O, resulting in a sharp decrease in desorption. Re‐sorption of IHP did not occur, which is probably due to its poor diffusion into goethite. The initial desorption rate of Pi with KCl and H2O decreased with increasing pre‐sorption time, whereas that of IHP was opposite. This study indicates that strong sorption on and weak desorption of IHP from iron (hydr)oxides may explain the accumulation of IHP in soils.  相似文献   

15.
Abstract

Surface horizons from Podzolic and Gleysolic soils were collected in various parts of the province of Quebec, Canada, and equilibrated with various amounts of KH2PO4 in 0.01 M CaCl2 for 48 hours. P sorption data conformed to the linear form of the Langmuir and Freundlich equations. P solubility isotherms showed evidence of hydroxyapatite formation in most samples studied, whereas equilibration solutions of only few samples were saturated with respect to either dicalcium phoshate dihydrate or octocalcium phosphate. These reaction products were associated to soil pH and levels of added phosphate. The average values of the Langmuir sorption maximum for these studied Gleysolic and Podzolic samples were 763 and 1096 μg/g respectively. These values were higher than those obtained by the segmented and modified Freundlich models.

Relationships between the soil characteristics and P sorption parameters were evaluated by regression analysis. Among all variables, oxalate‐extractable Fe plus Al content of the Podzolic samples and the ratio of oxalate—extractable Al to clay of the Gleysolic samples gave the best significant correlation coefficients. Furthermore, soil pH and various ratios such as pyrophosphate‐extractable Fe and Al, oxalate‐extractable Fe and organic matter to clay were found to be significantly correlated only with the P sorption parameters of the Gleysolic samples.  相似文献   

16.
Phosphate sorption on topsoil and subsoil samples from different soils located in the eastern part of Germany was studied. Two models were fitted to sorption data obtained after 4 and 40 d of gentle shaking. The models differ with respect to the fractions of iron and aluminium (hydr)oxides that are considered and whether the phosphate initially sorbed in the soil is taken into zccount. Oxalate-extractable P, (Pox), appears to be a major part of the total soil P. The total P sorption measured, F, was predominantly related to the amounts of amorphous iron (Feox) and aluminium (Alox). A significant relation between crystalline iron (Fed– Feox) and total P sorption was not found. Reversibly adsorbed phosphate (Pi), measured after 40 d reaction time, was a function of clay content and content of amorphous iron and aluminium (hydr)oxides.  相似文献   

17.
The phosphate adsorption capacity (Pmax) of samples from various horizons of five Danish podzolized soils were investigated before and after organic matter removal. Removal of organic matter had no direct influence on Pmax suggesting that organic matter did not compete with phosphate for adsorption sites. In the soils investigated aluminium and iron oxides were the main phosphate adsorbents. Thus, more than 96% of the variation in Pmax could be accounted for by poorly crystalline aluminium and iron oxides (extractable by oxalate) and by well-crystallized iron oxides (taken as the difference between dithionite-citrate-bicarbonate-extractable iron and oxalate-extractable iron). Organic matter affected phosphate adsorption indirectly by inhibiting aluminium oxide crystallization. The resulting poorly crystalline oxides had high Pmax. In contrast, the influence of organic matter on the crystallinity of the iron oxides, and therefore on their capacity to adsorb phosphate, seemed limited.  相似文献   

18.
Partial reduction of iron(III) oxides with hydrogen in the presence of a platinum catalyst leads to an equilibrium state after 4–20 h. From the measured Eh, pH, and Fe2+ concentration conditional standard potentials can be calculated using the formula Eo (volt) = Eh + 0.059 lg(Fe2+) + 0.18 pH which indicate the stability of Fe oxides against reduction. The reduceability decreases following the order ferrihydrite > lepidocrocite > hematite > goethite. The difference between hematite and goethite was more pronounced than that predicted from thermodynamic data.  相似文献   

19.
Abstract

Twelve soils with low‐medium phosphorus (P) retention capacities were equilibrated for 3 months with soluble phosphate at a rate of 100 mg P kg‐1 soil. The P sorption properties of these soils both with and without added P were studied, including equilibrium P concentration (EPCo), standard P requirement (SPR), soil P sorption capacity (b), maximum buffer capacity (MBC), and P sorption index (P‐SI). In general, the soils with no added P showed low values of all the above parameters. Oxalate extractable aluminum appeared to be the major responsible element for the control of P sorption in these soils. The addition of P to these soils had a considerable effect on their P sorption properties. The changes in EPCo were well correlated with P sorption index (r=0.80; p≤0.01 ). The EPCo values of the soils with and without added P were closely correlated to bicarbonate extractable P (P0lsen) and calcium chloride extractable P (PCaC12), with r=0.80, and r=0.99 (p≤0.001), respectively. Ninety percent of the variability in EPCo was explained by the corresponding variability in POlsen when a curvilinear relationship was adopted. The P sorption properties examined appear to be useful parameters to assess the environmental impact of soil P on the quality of surface waters.  相似文献   

20.
The iron oxides of soils of two river terrace sequences in Spain which show an increasing degree of redness with age were studied. Clay fractions contained only small amounts of oxalate-extractable Fe. Goethite and hematite, the only crystalline Fe-oxides identified, were determined quantitatively by X-ray diffraction (XRD) after concentrating the Fe-oxides by boiling in 5N NaOH and subtracting the step-counted diffractogram of the deferrated clay from that of the non-deferrated clay, obtaining thus a “pure” Fe-oxide diffractogram. EDTA extracted hematite preferentially to goethite as is seen by loss of red colour and by XRD. A good correlation was found between the content of hematite in the fine earth and a redness rating based on Munsell notations.In the Guadalquivir River sequence, Fed and Fed/Fet increased with age. The amount of both goethite and hematite formed from silicate-Fe increased with soil age but hematite increased more than goethite, possibly due to the xeric soil environment. Also, goethite increased in crystallinity as indicated by a decrease in XRD line broadening and Feo/Fed ratios. No such trends were found in the Esla River sequence, possibly because the initial alluvium was already highly weathered as shown by high Fed/Fet values (0.8) irrespective of terrace level.Al substitution in goethite calculated from XRD increased with soil age, reflecting the increasing acidity of the soils. Al substitution in hematite was markedly lower.  相似文献   

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