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Non-stable Fe minerals in waterlogged soils
Applied Geochemistry 110 (2019) 104424
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Non-stable Fe minerals in waterlogged soils Yuri N. Vodyanitskii a, Tatiana M. Minkina b, * a b
Lomonosov Moscow State University, Moscow, 119991, Russia Southern Federal University, Rostov-on-Don, 344006, Russia
A R T I C L E I N F O
A B S T R A C T
Editorial handling by Prof. M. Kersten
The hydromorphic soil profile is characterised by the presence of a gleyed horizon with specific features in different soils. The content and composition of non-stable Fe minerals can serve as important criterion for defining the gley type. Iron minerals soluble in the acid ammonium oxalate are assigned to the non-stable type. They can be divided into two groups depending on the moisture content in the analysed hydromorphic soils. The Fe(III) non-stable group includes minerals that are soluble in the oxalate from dry soils. The Fe(III)–Fe(II) nonstable group includes minerals that cannot resist the hydromorphic soil drying in laboratory. Their amount is determined by difference between the content of Fe extracted by the acid ammonium oxalate from the wet soil and the amount of Fe extracted from the dry soil stored for 1 or 36 months. In the hydromorphic Vertisols from the southern European part of Russia, the Fe content accounts for 7–10 and 3–6% of the bulk Fe content, respectively, in the Fe(III)–Fe(II) and Fe(III) non-stable minerals. Thus, a high share of non-stable Fe minerals oxidised during the drying of hydromorphic soils under laboratory conditions is established. To discriminate the hydromorphic soil types, we propose index I that characterises the interaction of Fe(III) non-stable minerals (Fefree) with colloids: I ¼ Fefree/CEC, where CEC is the cation exchange capacity. The fer ruginated and reduced gley (Gr) has a high index (I ~ 10). In Gr with high increment of redness (Δа* � 9) under drying the non-stable green rust is converted into brown lepidocrocite in the course of drying. Decrease of the moisture content in this process is also accompanied by change of the green rust composition: fougerite is transformed into tr�eburdenite, i.e., Fe-ephemer with a higher degree of Fe oxidation, leading to small increment of redness (Δа* � 3) in the greyish matrix. In the oxidised gley (Go) with I ¼ 0.05–0.5, the rate of green rust transformation is retarded by soil colloids. In the deferruginated gley (Gdf) with low I ¼ 0/04–0.17 and zero growth of Δа*, active Fe minerals are not formed in the course of drying.
Keywords: Sample preparation Soil humidity Oxalate-soluble fe CIE-Lab optical system Green rust Lepidocrocite Drying of soils
1. Introduction In the International Database of Soil Resources (World Reference …, 2006), hydromorphic mineral soils ‘situated under the influence of groundwaters’ make up the reference Gleysols group and occupy a vast area in the world (720 mln ha), with their largest terrains confined to the boreal regions. The hydromorphic soil profile is characterised by the presence of a gleyed horizon with specific features in different soils. The profile is usually identified under field conditions based on morphology, hue difference, abundance and shape of the Fe-bearing neoformations. Since the formation of gley is related to variation in the degree of Fe oxidation, it is expedient to define gley types based on the composition of Fe minerals. Proceeding from this viewpoint, three types of gley are defined in (Zaidelman, 1998). Type 1, reduced gley (Gr), is marked by greyish colour owing to Fe(II) minerals of the green rust group. Precisely
green rusts are responsible for the rapid colour alteration of gley after recovery of its sample from the section (Feder et al., 2005, 2018). Type 2, oxidised gley (Go), is characterised by brown mottles in the greyish matrix. The hue of brown mottles is governed by the Fe hydroxide type: ferrihydrite, lepidocrocite and goethite make up red-brown, orange and yellowish-brown mottles, respectively (World Reference …, 2006). Type 3 is represented by the heavy-textured deferruginated gley (Gdf), with the greyish colour defined by clay minerals without the Fe coating. The most reactive minerals are represented by the thermodynami cally non-stable and X-ray amorphous Fe(III) hydroxide (ferrihydrite) and Fe(II) minerals. Ferruginous minerals in Gleysols are rather diverse. In some places, the whole Fe is represented by Fe(II) (siderite, vivianite, chukanovite); in other places, by both Fe(II) and Fe(III) (magnetite and green rusts) (Etique et al., 2015). Many soils of the humid zone are characterised by a reversibility of
* Corresponding author. E-mail addresses: [email protected] (Y.N. Vodyanitskii), [email protected] (T.M. Minkina). https://doi.org/10.1016/j.apgeochem.2019.104424 Received 29 July 2019; Received in revised form 18 September 2019; Accepted 21 September 2019 Available online 23 September 2019 0883-2927/© 2019 Elsevier Ltd. All rights reserved.
Y.N. Vodyanitskii and T.M. Minkina
Applied Geochemistry 110 (2019) 104424
redox reactions (Кaurichev, Orlov, 1982). Oxidation of Fe(II) minerals and significant morphological alteration of the gleyed horizon are possible during the oxidizing period. The content and composition of non-stable Fe minerals can serve as important criterion for defining the gley type. Methods of the chemical extraction are used to determine Fe minerals of different stability (Kodama et al., 1977). In general, the total content of Fe oxides/hydr oxides is determined based on the dithionite-citrate-bicarbonate (FeDCB) solubility according to Mera-Jackson, assuming that the Fe-bearing sil icates can resist such influence and preserve the X-ray amorphous Fe minerals during treatment with the acid ammonium oxalate by the Tamm method. However, the existing methods for the chemical fractionation of Fe compounds based on the analysis of soils dried under laboratory con ditions ignore the fact of decomposition or crystallization of a part of Fe minerals during the hydromorphic soil drying. Scale of this distortion can be judged by comparing the content of extracted Fe(II) in the watersaturated and dried sediments. Data on the content of Fe(II) extracted by the ammonium oxalate from the water-saturated sediments from the bottom of the Potomac River flowing into the Atlantic Chesapeake Bay, USA, is described in (Phillips, Lovley, 1987, 1993). In the course of drying for 4 days at room temperature, content of the oxalate-soluble Fe decreased from 510 to 280 mmol/kg, i.e., by 230 mmol/kg that corre sponds to the content of non-stable Fe minerals. It is noteworthy that the water-saturated sediments yielded 210 mmol/kg Fe(II) and 300 mmol/kg Fe(III), while the dry variety only yielded Fe(III). Thus, the drying procedure changes not only the amount of the extracted Fe, but also its quality. Decrease of the content of mobile Fe compounds was also recorded during the aeration of bottom sediments recovered from the North Sea sediments (Kersten, Forstner, 1986). The process of drying exerts a similar influence on reduction of the share of mobile fractions of Cu, Cd and Pb in river sediments (Bordas, Bourg, 1998). Thus, drying of the sulphide-barren marine and river sediments increases the content of readily soluble metals. As for metal sulphides, the drying of sediments promotes transformation of the nonsoluble sulphide particles into the readily soluble sulphates. We have less data on the influence of drying on metals in soils, relative to river marine sediments. Therefore, a new method is obviously needed for the chemical fractionation of Fe min erals in the hydromorphic soil during its natural humidity. At present, the sulphide-barren gleys are divided into three large groups: (1) ferruginated reduced gley (Gr) marked by rapid yellowing in atmosphere due to the oxidation of non-stable Fe compounds defined as Green Rust (Trolard, Bourrie, 2008); (2) oxidised gley (Go) formed in a variable redox setting because of the precipitation of Fe as ferrihydrate and other Fe hydroxides at oxidation microbarriers within the gleyed horizon; and (3) deferruignated (Gdf) formed during washing - term “deferruignated” is rather inadequate for this variety, because it is al ways humidifed and usually contains a small amount of Fefree (commonly <50 mmol/kg). These three varieties of gley differ in terms of the content of both Fe(III) non-stable minerals and colloids. Thus, gleys of different genesis can be differentiated based on the content of Fe (III) non-stable minerals adjusted to the content of colloids. The aim of the present paper is to propose a new chemical frac tionation method for determining the contents of two forms of nonstable Fe minerals and to offer an indicator of different gley types based on the content of Fe(III) non-stable minerals per colloid unite.
2.1. Objects Choice of objects was dictated by the necessity to embrace hydro morphic soils over a vast European space formed on different bedrocks. We analysed Gleyed Cambisols on the granitic delluvium, gleysoils in palaeoalluvial deposits of the Kama River, gleysoils in alluvial deposits of the Klyaz’ma River, as well as hydromorphic Technosols and Vertisols in the alluvial clay deposits in the Rostov region. 2.1.1. Gleyed Cambisols, France Samples were taken in the Fougere area (western France) from Gleyed Cambisols marked by the seasonal colour variation (Feder et al., 2018). The area is situated at a height of 180 masl with numerous thalwegs up to 800 m long. Soils were formed here on a meter-scale granodioritic saprolite bed. The chosen profile is located on a hillslope and comprises the following horizons (from the top to bottom): (0–15 cm) black organo genic horizon without redox manifestations; (15–50 cm) oxidised silty gley with the greyish matrix (hue of 5Y6/4, according to Munsell) and brown mottles (hue of 2.5Y5/6); (50–80 cm) reduced silty gley, homo geneously blue (hue of 5BG6/1), almost always (10 months per year) inundated, without brown mottles; and (80–120 cm) granitic saprolite with signs of reduction and always moist. In the soil profile, content of the clay fraction is maximum at a depth of 70 cm in the upper part of the granitic saprolite. The organic horizon is underlain by the gleyed horizon with colour related to fougerite. The fougerite has a bluish-green colour. Boundary between the fougerite-bearing gleyed horizon and the oxidised lepidocrocite-bearing horizon varies seasonally at depths of 40–100 cm: rises during the rain period in winter and descends in dry summer. 2.1.2. Soils in palaeoalluvial deposits of the Cis-Ural region, Russia Hydromorphic soils of different degrees of gleyzation were studied in two catenas on the Middle Kama Plain, Krasnokamsk area of the central Perm Territory (Sataev, 2005). Sections were exposed on supra-floodplain terraces V and VII of the Kama River. On terrace VIII of the Bekryata Catena, the dark heavy-textured humic-gley soil was ana lysed. The mottled gleysolic horizons adjoin the greyish and brown-rusty mottles. Samples from horizons AUg (6–23 cm), G (23–43 cm), and B1g (43–80 cm) were scrutinised. On the supra-floodplain terrace V (Las’va Catena), the soddy pod zolised light-textured gley soil was analysed. The greyish and brownrusty mottles were found side by side in the gleysolic horizons. Sam ples from horizons ВТ1g (25–38 cm) and ВТ2g (38–60 cm) were scrutinised. 2.1.3. Hydromorphic Technosols and Vertisols, Rostov region, Russia Soils were studied in the Kamensk area 500 m SE of the Fillipenkov Settlement. We analysed soils of two types on the Glubokaya River terrace. Type 1 soils classified as hydromorphic Technosols They are intensely waterlogged and polluted mainly with inorganic materials transported from Lake Atamanskoe that was exploited as sludge col lector in the 1950’s. Samples were taken from the upper humus horizon (0–20 cm). The analytical results are as follows: carbonates 0.4–4.0%, рНaq 7.2–8.0, Corg 1.4–1.7%, silty particles (<1 μm) 12–18%. Type 2 represents the hydromorphic carbonate vertisol (Vertisols) developed on alluvial clays. We analysed the following horizons in the section: Ad (0–8 cm), A (8–32 cm), ABsa (32–71 cm), Bf (71–82 cm), BCf (82–100 cm). Effervescence of the soil is weak on the surface, vigorous beginning from a depth of 10 cm and again weak at 86 cm. Commencing from a depth of 32 cm, the section is marked by the deposition of salts as fari naceous and less common compact concretions. Approximately 10% of concretions do not effervesce with the hydrochloric acid. Iron segrega tions are recorded below a depth of 71 cm. Horizons Bf (71–82 cm) and BCf (82–100 cm) are marked by inhomogeneous dark-grey hue with a
2. Objects and methods Choice of objects was dictated by the necessity to embrace hydro morphic soils over a vast European space formed on different bedrocks. We analysed Gleyed Cambisols on the granitic delluvium, gley soils in palaeoalluvial deposits of the Kama River, gley soils in alluvial deposits of the Klyaz’ma River, as well as hydromorphic Technosols and Vertisols in the alluvial clay deposits of the Rostov region. 2
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Applied Geochemistry 110 (2019) 104424
pale-yellow tint along with white salt mottles and rusty Fe oxide mottles. These horizons (particularly, Bf) are more ferruginated than the upper horizons: the bulk Fe content is as much as 802–848 mmol/kg, in contrast to 741–780 mmol/kg in the upper Vertisols. Rusty Fe oxide mottles influence the chemical fractionation. The grain size composition of all horizons corresponds to the clay fraction. The upper horizons down to a depth of 71 cm are wet, whereas the deeper horizons are moist. Beginning from this depth, the grey background is marked by rusty Fe hydroxide mottles.
conditions. This approach is based on comparison of the content of the oxalate-extracted Fe in the wet sample (Feox-wet) with its content in the dried sample (Feox-dry). The discrepancy corresponds to the amount of Fe (III)–Fe(II) non-stable compounds. Previously, the content of oxalate-soluble Fe(II) and Fe(III) in the water-saturated alluvial sediments were determined during the analysis and cultivation of anaerobic bacteria (Phillips et al., 1987, 1993). A hermetically sealed sample was saturated with inert gas N2 to prevent the oxygen input from atmosphere. Preservation of reducing conditions in the sample is an obligatory condition in this method. We had a different task: differentiation of the Fe(III)–Fe(II) nonstable and Fe(III) non-stable minerals in terms of their ability to crys tallise during the hydromorphic soil drying. Table 1 demonstrates the scheme of chemical fractionation of Fe minerals according to their sta bility in hydromorphic soils. The group of Fe minerals that can resist the soil sample drying and acid ammonium oxalate treatment includes the coarse-crystalline he matite, goethite and many Fe-phyllosilicates. Green rusts belong to lowstable minerals that cannot resist the drying. Reaction to the chemical treatment of other minerals (ferrihydrite, ferroxyhyte, dispersed goethite) depends on the redox conditions of soils during the sampling. At low ЕН values, as in the water-saturated fresh-water sediments, the sample is saturated with Fe(II). In this case, treatment of the wet sample by the ammonium oxalate provokes the reduction of Fe(III) hydroxides (Blesa et al., 1987). In contrast, at a higher EH value typical of the oxidised gley soil (Gleysols), the sample is less saturated with Fe (II), leading to decrease of the content of non-stable Fe minerals that are sensitive to treatment of the wet soil sample by the ammonium oxalate (Vodyanitskii, 2001). In the dried (particularly, stored for a long time) sample, particles of Fe(III) hydroxides (ferrihydrite, ferroxyhyte, dispersed goethite) are crystallised gradually, leading to decrease in solubility. Therefore, the content of the oxalate-soluble Fe in the wet soil (Feox-wet) is higher than in the dry soil (Feox-dry). Method of determination of the oxalate-soluble Fe (Feox-wet) is as follows. The wet soil sample extracted from the section is placed in a sealed polyethylene packet and transported to laboratory. Material from the packet is placed in two similar boxes. Soil from one box is imme diately treated with the Tamm reagent according to the conventional method. Soil from the second box is dried at room temperature, with the subsequent determination of the dry mass. This value of mass is used for correcting the value if it differs from 1 g accepted for calculation by the Tamm method.
2.1.4. Alluvial boggy gley soil, Moscow region, Russia The section was exposed beneath the bog vegetation on the Klyaz’ma River floodplain (Vodyanitskii et al., 2018). The mucky horizon A02 is situated at a depth of 0–50 cm. It is underlain by a greyish gley (G) horizon at 50–95 cm. In terms of the grain size composition, the gley corresponds to medium loam. Four gley samples were studied. Samples 1 and 2 were taken from the upper part (50–55 cm); samples 3 and 4, from the deeper (moister) part of the horizon (90–95 cm). The ground water table was recorded at a depth of 1 m. The upper and lower parts of the gley horizon differ in many chemical parameters. Its lower part is depleted in carbon and nitrogen as compared to the upper part: Corg ¼ 0.74 � 0.015% versus 1.41 � 0.24%; N ¼ 0.024 � 0.0025% versus 0.096 � 0.017%. The bulk Fe content is low: Febulk ¼ 414–612 mmol/kg, which is two times lesser than the average content in the Earth’s crust (1110 mmol/ kg). The content of non-silicate Fe compounds is very low (dithionitecitrate-bicarbonate extractable Fe, FeDCB ¼ 41–55 mmol/kg), leading to low values of the ratio FeDCB/Febulk ¼ 0.08–0.13. All these facts testify to a weak ferrugination of the given gley type. A significant amount of nonsilicate Fe compounds in the dry soil is dissolved in the acid ammonium oxalate (Feox-dry): Feox-dry ¼ 34–43 mmol/kg, leading to a high value of the ratio Feox-dry/FeDCB ¼ 0.67–0.88. 2.2. Methods The bulk Fe content in the soils was determined with a Spectroscan MAKC-GV X-ray fluorescent spectrometer. According to the manual (Vorob’eva 2006), the pH value was measured by a pHS-3C potenti ometer in the soil-water suspension (1 : 2.5); the organic matter content was determined by the titrimetric method (bichromate oxidation); the carbonate content was determined by the volumetric method; and silt particles were determined by pipetting with the pyrophosphate sample preparation. The cation exchange capacity (CEC) was determined by the ammonium acetate method. The Fe content in the X-ray amorphous and low-crystalline particles were determined after the soil treatment by the acid ammonium oxalate at pH ¼ 3.25. Examination of mineralogical transformations of Fe compounds in the Gleyed Cambisol profile (France) by the method of nuclear gamma €ssbauer analyser made it possible to determine resonance with a field Mo spatiotemporal changes of the Fe composition in the soil solid phase for the first time (Feder et al., 2005). To reveal small changes in gley colour in the alluvial boggy gley soil of the Moscow region, we measured the reflection spectra in the course of gley drying to the air-dry state at room temperature (Vodyanitskii et al., 2018). The colour characteristics are presented in the CIE-Lab system.
3.2. Contents of two forms of non-stable Fe minerals in the hydromorphic Vertisols Table 2 demonstrates schematically the principle of chemical frac tionation of Fe minerals according to their stability in hydromorphic soils, with the hydromorphic Vertisols in southern Russia as example. The oxalate extracts from the wet soil a higher amount of Fe (Feoxwet) than from the dry soil (Feox-dry) stored for 1 month and the more so from a soil stored for 36 months. The oxalate recovered 11.6–14.0% of the bulk Fe content from the wet soil, but only 5.3–7.2% from the soil Table 1 Scheme of the chemical fractionation of Fe minerals based on their stability.
3. Results and discussion 3.1. Determination of two forms of non-stable Fe minerals in the hydromorphic soils Since the conventional chemical fractionation of Fe compounds in the dried samples is inapplicable for hydromorphic soils, we propose a new approach that can take into consideration the content of non-stable Fe compounds, which cannot resist the drying under laboratory 3
Category of Fe mineral stability
Determination method
Soluble Fe minerals
Fe(III)–Fe(II) nonstable compounds Fe(III) non-stable compounds
Feox-wet-Feox-dry
Stable Fe compounds
Febulk-Feox-wet
Green rusts. Reducible: ferrihydrite, ferroxyhyte, dispersed goethite Siderite. Minerals able to crystallise during drying: ferrihydrite, ferroxyhyte, dispersed goethite Coarse-crystalline goethite, hematite, several Fe- phyllosilicates
Feox-dry
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Applied Geochemistry 110 (2019) 104424
Table 2 Fe content in compounds of different stability in the hydromorphic Atamanskoe Vertisols (Fe content is given in mmol/kg, values in parentheses show percentage from Febulk). Horizon, depth (cm)
Fe fractions Febulk
Wet samples Feox-wet
Ad (0–8) A (8–32) ABs (32–40) ABs (40–71) Bf (71–82) BCfg (82–100)
751 780 741 768 848 802
87.4 102.8 89.2 88.2 114.0 112.1
Sample storage time - 1 month
Sample storage time - 36 months
Febulk –Feox-bulk
Feox-dry
Feox-wet – Feox-dry
Feox-dry
Feox-wet – Feox-dry
Stable Fe
Fe(III) non-stable
Fe(III)–Fe(II) non-stable
Fe(III) non-stable
Fe(III)–Fe(II) non-stable
663.6 677.2 651.8 679.8 734.0 689.9
51.9 (6.9) 51.4 (6.6) 39.4 (5.3) 48.4 (6.3) 61.2 (7.2) 56.6 (7.1)
35.5 (4.7) 51.4 (6.6) 49.8 (6.7) 39.8 (5.2) 52.8 (6.2) 55.5 (6.9)
25.1 (3.3) 25.2 (3.2) 22.8 (3.1) 26.5 (3.4) 52.2 (6.2) 33.3 (4.1)
62.6 (8.3) 77.6 (10.0) 66.1 (8.9) 61.7 (8.0) 61.8 (7.3) 78.8 (9.8)
(88.4) (86.8) (88.0) (88.5) (86.5) (86.0)
dried for 1 month and just 3.1–6.2% from the soil stored for 36 months. Thus, even in the course of a short-term storage of dry samples, a sig nificant amount of Fe hydroxides is crystallised and they can withstand the oxalate treatment. The amount of Fe extracted from the briefly dried soil is assigned to the Fe(III) non-stable type, and the Fe content defined from the discrepancy Feox-wet – Feox-dry is assigned to the Fe(III)–Fe(II) non-stable compounds. Let us note that our data and the results reported in (Phillips et al., 1987, 1993) differ basically from the results obtained by the extraction of Fe from wet soils by other reagents. These discrepancies are related to different properties of reagents. For example, antagonistic data were obtained during the extraction of Fe from wet soils using the ammonium acetate buffer (AAB) with pH ¼ 4.8 (Bartlett, James, 1980) or com plexons such as ethylenediamine tetraacid, EDTA (Haynes, Swift, 1991) and its homologue diethylenetriamine penta-acetic acid, DTPA (Muel ler, 2013). Naturally, efficiency of AAB is several orders of magnitude lower than that of complexons, because AAB only extracts the loosely bonded Fe particles, while complexons extract Fe after the formation of a robust complex with this element. For example, bond strength of EDTA with Fe (II) is appreciably weaker than with Fe(III): log K1 ¼ 14.2 and 24.2, respectively (Lurie, 1979). It is very important that the bond strength of complexon with Fe(III) is much stronger than with Fe(II). Therefore, efficiency of complexons as extragents is boosted in the course of soil drying and oxidation of Fe(III) to Fe(II). Experiment reported in (Leg gett, Argyle, 1983) demonstrated that increase of temperature during the soil drying fostered the extractability of Fe by complexon. Second reason for the increase of Fe extractability by the complexon lies in the acidification of soil in the course of its drying (Vodyanitskii, Mineev, 2016). The point is that the formation of more robust Fe(III) complexes needs a more acid medium, relative to the less robust Fe(II) complexes (Dyatlova et al., 1988). In contrast, efficiency of the oxalate is promoted if the wet soil contains Fe(II) that catalyses the dissolution of Fe(III) minerals (Blesa et al., 1987; Vodyanitskii, 2001). Thus, efficiency of complexons de creases in the initially wet soils, but efficiency of the oxalate increases if the reduced Fe is present.
capacity. High value of index I reflects a high probability of the formation of Fe (III) non-stable minerals in the gley. On the contrary, interaction of these different Fe compounds with soil colloids inhibits their formation when I → min. At the same time, interaction of active forms of Fe compounds with soil colloids is manifested, in particular, in the capacity of Fe hy droxide particles to precipitate on clay mineral edges, blockade some exchange sites, and thus decrease the CEC value (Oscarson; Heimann, 1988). Probably, cation exchange capacity of the intensely ferruginated gley could be higher if the clay mineral edges were not blockaded by Fe hydroxide particles. The three gley types with diverse aquatic settings and morphologies should also differ in terms of index I that reflects different probabilities of the formation of Fe(II) non-stable minerals in the gley. 3.3.1. Ferruginated reduced gley (Gr) with a high value of index I 3.3.1.1. Gleyed Cambisols, western France. Data on the colour of different Gleyed Cambisol horizons expressed in the Munsell system are presented in (Feder et al., 2005). Since the peculiarity of this system is the cylindrical coordinates, hampering statistical calculations is less convenient, we converted the original colour data into the more modern CIE-L*a*b* optical system, which is a universal color space in Cartesian coordinates. Table 3 presents colour parameters of two Cambisol horizons after converting them from the Munsell system into the CIE-Lab system ac cording to algorithm in (Vodyanitskii, Kirillova, 2016). As is evident, redness of the greyish mottles in the oxidised gley increases from 2.5 to €ssbauer field 5.2; i.e., colour difference Δа* ¼ 2.7. Data on the Mo measurements revealed that increase of redness is caused by the oxidation of Fe ephemers of the green rust group. Fougerite, a double layered hydromica with Fe2þ: Fe3þ ¼ 2 : 1, is transformed into another hydroxide (tr�eburdenite) belonging to the same but more oxidised group of green rusts with Fe2þ: Fe3þ ¼ 1 : 2. Redness of brown mottles in the oxidised gley, where green rust was transformed into lepidocrocite, increases even more: from 3.5 to 5.2, i.e., Δа* ¼ 8.7. The reduced gley in Gleyed Cambisols is marked by low CEC values ranging from 4.5 to 6.1 cmol (þ)/kg (Feder et al., 2018). On the other hand, approximately 90% Fe is extracted by
3.3. Diagnostics of gley types based on the content of Fe(III) non-stable minerals per colloid unit Our approach is based the relationship of two indicators of gley: cation exchange capacity (CEC), a proxy of soil colloids; content of free Fe compounds in the dried soil, i.e., Fe(III) non-stable compounds. To dissolve free Fe compounds, researchers commonly use the dithionitecitrate in the case of long-term (up to 200 h) treatment (Feder et al., 2018) or the acid ammonium oxalate according to the Tamm method (Kodama et al., 1977; Vodyanitskii, 2001). The content of non-silicate Fe compounds Fefree is presented in mol/kg. To characterize the gley types based on the content of Fe (III) non-stable compounds per colloid unit, we propose index I: I ¼ Fefree/CEC, where CEC is the cation exchange
Table 3 Colour of Gelyed Cambisols (France) expressed in the CIE-Lab optical system, initial data from (Feder et al., 2005). Horizon part Reduced gley Greyish matrix Oxidised gley Greyish matrix Brown mottles
4
L*
a*
b*
61.7
5.2
0.6
61.7 51.6
25 5.2
29.0 42.0
Y.N. Vodyanitskii and T.M. Minkina
Applied Geochemistry 110 (2019) 104424
dithionite-citrate-bicarbonate and 60% Fe by citrate-bicarbonate. In absolute values, the content of active Fe extracted by citrate-bicarbonate is very high (680 mmol/kg) (Feder et al., 2018). Very highvalues of index I ¼ 11–14 are reflected favourable settings for the formation of non-stable green rusts in the gley. Thus, the gley containing green rusts is marked by highindex I values (~0.1–0.5) (~5–15).
(þ)/kg). Thus, the deferruginated gley shows a low index I ¼ 0.07–0.09. Colour indicators of the deferruginated gley before and after the drying are presented in Table 5. After the drying, lightness and yel lowness of gley increased, in general, significantly (ΔL* ¼ 15, Δb* ¼ 6.2), but redness did not practically change (Δа*is 0.1). Preservation of redness (Δа* � 0) owing to the evaporation of moisture from any soil (automorphic variety included) should be discriminated from the slight increase of redness (Δа* ¼ 2.7) because of the transformation of green rust minerals in the dehydrated Gleyed Cambisols in western France. Colour alteration due to the evaporation of moisture in 41 soil samples of genetically different colour is described in the Munsell optical system (Post et al., 1993). Conversion of colour indicators to the more convenient orthogonal CIE-L*a*b* optical system revealed: in general, lightness of soils increased significantly after drying (ΔL* ¼ 15), but redness increased slightly (Δа* ¼ 1.3). This result is consistent with the colour alteration in dry samples of the heavy-textured Cambisols in South Carolina, USA (Stiglitz et al., 2017), where lightness increased appreciably (ΔL* ¼ 16), but redness only slightly (Δа* ¼ 1.6). Thus, water evaporation in the course of drying deciphered by a minor increase of redness (Δа* < 1.7) can be discriminated from the transformation of green rust minerals accompanied by a stronger increase of redness (Δа* ¼ 2.7). The obtained results are summarised in Table 6. Among the three gley types, only the reduced gley (Gr) is marked by a high value of index I. Based on this index, the oxidised gley (Go), however, cannot be distinguished reliably from the deferruignated gley (Gdf). Wide variation of index I in the oxidised gley (Go) is related to a shortcoming of its identification based on morphology. It is rather difficult to detect a few tiny brown mottles in the grayish brown matrix to define this horizon as oxidised gley (Gо). At the same time, chemical properties (Feox and CEC) of sample, on the whole, determine in labo ratory after the drying. Another important shortcoming of the morphological identification of gley types lies in different nature of criteria. The reduced and oxidised gley is identified based on the phys icochemical properties of Fe compounds, whereas the deferruignated gley is recognised based on the degree of Fe evacuation from the horizon. In connection with shortcomings of the morphological identification of gley types, Table 6 presents the theoretical values of index I lacking any strict dependence on the gley morphology. Theoretical domains do not differ from the factual boundaries only for the reduced gley (Gr). However, the range of index I is 0.08–0.5 for the oxidised gley and <0.08 for the deferruignated gley (Gdf). Database augmentation can refine the critical value of index I ¼ 0.08.
3.3.2. Oxidised gley (Go) with moderate and low values of index I 3.3.2.1. Gleyed soils in palaeoalluvial deposits, Cis-Ural region, Russia. In the Bekryata Catena, examination of the heavy textured dark humic-gley soil using the TEM method revealed that horizon AUg (6–23 cm) con tains Fе-vernadite and Mn-ferroxyhyte (Sataev, 2005). The content of active oxalate-soluble Fe compounds, Feox-dry ¼ 85 mmol/kg; CEC ¼ 29.5 cmol (þ)/kg; I ¼ 0.29. In the lower horizons G (23–43 cm) and B1g (43–80 cm), I ¼ 0.16 and 0.24, respectively (Table 4). In the Las’va Catena, the soddy podzolised light-textured gley soil in horizon ВТ1g (25–38 cm) contains ferrihydrite and siderite. The brown mottles therein are likely related to Fe-pigment (ferrihydrite). The content of active oxalate-soluble Fe compounds, Feox ¼ 31 mmol/kg, CEC ¼ 6.9 cmol (þ)/kg, I ¼ 0.45. Horizon ВТ2g (38–60 cm) contains ferrihydrite, siderite and ferroxyhyte. The gleysolic horizons are char acterised by a mottled hue and the presence of greyish and brown-rusty mottles side by side. The brown mottles likely include Fe-pigments (ferrihydrite and ferroxyhyte). The content of active oxalate-soluble Fe compounds, Feox ¼ 48 mmol/kg, CEC ¼ 9.9 cmol (þ)/kg, I ¼ 0.48. Thus, the gleysolic soils in the Cis-Ural region are marked by I ¼ 0.16–0.48 (Table 4). 3.3.2.2. Hydromorphic technosols, Rostov region, Russia. In the studied soil samples, CEC ¼ 32–36 cmol (þ)/kg, Feox-dry ¼ 75–139 mmol/kg. The content of Fe (III) non-stable compounds per colloid unit is, on the average, I ¼ 0.26–0.43. 3.3.2.3. Hydromorphic Vertisols, Rostov region, Russia. In soil from sec tion 9, CEC ¼ 27–49 cmol (þ)/kg, Feox-dry ¼ 23–52 mmol/kgThe content of Fe (III) non-stable compounds per colloid unit is low, I ¼ 0.05–0.19. 3.3.3. Deferruginated gley (Gdf) with lowvalue of index I This gley is always intensely wetted. Term ‘deferruginated’ is not quite accurate for this gley, because it always contains a small amount of Fefree (usually <50 mmol/kg). Moreover, one should bear in mind that all reagents described above are insufficiently selective because of the extraction of a part of Fe from the phyllosilicate lattice.
4. Conclusions
3.3.3.1. Alluvial boggy mucky-gley soil, Moscow region, Russia. In this gley, the content of semi-stable Fe minerals is low: Feox-dry varies from 34 to 42 to 36–43 mmol/kg in the lower part and CEC is high (~50 cmol
Methods of the chemical extraction are used to determine Fe min erals with different stability, whereas the oxalate treatment of soils is applied most commonly to determine Fe in the composition of nonstable minerals. Methods of the chemical extraction, however, are
Table 4 Chemical and mineral compositions of Fe minerals in gleyed soils from palae oalluvial deposits in the Kama region, Russia (Sataev, 2005). Horizon, depth (cm)
Febulk mmol/kg
Feox-dr mmol/kg
Fe minerals
Dark humic gley of heavy texture, Bekryata Catena AUg (6–23) 662 85 Fe-vernadite, Mn-ferroxyhyte G (23–43) 1009 56 Fe-vernadite, goethite В1g (43–80) 729 73 Not determined Soddy podzolised gleysolic with light texture, Las’va Catena BT1g 516 31 Ferrihydrite, (25–38) siderite BT2g 339 48 Ferrihydrite, (38–60) siderite, ferroxyhyte
CEC cmol/ kg
Index I
29.5
0.29
35.3
0.16
31.0
0.24
6.9
0.45
10.0
0.48
Table 5 Light (L*), red (a*) and yellow (b*) hues of the wet and dry gley, alluvial peaty mucky-gley soil, Moscow region, Russia. Parameter L*wet L*dry ΔL* a*wet a*dry Δa* b*wet b*dry Δb*
5
Horizon top
Horizon bottom
1
2
3
4
33.9 48.7 14.8 3.75 3.75 0.00 7.41 14.47 7.06
40.0 55.4 15.4 3.12 3.75 0.63 8.53 13.55 5.02
40.8 55.5 14.7 2.50 1.35 1.15 8.09 15.68 7.59
41.8 57.0 15.2 1.87 1.87 0.00 7.59 12.73 5.14
Y.N. Vodyanitskii and T.M. Minkina
Applied Geochemistry 110 (2019) 104424
Declaration of interests
Table 6 Values of index I in different gley types. Gley
Reduced gley (Gr)
Oxidised gley (Gо)
Deferruginated gley (Gdf)
Index I* Index I** Transformation of Fe minerals
>2 >2 Green rusts → lepidocrocite Fougerite → tr�eburdenite Gleyed Cambisols
0.08–0.5 0.05–0.5 Green rusts → lepidocrocite, ferrihydrite. Goethite Gleyed soils of the podzolised series and agrosoddy soils. Floodplain soils of the forest and steppe zones
0.07–0.08 <0.08 No
Soils
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References Bartlett, R., James, B., 1980. Studying dried, stored soil samples—some pitfalls. Soil Sci. Soc. Am. J. 44, 721–724. https://doi.org/10.2136/ sssaj1980.03615995004400040011x. Blesa, M.A., Marinovich, H.A., Baumgrater, E.C., Maroto, A.J.G., 1987. Mechanism of dissolution of magnetite by oxalic acid-ferrous ion solutions. Inorg. Chem. 26, 3713–3717. https://doi.org/10.1021/ic00269a019. Bordas, F., Bourg, A.C.M., 1998. A critical evaluation of sample pretreatment for storage of contaminated sediments to be investigated for the potential mobility of their heavy metal load. Water Air Soil Pollut. 103, 137–149. Dyatlova, N.V., Temkina, D.Z., Popov, K.I., 1988. Complexons and Complexonates of Metals. Khimiya, Moscow, p. 544 (in Russian). Etique, M., Jorand, F.P.A., Rubi, C., 2015. Magnetite as a precursor for green rust through the hydrogenotrophic activity of the iron-reducing bacteria Shewanella putrefaciens. Geobiology 14 (3), 237–254. https://doi.org/10.1111/gbi.12170. Feder, F., Trolard, F., Klingelhofer, G., Bourrie, G., 2005. In situ M€ ossbauer spectroscopy – evidence for green rust (fougerite) in gleysol and its mineralogical transformation with time and depth. Geochem. Cosmochim. Acta 69, 4463–4483. https://doi. org/10.1016/j.gca.2005.03.042. Feder, F., Trolard, F., Bourri�e, G., Klingelh€ ofer, G., 2018. Quantitative estimation of fougerite green rust in soils and sediments by citrate-bicarbonate kinetic extractions. Soil Systems 2, 54. https://doi.org/10.3390/soilsystems2040054. Haynes, R.J., Swift, R.S., 1991. Concentrations of extractable Cu, Zn, Fe and Mn in a group of soils as influenced by air- and oven-drying and rewetting. Geoderma 49, 319–333. https://doi.org/10.1016/0016-7061(91)90083-6. Kersten, M., Forstner, U., 1986. Chemical fractionation of heavy metals in anoxic estuarine and coastal sediments. Water Sci. Technol. 18, 121–130. Leggett, G.E., Argyle, D.P., 1983. The DTPA-extractable iron, manganese, copper, and zinc from neutral and calcareous soils dried under different conditions. Soil Sci. Soc. Am. J. 47, 518–522. https://doi.org/10.2136/sssaj1983.03615995004700030025x. Lurie, YuYu, 1979. Handbook of Аnalytical Сhemistry. Khimiya, Moscow, p. 480 (in Russian). Mueller, A., 2013. The effect of drying and drying temperature on soil analytical test values. Bachelor of Science in Environmental Science, Oklahoma State University, USA, p. 73. Oscarson, D.W., Heimann, R.B., 1988. The effect of an Fe(II)-silicate on selected properties of a montmorillonitic clay. Clay Miner. 23, 81–90. https://doi.org/10.118 0/claymin.1988.023.1.08. Phillips, E.J.P., Lovley, D.R., 1987. Determination of Fe(III) and Fe(II) in oxalate extracts of sediment. Soil Sci. Soc. Am. J. 51 (4), 938–941. https://doi.org/10.2136/sssaj1 987.03615995005100040021x. Phillips, E.J.P., Lovley, D.R., Roden, E.E., 1993. Composition of non-microbially reducible Fe(III) in aquatic sediments. Appl. Environ. Microbiol. 59 (8), 2727–2729. Post, D.R., Bryant, R.B., Batchily, A.K., Huete, A.R., Levine, S.J., Mays, M.D., Escadafal, R., 1993. Correlation between field and laboratory measurements of soil color. In: Soil Color, vol. 31. SSSA Special Publication, pp. 35–49. https://doi.org/1 0.2136/sssaspecpub31.c3. Sataev, E.F., 2005. Setting and Oxidogenesis of Soils in Palaeoalluvial Deposits on the Middle Kama Lowland (Abstract of the Ph.D. (Agricult. Sci.) Dissertation). Moscow, pp. 16 (in Russian). Stiglitz, R., Michailova, E., Post, Ch, Schlautman, M., Sharp, J., 2017. Using an inexpensive color sensor for rapid assessment of soil carbon. Geoderma 286, 98–103. https://doi.org/10.1016/j.geoderma.2016.10.027. Trolard, F., Bourrie, G., 2008. Geochemistry of green rusts and fougerite: a reevaluation of Fe cycle in soils. Adv. Agron. 99, 227–288 (chapter 5). https://doi. org/10.1016/S0065-2113(08)00405-7. Vodyanitskii, YuN., 2001. On the dissolution of iron minerals in Tamm’s reagent. Eurasian Soil Sci. 34 (10), 1086–1096. Vodyanitskii, YuN., Kirillova, N.P., 2016. Conversion of Munsell color coordinates to the CIE-L*a*b* system: tables and calculation examples. Mosc. Univ. Soil Sci. Bull. 4, 139–146. Vodyanitskii, YuN., Mineev, V.G., 2016. Difference in the pH values of hydromorphic soils in field and laboratory analyses. Mosc. Univ. Soil Sci. Bull. 71 (1), 1–6. Vodyanitskii, YuN., Kirillova, N.P., Savichev, A.T., Sileva, T.M., 2018. Color change of alluvial gley slightly ferruginous soil during drying. Mosc. Univ. Soil Sci. Bull. 73 (4), 142–148. Vorobeva, L.A., 2006. Theory and Practice of the Chemical Analysis of Soils. GEOS, Moscow, p. 400 (in Russian). World Reference Base for Soil Resources, 2006. A world soil Resources report no. 103. FAO, Rome, p. 132. https://doi.org/10.1017/S0014479706394902. Zaidelman, F.R., 1998. Gley and its Role in the Soil Formation. Lomonosov Moscow State University, Moscow, p. 316 (in Russian). Кaurichev, I.S., Orlov, D.S., 1982. Redox Processes and Their Role in the Genesis and Soil Fertility. Kolos, Moscow, p. 246 (in Russian). Кodama, H., McKeage, J.A., Tremblay, R.J., Gosselin, J.R., Townsend, M.G., 1977. Characterization of iron oxide compounds in soils by Mossbauer and other methods. Can. J. Earth Sci. 14, 1–15. https://doi.org/10.1139/e77-001.
Floodplain soils of the forest and steppe zones
* real values. ** theoretically calculated.
based on the analysis of soils dried under laboratory conditions that ignores the fact of decomposition or crystallization of a part of Fe minerals in the course of soil drying. Scale of this distortion can be quite significant. Iron compounds dissolvable in the acid ammonium oxalate are assigned to the non-stable type. We propose to differentiate them into two groups 1) the Fe(III) non-stable compounds that are dissolved in the oxalate from the dry soil and 2) the Fe(III)–Fe(II) non-stable non-stable minerals that do not resist the hydromorphic soil drying in laboratory. Their amount is determined based on the discrepancy between the amount of Fe extracted by the acid ammonium oxalate from the wet soil and the content of Fe recovered from the dry soil (stored for 1 or 36 months). Thus, influence of oxalate reacting with Fe(II) in the wet soil is opposite to that of complexons extracting a lesser amount of Fe from the wet soil than from the dry variety. Hence, based on the amount of Fe extracted by the oxalate, we can define the share of non-stable Fe minerals that are oxidised during a short-term drying of hydromorphic soils under laboratory conditions. To describe hydromorphic soil types based on the content of Fe(III) non-stable minerals, we propose to apply index (I)that characterises the interaction of Fe(III) non-stable minerals with colloids and can be pre sented as I ¼ Fefree/CEC, where CEC is the cation exchange capacity. It has been established that the non-stable green rust in the ferruginated and reduced gley (Gr) with a high value of index I (~10) and intense increase of redness (Δа* � 9) is transformed into the brown lep idocrocite in the course of soil drying. Decrease of the moisture content is also accompanied by changes of the green rust composition: fougerite is converted into tr�eburdenite, i.e., Fe-ephemer with a higher degree of Fe oxidation and weak increase of redness (Δа* � 3). In the oxidised gley (Go) with I ¼ 0.05–0.5, the rate of green rust transformation is retarded by soil colloids. In the deferruginated gley (Gdf) with high I ¼ 0.04–0.17 and zero growth of Δа*, active Fe minerals are not formed in the course of drying. Acknowledgments The authors thank N.P. Kirillova and A.T. Savichev for help in the work. This work was supported by the Russian Foundation for Basic Research (grant no. 19-29-05265_mk) and Grant of the President of Russian Federation to support Leading scientific schools (no. NSh3464.2018.11). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.apgeochem.2019.104424.
6
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Applied Geochemistry journal homepage: http://www.elsevier.com/locate/apgeochem
Non-stable Fe minerals in waterlogged soils Yuri N. Vodyanitskii a, Tatiana M. Minkina b, * a b
Lomonosov Moscow State University, Moscow, 119991, Russia Southern Federal University, Rostov-on-Don, 344006, Russia
A R T I C L E I N F O
A B S T R A C T
Editorial handling by Prof. M. Kersten
The hydromorphic soil profile is characterised by the presence of a gleyed horizon with specific features in different soils. The content and composition of non-stable Fe minerals can serve as important criterion for defining the gley type. Iron minerals soluble in the acid ammonium oxalate are assigned to the non-stable type. They can be divided into two groups depending on the moisture content in the analysed hydromorphic soils. The Fe(III) non-stable group includes minerals that are soluble in the oxalate from dry soils. The Fe(III)–Fe(II) nonstable group includes minerals that cannot resist the hydromorphic soil drying in laboratory. Their amount is determined by difference between the content of Fe extracted by the acid ammonium oxalate from the wet soil and the amount of Fe extracted from the dry soil stored for 1 or 36 months. In the hydromorphic Vertisols from the southern European part of Russia, the Fe content accounts for 7–10 and 3–6% of the bulk Fe content, respectively, in the Fe(III)–Fe(II) and Fe(III) non-stable minerals. Thus, a high share of non-stable Fe minerals oxidised during the drying of hydromorphic soils under laboratory conditions is established. To discriminate the hydromorphic soil types, we propose index I that characterises the interaction of Fe(III) non-stable minerals (Fefree) with colloids: I ¼ Fefree/CEC, where CEC is the cation exchange capacity. The fer ruginated and reduced gley (Gr) has a high index (I ~ 10). In Gr with high increment of redness (Δа* � 9) under drying the non-stable green rust is converted into brown lepidocrocite in the course of drying. Decrease of the moisture content in this process is also accompanied by change of the green rust composition: fougerite is transformed into tr�eburdenite, i.e., Fe-ephemer with a higher degree of Fe oxidation, leading to small increment of redness (Δа* � 3) in the greyish matrix. In the oxidised gley (Go) with I ¼ 0.05–0.5, the rate of green rust transformation is retarded by soil colloids. In the deferruginated gley (Gdf) with low I ¼ 0/04–0.17 and zero growth of Δа*, active Fe minerals are not formed in the course of drying.
Keywords: Sample preparation Soil humidity Oxalate-soluble fe CIE-Lab optical system Green rust Lepidocrocite Drying of soils
1. Introduction In the International Database of Soil Resources (World Reference …, 2006), hydromorphic mineral soils ‘situated under the influence of groundwaters’ make up the reference Gleysols group and occupy a vast area in the world (720 mln ha), with their largest terrains confined to the boreal regions. The hydromorphic soil profile is characterised by the presence of a gleyed horizon with specific features in different soils. The profile is usually identified under field conditions based on morphology, hue difference, abundance and shape of the Fe-bearing neoformations. Since the formation of gley is related to variation in the degree of Fe oxidation, it is expedient to define gley types based on the composition of Fe minerals. Proceeding from this viewpoint, three types of gley are defined in (Zaidelman, 1998). Type 1, reduced gley (Gr), is marked by greyish colour owing to Fe(II) minerals of the green rust group. Precisely
green rusts are responsible for the rapid colour alteration of gley after recovery of its sample from the section (Feder et al., 2005, 2018). Type 2, oxidised gley (Go), is characterised by brown mottles in the greyish matrix. The hue of brown mottles is governed by the Fe hydroxide type: ferrihydrite, lepidocrocite and goethite make up red-brown, orange and yellowish-brown mottles, respectively (World Reference …, 2006). Type 3 is represented by the heavy-textured deferruginated gley (Gdf), with the greyish colour defined by clay minerals without the Fe coating. The most reactive minerals are represented by the thermodynami cally non-stable and X-ray amorphous Fe(III) hydroxide (ferrihydrite) and Fe(II) minerals. Ferruginous minerals in Gleysols are rather diverse. In some places, the whole Fe is represented by Fe(II) (siderite, vivianite, chukanovite); in other places, by both Fe(II) and Fe(III) (magnetite and green rusts) (Etique et al., 2015). Many soils of the humid zone are characterised by a reversibility of
* Corresponding author. E-mail addresses: [email protected] (Y.N. Vodyanitskii), [email protected] (T.M. Minkina). https://doi.org/10.1016/j.apgeochem.2019.104424 Received 29 July 2019; Received in revised form 18 September 2019; Accepted 21 September 2019 Available online 23 September 2019 0883-2927/© 2019 Elsevier Ltd. All rights reserved.
Y.N. Vodyanitskii and T.M. Minkina
Applied Geochemistry 110 (2019) 104424
redox reactions (Кaurichev, Orlov, 1982). Oxidation of Fe(II) minerals and significant morphological alteration of the gleyed horizon are possible during the oxidizing period. The content and composition of non-stable Fe minerals can serve as important criterion for defining the gley type. Methods of the chemical extraction are used to determine Fe minerals of different stability (Kodama et al., 1977). In general, the total content of Fe oxides/hydr oxides is determined based on the dithionite-citrate-bicarbonate (FeDCB) solubility according to Mera-Jackson, assuming that the Fe-bearing sil icates can resist such influence and preserve the X-ray amorphous Fe minerals during treatment with the acid ammonium oxalate by the Tamm method. However, the existing methods for the chemical fractionation of Fe compounds based on the analysis of soils dried under laboratory con ditions ignore the fact of decomposition or crystallization of a part of Fe minerals during the hydromorphic soil drying. Scale of this distortion can be judged by comparing the content of extracted Fe(II) in the watersaturated and dried sediments. Data on the content of Fe(II) extracted by the ammonium oxalate from the water-saturated sediments from the bottom of the Potomac River flowing into the Atlantic Chesapeake Bay, USA, is described in (Phillips, Lovley, 1987, 1993). In the course of drying for 4 days at room temperature, content of the oxalate-soluble Fe decreased from 510 to 280 mmol/kg, i.e., by 230 mmol/kg that corre sponds to the content of non-stable Fe minerals. It is noteworthy that the water-saturated sediments yielded 210 mmol/kg Fe(II) and 300 mmol/kg Fe(III), while the dry variety only yielded Fe(III). Thus, the drying procedure changes not only the amount of the extracted Fe, but also its quality. Decrease of the content of mobile Fe compounds was also recorded during the aeration of bottom sediments recovered from the North Sea sediments (Kersten, Forstner, 1986). The process of drying exerts a similar influence on reduction of the share of mobile fractions of Cu, Cd and Pb in river sediments (Bordas, Bourg, 1998). Thus, drying of the sulphide-barren marine and river sediments increases the content of readily soluble metals. As for metal sulphides, the drying of sediments promotes transformation of the nonsoluble sulphide particles into the readily soluble sulphates. We have less data on the influence of drying on metals in soils, relative to river marine sediments. Therefore, a new method is obviously needed for the chemical fractionation of Fe min erals in the hydromorphic soil during its natural humidity. At present, the sulphide-barren gleys are divided into three large groups: (1) ferruginated reduced gley (Gr) marked by rapid yellowing in atmosphere due to the oxidation of non-stable Fe compounds defined as Green Rust (Trolard, Bourrie, 2008); (2) oxidised gley (Go) formed in a variable redox setting because of the precipitation of Fe as ferrihydrate and other Fe hydroxides at oxidation microbarriers within the gleyed horizon; and (3) deferruignated (Gdf) formed during washing - term “deferruignated” is rather inadequate for this variety, because it is al ways humidifed and usually contains a small amount of Fefree (commonly <50 mmol/kg). These three varieties of gley differ in terms of the content of both Fe(III) non-stable minerals and colloids. Thus, gleys of different genesis can be differentiated based on the content of Fe (III) non-stable minerals adjusted to the content of colloids. The aim of the present paper is to propose a new chemical frac tionation method for determining the contents of two forms of nonstable Fe minerals and to offer an indicator of different gley types based on the content of Fe(III) non-stable minerals per colloid unite.
2.1. Objects Choice of objects was dictated by the necessity to embrace hydro morphic soils over a vast European space formed on different bedrocks. We analysed Gleyed Cambisols on the granitic delluvium, gleysoils in palaeoalluvial deposits of the Kama River, gleysoils in alluvial deposits of the Klyaz’ma River, as well as hydromorphic Technosols and Vertisols in the alluvial clay deposits in the Rostov region. 2.1.1. Gleyed Cambisols, France Samples were taken in the Fougere area (western France) from Gleyed Cambisols marked by the seasonal colour variation (Feder et al., 2018). The area is situated at a height of 180 masl with numerous thalwegs up to 800 m long. Soils were formed here on a meter-scale granodioritic saprolite bed. The chosen profile is located on a hillslope and comprises the following horizons (from the top to bottom): (0–15 cm) black organo genic horizon without redox manifestations; (15–50 cm) oxidised silty gley with the greyish matrix (hue of 5Y6/4, according to Munsell) and brown mottles (hue of 2.5Y5/6); (50–80 cm) reduced silty gley, homo geneously blue (hue of 5BG6/1), almost always (10 months per year) inundated, without brown mottles; and (80–120 cm) granitic saprolite with signs of reduction and always moist. In the soil profile, content of the clay fraction is maximum at a depth of 70 cm in the upper part of the granitic saprolite. The organic horizon is underlain by the gleyed horizon with colour related to fougerite. The fougerite has a bluish-green colour. Boundary between the fougerite-bearing gleyed horizon and the oxidised lepidocrocite-bearing horizon varies seasonally at depths of 40–100 cm: rises during the rain period in winter and descends in dry summer. 2.1.2. Soils in palaeoalluvial deposits of the Cis-Ural region, Russia Hydromorphic soils of different degrees of gleyzation were studied in two catenas on the Middle Kama Plain, Krasnokamsk area of the central Perm Territory (Sataev, 2005). Sections were exposed on supra-floodplain terraces V and VII of the Kama River. On terrace VIII of the Bekryata Catena, the dark heavy-textured humic-gley soil was ana lysed. The mottled gleysolic horizons adjoin the greyish and brown-rusty mottles. Samples from horizons AUg (6–23 cm), G (23–43 cm), and B1g (43–80 cm) were scrutinised. On the supra-floodplain terrace V (Las’va Catena), the soddy pod zolised light-textured gley soil was analysed. The greyish and brownrusty mottles were found side by side in the gleysolic horizons. Sam ples from horizons ВТ1g (25–38 cm) and ВТ2g (38–60 cm) were scrutinised. 2.1.3. Hydromorphic Technosols and Vertisols, Rostov region, Russia Soils were studied in the Kamensk area 500 m SE of the Fillipenkov Settlement. We analysed soils of two types on the Glubokaya River terrace. Type 1 soils classified as hydromorphic Technosols They are intensely waterlogged and polluted mainly with inorganic materials transported from Lake Atamanskoe that was exploited as sludge col lector in the 1950’s. Samples were taken from the upper humus horizon (0–20 cm). The analytical results are as follows: carbonates 0.4–4.0%, рНaq 7.2–8.0, Corg 1.4–1.7%, silty particles (<1 μm) 12–18%. Type 2 represents the hydromorphic carbonate vertisol (Vertisols) developed on alluvial clays. We analysed the following horizons in the section: Ad (0–8 cm), A (8–32 cm), ABsa (32–71 cm), Bf (71–82 cm), BCf (82–100 cm). Effervescence of the soil is weak on the surface, vigorous beginning from a depth of 10 cm and again weak at 86 cm. Commencing from a depth of 32 cm, the section is marked by the deposition of salts as fari naceous and less common compact concretions. Approximately 10% of concretions do not effervesce with the hydrochloric acid. Iron segrega tions are recorded below a depth of 71 cm. Horizons Bf (71–82 cm) and BCf (82–100 cm) are marked by inhomogeneous dark-grey hue with a
2. Objects and methods Choice of objects was dictated by the necessity to embrace hydro morphic soils over a vast European space formed on different bedrocks. We analysed Gleyed Cambisols on the granitic delluvium, gley soils in palaeoalluvial deposits of the Kama River, gley soils in alluvial deposits of the Klyaz’ma River, as well as hydromorphic Technosols and Vertisols in the alluvial clay deposits of the Rostov region. 2
Y.N. Vodyanitskii and T.M. Minkina
Applied Geochemistry 110 (2019) 104424
pale-yellow tint along with white salt mottles and rusty Fe oxide mottles. These horizons (particularly, Bf) are more ferruginated than the upper horizons: the bulk Fe content is as much as 802–848 mmol/kg, in contrast to 741–780 mmol/kg in the upper Vertisols. Rusty Fe oxide mottles influence the chemical fractionation. The grain size composition of all horizons corresponds to the clay fraction. The upper horizons down to a depth of 71 cm are wet, whereas the deeper horizons are moist. Beginning from this depth, the grey background is marked by rusty Fe hydroxide mottles.
conditions. This approach is based on comparison of the content of the oxalate-extracted Fe in the wet sample (Feox-wet) with its content in the dried sample (Feox-dry). The discrepancy corresponds to the amount of Fe (III)–Fe(II) non-stable compounds. Previously, the content of oxalate-soluble Fe(II) and Fe(III) in the water-saturated alluvial sediments were determined during the analysis and cultivation of anaerobic bacteria (Phillips et al., 1987, 1993). A hermetically sealed sample was saturated with inert gas N2 to prevent the oxygen input from atmosphere. Preservation of reducing conditions in the sample is an obligatory condition in this method. We had a different task: differentiation of the Fe(III)–Fe(II) nonstable and Fe(III) non-stable minerals in terms of their ability to crys tallise during the hydromorphic soil drying. Table 1 demonstrates the scheme of chemical fractionation of Fe minerals according to their sta bility in hydromorphic soils. The group of Fe minerals that can resist the soil sample drying and acid ammonium oxalate treatment includes the coarse-crystalline he matite, goethite and many Fe-phyllosilicates. Green rusts belong to lowstable minerals that cannot resist the drying. Reaction to the chemical treatment of other minerals (ferrihydrite, ferroxyhyte, dispersed goethite) depends on the redox conditions of soils during the sampling. At low ЕН values, as in the water-saturated fresh-water sediments, the sample is saturated with Fe(II). In this case, treatment of the wet sample by the ammonium oxalate provokes the reduction of Fe(III) hydroxides (Blesa et al., 1987). In contrast, at a higher EH value typical of the oxidised gley soil (Gleysols), the sample is less saturated with Fe (II), leading to decrease of the content of non-stable Fe minerals that are sensitive to treatment of the wet soil sample by the ammonium oxalate (Vodyanitskii, 2001). In the dried (particularly, stored for a long time) sample, particles of Fe(III) hydroxides (ferrihydrite, ferroxyhyte, dispersed goethite) are crystallised gradually, leading to decrease in solubility. Therefore, the content of the oxalate-soluble Fe in the wet soil (Feox-wet) is higher than in the dry soil (Feox-dry). Method of determination of the oxalate-soluble Fe (Feox-wet) is as follows. The wet soil sample extracted from the section is placed in a sealed polyethylene packet and transported to laboratory. Material from the packet is placed in two similar boxes. Soil from one box is imme diately treated with the Tamm reagent according to the conventional method. Soil from the second box is dried at room temperature, with the subsequent determination of the dry mass. This value of mass is used for correcting the value if it differs from 1 g accepted for calculation by the Tamm method.
2.1.4. Alluvial boggy gley soil, Moscow region, Russia The section was exposed beneath the bog vegetation on the Klyaz’ma River floodplain (Vodyanitskii et al., 2018). The mucky horizon A02 is situated at a depth of 0–50 cm. It is underlain by a greyish gley (G) horizon at 50–95 cm. In terms of the grain size composition, the gley corresponds to medium loam. Four gley samples were studied. Samples 1 and 2 were taken from the upper part (50–55 cm); samples 3 and 4, from the deeper (moister) part of the horizon (90–95 cm). The ground water table was recorded at a depth of 1 m. The upper and lower parts of the gley horizon differ in many chemical parameters. Its lower part is depleted in carbon and nitrogen as compared to the upper part: Corg ¼ 0.74 � 0.015% versus 1.41 � 0.24%; N ¼ 0.024 � 0.0025% versus 0.096 � 0.017%. The bulk Fe content is low: Febulk ¼ 414–612 mmol/kg, which is two times lesser than the average content in the Earth’s crust (1110 mmol/ kg). The content of non-silicate Fe compounds is very low (dithionitecitrate-bicarbonate extractable Fe, FeDCB ¼ 41–55 mmol/kg), leading to low values of the ratio FeDCB/Febulk ¼ 0.08–0.13. All these facts testify to a weak ferrugination of the given gley type. A significant amount of nonsilicate Fe compounds in the dry soil is dissolved in the acid ammonium oxalate (Feox-dry): Feox-dry ¼ 34–43 mmol/kg, leading to a high value of the ratio Feox-dry/FeDCB ¼ 0.67–0.88. 2.2. Methods The bulk Fe content in the soils was determined with a Spectroscan MAKC-GV X-ray fluorescent spectrometer. According to the manual (Vorob’eva 2006), the pH value was measured by a pHS-3C potenti ometer in the soil-water suspension (1 : 2.5); the organic matter content was determined by the titrimetric method (bichromate oxidation); the carbonate content was determined by the volumetric method; and silt particles were determined by pipetting with the pyrophosphate sample preparation. The cation exchange capacity (CEC) was determined by the ammonium acetate method. The Fe content in the X-ray amorphous and low-crystalline particles were determined after the soil treatment by the acid ammonium oxalate at pH ¼ 3.25. Examination of mineralogical transformations of Fe compounds in the Gleyed Cambisol profile (France) by the method of nuclear gamma €ssbauer analyser made it possible to determine resonance with a field Mo spatiotemporal changes of the Fe composition in the soil solid phase for the first time (Feder et al., 2005). To reveal small changes in gley colour in the alluvial boggy gley soil of the Moscow region, we measured the reflection spectra in the course of gley drying to the air-dry state at room temperature (Vodyanitskii et al., 2018). The colour characteristics are presented in the CIE-Lab system.
3.2. Contents of two forms of non-stable Fe minerals in the hydromorphic Vertisols Table 2 demonstrates schematically the principle of chemical frac tionation of Fe minerals according to their stability in hydromorphic soils, with the hydromorphic Vertisols in southern Russia as example. The oxalate extracts from the wet soil a higher amount of Fe (Feoxwet) than from the dry soil (Feox-dry) stored for 1 month and the more so from a soil stored for 36 months. The oxalate recovered 11.6–14.0% of the bulk Fe content from the wet soil, but only 5.3–7.2% from the soil Table 1 Scheme of the chemical fractionation of Fe minerals based on their stability.
3. Results and discussion 3.1. Determination of two forms of non-stable Fe minerals in the hydromorphic soils Since the conventional chemical fractionation of Fe compounds in the dried samples is inapplicable for hydromorphic soils, we propose a new approach that can take into consideration the content of non-stable Fe compounds, which cannot resist the drying under laboratory 3
Category of Fe mineral stability
Determination method
Soluble Fe minerals
Fe(III)–Fe(II) nonstable compounds Fe(III) non-stable compounds
Feox-wet-Feox-dry
Stable Fe compounds
Febulk-Feox-wet
Green rusts. Reducible: ferrihydrite, ferroxyhyte, dispersed goethite Siderite. Minerals able to crystallise during drying: ferrihydrite, ferroxyhyte, dispersed goethite Coarse-crystalline goethite, hematite, several Fe- phyllosilicates
Feox-dry
Y.N. Vodyanitskii and T.M. Minkina
Applied Geochemistry 110 (2019) 104424
Table 2 Fe content in compounds of different stability in the hydromorphic Atamanskoe Vertisols (Fe content is given in mmol/kg, values in parentheses show percentage from Febulk). Horizon, depth (cm)
Fe fractions Febulk
Wet samples Feox-wet
Ad (0–8) A (8–32) ABs (32–40) ABs (40–71) Bf (71–82) BCfg (82–100)
751 780 741 768 848 802
87.4 102.8 89.2 88.2 114.0 112.1
Sample storage time - 1 month
Sample storage time - 36 months
Febulk –Feox-bulk
Feox-dry
Feox-wet – Feox-dry
Feox-dry
Feox-wet – Feox-dry
Stable Fe
Fe(III) non-stable
Fe(III)–Fe(II) non-stable
Fe(III) non-stable
Fe(III)–Fe(II) non-stable
663.6 677.2 651.8 679.8 734.0 689.9
51.9 (6.9) 51.4 (6.6) 39.4 (5.3) 48.4 (6.3) 61.2 (7.2) 56.6 (7.1)
35.5 (4.7) 51.4 (6.6) 49.8 (6.7) 39.8 (5.2) 52.8 (6.2) 55.5 (6.9)
25.1 (3.3) 25.2 (3.2) 22.8 (3.1) 26.5 (3.4) 52.2 (6.2) 33.3 (4.1)
62.6 (8.3) 77.6 (10.0) 66.1 (8.9) 61.7 (8.0) 61.8 (7.3) 78.8 (9.8)
(88.4) (86.8) (88.0) (88.5) (86.5) (86.0)
dried for 1 month and just 3.1–6.2% from the soil stored for 36 months. Thus, even in the course of a short-term storage of dry samples, a sig nificant amount of Fe hydroxides is crystallised and they can withstand the oxalate treatment. The amount of Fe extracted from the briefly dried soil is assigned to the Fe(III) non-stable type, and the Fe content defined from the discrepancy Feox-wet – Feox-dry is assigned to the Fe(III)–Fe(II) non-stable compounds. Let us note that our data and the results reported in (Phillips et al., 1987, 1993) differ basically from the results obtained by the extraction of Fe from wet soils by other reagents. These discrepancies are related to different properties of reagents. For example, antagonistic data were obtained during the extraction of Fe from wet soils using the ammonium acetate buffer (AAB) with pH ¼ 4.8 (Bartlett, James, 1980) or com plexons such as ethylenediamine tetraacid, EDTA (Haynes, Swift, 1991) and its homologue diethylenetriamine penta-acetic acid, DTPA (Muel ler, 2013). Naturally, efficiency of AAB is several orders of magnitude lower than that of complexons, because AAB only extracts the loosely bonded Fe particles, while complexons extract Fe after the formation of a robust complex with this element. For example, bond strength of EDTA with Fe (II) is appreciably weaker than with Fe(III): log K1 ¼ 14.2 and 24.2, respectively (Lurie, 1979). It is very important that the bond strength of complexon with Fe(III) is much stronger than with Fe(II). Therefore, efficiency of complexons as extragents is boosted in the course of soil drying and oxidation of Fe(III) to Fe(II). Experiment reported in (Leg gett, Argyle, 1983) demonstrated that increase of temperature during the soil drying fostered the extractability of Fe by complexon. Second reason for the increase of Fe extractability by the complexon lies in the acidification of soil in the course of its drying (Vodyanitskii, Mineev, 2016). The point is that the formation of more robust Fe(III) complexes needs a more acid medium, relative to the less robust Fe(II) complexes (Dyatlova et al., 1988). In contrast, efficiency of the oxalate is promoted if the wet soil contains Fe(II) that catalyses the dissolution of Fe(III) minerals (Blesa et al., 1987; Vodyanitskii, 2001). Thus, efficiency of complexons de creases in the initially wet soils, but efficiency of the oxalate increases if the reduced Fe is present.
capacity. High value of index I reflects a high probability of the formation of Fe (III) non-stable minerals in the gley. On the contrary, interaction of these different Fe compounds with soil colloids inhibits their formation when I → min. At the same time, interaction of active forms of Fe compounds with soil colloids is manifested, in particular, in the capacity of Fe hy droxide particles to precipitate on clay mineral edges, blockade some exchange sites, and thus decrease the CEC value (Oscarson; Heimann, 1988). Probably, cation exchange capacity of the intensely ferruginated gley could be higher if the clay mineral edges were not blockaded by Fe hydroxide particles. The three gley types with diverse aquatic settings and morphologies should also differ in terms of index I that reflects different probabilities of the formation of Fe(II) non-stable minerals in the gley. 3.3.1. Ferruginated reduced gley (Gr) with a high value of index I 3.3.1.1. Gleyed Cambisols, western France. Data on the colour of different Gleyed Cambisol horizons expressed in the Munsell system are presented in (Feder et al., 2005). Since the peculiarity of this system is the cylindrical coordinates, hampering statistical calculations is less convenient, we converted the original colour data into the more modern CIE-L*a*b* optical system, which is a universal color space in Cartesian coordinates. Table 3 presents colour parameters of two Cambisol horizons after converting them from the Munsell system into the CIE-Lab system ac cording to algorithm in (Vodyanitskii, Kirillova, 2016). As is evident, redness of the greyish mottles in the oxidised gley increases from 2.5 to €ssbauer field 5.2; i.e., colour difference Δа* ¼ 2.7. Data on the Mo measurements revealed that increase of redness is caused by the oxidation of Fe ephemers of the green rust group. Fougerite, a double layered hydromica with Fe2þ: Fe3þ ¼ 2 : 1, is transformed into another hydroxide (tr�eburdenite) belonging to the same but more oxidised group of green rusts with Fe2þ: Fe3þ ¼ 1 : 2. Redness of brown mottles in the oxidised gley, where green rust was transformed into lepidocrocite, increases even more: from 3.5 to 5.2, i.e., Δа* ¼ 8.7. The reduced gley in Gleyed Cambisols is marked by low CEC values ranging from 4.5 to 6.1 cmol (þ)/kg (Feder et al., 2018). On the other hand, approximately 90% Fe is extracted by
3.3. Diagnostics of gley types based on the content of Fe(III) non-stable minerals per colloid unit Our approach is based the relationship of two indicators of gley: cation exchange capacity (CEC), a proxy of soil colloids; content of free Fe compounds in the dried soil, i.e., Fe(III) non-stable compounds. To dissolve free Fe compounds, researchers commonly use the dithionitecitrate in the case of long-term (up to 200 h) treatment (Feder et al., 2018) or the acid ammonium oxalate according to the Tamm method (Kodama et al., 1977; Vodyanitskii, 2001). The content of non-silicate Fe compounds Fefree is presented in mol/kg. To characterize the gley types based on the content of Fe (III) non-stable compounds per colloid unit, we propose index I: I ¼ Fefree/CEC, where CEC is the cation exchange
Table 3 Colour of Gelyed Cambisols (France) expressed in the CIE-Lab optical system, initial data from (Feder et al., 2005). Horizon part Reduced gley Greyish matrix Oxidised gley Greyish matrix Brown mottles
4
L*
a*
b*
61.7
5.2
0.6
61.7 51.6
25 5.2
29.0 42.0
Y.N. Vodyanitskii and T.M. Minkina
Applied Geochemistry 110 (2019) 104424
dithionite-citrate-bicarbonate and 60% Fe by citrate-bicarbonate. In absolute values, the content of active Fe extracted by citrate-bicarbonate is very high (680 mmol/kg) (Feder et al., 2018). Very highvalues of index I ¼ 11–14 are reflected favourable settings for the formation of non-stable green rusts in the gley. Thus, the gley containing green rusts is marked by highindex I values (~0.1–0.5) (~5–15).
(þ)/kg). Thus, the deferruginated gley shows a low index I ¼ 0.07–0.09. Colour indicators of the deferruginated gley before and after the drying are presented in Table 5. After the drying, lightness and yel lowness of gley increased, in general, significantly (ΔL* ¼ 15, Δb* ¼ 6.2), but redness did not practically change (Δа*is 0.1). Preservation of redness (Δа* � 0) owing to the evaporation of moisture from any soil (automorphic variety included) should be discriminated from the slight increase of redness (Δа* ¼ 2.7) because of the transformation of green rust minerals in the dehydrated Gleyed Cambisols in western France. Colour alteration due to the evaporation of moisture in 41 soil samples of genetically different colour is described in the Munsell optical system (Post et al., 1993). Conversion of colour indicators to the more convenient orthogonal CIE-L*a*b* optical system revealed: in general, lightness of soils increased significantly after drying (ΔL* ¼ 15), but redness increased slightly (Δа* ¼ 1.3). This result is consistent with the colour alteration in dry samples of the heavy-textured Cambisols in South Carolina, USA (Stiglitz et al., 2017), where lightness increased appreciably (ΔL* ¼ 16), but redness only slightly (Δа* ¼ 1.6). Thus, water evaporation in the course of drying deciphered by a minor increase of redness (Δа* < 1.7) can be discriminated from the transformation of green rust minerals accompanied by a stronger increase of redness (Δа* ¼ 2.7). The obtained results are summarised in Table 6. Among the three gley types, only the reduced gley (Gr) is marked by a high value of index I. Based on this index, the oxidised gley (Go), however, cannot be distinguished reliably from the deferruignated gley (Gdf). Wide variation of index I in the oxidised gley (Go) is related to a shortcoming of its identification based on morphology. It is rather difficult to detect a few tiny brown mottles in the grayish brown matrix to define this horizon as oxidised gley (Gо). At the same time, chemical properties (Feox and CEC) of sample, on the whole, determine in labo ratory after the drying. Another important shortcoming of the morphological identification of gley types lies in different nature of criteria. The reduced and oxidised gley is identified based on the phys icochemical properties of Fe compounds, whereas the deferruignated gley is recognised based on the degree of Fe evacuation from the horizon. In connection with shortcomings of the morphological identification of gley types, Table 6 presents the theoretical values of index I lacking any strict dependence on the gley morphology. Theoretical domains do not differ from the factual boundaries only for the reduced gley (Gr). However, the range of index I is 0.08–0.5 for the oxidised gley and <0.08 for the deferruignated gley (Gdf). Database augmentation can refine the critical value of index I ¼ 0.08.
3.3.2. Oxidised gley (Go) with moderate and low values of index I 3.3.2.1. Gleyed soils in palaeoalluvial deposits, Cis-Ural region, Russia. In the Bekryata Catena, examination of the heavy textured dark humic-gley soil using the TEM method revealed that horizon AUg (6–23 cm) con tains Fе-vernadite and Mn-ferroxyhyte (Sataev, 2005). The content of active oxalate-soluble Fe compounds, Feox-dry ¼ 85 mmol/kg; CEC ¼ 29.5 cmol (þ)/kg; I ¼ 0.29. In the lower horizons G (23–43 cm) and B1g (43–80 cm), I ¼ 0.16 and 0.24, respectively (Table 4). In the Las’va Catena, the soddy podzolised light-textured gley soil in horizon ВТ1g (25–38 cm) contains ferrihydrite and siderite. The brown mottles therein are likely related to Fe-pigment (ferrihydrite). The content of active oxalate-soluble Fe compounds, Feox ¼ 31 mmol/kg, CEC ¼ 6.9 cmol (þ)/kg, I ¼ 0.45. Horizon ВТ2g (38–60 cm) contains ferrihydrite, siderite and ferroxyhyte. The gleysolic horizons are char acterised by a mottled hue and the presence of greyish and brown-rusty mottles side by side. The brown mottles likely include Fe-pigments (ferrihydrite and ferroxyhyte). The content of active oxalate-soluble Fe compounds, Feox ¼ 48 mmol/kg, CEC ¼ 9.9 cmol (þ)/kg, I ¼ 0.48. Thus, the gleysolic soils in the Cis-Ural region are marked by I ¼ 0.16–0.48 (Table 4). 3.3.2.2. Hydromorphic technosols, Rostov region, Russia. In the studied soil samples, CEC ¼ 32–36 cmol (þ)/kg, Feox-dry ¼ 75–139 mmol/kg. The content of Fe (III) non-stable compounds per colloid unit is, on the average, I ¼ 0.26–0.43. 3.3.2.3. Hydromorphic Vertisols, Rostov region, Russia. In soil from sec tion 9, CEC ¼ 27–49 cmol (þ)/kg, Feox-dry ¼ 23–52 mmol/kgThe content of Fe (III) non-stable compounds per colloid unit is low, I ¼ 0.05–0.19. 3.3.3. Deferruginated gley (Gdf) with lowvalue of index I This gley is always intensely wetted. Term ‘deferruginated’ is not quite accurate for this gley, because it always contains a small amount of Fefree (usually <50 mmol/kg). Moreover, one should bear in mind that all reagents described above are insufficiently selective because of the extraction of a part of Fe from the phyllosilicate lattice.
4. Conclusions
3.3.3.1. Alluvial boggy mucky-gley soil, Moscow region, Russia. In this gley, the content of semi-stable Fe minerals is low: Feox-dry varies from 34 to 42 to 36–43 mmol/kg in the lower part and CEC is high (~50 cmol
Methods of the chemical extraction are used to determine Fe min erals with different stability, whereas the oxalate treatment of soils is applied most commonly to determine Fe in the composition of nonstable minerals. Methods of the chemical extraction, however, are
Table 4 Chemical and mineral compositions of Fe minerals in gleyed soils from palae oalluvial deposits in the Kama region, Russia (Sataev, 2005). Horizon, depth (cm)
Febulk mmol/kg
Feox-dr mmol/kg
Fe minerals
Dark humic gley of heavy texture, Bekryata Catena AUg (6–23) 662 85 Fe-vernadite, Mn-ferroxyhyte G (23–43) 1009 56 Fe-vernadite, goethite В1g (43–80) 729 73 Not determined Soddy podzolised gleysolic with light texture, Las’va Catena BT1g 516 31 Ferrihydrite, (25–38) siderite BT2g 339 48 Ferrihydrite, (38–60) siderite, ferroxyhyte
CEC cmol/ kg
Index I
29.5
0.29
35.3
0.16
31.0
0.24
6.9
0.45
10.0
0.48
Table 5 Light (L*), red (a*) and yellow (b*) hues of the wet and dry gley, alluvial peaty mucky-gley soil, Moscow region, Russia. Parameter L*wet L*dry ΔL* a*wet a*dry Δa* b*wet b*dry Δb*
5
Horizon top
Horizon bottom
1
2
3
4
33.9 48.7 14.8 3.75 3.75 0.00 7.41 14.47 7.06
40.0 55.4 15.4 3.12 3.75 0.63 8.53 13.55 5.02
40.8 55.5 14.7 2.50 1.35 1.15 8.09 15.68 7.59
41.8 57.0 15.2 1.87 1.87 0.00 7.59 12.73 5.14
Y.N. Vodyanitskii and T.M. Minkina
Applied Geochemistry 110 (2019) 104424
Declaration of interests
Table 6 Values of index I in different gley types. Gley
Reduced gley (Gr)
Oxidised gley (Gо)
Deferruginated gley (Gdf)
Index I* Index I** Transformation of Fe minerals
>2 >2 Green rusts → lepidocrocite Fougerite → tr�eburdenite Gleyed Cambisols
0.08–0.5 0.05–0.5 Green rusts → lepidocrocite, ferrihydrite. Goethite Gleyed soils of the podzolised series and agrosoddy soils. Floodplain soils of the forest and steppe zones
0.07–0.08 <0.08 No
Soils
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References Bartlett, R., James, B., 1980. Studying dried, stored soil samples—some pitfalls. Soil Sci. Soc. Am. J. 44, 721–724. https://doi.org/10.2136/ sssaj1980.03615995004400040011x. Blesa, M.A., Marinovich, H.A., Baumgrater, E.C., Maroto, A.J.G., 1987. Mechanism of dissolution of magnetite by oxalic acid-ferrous ion solutions. Inorg. Chem. 26, 3713–3717. https://doi.org/10.1021/ic00269a019. Bordas, F., Bourg, A.C.M., 1998. A critical evaluation of sample pretreatment for storage of contaminated sediments to be investigated for the potential mobility of their heavy metal load. Water Air Soil Pollut. 103, 137–149. Dyatlova, N.V., Temkina, D.Z., Popov, K.I., 1988. Complexons and Complexonates of Metals. Khimiya, Moscow, p. 544 (in Russian). Etique, M., Jorand, F.P.A., Rubi, C., 2015. 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Geochemistry of green rusts and fougerite: a reevaluation of Fe cycle in soils. Adv. Agron. 99, 227–288 (chapter 5). https://doi. org/10.1016/S0065-2113(08)00405-7. Vodyanitskii, YuN., 2001. On the dissolution of iron minerals in Tamm’s reagent. Eurasian Soil Sci. 34 (10), 1086–1096. Vodyanitskii, YuN., Kirillova, N.P., 2016. Conversion of Munsell color coordinates to the CIE-L*a*b* system: tables and calculation examples. Mosc. Univ. Soil Sci. Bull. 4, 139–146. Vodyanitskii, YuN., Mineev, V.G., 2016. Difference in the pH values of hydromorphic soils in field and laboratory analyses. Mosc. Univ. Soil Sci. Bull. 71 (1), 1–6. Vodyanitskii, YuN., Kirillova, N.P., Savichev, A.T., Sileva, T.M., 2018. Color change of alluvial gley slightly ferruginous soil during drying. Mosc. Univ. Soil Sci. Bull. 73 (4), 142–148. Vorobeva, L.A., 2006. Theory and Practice of the Chemical Analysis of Soils. GEOS, Moscow, p. 400 (in Russian). World Reference Base for Soil Resources, 2006. 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Floodplain soils of the forest and steppe zones
* real values. ** theoretically calculated.
based on the analysis of soils dried under laboratory conditions that ignores the fact of decomposition or crystallization of a part of Fe minerals in the course of soil drying. Scale of this distortion can be quite significant. Iron compounds dissolvable in the acid ammonium oxalate are assigned to the non-stable type. We propose to differentiate them into two groups 1) the Fe(III) non-stable compounds that are dissolved in the oxalate from the dry soil and 2) the Fe(III)–Fe(II) non-stable non-stable minerals that do not resist the hydromorphic soil drying in laboratory. Their amount is determined based on the discrepancy between the amount of Fe extracted by the acid ammonium oxalate from the wet soil and the content of Fe recovered from the dry soil (stored for 1 or 36 months). Thus, influence of oxalate reacting with Fe(II) in the wet soil is opposite to that of complexons extracting a lesser amount of Fe from the wet soil than from the dry variety. Hence, based on the amount of Fe extracted by the oxalate, we can define the share of non-stable Fe minerals that are oxidised during a short-term drying of hydromorphic soils under laboratory conditions. To describe hydromorphic soil types based on the content of Fe(III) non-stable minerals, we propose to apply index (I)that characterises the interaction of Fe(III) non-stable minerals with colloids and can be pre sented as I ¼ Fefree/CEC, where CEC is the cation exchange capacity. It has been established that the non-stable green rust in the ferruginated and reduced gley (Gr) with a high value of index I (~10) and intense increase of redness (Δа* � 9) is transformed into the brown lep idocrocite in the course of soil drying. Decrease of the moisture content is also accompanied by changes of the green rust composition: fougerite is converted into tr�eburdenite, i.e., Fe-ephemer with a higher degree of Fe oxidation and weak increase of redness (Δа* � 3). In the oxidised gley (Go) with I ¼ 0.05–0.5, the rate of green rust transformation is retarded by soil colloids. In the deferruginated gley (Gdf) with high I ¼ 0.04–0.17 and zero growth of Δа*, active Fe minerals are not formed in the course of drying. Acknowledgments The authors thank N.P. Kirillova and A.T. Savichev for help in the work. This work was supported by the Russian Foundation for Basic Research (grant no. 19-29-05265_mk) and Grant of the President of Russian Federation to support Leading scientific schools (no. NSh3464.2018.11). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.apgeochem.2019.104424.
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