Seasonally Precipitated Iron Oxides in a Yertisol of Southeast TexasD. C. Golden, F. T. Turner, H. Sittertz-Bhatkar, and J. B. Dixon*

ABSTRACT

The composition and crystallinity of Fe oxides in soils determinesthe reactivity and toxicity of Fe through redox and solubility reactions.The mineralogy and crystallinity of Fe oxides seasonally precipitatingon ped surfaces and within soil pores and those forming around rice(Oryza saliva L.) roots were investigated by x-ray diffraction, electronmicroscopy, and electron microprobe analyses. Iron oxides precipi-tated on exposed surfaces of the League soil (fine, montmorillonitic,hyperthermic Oxyaquic Dystrudert), which is flooded during rice pro-duction, differed from Fe phases precipitated around rice roots. Ironoxides precipitated on ped surfaces and within soil pores were rela-tively poorly crystallized while those precipitated on rice-root surfaceswere well crystallized. The presence of soluble Si and P during floodingmay be responsible for precipitation of the less crystalline Fe oxides.Infrared and electron-diffraction data on the precipitate suggest thepresence of PO4 groups either adsorbed or coprecipitated with Feoxide. Depletion of Si and P from the rhizosphere is believed tocontribute to the formation of well-crystallized lepidocrocite on rootsurfaces. The poorly crystalline Fe-oxide precipitate that forms onped surfaces upon draining and oxidation of the League soil hasadsorbed or occluded Si and P. Thus, Fe oxides may influence themobility of Si and P in alternately flooded and drained soils. Electron-diffraction data suggests that some of the Fe may be precipitated asstrengite. Upon reduction and dissolution, these oxides release Fe,Si, and P into the soil solution and influence the nutrient dynamicsin the rhizosphere of the rice plant.

A:EDDISH-BROWN (SYR to 7.5YR) Fe oxide precipi-tates on ped surfaces, desiccation cracks, and

within pores in the League clay soil upon drying. Crys-talline Fe oxides also form on the roots of rice plantsgrown in flooded soil (Chen et al., 1980). The reactivity,availability, mobility, and solubility of Fe in rice soils isrelated to changes in redox conditions during floodingand draining of the soil (Ponnamperuma, 1972). Al-though nutrient release due to soil reduction has beeninvestigated (Ponnamperuma, 1972), the mineralogy ofseasonally precipitated Fe-containing phases found in

D.C. Golden, MailCode SN4, NASA/JSC, Houston, TX 77058; F.T.Turner, Texas A&M Univ. Agricultural Research and Extension Center,Route 7, Box 999, Beaumont, TX 77713; H. Sittertz-Bhatkar, ElectronMicroscopy Center, Texas A&M Univ., College Station, TX 77843; andJ.B. Dixon, Soil and Crop Sciences Dep., Texas A&M Univ., CollegeStation, TX 77843. Received 27 May 1994. *Corresponding author (j-dixon@tamu.edu).

Published in Soil Sci. Soc. Am. J. 61:958-964 (1997).

these soils has received relatively little attention (Bachaand Hossner, 1977; Chen et al., 1980; Karim and New-man, 1986). Iron-containing phases present in rice soilsare important in that they control the solution concen-tration of Fe through redox and solubility relationships.Several factors affect the crystallinity and mineralogyof Fe oxides formed in soils, e.g., the presence in solutionof Si, P, and dissolved carbonate species (influenced byCO2 partial pressure), the rate of oxidation, the concen-tration of Fe in solution, the pH, and the Eh (Schwert-mann and Taylor, 1989). The purpose of this researchwas to compare the mineralogy of Fe oxides formed onrice roots with those precipitated on ped surfaces andto relate their properties to current knowledge of Fe-oxide chemistry.

MATERIALS AND METHODS

Many large clods (10 kg) having a reddish-brown precipitate(SYR) on their surfaces were collected after draining andplowing the League clay soil (formerly Beaumont soil series)at the Texas A&M University Agriculture Research Centernear Beaumont, TX. These clods were air dried for 2 wk,broken into smaller pieces (=5 cm), and used without fur-ther processing.

Experiment 1

Two kilograms of air-dry soil was rewetted and floodedwith a 5-cm-deep layer of water in a tall 2-L plastic beaker.Prior to mixing, 1 g of glucose was dissolved in the water toprovide a source of C. Soil Eh and pH were monitored dailyusing a polished platinum electrode, a hydrogen electrode, anda calomel reference electrode. Soil solutions were periodicallysampled at a 10-cm soil depth using a suction-cup soil-solutionsampler (Wood, 1973), and analyzed for total concentrationsof Mn [Mn(T)J, Fe [Fe(T)], and Si [Si(T)J after filtrationthrough a Whatman no. 42 filter. The amounts of Mn(T) andFe(T) were determined by atomic absorption spectrometry,whereas Si(T) was determined by the colorimetric procedureof Hallmark et al. (1982).

At 17 d, a 500-mL sample of soil solution was obtainedusing a suction-cup sampler (pore size = 2.1 jxm) and oxidizedby sparging with air at approximately 10 mL min""1 for 10 h.The control for this experiment was the oxidation of a 50 |Ag

Abbreviations: Eh, redox potential; XRD, x-ray diffraction; TEM, trans-mission-electron micrograph or microscopy; EDXRA, energy-dispersivex-ray analysis.

GOLDEN ET AL.: SEASONALLY PRECIPITATED IRON OXIDES 959

mLr1 solution of Fe2+ [as Fe(ClO4)2] by aerating at the samerate as for the sample solution at 22°C. The resulting precipi-tates were analyzed using x-ray diffraction (XRD).

Experiment 2A plastic beaker with 2 kg of League clay soil was set up

similarly to Experiment 1 and 10 rice seedlings (cv. Labelle)were planted in it. The rice roots were harvested after main-taining flooded conditions for 60 d. A brown (7.5YR 6/8)precipitate around the roots was carefully collected byscraping with a razor blade, and an aliquot was freeze-dried.The remaining Fe oxide was fractionated into coarse (2-0.2(jun) and fine (<0.2 (Jim) clay fractions by the procedure ofJackson (1969, p. 127-141).

Iron-Oxide CharacterizationIron oxides isolated in Experiments 1 and 2 were mounted

on holey C-film substrate over a Cu grid and analyzed bytransmission-electron microscopy (TEM) using a Philips 400Ttransmission-electron microscope (120 kV) equipped with anenergy-dispersive x-ray (EDXRA) spectrometer (Philips,Eindhoven, the Netherlands). Higher magnification TEM ex-amination was performed using a Zeiss IOC electron micro-scope operated at 100 kV (Carl Zeiss Inc., Thornwood, NY),and later by a JEOL 200FX scanning transmission-electronmicroscope operated at 200 kV (Jeol USA, Peabody, MA).Oriented mounts of powders of the Fe oxides were rate-scanx-rayed using a Philips Norelco x-ray diffractometer with agraphite monochromator (Cu-Ka radiation) from 2 to 70°26(only 2-50°26 is shown in Fig. 2). Scanning-electron micros-copy was performed on C-coated pieces of clods (2-3 mm) thatwere mounted on Al stubs, using a JEOL JSM 35 operating at15 kV. Specimens of selected precipitates were diluted in KBrto 3 g kg"1 concentration, pressed into pellets, and analyzedby infrared spectrometry using a Perkin-Elmer Model 283spectrometer (Perkin-Elmer, Norwalk, CT).

An approximately 2 by 2 by 2 mm piece of ped with a surfacecoating of Fe oxide was prepared for elemental analysis byembedding the ped in Embed-812 epoxy (Electron MicroscopySciences, Philadelphia, PA) and then cutting the whole pedin a direction perpendicular to the Fe coating using a diamondknife. The cut surface was C coated and examined under a

200

-20018 20

Fig. 1. Release of Fe and Si to soil solution and change in redoxpotential (Eh) during reduction of League clay soil.

Cameca 100CX electron microprobe operated at 20 kV and10 nA beam current (Cameca Instruments, Stamford, CT).

RESULTS• Reductive Dissolution and Precipitation

from SolutionAnalyses of soil solutions from Experiment 1 indi-

cated an increase in Fe(T) and Si(T) during the courseof 17 d (Fig. 1). An initial increase in Mn(T) from 4 to5 mg kg"1 was observed after 1 d, followed by a slightdecrease to 3 mg kg"1 at the end of 17 d (data notshown). When air was passed through the clear filteredsoil solution (pH 4.9) a yellowish-brown (7.5YR 8/6)precipitate was formed that exhibited no clearly definedXRD peaks (Fig. 2c). In contrast, the oxidized Fe(ClO4)2control solution (pH adjusted to 4.9, not shown) andthe Fe-oxide precipitate on root surfaces (Fig. 2a and2b) each yielded XRD peaks for lepidocrocite.

Iron Oxide Formed by the Oxidationof the Soil Extract

Thin deposits of aggregated Fe-oxide particles ob-tained by oxidation of the soil solution (extracted on

3.34 Q

1.93 L J\L _Av——/WvA*V ^V^^

Fig. 2. X-ray diffractograms of Fe-oxide precipitates: (a) preferentially oriented mount of the 0.2-jjmi fraction separated from rice root surfaces;(b) randomly oriented mount of material in (a); (c) poorly crystalline precipitate formed by oxidizing the reduced soil solution.

960 SOIL SCI. SOC. AM. J., VOL. 61, MAY-JUNE 1997

the 17th d) reveal a mixture of fine granules and smalllaths (Fig. 3a, inset, and 3b). Larger aggregates are usu-ally too electron dense to reveal their interior fabric(Fig. 3a, 4c, and 4d). Effective dispersion of these lathand granule associations was difficult to obtain despitethe relatively poor crystallinity of this material. Elemen-tal analysis of this precipitate by EDXRA indicatedthe presence of Fe, Si, and P (Fig. 4f). The electron-diffraction pattern (Fig. 4e) has three diffuse bands withd-spacings of about 3.37, 2.96, and 1.61 A that do notcorrespond with those of common Fe oxides. They cor-respond reasonably well with strengite spacings (JointCommittee on Powder Diffraction Standards, 1983, Fileno. 15-513) of 3.28 A (202), 2.95 A (131), and 1.62 A(no index given). Although the strengite interpretationis not conclusive, the presence of P and Fe in the precipi-tate, the slight solubility of FePO4 under oxidizing condi-tions, and the closeness of the diffuse electron-diffrac-tion bands observed to some of the main strengite linessuggests that the precipitate may be a poorly crystallineferric phosphate.

The infrared spectrum of the soil Fe precipitate (datanot shown) indicated the presence of a broad band near1030 cm"1 due to silicate and/or phosphate, Fe-Ostretching bands at 510 cm"1, and a shoulder near 1190cm"1 possibly due to P=O stretching of adsorbed phos-phate (Parfitt et al., 1976). Conceivably some of thesebands may be influenced by the presence of strengite(Ross, 1974). Other investigations of similar Fe-oxidecoatings on League (Beaumont) clay soil identified fer-rihydrite and very small crystals of lepidocrocite basedon differential x-ray diffraction and lattice fringes ob-tained by high-resolution TEM (Dixon et al., 1989;Dixon, 1994; Wang et al, 1993).

Iron-Oxide Precipitate on Fed SurfacesThe morphology of the natural precipitate observed

on ped surfaces of the plowed soil consisted of globularaggregates about 50 to 150 nm in diameter (Fig. 4a and4b). Some of the aggregates were in elongated worm-like shapes (Fig. 4a) and others covered larger areas onlayer-silicate surfaces (Fig. 4b). Elemental mapping ofa cross section of the ped in Fig. 5a shows that Fe ismainly concentrated near the surface in a layer about200 |xm thick (Fig. 5b). The Si distribution in this pedclearly shows the presence of rounded quartz grainswithin the clay-rich matrix (Fig. 5c). The elemental dis-tribution of K, Ca, and Al is fairly uniform throughoutthe clay-rich matrix (Fig. 5d-5f). A sample of the Fe-rich surface layer in ultrathin section indicates that it ismade up of a mixture of fine laths distributed in a veryfine-grained material (Fig. 6a). This observation sup-ports those reported by Dixon (1994), Dixon et al.(1989), and Wang et al. (1993). Elemental analyses ofthe ultrathin section by EDXRA show very high Fe,some K and Si, and a trace of P (Fig. 6c). The interior ofthe ped in thin section shows well-ordered quasicrystals(tactoids) of smectite (Fig. 6b). This assignment is con-firmed by EDXRA (Fig. 6d). The small Fe peak in Fig.6d is probably due to structural Fe in the smectite.

Iron-Oxide Precipitate on Rice RootsThe precipitates of Fe oxide found on the harvested

rice roots consisted of larger (i.e., several micrometers)particle aggregates (Fig. 7a and 7b) than those formedby oxidation of the soil solution. These large-particleaggregates were also electron dense and completely ab-

Fig. 3. Transmission electron micrographs of poorly crystalline Fe oxides precipitated by oxidation of soil solution for a 17-d period.

GOLDEN ET AL.: SEASONALLY PRECIPITATED IRON OXIDES 961

Fig. 4. (a) and (b) Scanning electron micrographs of an Fe-oxide precipitate on layer silicates at the ped surface; (c) and (d) transmission electronmicrographs of the precipitate obtained by oxidizing soil-solution extract under laboratory conditions for 17 d; (e) electron diffraction patternof material in (d); and (f) energy dispersive x-ray spectrum of material in (d) showing Si, P, and Fe in the precipitate.

962 SOIL SCI. SOC. AM. J., VOL. 61, MAY-JUNE 1997

Fig. 5. Electron-microprobe analysis data for a League clay soil ped: (a) backscattered electron image; (b) Fe distribution, note the highconcentration of Fe near the ped surface; (c) Si distribution, subrounded quartz grains are visible within the clay matrix; (d) K distribution;(e) Ca distribution; and (f) Al distribution. Potassium and Ca are in exchange positions of the clay particles and Al is in the clay structures.Horizontal scales: 200 u.m.

sorbed the electron beam. The particle in Fig. 7a con-tained Al, Si, and Fe (Fig. 7c), indicating the presenceof layer silicates and Fe oxides, whereas no Si or P wasdetected in the particle in Fig. 7b, indicating a pure-Femineral. X-ray diffraction patterns (Fig. 2a and 2b) offine clay from the rice-root surfaces indicated the pres-ence of some smectite (broad peak near 14.2 A), kaolin-ite (7.13 A), and appreciable lepidocrocite (6.23 A). Inthe coarse clay, mostly lepidocrocite and quartz wereidentified (data not shown). The layer-silicate clay peakswere less distinct, probably due to their poor orien-tation.

DISCUSSIONThe League clay soil is alternately flooded and

drained due to seasonal rains. Under rice production itis flooded for 80 to 90 d and drained afterward. As thesoil dries, a red precipitate forms on the freshly exposeddesiccation cracks and on pore surfaces, presumably dueto the oxidation of Fe2+ transported to exposed soilsurfaces by evaporative mass flow and diffusion. ThisFe-oxide precipitate is poorly crystalline and containsP and Si, which are known to adsorb to crystal-growthsites of Fe oxides (Quinn et al., 1988; Schwertmann andThalmann, 1976). The poorly crystalline Fe oxide thatwe precipitated directly from the soil solution also con-tains Si and P as these anions were soluble in the reduced

medium. In contrast, the precipitate that forms due tooxidizing conditions near the rice-root surface (Greenand Etherington, 1977) contains abundant, well-crystal-lized lepidocrocite that is free of P and Si. An activeuptake of Si by rice roots is yet to be confirmed (Okudaand Takahashi, 1964), although it is known that riceplants accumulate large quantities of Si. The well-formed crystals of Fe oxides near roots may stem fromthe uptake of Si and P by rice roots, creating a Si- andP-depleted rhizosphere.

Other factors that may influence the formation oflarge crystals are the rate of oxidation of Fe2+, the partialpressure of CO2, the local pH, and the presence of amineral surface suitable for epitaxial growth of crystals.In these experiments, the first three of these factorswere held constant and as close to the conditions foundin the field as possible, yet the crystallinity of the precipi-tates differed substantially. For example, at the samerate of oxidation, a poorly crystalline precipitate formedin the extracted Si-rich soil solution, whereas a crystal-line precipitate of lepidocrocite was produced by oxidiz-ing the Si- and P-free Fe(ClO4)2 solution. Because of itshigh clay content, the buffering capacity of League claysoil is large and the pH will not vary much in responseto the acidity released by hydrolysis of ferric ions. Fur-thermore, although sufficient surface area for epitaxialgrowth should have been present in the suction-cup-sampled soil solution because the pore size of the suction

GOLDEN ET AL.: SEASONALLY PRECIPITATED IRON OXIDES 963

Counts ( x l O ) Counts ( X l O )

3 -

Z -

1 -

2 -

1 -

0 -V w JL A A

8 10Range ( k e V )

———|———!———I———I———|———I———!———I———|-

0 E 4 6 8 10Range ( k e V )

Fig. 6. Transmission electron micrograph of thin sections of League clay soil: (a) Fe-rich layer near the ped surface with a lath-shaped mineraldispersed in a ferrihydrite matrix; (b) smectite-rich interior of the ped; (c) EDXRA spectrum of the specimen in (a); (d) EDXRA spectrumof the specimen in (b). The small Fe peak probably results from structural Fe of smectite.

cup filter (2.1 |xm) would allow uptake of fine-clay (i.e.,<0.2-|xm) particles during the sampling procedure, crys-talline oxides did not form. It seems, therefore, thatregardless of the other environmental factors, the pres-ence of soluble Si is the only factor that always causespoor crystallinity.

The data presented here suggest that the crystallinityof Fe oxides formed in rice-producing League clay soilis influenced by the local concentrations of silicate andphosphate anions present in the soil solution. Crystallinelepidocrocite is formed near the root surface, whichapparently is depleted in Si and P compared with thesoil solution. Poorly crystalline Fe oxides result fromthe oxidation of the bulk solution. These poorly crystal-line Fe phases containing Si and P may influence thenutrient dynamics of flooded soils by preventing Si frompolymerizing and by readily releasing Si and P in a plant-available form under reducing conditions.

ACKNOWLEDGMENTS

The authors thank Karen Caroll and Christine Wallace fortheir word-processing assistance.

964 SOIL SCI. SOC. AM. J., VOL. 61, MAY-JUNE 1997

102 1 0 4 - 106

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08 |10 102 104 106

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Fig. 7. Transmission elctron micrographs of an Fe-oxide precipitate on rice-root surface: (a) layer silicate and Fe oxide, (b) Fe oxide, (c) EDXRAspectrum of specimen in (a), (d) EDXRA spectrum of specimen in (b). Sample mounted on holey-C film.