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Published online 22 August 2006
Published in Soil Sci Soc Am J 70:1731-1740 (2006)
DOI: 10.2136/sssaj2005.0414
© 2006 Soil Science Society of America
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Soil Chemistry

Phosphate Desorption from Goethite in the Presence of Galacturonate, Polygalacturonate, and Maize Mucigel (Zea mays L.)

Christian Mikuttaa,*, Günter Neumannb and Friederike Langa

a Institute of Ecology, Berlin Univ. of Technology, Salzufer 12, D-10587 Berlin, Germany
b Institute for Plant Nutrition, Univ. of Hohenheim, Fuwirthstr. 20, D-70593 Stuttgart, Germany

* Corresponding author (christian.mikutta{at}tu-berlin.de)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Uronates are important constituents of maize mucilage and polyuronates are used as a simplified model of the soil–root interface. We tested whether galacturonate (GA) and polygalacturonate (PGA) impair the diffusion of phosphate (PO4) into and out of pores of a synthetic goethite (147 m2 g–1) and whether the effect of maize mucigel (MU) is comparable to PGA. We measured the PO4 desorption kinetics of goethites in batch experiments over 2 wk at pH 5. One part of the goethite was equilibrated with organic substances before PO4 addition, another part after addition of PO4. Before the desorption experiments, the porosity of our samples was analyzed by N2 gas adsorption. In each treatment a rapid initial desorption was followed by a slow desorption reaction, which is assigned to the diffusion of PO4 out of mineral pores. No consistent relation between the micro- and mesoporosity and the rate of the slow PO4 desorption was observed. Compared with the C-free control, only PGA and MU affected the fraction of PO4 mobilized by the fast and slow desorption reaction: when PGA was sorbed to goethite before PO4, twice as much PO4 was mobilized via the fast reaction than in the treatment where PO4 was sorbed before PGA, suggesting a decreased accessibility of goethite pores to PO4. Mucigel, however, showed reversed effects, which is ascribed to its differing chemical composition. In conclusion, PGA seems inappropriate as a model substance for maize MU collected from non-axenic sand cultures. Under the experimental conditions chosen, the efficacy of all organic substances to increase PO4 solution concentrations by pore clogging and sorption competition is small.

Abbreviations: DOM, dissolved organic matter • GA, galacturonate • MU, mucigel • OM, organic matter • PE, polyethylene • PO4, phosphate • PGA, polygalacturonate


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE P reservoir of soils is usually large (Foth and Ellis, 1997), but PO4 concentrations found in the soil solution are generally smaller than 20 µM (Reisenauer, 1964; Barber, 1974). The reason for the low PO4 concentrations under acid conditions is the strong sorption of PO4 to soil minerals, especially hydrous Fe and Al oxides. Phosphate is known to adsorb either on outer mineral surfaces or in nanometer-pores of soil minerals and aggregates, respectively, that are accessible to PO4 by intraparticle diffusion (Willet et al., 1988; Torrent et al., 1990; Strauss et al., 1997; Mikutta et al., 2006a).

In soils, mineral surfaces are partly covered with organic matter (OM) (Fontes et al., 1992; Mayer and Xing, 2001; Gerin et al., 2003). Organic coatings of the soil rhizosphere may consist primarily of microbe-derived and phytogenic OM, with polysaccharides being important components. Especially mucilages have been implicated to strongly bind soil particles together, thus coating mineral surfaces (Vermeer and McCully, 1982; Watt et al., 1993). Mucilages are pectin-like high molecular weight root exudates, which are primarily secreted by root cap cells (Paull and Jones, 1975; Rougier, 1981) and comprise about 90 to 95% polysaccharides with about 20 to 35% of uronic acids (Cortez and Billes, 1982; Morel et al., 1986).

The sorption of PGA—as a model substance for root mucilage- to goethite has been shown to reduce the pore volume of <10-nm pores and the amount of PO4 sorbed within 2 wk (Mikutta et al., 2006b). Gaume et al. (2000) explained the higher PO4 exchangeability after PO4 addition to PGA- and mucilage-treated ferrihydrite by microaggregation of ferrihydrite particles that decreased the accessibility of sorption sites for PO4. No further studies on the effect of mucilage or mucilage-like substances on the accessibility of mineral pores to PO4 or other oxyanions are available. In addition, only one study is available relating the ‘pore clogging’ of hydrous Fe oxides by organic sorbates to the desorption kinetics of oxyanions. Lang and Kaupenjohann (2003) studied the effect of residence time of molybdate on the molybdate desorption kinetics using pure goethites and goethites incubated with dissolved OM (DOM). They found that OM coatings prevented molybdate from diffusion into intraparticle pores, thus favoring its enrichment on outer goethite surfaces and hence its fast desorption compared with pure goethites.

Until now it has not been studied whether oxyanions might be trapped in micro- (Ø < 2 nm) and mesopores (Ø 2–50 nm) by organic coatings comprising macromolecular root exudates. This situation likely occurs when oxyanions sorb to porous minerals before OM. We hypothesized that high molecular weight OM entraps PO4 in goethite pores when added after PO4 and, conversely, that the diffusion of PO4 into goethite pores is impaired when OM is sorbed before PO4. Polygalacturonate is commonly used as a model of the soil–root interface (Morel et al., 1987; Gessa and Deiana, 1992; Ciurli et al., 1996; Gaume et al., 2000; Grimal et al., 2001; Mikutta et al., 2006b). However, well-defined root exudates seldom exist in the rhizosphere, and the ability of PGA or mucilage to affect the kinetics of PO4 sorption and desorption of hydrous Fe oxides might depend on the degree of alteration of these substances, for example, by microbial activity or by complexation with polyvalent cations (Deiana et al., 2001; Mimmo et al., 2003; Gessa et al., 2005). To more realistically test the effect of macromolecular root exudates on the PO4 desorption kinetics of hydrous Fe oxides, we additionally used mucigel of maize plants. Mucigel is a gelatinous material at root surfaces of plants grown under non-axenic conditions (Jenny and Grossenbacher, 1963). It includes pure and modified mucilage, bacterial cells, their metabolic products as well as colloidal mineral and/or organic matter inherited from the sampling environment. We hypothesized that the PO4 desorption kinetics of goethite treated with PGA is comparable with mucigel-treated goethite.

Goethite was used because it is the most prominent hydrous Fe oxide in soils (Cornell and Schwertmann, 2003). Polygalacturonate and MU were taken as phytogenic macromolecular organic sorbates, whereas GA was used to identify effects arising from the polymeric nature of PGA only. For example, in contrast to GA, sorption of PGA in micropores is unlikely due to size-constraints (Gaume et al., 2000). All experiments were conducted at pH 5 to resemble the acidic conditions in the growth media of PO4 starved plants (Neumann and Römheld, 1999) and to minimize the influence of bicarbonate.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Preparation of Goethite
The goethite was synthesized by oxidative hydrolysis of Fe(II) (FeSO4·7H2O, Merck, extra pure) at pH 7 using H2O2 as an oxidant. The precipitate was washed until the electric conductivity was below 10 µS cm–1, freeze-dried, softly ground and sieved to a particle size < 200 µm. The oxalate-soluble Fe content according to Blakemore et al. (1987) was 4.9%. Powder X-ray diffractograms of the goethites were obtained using a Siemens D5005 instrument (Siemens AG, Germany) with CuK{alpha}-radiation of wavelength 0.15406 nm (40 kV, 30 mA). The scans indicated pure goethite with no detectable contamination. Differential X-ray analysis after oxalic acid-ammonium oxalate treatment (Schwertmann, 1964) did not reveal ferrihydrite contaminations. Scanning electron micrographs of pure and OM-coated goethite were obtained with a Hitachi S-4000 microscope fitted with an energy-dispersive X-ray detector.

Organic Substances
The polygalacturonic acid used comprised 37.2% C and 0.05% N [(C6H8O6)n, > 95%, M = 25–50 kDa, Fluka P81325]. The most prominent cations of the PGA were Na (192 mmolc kg–1) and Ca (11.4 mmolc kg–1). Polygalacturonate solutions were prepared by dissolving PGA in 0.01 M KNO3 with the help of 10 µL of 1 M KOH mg–1 PGA. The pH of the solutions was then readjusted to pH 5 with 0.01 M HNO3 without any visible flocculation occurring. Galacturonic acid was used in the form of D(+) galacturonic acid (C6H10O7·H2O, > 93%, Fluka P48280).

Mucigel from maize plants (Zea mays L., cv. Marshal) was obtained by the method outlined in Neumann et al. (1999). Twelve maize plants were grown in 2.8-L glass tubes filled with quartz sand under greenhouse conditions with a light period of 16 h. Using a wick-irrigation system with two drippers per culture vessel for continuous percolation of nutrient solution (1 L per plant, replaced every second day), a nutrient solution was constantly percolated through the tubes containing 2 mM Ca(NO3)2, 0.1 mM KCl, 0.7 mM K2SO4, 0.5 mM MgSO4, 0.5 µM KH2PO4, 100 µM Fe ethylene diamine tetraacetic acid (EDTA), 10 µM H3BO3, 0.5 µM MnSO4, 0.5 µM ZnSO4, 0.2 µM CuSO4, and 0.01 µM (NH4)Mo7O24. After 3 wk, the concentration of all components was doubled, except for Fe, which was raised to 150 µM of Sequestren (Syngenta, Wilmington, DE) instead of Fe EDTA. After approximately 8 wk, fertilization was raised to the maximum level, reaching the five-fold concentration of the initial nutrient supply. Sequestren application was increased to 450 µM. From the middle of November 2004 until the end of January 2005 (begin of flowering) root MU was collected alternately from six plants every 2 d; the glass tubes were percolated twice with 500 mL of distilled H2O to remove accumulated salts originating from the nutrient solution as far as possible and to induce swelling of the mucilage by flushing the roots with water. After 2 h, the draining tubes were closed and the tubes were incubated with 500 mL of warm (35–38°C) distilled H2O for solubilization of mucilage. Thereafter, the solution was collected in polyethylene (PE) bottles and percolated twice through the glass tubes. Percolates (approx. 4 L per 6 plants) were subsequently concentrated by rotary-evaporation at 45°C to a volume of approx. 100 mL and stored at –20°C. At the end of the collection period, all preconcentrated samples were combined and lyophilized to complete dryness.

The mixed lyophilized samples were extracted with ice-cold methanol (80% v/v) to solubilize salts and low-molecular weight organic compounds. Repeated washings of the 80% (v/v) methanol-insoluble high molecular weight fraction were performed by resuspending in 80 mL 80% (v/v) methanol and subsequent centrifugation (5 min, 3500 x g). Washings were performed until the electric conductivity was below 50 µS cm–1.

An attempt to remove cations originating from the nutrient solution and potentially bound to cation-exchange sites of the MU was performed by resuspending the MU in 200 mL of 80% methanol. Five membrane bags each containing two grams of a cation exchange resin (Dowex 50 WX8'' , 20–50 mesh, Na+ form) were added, and the suspension was shaken for 6 h at room temperature. After final centrifugation, the MU was air-dried at 27°C (approx. 6 g dry matter). For the experiments, we prepared MU solutions by dispersing MU in 0.01 M KNO3, sonicating the suspension for 40 min, and readjusting the pH to 5 with 0.01 M HNO3.

Characterization of Mucigel
Total C and N content of the MU were determined with an Elementar Vario EIII C/N/S Analyzer. The MU was analyzed in triplicate for the content of sugars, uronic acids, and proteins. For the analysis of sugars and uronic acids, 10 mg of the dry MU were hydrolyzed for 3 h at 100°C with 1 mL of 2 M H2SO4 (Harborne, 1984). Insoluble material was removed by centrifugation, and the hydrolysate was analyzed for reducing sugars after 10-fold dilution and pH adjustment to 4 to 5 using glucose as a standard (Blakeney and Mutton, 1980). Uronic acids were determined with glucuronic acid as standard (Blumenkrantz and Asboe-Hansen, 1973). Proteins were quantified according to Bradford (1976) after resuspending 10 mg of the dry MU in 0.5 mL distilled H2O. Whereas no soluble proteins could be detected in the MU suspension, a clearly positive Coomassie-blue staining indicated the presence of insoluble proteins.

Cations bound by MU were measured with atomic absorption spectrometry (PerkinElmer 1100B) after acid digestion of the MU in concentrated HNO3 (10:1/w:v). Total P in the MU was determined photometrically at 710 nm (Murphy and Riley, 1962) after acid hydrolysis in conc. HCl (50:1/w:v). The amount of organic P (Porg) was operationally defined as the difference between total P and the amount of P complex bound by MU (Pinorg). Pinorg was determined after ultracentrifugation (440 000 x g, 1 h) of an aqueous suspension of MU containing 1 g C L–1. Note that the Porg fraction may also contain inorganic PO4 forms that are not or poorly soluble in water. Mineral phases in the MU were identified with powder X-ray diffraction (Siemens D5000). Table 1 summarizes the chemical composition of the maize MU used.


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  		Table 1. Composition of the maize mucigel used in this study. Values in parentheses indicate standard error of triplicate measurements.

 
Sorption of Phosphate and Organic Matter
Goethite (18 g) was weighed into a 2-L HD-PE bottle and equilibrated for 20 h in 1 L of 0.01 M KNO3 solution at pH 5 on a reciprocating shaker at 130 rev min–1 using 0.01 M HNO3 for pH re-adjustments. To impair microbial activity the bottle was coated with Al foil and 100 µL of 0.1 M AgNO3 solution were added. After 20 h, 1 L of 0.01 M KNO3 solution (pH 5) containing a PO4 concentration of 8 mM was added. Phosphate was provided as KH2PO4 (Merck, p.a.). The sample was then transferred onto a rotary shaker running at 10 rev min–1. The pH was manually maintained at 5 ± 0.2 by addition of 0.01 M HNO3. After 15 d the suspension was 0.2-µm membrane filtered, and the filter residue was washed with 500 mL of doubly deionized water, freeze-dried, and softly homogenized in an agate mortar.

Eight grams of phosphated goethite were then weighed into a 2-L HD-PE bottle and 1 L of 0.01 M KNO3 solution (pH 5) containing 1 g C L–1 of GA or PGA was added. Because we had only a few grams of MU material at hand, 280 mL of MU solution with 1 g C L–1 were given to 2.24 g of phosphated goethite. The samples were shaken on a rotary shaker at 20 rev min–1 in the dark at 20 ± 2°C for 12 h. Afterward, the samples were 0.2-µm membrane filtered, washed with 500 mL of doubly deionized water to remove excess PO4 and organic C, freeze-dried, softly homogenized and stored in the dark until use.

In another treatment we reversed the sequence of OM and PO4 addition to pure goethite. Details of this procedure were identical to the first treatment. After PO4 and/or OM sorption and subsequent freeze-drying of the samples, organic C contents were determined with an Elementar Vario EIII C/N/S Analyzer. Before the freeze-drying commenced, all samples were frozen at –80°C. The dried samples were further examined with N2 adsorption.

Phosphate Desorption Kinetics
To avoid product limitation in the desorption experiments, we used synthetic Al2O3 (Merck, pH 6.0 ± 0.5, 50–150 µm) as an infinitive sink for PO4 that is mobilized in our batch-desorption experiments. The Al2O3 was sieved to a particle size > 100 µm. The PO4 sorption capacity of the sieved Al2O3 in 0.01 M KNO3 solution at pH 5 was ~410 µmol PO4 g–1 as judged from the PO4 adsorption isotherm. The Al2O3 ‘sink’ ensured PO4 solution concentrations of <2.5 µM, which corresponds to <2% of the total PO4 desorbed. After 2 wk of PO4 desorption, PO4 solution concentrations were typically << 0.7 µM in all samples.

Six grams of Al2O3 were packed into a 16 x 5 cm polyamide net of 50-µm mesh size, which was sealed with an impulse sealer ME-200HI (MoFix GmbH, Bad Rappenau, Germany). To avoid H+ buffering by dry Al2O3, the Al2O3–bags were equilibrated at pH 5 in doubly deionized water until the pH was constant. At the beginning of the desorption run, the nets were added simultaneously with the background electrolyte solution (1 L of 0.01 M KNO3, pH 5) to the reaction bottles that contained 2 g of differently treated goethite. Again, in the MU treatments the sample weight had to be reduced to 0.3 g. Therefore, 150 mL of background electrolyte solution were added simultaneously with 0.9 g of pre-equilibrated Al2O3 to MU-treated goethites.

Triplicate samples were shaken in the dark on a rotary shaker at 10 rev min–1 and at 20 ± 2°C. To inhibit microbial activity 100 µL of 0.1 M AgNO3 were added per liter background electrolyte solution. The pH was manually maintained at pH 5 with 0.01 M HNO3. Deviations from the target pH were <0.5 for times <24 h and <0.2 for times >24 h. After 1, 2, 4, 8, 24, 48, 96, 168, and 336 h a 10-mL aliquot (5 mL in MU treatments) was removed from the suspensions and 0.45-µm membrane filtered. The filter residue was freeze-dried. Subsequently, 5.0 ± 0.05 mg of the filter residue were weighed into a glass vial and dissolved in 100 µL conc. HCl. To ensure a rapid dissolution of the solids, the vials were placed in an oven at 105°C for 3 min. Afterward, 10 mL doubly deionized water were added and the PO4 concentration was analyzed photometrically by the ascorbic-molybdenum blue method at 710 nm (Murphy and Riley, 1962). Solutions containing MU components were additionally 0.2-µm membrane filtered (cellulose nitrate) to obtain clear solutions for the PO4 measurement. Standards were prepared to resemble the matrix of the samples analyzed. The analytical precision of photometric determination of PO4 was ~1%. Subsample variability was 1.4% on average. At the end of the desorption experiments the organic C content of the freeze-dried goethites was measured with an Elementar Vario EIII C/N/S Analyzer. Carbon desorption during the PO4 desorption experiments was less than 9% in all treatments (not shown). During the PO4 desorption run, the {zeta}-potential of OM-treated goethites was measured with a Zetasizer 2000 (Malvern Instruments, UK) by resuspending OM-treated goethites in 0.01 M KNO3 at pH 5. At each point in time, three to eight measurements were recorded and averaged. The {zeta}-potential was determined because it might control the colloidal properties of OM-treated goethites and hence their PO4 desorption kinetics.

Desorption Data Evaluation
Desorption of PO4 from goethite was modeled using a linear combination of a first-order rate equation and the parabolic rate law (Lang and Kaupenjohann, 2003). While the first term describes the fast desorption of PO4 from external goethite surfaces, the diffusion term models the slow transport-controlled desorption of PO4:

Formula 1[1]
where P/Pinitial (t) is the fractional amount of PO4 desorbed at time t, a0 is the fractional amount of PO4 desorbed by the fast reaction, k is the rate constant of the fast desorption (h–1), and b is the rate constant of the slow PO4 desorption (h–0.5). The parameters a0, k, and b were determined by minimizing the sum of squared differences between the observed and predicted values of the dependent variable using the Marquardt-Levenberg algorithm implemented in SigmaPlot for Windows Version 7.0 (SPSS, Inc.). Parameters were evaluated with the t-statistics, which tests the null hypothesis that the parameter is zero by comparing the parameter value with its standard error. The rate constant of the slow PO4 desorption, b, is related to the apparent diffusion constant (D/r2)app (h–1):

Formula 2[2]
where q{infty} is the fractional amount of PO4 diffused at infinite time, D is the apparent diffusion coefficient (m2 h–1), and r is the radius of diffusion (m). To calculate the apparent diffusion constant, (D/r2)app, we used the fraction of PO4 present at t = 0 h corrected for the fractional amount of PO4 desorbed rapidly (a0) as an approximation for q{infty} in Eq. [2].

Surface Area and Porosity Measurements
We used an Autosorb-1 gas sorption system (Quantachrome, Syosset, NY) to assess porosity and surface area of goethite after addition of PO4, organic compounds, or both. Helium was used as backfill gas, N2 was used as adsorbate. Approximately 100-mg sample (~15 m2) were degassed until the rate of pressure increase by vapor evolution was below about 1.3 Pa min–1 within a 0.5-min test interval. Nitrogen adsorption and desorption isotherms were obtained from 79 points in the partial pressure range 3.0 x 10–5 to 0.995 P/P0. Specific surface area was calculated from the BET equation (Brunauer et al., 1938).

Microporosity (<2 nm) and average micropore diameter were determined according to the Dubinin-Radushkevic method (Gregg and Sing, 1982). Mesopore (2–50 nm) size distribution was calculated on the desorption leg using the BJH method (Barrett et al., 1951). Separation between small (2–5 nm), medium (5–10 nm), and large mesopores (10–50 nm) was achieved by linear interpolation of the BJH desorption data. Total pore volume was taken at 0.995 P/P0 and the average pore diameter was calculated as Dp = 4Vliq/SSA, were Vliq is the liquid volume of N2 contained in pores at 0.995 P/P0 and SSA is the BET surface area. All isotherms were recorded in triplicate.

The MU contributed up to 31% to the sample's mass, which when unaccounted for would a priori decrease the adsorption of N2 at 77 K in these goethite samples because N2–specific surface areas of OM are generally < 5 m2 g–1 (De Jonge and Mittelmeijer-Hazeleger, 1996; Alvarez-Puebla and Garrido, 2005). Therefore, porosity and surface area of MU-treated goethites were corrected for the mass of MU present in the sample:

Formula 3[3]
where X determines specific pore volume (e.g., mesopore volume) or specific surface area measured with N2 adsorption, and Cgoethite and Cmucigel denote the organic C content (%) of the MU-treated goethite and the MU, respectively. In GA and PGA treatments, the organics contributed less than 4 wt% to the total sample mass. For this reason, the weight of PGA and GA was not considered for the determination of specific porosity and surface area.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Carbon Contents and Sorption Competition
The C content of 118 mg C g–1 (dry weight) of the maize mucigel was significantly lower than values published for maize mucilage (Morel et al., 1986; Gaume et al., 2000; Grimal et al., 2001). Powder diffraction analysis revealed contaminations by mineral matter, including quartz (SiO2), calcite (CaCO3), gypsum (CaSO4·2H2O), and 1:1 layer silicates (Fig. 1 ). The presence of gypsum near roots of maize plants has been observed by Malzer and Barber (1975) for conditions where ion supply by mass-flow exceeded ion uptake.


Figure 1
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  		Fig. 1. X-ray diffractogram of the maize mucigel used in this study. Abbreviations: C, calcite; G, gypsum; L, 1:1 layer silicate; Q, quartz.

 
Provided all S in the MU was bound as CaSO4, the contribution of carbonate-C to the total MU-C would be 2% (Table 1). Contrary, provided all Ca measured in the MU was bound as carbonate, the contribution of carbonate-C to the total MU-C would be 5% (Table 1). Chemical equilibrium calculations with VisualMINTEQ (Allison et al., 1991) showed that at pH 5 precipitation of Ca or Al phosphates was unlikely in our experiments.

Initial C contents of the goethite samples are presented in Table 2. For pure and phosphated goethite they increased in the order GA < PGA << MU. Compared with pure goethite systems, sorption of OM to goethite was inhibited by 0% (MU), 41% (PGA), and 89% (GA) by pre-sorbed PO4 (Table 2), showing a strong competition of presorbed PO4 with GA and PGA. No effect of presorbed PO4 on MU sorption can be explained by the fact that in the MU treatment MU-C presumably existed partially as a separate solid phase, which was not affected by the addition of PO4. Phosphate sorbed to OM-treated goethites within 15 d displaced 72% of GA-C, 56% of PGA-C, and 35% of MU-C, showing that PO4 strongly competed with sorbed OM (Table 2). The result shows that PO4 was less able to desorb PGA-C compared with GA-C, which can be explained by the multi-site attachment of polymers to mineral surfaces (‘octopus’ effect, Podoll et al., 1987). Our results comply with Nagarajah et al. (1970) who reported that PGA markedly decreased PO4 sorption to goethite, gibbsite, and kaolinite whereas the monomer hardly influenced PO4 sorption.


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  		Table 2. Carbon content, specific surface area, and porosity characteristics of goethite after sorption of PO4, organic matter, or both. Abbreviations used: P, phosphate; GA, galacturonate; PGA, polygalacturonate; MU, mucigel. The sequence of abbreviations indicates the sequence of sorbate addition to goethite. Values in parentheses represent standard error.

 
Porosity Changes
Sorption of PO4 to pure goethite decreased the average micropore diameter from 0.85 to 0.78 nm and the volume of 2- to 5-nm pores (Table 2), indicating that PO4 penetrated into pores < 5 nm. The diffusion of PO4 into micropores of goethite and drinking-water treatment residuals has been reported before (Strauss et al., 1997; Makris et al., 2004; Mikutta et al., 2006a). The decrease in volume of small mesopores after PO4 addition might also be explained by an aggregation of goethite crystallites by PO4 (Anderson et al., 1985).

Addition of all organics to pure goethite reduced the pore volume of <5-nm pores (Table 2). Microporosity decreased up to 16% in the order PGA < GA << MU, while the mesoporosity of <5-nm pores decreased up to 21% in the order MU < GA = PGA (Table 2). These results indicate that all organics partially clogged the pores < 5 nm of goethite, that is, decreased the accessibility of pores to N2 at 77 K. Decreased micro- and mesopore volumes are in line with studies showing that OM penetrates into mineral pores (Kaiser and Guggenberger, 2003; Mayer et al., 2004; Zimmerman et al., 2004).

In the presence of PO4, organic substances decreased the micropore volume up to 22% and < 5-nm mesopore volume up to 12% compared with C-free phosphated goethite (Table 2). Mucigel addition to pure and phosphated goethite increased the volume of mesopores > 10 nm and hence, the average pore diameter in these treatments was significantly larger than in the GA and PGA treatments (Table 2). Both PGA and MU did not prevent PO4 from diffusion into micropores, as shown by decreasing micropore volumes after PO4 addition when compared with the treatments without PO4 (Table 2).

Effects of Galacturonate and Polygalacturonate on the Phosphate Desorption Kinetics
Treatments ‘P+OM’
Figure 2 shows the PO4 desorption kinetics of pure and OM-treated goethites. In all cases except from the ‘PGA+P’ treatment, desorption continued and did not reach an equilibrium within 2 wk (Fig. 2). While 36% of PO4 sorbed to pure goethite was mobilized within 2 wk, 31 and 29% were desorbed from phosphated goethite to which GA or PGA had been added (Fig. 2a). Kinetic modeling indicated that a similar fraction of PO4 was rapidly desorbed from GA- and PGA-coated samples compared with C-free goethite (Table 3, a0). In contrast, the apparent diffusion constants, (D/r2)app, of GA- and PGA-coated goethites were significantly smaller than that of C-free goethite (Table 3). This may indicate a successful entrapment of PO4 in goethite pores < 5 nm by GA and PGA as suggested by porosity measurements (Table 2). However, on the basis of small differences in apparent diffusion constants, with unknown radii of diffusion r that are probably different in the GA and PGA treatments, an entrapment of PO4 in pores by GA and PGA cannot be concluded without ambiguity.


Figure 2
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  		Fig. 2. Fractional desorption of PO4 in 0.01 M KNO3 background electrolyte at pH 5 with a solid concentration of 2 g L–1: (a) organic matter (OM) sorbed to phosphated goethite and (b) PO4 sorbed to OM-treated goethite. Abbreviations used: P, phosphate; GA, galacturonate; PGA, polygalacturonate; MU, mucigel. Sequence of abbreviations indicates the sequence of sorbate addition. Dashed lines are the model fits using Eq.[1]. Error bars are given as standard errors of three replicate measurements.

 

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  		Table 3. Amount of PO4 initially present (Pinitial) in the samples, PO4 desorbed after 2 wk (Pdesorbed), and parameter estimates obtained by fitting Eq. [1] to the PO4 desorption data. Also given is the apparent diffusion constant (D/r2)app. Abbreviations used: P, phosphate; GA, galacturonate; PGA, polygalacturonate; MU, mucigel. The sequence of abbreviations indicates the sequence of sorbate addition. Values in parentheses denote standard error.

 
Treatments ‘OM+P’
When OM was sorbed to goethite before PO4, the fractional amount of PO4 desorbed after 2 wk varied only between 34 and 30% for GA- and PGA-treated goethite, respectively (Fig. 2b). The kinetics of PO4 release in the GA treatment were similar to that of C-free goethite (Table 3).

The desorption of PO4 from PGA-coated goethite showed a near two-fold increase in the fraction of PO4 rapidly desorbed in comparison with the ‘P+PGA’ treatment (Table 3). Although about 86% of the total PO4 desorbed was desorbed by the fast desorption reaction (Table 3, ‘PGA+P’), the rate constant of the fast desorption k was only one-forth of that of the C-free control (Table 3).

The increase in the fraction of PO4 rapidly desorbed was coupled with a strong decrease in the rate constant of the slow PO4 desorption and the apparent diffusion constant (Table 3, ‘PGA+P’), showing that the diffusion resistance for PO4 increased. This result does not comply with Lang and Kaupenjohann (2003) who reported increased (D/r2)app values for molybdate desorption from goethites that were pre-incubated with DOM. This inconsistency may be caused by higher C loadings (0.12 and 0.77 mg C m–2) in the study of Lang and Kaupenjohann (2003), and OM differing in structure and reactivity. Decreased (D/r2)app values in the ‘PGA+P’ treatment suggest that PO4 diffusion out of goethite pores was impeded by PGA coatings.

Pore clogging by PGA and/or aggregation of goethite particles may account for differences in the PO4 desorption kinetics of both PGA treatments. However, the <5-nm porosity was nearly equal in both PGA-treatments (Table 2). Therefore, our porosity data cannot explain differences in the PO4 desorption kinetics of both PGA treatments. This inconsistency might be due to the fact that N2 porosity data do not reflect the accessibility of pores to PO4.

Aggregation of goethite crystallites by PGA in the ‘PGA+P’ treatment might have caused a partial occlusion of accessible sorption sites, thus limiting or preventing the transfer of PO4 into aggregates during PO4 sorption before the desorption experiment (Willet et al., 1988; Linquist et al., 1997). Polygalacturonate is also capable of increasing the cohesion of soil particles (Traoré et al., 2000) and decreasing the wetting rate of soil (Czarnes et al., 2000), which might explain the low rate constant of the fast PO4 release (Table 3, k). Aggregation of ferrihydrite particles by PGA at a similar C loading compared with our ‘PGA+P’ treatment (~0.03 mg C m–2) decreased the transfer rate of 33P from solution to phosphated ferrihydrite surfaces within aggregates (Gaume et al., 2000). In accordance with these studies, our results suggest that in the ‘PGA+P’ treatment PO4 was enriched on outer aggregate surfaces, and thus a decreased (D/r2)app value rather reflects a decreased supply of PO4 from intra-aggregate pores.

Effect of Mucigel on the Phosphate Desorption Kinetics
A significantly larger fraction of PO4 was desorbed in the ‘P+MU’ treatment compared with pure goethite (42%, Fig. 2a). Table 3 states the initial PO4 content of the solids used. The MU contained a large amount of inorganic PO4 (Table 1), which matched the surplus of PO4 present in the MU treatment compared with pure goethite (Table 3, Pinitial). Consequently, the MU-bound PO4 likely contributed to the PO4 desorption kinetics and explains the offset of 0.1 in the fraction of PO4 desorbed from MU samples in relation to pure goethite (Fig. 2a). Organic C measurements indicated no net release of C in this treatment. Therefore, it seems unlikely that organically bound P contributed significantly to the increase in the rate constant and the fraction of the fast desorbing PO4 (Table 3, k, a0).

The slow PO4 desorption reaction of both MU treatments did not statistically differ from the C-free control, indicating a similar diffusion resistance for PO4 [Table 3, b, (D/r2)app]. This result contrasts our N2 adsorption measurements because the micropore volume of pure and phosphated goethites was effectively reduced by MU (Table 2). Our findings therefore imply that the slow PO4 mobilization from OM-treated goethite is either not primarily controlled by micropore diffusion, or that reduced micropore volumes measured with N2 adsorption are only confined to the dry state due to occlusion of mineral surfaces by OM after freeze-drying, which is reversible after rehydration.

Comparison of Mucigel with Polygalacturonate
Figure 3 shows SEM images of PGA- and MU-treated goethites with representative EDX spectra. Microaggregates of both treatments differed in their surface morphology, being more ‘frayed’ in the case of MU. EDX-spectra of MU-treated goethite also supported the presence of layered silicates as indicated by the Al and Si peaks (Fig. 3).


Figure 3
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  		Fig. 3. Scanning electron microscopy images of microaggregates of freeze-dried PGA-treated goethite (top) and mucigel-treated goethite (bottom). Insets show representative EDX-spectra of polygalcturonate- and mucigel-treated goethites.

 
On a C basis, MU reduced the micro- and small mesopore volume of pure and phosphated goethite far less effectively than PGA (Table 2). In accordance, MU affected the kinetics of the slow PO4 desorption less effective than PGA (Table 3). Mucigel and PGA showed opposite effects on the PO4 desorption kinetics (Table 3). No effect of MU on the slow desorption kinetics of PO4 implies that MU was not as strongly associated with goethite than PGA, and probably existed primarily as a second solid phase. This reasoning is supported by a lower affinity of maize mucilage to goethite at pH 5.5 compared with PGA (Grimal et al., 2001)—a circumstance that might have been amplified in our experiment due to the presence of mineral matter other than goethite (Fig. 1). In addition, the assemblage of polyvalent cations differed significantly between PGA and MU. For example, about two orders of magnitude more Ca was present in the MU, While Al is another dominant polyvalent cation in the MU (Table 1), PGA contained almost no Al. Larger amounts of polyvalent cations in the MU compared with PGA might explain why the {zeta}-potential of MU-treated goethites is generally higher than that of PGA-treated goethites (Fig. 4 ).


Figure 4
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  		Fig. 4. {zeta}-Potential of C-free, polygalcturonate- and mucigel-treated goethites during phosphate desorption. After each reaction time about 200 µg of freeze-dried 0.45-µm filter residue were resuspended into 4 mL of 0.01 M KNO3 at pH 5. Sequence of abbreviations indicates the sequence of sorbate addition. Error bars are given as standard errors.

 
Further, the uronic acid content of the MU used was small compared with maize mucilage (Table 1). For example, Morel et al. (1986) reported uronic acid contents of 219 and 340 mg g–1 (dry weight) for maize mucilage obtained under non-axenic (nodal roots) and axenic conditions, respectively. Assuming a C content of nodal root mucilage of approximately 400 mg g–1 (Morel et al., 1986), our MU would have an uronic acid content of about 21 mg g–1, which is closer to the 30 mg g–1 of axenically secreted mucilage given by Bacic et al. (1986). Our findings imply that PGA is inappropriate as a model substance for maize mucigel, and that maize mucilage collected under axenic conditions (Morel et al., 1986; Watt et al., 1993; Read et al., 1999) or from nodal aerials (Morel et al., 1986) may differ in reactivity from maize mucigel.

Ecological Implications
To prepare organic coatings on phosphated goethite, we used initial C concentrations corresponding to about 71 µmol C m–2. Despite these high C concentrations in solution, both GA and PGA displaced only up to 7% of presorbed PO4 within 12 h of equilibration, showing the low competitiveness of both compounds (Table 2). When organic sorbates were added before PO4, they inhibited PO4 sorption by only up to 13% (Table 3). Compared with low-molecular-weight organic acid anions, the ability of the organics used to impair the sorption of PO4 to goethite at pH 5 was small (Geelhoed et al., 1998; Mikutta et al., 2006a). Mucigel was capable of storing organic and inorganic P, the latter likely complexed by polyvalent cations (Table 1) or molecules possessing anion exchange sites like protonated amino groups. This ‘trapping’ of PO4 by MU might be of environmental importance when PO4 becomes bioavailable due to its rapid desorption (Fig. 2a) and organically bound P is released on mineralization of MU in the rhizosphere. The ability of ternary PO4–Fe(Al)–OM complexes to contribute to P retention in soils has been documented by several researchers (Appelt et al., 1975; Bloom, 1981; Borie and Zunino, 1983; Gerke and Hermann, 1992; Gerke et al., 1995). In Fig. 5 we plotted the difference ({Delta}P, µmol g–1) in the amount of PO4 sorbed (desorbed) between pure and each OM-treated goethite. A comparison of {Delta}P of PO4 sorption with {Delta}P of PO4 desorption indicates the net bioavailability of PO4 after one sorption (15 d) and desorption run (14 d). In four out of six treatments {Delta}P of PO4 sorption and desorption were equal, indicating a zero net effect on the bioavailability of PO4. Only in the ‘PGA+P’ and ‘MU+P’ treatments {Delta}P of PO4 sorption was larger than {Delta}P of PO4 desorption, showing a slightly positive net effect on the bioavailability of PO4.


Figure 5
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  		Fig. 5. Difference in the amount of PO4 sorbed (15 d) and desorbed (14 d) between pure goethite and organic matter-treated goethites ({Delta}P). Abbreviations used: P, phosphate; GA, galacturonate; PGA, polygalacturonate; MU, mucigel. Sequence of abbreviations indicates the sequence of sorbate addition. Error bars denote standard error.

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The order of PGA and PO4 addition significantly affected the fraction of PO4 that was desorbed by the fast and slow desorption reaction. This phenomenon can be explained by pore clogging and aggregation.

The release kinetics of PO4 in MU-treated goethite samples was contrary to that of PGA, which is ascribed to its differing chemical composition. Our results indicate that in contrast to PGA, MU has to be treated as a separate phase rather than a coating of the mineral. Accordingly, PGA seems inappropriate as a model substance for maize MU collected from sand cultures under non-axenic conditions.

An entrapment of PO4 in <5-nm pores of goethite could not be verified without ambiguity, when organic substances were added to goethite after PO4.

We conclude that due to the high competitiveness of PO4 under the experimental conditions chosen (pH 5, I = 0.01 M, C loadings < 21 µmol m–2), the net effects of root exudates on the bioavailability of PO4 are small.


   ACKNOWLEDGMENTS
 
We are grateful to Tsehaye Tesfamariam who supplied us with the maize mucigel, to Elisabeth Irran for the XRD analysis of the mucigel, and to Andrea Herre for helpful comments on the manuscript. This study was funded by the German Research Foundation (DFG, KA 1139/8).

Received for publication December 20, 2005.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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