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Soil Science Society of America Journal 66:464-473 (2002)
© 2002 Soil Science Society of America

DIVISION S-2—SOIL CHEMISTRY

Phosphate and Sulfate Sorption in Spodosols with Albic Horizon from Northern Spain

M. Camps Arbestain*, M. E. Barreal and F. Macías

Departamento de Edafoloxía e Química Agrícola. Facultade de Bioloxía. Universidade de Santiago de Compostela. 15782-Santiago de Compostela, Spain

* Corresponding author (edmcamps{at}usc.es)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The affinity of PO4 and SO4 for reactive Al and Fe compounds may differ between the Bh and Bs horizons of Spodosols. Sorption of SO4 and PO4 in soils was investigated in four Spodosols from Northern Spain. Sorption experiments were carried out by adding 1.6 and 16.1 mM KH2PO4 or 0.4 and 1.6 mM K2SO4 to soils (soil/solution, 1:10). Most (98.5–99.8%) of the PO4 added at 1.6 mM was retained by the illuvial Bh, Bhs, Bsh, and Bs horizons. The SO4 retained by the same horizons at the same initial concentration was below 28%. Phosphate sorption at 16.1 mM in the Bh and Bhs horizons was significantly (P < 0.05) greater than in the Bsh and Bs, corresponding to their greater oxalate extractable-Al and –Fe contents, whereas the opposite was observed for SO4 absorption. Sorption of PO4 primarily displaced mineral-bound organic C and, to a lesser extent, Si into solution, whereas sorption of SO4 increased solution pH. The molar ratios of organic C released into solution and PO4 sorbed were greater in the Bsh and Bs horizons than in the Bh and Bhs horizons. The differences observed in PO4 and SO4 sorption between Bh and Bs horizons were mainly attributed to the different organic C contents of the horizons and to the dissimilar affinities of the inorganic anions studied for reactive sites.

Abbreviations: Alcu, CuCl2-extractable Al • Alla, LaCl3-extractable Al • Aln, NaOH-extractable Al • Alo, ammonium oxalate-extractable Al • Alp, sodium pyrophosphate-extractable Al • Cp, sodium pyrophosphate-extractable C • DOC, dissolved organic C • ECEC, effective cation-exchange capacity • Feo, ammonium oxalate-extractable Fe • Fed, dithionite citrate-extractable Fe • FTIR, Fourier-transform infrared spectroscopy • ODOE, optical density of the oxalate extract • Sin, NaOH-extractable Si • Sio, ammonium-extractable Si • XRD, x-ray diffractometry


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
SPODOSOLS ARE MORPHOLOGICALLY diverse, in response to differences in climate, parent material, and drainage conditions. In Spain, podzolization processes occur in soils developed from quartz-rich parent materials (sandstones, quartzites, and quartz-rich coarse sediments). Less frequently, podzolization occurs in soils developed from materials containing greater amounts of Al and Fe (phyllites, slates, granodiorites, and granites) in cold areas, where only small amounts of these elements are released through weathering. Podzolized soils are usually located in well-drained areas of Spain with mean annual rainfall >800 mm and under acidifying vegetation. Spodosols that develop from materials in which weatherable minerals, basic cations, and Fe and Al are scarce tend to have a greater degree of profile differentiation than those developed from materials with a greater Fe and Al content, in which the albic horizon is thin or absent.

Two contrasting geochemical compartments are identified in well differentiated Spodosols (Ugolini and Dahlgren, 1991). The upper horizons (O, A, E, Bh) contain large amounts of organic acids, which cause weathering through acid complexolysis and which limit the dissociation of H2CO3. Under such conditions, mineral formation is practically nonexistent and only quartz and, in some cases, kaolinite is stable. Organically complexed-Fe and, to a lesser extent, Al compounds precipitate in the Bh horizons as coatings and aggregates when the C/metal ratio of the complexes decreases. Weathering in the lower horizons (Bhs, Bsh, Bs, BC, and C) is mainly caused by weaker acids, such as H2CO3 and hydroxy-Al ions. Short-range Al and aluminosilicate compounds, such as gibbsite of low crystallinity and imogolite, may be present in these horizons. In the Spodosols developed from materials with higher Fe and Al contents than quartzites and sandstones, the trends are similar to those described above, although the degree of podzolization tends to be less accentuated, and in situ weathering may be an important mechanism in the formation of short-range ordered Fe and Al compounds in B horizons (Macías et al., 1988).

The upper (O, A, and E) horizons differ greatly from the spodic (Bh and Bs) horizons in the content, nature and reactivity of the Fe and Al compounds. This explains their contrasting anion sorption capacities, the spodic horizons being those which display the greatest PO4 and SO4 sorption capacity (Fuller et al., 1985; MacDonald and Hart, 1990; Yuan and Lavkulich, 1994). Within the spodic horizons, the ability of the different Fe and Al fractions to sorb either PO4 or SO4 may differ according to the organic matter content or the nature of the Fe and Al compounds. The different abilities of PO4 and SO4 to compete with organic anions are related to their individual affinities for adsorption sites. Kaiser and Zech (1999) showed that desorption by SO4 of organic matter from minerals was low and mainly confined to the hydrophilic fraction. Phosphate was more efficient in desorbing all of the hydrophilic and part of the hydrophobic fractions. Liu et al. (1999) indicated that the order of addition of oxalate and SO4 had a strong influence on the adsorption of these anions on goethite at low pH values, the anion added first prevented the adsorption of the anion added second. When PO4 was added, SO4 desorption was strongly promoted as was, to a lesser extent, that of oxalate.

The different abilities of SO4 and PO4 to compete with organic anions may explain the different sorption reactions of these anions with Fe and Al of Bh and Bs horizons. In a study of spodic B horizons in Sweden, Karltun and Gustafsson (1993) found that as the organically bound Fe and Al fraction increased, the SO4 sorption capacity of soils decreased. Similarly, Fuller et al. (1985) observed that the Bs horizons retained more SO4 than did Bh and Bhs horizons. In contrast to SO4, Zhou et al. (1997) reported that Fe- and Al-humus complexes were the primary source of the PO4 sorption capacity of Bh horizons in sandy Coastal Plain soils in Florida. Aluminum-organic coprecipitates have been shown to retain large amounts of PO4 and low molecular mass organic acids (De Cristofaro et al., 2000). These authors observed that more PO4 than tartrate was sorbed on the hydroxy-Al-oxalate precipitates, although the sorbed PO4/sorbed tartrate ratio decreased when the pH was decreased and the content of oxalate in the precipitates was increased; tartrate also appeared to be more specific than PO4 in replacing oxalate.

The objective of this research was to study the PO4 and SO4 sorption capacities of some Spanish Spodosols with albic horizon. The main aim was to investigate how differences in the nature of Al and Fe compounds and organic C contents of Bh and Bs horizons affect PO4 and SO4 sorption reactions.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soils Studied
Four Spodosols, developed from quartzites, sandstones, a colluvium of quartz blocks, and a glacial till of granites were sampled by horizon at locations shown in Fig. 1 . Soil classification, according to Soil Survey Staff (1999), and site characteristics such as vegetation, parent material, mean annual air temperature, and total annual precipitation are given in Table 1. The profile from the Pyrenees (CER) is an Aquic Haplocryod and the other three (C45, L40, and PS), all from Galicia, are Typic Haplorthods.



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  		Fig. 1. Sampling sites (dashed area has a total annual precipitation >=800 mm).

 

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  		Table 1. Site descriptions.

 
Soil Characterization
Soil samples were collected from individual horizons, air-dried, and passed through a 2-mm sieve prior to analysis. Soil pH was measured in H2O and in 0.1 M KCl (soil/solution, 1:2.5). Soil pH in NaF at 2 min (Fieldes and Perrot, 1966) was also measured to identify reactive Al. Organic C content was analyzed by combustion with a LECO carbon analyzer (Model CHN-1000, LECO Corp., St. Joseph, MI). Exchangeable cations were extracted with 1 M NH4Cl (Peech et al., 1947). Exchangeable H+ was titrated to pH 4.5 with 0.005 M NaOH using an automatic titrator (Model TT2022, Crison Instruments, Alella, Spain). Effective cation-exchange capacity (ECEC) was calculated as the sum of base cations, Al, and H+. Separate acid ammonium oxalate (Blakemore, 1978) and sodium pyrophosphate (Bascomb, 1968) extractable Fe and Al (Feo, Fep, and Alo, Alp, respectively) was measured for each sample. Dithionite-citrate-extractable Fe (Fed) content (Holmgren, 1967) was also determined. Optical density of the oxalate extracts was measured as described by the USDA (1996). Aluminum was also extracted with 0.5 M NaOH (Aln) (Borggaard, 1985), with 0.33 M LaCl3 (Alla) (Hargrove and Thomas, 1981) and with 0.5 M CuCl2 (Alcu) (Juo and Kamprath, 1979). Silicon was determined in both the oxalate and the NaOH extracts (Sio, Sin). Cations were determined by atomic absorption or emission spectrophotometry (Perkin-Elmer 2380, Norwalk, CT). Carbon in the sodium pyrophosphate extract (Cp) was determined using a TOC-5000 analyzer (Shimadzu Corp., Tokyo, Japan). All extractions and analyses were performed in duplicate. The clay fraction was collected after treating the soil samples with 6% (vol./vol.) H2O2 followed by repeated dispersion and sedimentation. The mineralogy of the clay fraction was determined on untreated samples by x-ray diffractometry (XRD) using CuK{alpha} radiation with a Philips PM8203 diffractometer (Philips, Wavre, Belgium). The Fourier transform infrared (FTIR) spectra of both the clay fraction and untreated soil samples were obtained using a Brucker IFS-66V spectrometer (Brucker Daltonik GmbH, Karlsruhe, Germany). Particle-size distribution was determined by the pipette method (Gee and Bauder, 1986).

Phosphate and Sulfate Sorption Studies
Native PO4 was determined by extracting the soil with distilled deionized water (Olsen and Sommers, 1982) and with acid ammonium oxalate (Wolf and Baker, 1990). Dissolved organic C (DOC) concentration in the acid ammonium oxalate extract was determined by a TOC-5000 analyzer (Shimadzu Corp., Tokio, Japan). Phosphate adsorption experiments were carried out by adding 30 mL of 1.6 and 16.1 mM KH2PO4 solutions, with a CaCl2 0.01 M concentration, to 3 g soil. Several drops of toluene were added to inhibit microbial activity. The change in DOC concentration of the KH2PO4 solutions after the addition of toluene was <=0.3 mM. Suspensions were left to equilibrate for 4 to 5 d (shaken twice a day) before measuring pH. Solutions were separated by centrifugation at 2100 x g, filtered through 0.45-µm membrane filter, and the PO4 concentration in the extract was determined colorimetrically (Murphy and Riley, 1962). In the same extract, the concentrations of Si and Al were determined by atomic absorption, SO4 by ion chromatography (Model Dionex-4500i, Dionex Corp., Sunnyvale, CA), and DOC as described above.

Native SO4 was determined by sequential extraction of soil samples with distilled deionized water and 0.016 M KH2PO4 (Camps Arbestain et al., 1999). The SO4 concentration in the extracts was determined as described above. Sulfate sorption studies were conducted by adding 30 mL of an acidified solution (pH 3.0) of either 0.4 or 1.6 mM SO4 (K2SO4) solution to 3 g of soil. The ionic strength was adjusted to 5.73 mM with KCl, as detailed by Camps Arbestain et al. (1999). This pH was selected after having examined the pH values of wet deposition in the study areas in recent years. In contrast, the PO4 solutions were not acidified, thus simulating the most common route of entry of PO4 to soils as fertilizer. The suspensions were shaken for 24 h and the pH was then measured. Solution was centrifugated and filtered through a 0.45-µm membrane filter. Concentrations of SO4, Si, Al, and DOC in the extracts were determined as described above. The term sorption is used throughout the text to refer to either PO4 and SO4 removal from solution. Sorbed PO4 and SO4 were both calculated as the difference between initial and equilibrium concentrations. All sorption experiments and analytical analyses were performed in duplicate. A Student's t test was used to detect significant differences between the different treatments studied (0.4 mM SO4, 1.6 mM SO4, 1.6 mM PO4, and 16.1 mM PO4) and between the different horizons.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soil Characterization
Amounts of Fep and Alp were generally equal to or greater than those of Feo and Alo (Table 2). Sodium pyrophosphate is assumed to recover organically bound Al and Fe, although it may also extract some amounts of both crystalline and short-range ordered inorganic compounds (Kaiser and Zech, 1996a). The oxalate reagent extracts organically bound and noncrystalline oxy-hydroxides of Al and Fe, allophane, imogolite, and allophane-like constituents (Wada, 1989). Accordingly, the results suggest that most of the Feo and Alo were complexed with organic matter. The capacity of organic matter to complex metals was estimated by calculating the molar (Alp + Fep)/Cp ratio. According to Dahlgren et al. (1993), this ratio ranges from 0.1 to 0.2 when the complexing capacity of organic matter is saturated, as with the Bh and Bhs horizons (mean value: 0.18). These ratios were generally below the saturation capacity in the A and E horizons (mean value: 0.09) whereas they surpassed that capacity in the Bsh and Bs (mean value: 0.27) and C horizons (mean value: 0.63). The high ratios observed in the deeper horizons may be related to the low specificity of the pyrophosphate extraction as it may also extract small amounts of both crystalline and short-range ordered inorganic compounds (Kaiser and Zech, 1996a). Copper (II) chloride is a more specific extractant of relatively stable Al-humus complexes compared with sodium pyrophosphate (Juo and Kamprath, 1979; Kaiser and Zech, 1996a). Values of CuCl2 extractable-Al (Alcu) were greatest in the Bh and Bhs horizons (Table 2).


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  		Table 2. Selected properties of the soils studied.

 
The Fourier transform infrared spectra of untreated soil samples (<2 mm) of the spodic horizons (Fig. 2) showed more intense bands at 1625 and 1400 cm-1 in the upper rather than in the lower spodic horizons. Gu et al. (1994) and Kaiser et al. (1997) attributed the bands at 1625 and at 1400 cm-1 to the presence of carboxylate and complexed carboxylated groups, respectively. These authors reported an increase in the intensity of these bands (and a shift of the 1625 cm-1 band to 1600 cm-1) when dissolved organic matter sorbed onto Fe and Al inorganic compounds. The 1695 cm-1 band displayed by the Bh horizon of Soil PS further indicates the presence of protonated carboxyl groups (Fig. 2). The identification of protonated carboxyl groups in this horizon seems consistent with the fact that this horizon is the most acidic and organic C rich of the spodic horizons studied (Table 2) and has a relatively small (Alp + Fep)/Cp ratio (value 0.10). Moreover, the upper and lower spodic horizons also differed in the intensities of the 1033 and 1082 cm-1 bands. A stronger absorption at 1033 cm-1 was always observed in the upper compared with the lower spodic horizons. In the upper horizons of soils C45, PS, and CER the intensity of the 1033 cm-1 band was greater than that of 1082 cm-1, whereas the inverse occurred in the lower spodic horizons. Stone et al. (2001) assigned the peak at 1080 cm-1 to silicate minerals (quartz, see below), whereas that at 1033 cm-1 to either silicate minerals or methoxyl groups. Finally, White (1971) attributed the bands at 1172, 1082, 800, 780, 697, 512, and 462 cm-1 to the presence of quartz. These peaks were clearly identified in all the horizons studied given the predominance of quartz in the mineral fraction of these soils.



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  		Fig. 2. Infrared spectra of untreated soil samples (<2 mm) from the spodic horizons studied.

 
Maximum concentrations of extractable Fe (Fep, Feo, Fed) were always detected in the upper spodic horizon (Table 2). Given that the dithionite-citrate treatment additionally extracts the crystalline oxy-hydroxides of Fe unlike the ammonium oxalate treatment, the large Feo/Fed ratio detected in these horizons (mean value: 0.7) indicates a predominance of amorphous over crystalline Fe compounds. The low degree of crystallinity of Fe oxy-hydroxides was corroborated by the lack of XRD patterns for these compounds in the clay fraction (Table 3). The profile distribution of the different forms of extractable Al (Aln, Alo, and Alp) in the soils studied was more variable than that of Fe (Table 2). In the PS Spodosol, maximum concentrations of Aln, Alo, and Alp were all detected in the lower spodic horizon (a Bhs horizon). In the other three Spodosols, Alo and Alp peaked in the upper spodic horizon whereas the maximum concentrations of Aln were found in the Bs horizons. This indicates the presence of more inorganic Al compounds in the Bs horizons. The identification of gibbsite in the clay fraction of the Bs horizon of the L40 profile by XRD and FTIR techniques (Table 3) supports these results. In addition, the fact that Sio peaked in the lower spodic horizons (Table 2), which generally had an Alp/Alo ratio <1.0, indicates the presence of short-range-order aluminosilicates in these horizons. The uneven distribution of Fe and Al compounds with depth is common in Spodosols and supports the idea that during the genesis of these soils Al is accumulated independently of Fe in the lower part of the profile because of differences in the solubilities of Al- and Fe-organic complexes. Exceptionally high Aln values were detected throughout the CER profile and tended to increase with depth (Table 2). The NaOH treatment is assumed to extract poorly crystalline forms of both 1:1 phyllosilicates and gibbsite in addition to those compounds dissolved by the ammonium oxalate extractant. The high molar ratios of Aln/Sin detected in this soil (Table 2) suggest the presence of minerals rich in readily solubilized Al. However, neither gibbsite nor the polymorphs of this mineral were identified in these soils (Table 3). Finally, this profile also differed from the other three Spodosols studied in the dominant kandite present (Table 3); whereas halloysite dominated in the former, kaolinite did so in the latter three.


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  		Table 3. Mineralogy of the clay fraction of the spodic horizons identified by either x-ray diffractometry (XRD) or FTIR Fourier-transform infrared spectroscopy.

 
The pH values in H2O and in KCl were very low throughout the soil profiles, particularly in the A, E, and upper B horizons (Table 2). The acidity in these horizons is determined by free organic acids of low molecular weight. In the lower spodic horizons the pH increases as a consequence of a decrease in the activity of organic acids, and the acid-base equilibrium becomes controlled by CO2 and hydroxyaluminum ions (Macías et al., 1987; Ugolini and Dahlgren, 1991). The reactivity to the NaF test was low in the A, E, and upper B horizons (pH < 8.0), whereas the lower B horizons had values >9.0 (Table 2). The F anion should react with reactive Al surfaces and release OH- to the solution (Mizota and Wada, 1980), causing an increase in the solution pH from its initial value of ~7.8. Organic acids neutralize part of the released OH- and even decrease the initial pH of the solution (Perrot et al., 1976). This explains the low NaF reactivity of the upper spodic horizons despite their large contents of Al compounds.

Anion Sorption and Reactive Surfaces
Positive correlations were found between SO4 sorption and the presence of Alo, Aln, and Alp, and between PO4 and the presence of Alp, Alo, Alcu, Feo, Fed, and Fep (all reported in decreasing order) (Table 4). The correlation coefficients obtained reflect the more important role of reactive Al (in the form of Al-humus complexes and short-range ordered Al compounds), compared with that of Fe, on the anion sorption in these soils. The significant r value (P < 0.05) obtained for Cp for PO4 but not for SO4 (Table 4) indicates that sorption of the former anion was associated with horizons that contained large amounts of organic C, whereas this was not the case for SO4. Given that sorption of both anions were highly correlated with Alo + Feo (Table 4), both SO4 and PO4 sorbed were plotted against this sum (Fig. 3) , thereby providing an index of the reactive Al and Fe compounds present in these soils. Almost all of the PO4 added at 1.6 mM was sorbed by the spodic and C horizons, whereas sorption of PO4 in the A and E horizons was much lower (Fig. 3A). These results are consistent with contents of reactive Al and Fe of these horizons. In contrast to the PO4 sorption data, the percentage of SO4 retained at the same initial molar concentration (1.6 mM) was always below 28% (Fig. 3A). The lower affinity of SO4 compared with PO4 for reactive sites and the lower ability of the former to displace organic anions from those sites are both well known (Kaiser and Zech, 1996b, 1999; Liu et al., 1999). The Bh and Bhs spodic horizons displayed a smaller SO4 sorption than the Bsh and Bs horizons (significant at P < 0.05), in spite of the higher Alo + Feo levels of the former (Fig. 3A). These results suggests that the higher organic C present in the Bh and Bhs horizons (mean organic C: 41.7 g kg-1) compared with that in the Bsh and Bs horizons (mean organic C: 12.9 g kg-1) may have exerted a negative influence on the SO4 sorption in these horizons. The greater SO4 sorption observed whithin each soil profile in the spodic subhorizon with the lower organic C content (Fig. 4) further points towards the inhibitory effect of organic anions on SO4 sorption. Similar results were obtained by Fuller et al. (1985). These authors observed a greater SO4 sorption in the Bs horizons than in organic matter-rich Bh and Bhs horizons. Finally, the PO4 retained at 16.1 mM generally increased with increasing contents of Alo + Feo (Fig. 3B). In contrast to SO4, values of PO4 sorption were significantly (P < 0.05) greater in Bh and Bhs horizons than in Bsh and Bs horizons (Fig. 3B and 4), in accordance with their greater Alo + Feo contents.


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  		Table 4. Correlation coefficients (r) obtained between contents of different Fe, Al, and C fractions{dagger} of the soils studied and both SO4 (initial concentration, 1.6 mM) and PO4 (initial concentration, 16.1 mM) sorption (n = 10).

 


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  		Fig. 3. (A) Percentage of PO4 sorption from a 1.6 mM PO4 solution and percentage of SO4 sorbed from a 1.6 mM SO4 solution plotted against reactive Al and Fe (expressed as Alo + Feo; units in mmol kg-1). (B) Percentage of PO4 sorption from 16.1 mM PO4 solution plotted against reactive Al and Fe (expressed as Alo + Feo; units in mmol kg-1).

 


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  		Fig. 4. (A–D) Values of SO4 sorbed and Si and DOC in solution obtained for the 0.4 mM and 1.6 mM SO4 treatments (units in mmol kg-1). (E–H) Values of PO4 sorbed and Si and DOC in solution obtained for the 1.6 and 16.1 mM PO4 treatments (units in mmol kg-1).

 
Oxalate-extractable PO4 represents the PO4 originally associated with amorphous Fe and Al compounds, although it may also dissolve other P forms, including Fe and Al phosphates. Oxalate-extractable PO4 made up a small fraction of the total retained in this study (0–14%) (when considering the sum of oxalate-extractable PO4 + sorbed PO4 at 16.1 mM) (Table 2). Within each soil profile, this PO4 fraction always peaked in the lower spodic horizons (Table 2), in contrast to the above mentioned results obtained when PO4 was artificially added to the soils. Values of oxalate-extractable PO4 were especially low in the three Bh horizons studied (mean 1.5 mmol kg-1) and significantly different (P < 0.01) from those in Bhs, Bsh, and Bs horizons (mean 10.5 mmol kg-1) (Table 2). Natural conditions may differ greatly from those simulated in the laboratory. In this experiment, the molar ratio of DOC/PO4 in solution was greatly altered (average molar ratio of 1080 under natural conditions and of 0.2 when a 16.1 mM PO4 solution was added to the upper B horizons). Moreover, the pH values of the three Bh horizons were always smaller than those of the other spodic horizons (Table 2). De Cristofaro et al. (2000) indicated that the efficiency of tartaric acid in preventing PO4 sorption on organo-Al complexes was increased by decreasing the initial PO4/tartrate molar ratio. They also observed that the sorbed PO4/sorbed tartrate ratio tended to decrease as pH decreased and as the content of oxalate in the precipitates increased. Moreover, Liu et al. (1999) indicated that at low pH values even large amounts of PO4 could not desorb oxalate from goethite completely. Thus, the large DOC/PO4 ratio present in the Bh horizons under natural conditions and the low pH of these horizons (Table 2) may have had a negative influence on PO4 sorption, in spite of the presence of many reactive surfaces, mainly in the form of Al- and Fe-humus complexes. As this ratio decreases and pH increases with depth, the percolating PO4 would tend to accumulate in the lower spodic horizons, as was found in these soils, although further studies would be needed to confirm this hypothesis. Phosphate extractable-SO4 values, which were generally lower than those of oxalate extractable-PO4 (Table 2), peaked either in the upper or in the lower spodic horizons without showing a clear pattern between the two subhorizons.

Phosphate and Sulfate Sorption and Anion Release
As SO4 sorption increased with increasingly high concentrations of SO4 added (0.4 mM vs. 1.6 mM) (Fig. 4A–D) there was a significant (P < 0.01) increase in the pH of the solutions and a significant (P < 0.01) decrease in soluble Al (Table 5) related to the higher equilibrium solution pH (Table 5). There were no significant differences (P < 0.05) between means of soluble Si and DOC concentrations at the two SO4 levels studied (Fig. 4A–D). These results are thus consistent with the widely held view that ligand exchanges take place between SO4 and both OH and OH2 (Curtin and Syers, 1990; Guadalix and Pardo, 1991), and also with the well-known weak ability of SO4 to compete with organic anions for sorption sites (Kaiser and Zech, 1999; Liu et al., 1999). The results are also in accordance with those of Rajan (1979), who did not detect any significant release of Si by SO4 sorption. When concentrations of the anions released with the 1.6 mM SO4 treatment (Fig. 4A–D) were compared with those released with the 1.6 mM PO4 treatment (Fig. 4E–H), no significant differences (P < 0.05) were detected between treatments. A higher level of DOC released in the PO4 treatment because of the higher sorption values obtained would be expected, given the well-known high capacity of PO4 to displace sorbed organic C anions compared with SO4. It is possible that at an initial concentration of 1.6 mM PO4, there were enough binding sites for sorption of both PO4 and organic C anions.


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  		Table 5. pH values of the suspensions and concentrations of Al in solution for the different SO4 and PO4 treatments, in the spodic and C horizons studied.

 
In general, greater differences were observed between the two PO4 treatments than between the SO4 treatments, particularly for the Si and DOC concentrations in solution (Fig. 4E–H). However, no significant differences (P < 0.05) were observed for the SO4 concentrations in solution between the two PO4 treatments (data not shown). Means of soluble Si and DOC concentrations of the 16.1 mM PO4 treatment were significantly higher (P < 0.01) than those of the 1.6 mM PO4 treatment. Increases in soluble Si were evident in the spodic and C horizons (Fig. 4E–H). These horizons had higher Sio and Sin contents than the corresponding upper horizons (Table 2). However, from the data available, it is not possible to distinguish between the mechanism for Si desorption and the release of Si because of the breakdown of the microstructure of the minerals (Veith and Sposito, 1977; Imai et al., 1981). Increases in DOC were especially evident in the spodic and C horizons (Fig. 4E–H), confirming the well-known capacity of PO4 to displace sorbed organic C anions. In contrast, differences between means of pH values and soluble Al contents obtained with the 1.6 and 16.1 mM PO4 treatments were not as clear cut as those detected between the two SO4 treatments (Table 4). However, common pattern was observed in the three Bh horizons studied there being a sharp decrease in soluble Al with increasing PO4, although pH values did not increase (Table 5). These results suggest that the precipitation of an Al-PO4 mineral, apart from sorption processes, cannot be disregarded.

To estimate the net amount of anions exchanged in soils, the sorbed PO4 and the solution DOC, Si, and SO4 in the 1.6 mM PO4 treatment were subtracted from those of the 16.1 mM PO4 treatment. The data obtained in the SO4 sorption experiments was not included because no significant differences in these variables were detected between treatments. The net sum of Si + DOC + SO4 released was plotted against net PO4 sorbed (Fig. 5) . The results indicate that both variables were generally in a ratio of 1:1, except for the Bh and Bhs horizons that followed a different pattern. However, careful consideration should be made when balancing these anion exchanges, especially with DOC, as the exchanges with these organic compounds depend more on the amount of functional groups they have than on their total C content (Inskeep, 1989). In the Bh and Bhs horizons studied the mean of molar ratios of net DOC released/net PO4 sorbed were significantly lower (P < 0.01) than that of Bsh and Bs horizons, which contrast with the higher Cp and organic C contents of the former horizons (Table 2). Kaiser and Zech (1997) indicated that competition between DOC and SO4 or PO4, for sorption sites was more pronounced with hydrophilic than hydrophobic DOC, and could result in a higher mobility of the hydrophilic DOC, especially in soils with low sorption capacity. Thus, a higher hydrophilic DOC/hydrophobic DOC ratio in the Bs than in the Bh horizons, together with the lower number of binding sites of the former may explain the higher amounts of DOC released per unit per sorbed PO4 in the Bs horizons. These authors (in the works cited) suggested the competitive sorption of the different DOC fractions as a reason for the differentiation of Bh and Bs horizons of Spodosols. The present data is the result of the summation of several reactions (the possibility of Al-PO4 precipitation should also be considered and the lower pH of the three Bh horizons may also have influenced the lower displacement of organic C into solution) and thus, it is difficult to draw conclusions. Detailed information about OH and Al fluxes, the number of carboxyl groups and organic C composition would possibly aid in the interpretation of the results, and thus, are needed in a further investigation.



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  		Fig. 5. Net Si + DOC + SO4 released plotted against net PO4 sorbed (data from the 16.1 mM PO4 treatment minus data from 1.6 mM PO4 solution), in the four Spodosols studied.

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The identification of the mechanisms of anion sorption from soil studies is a difficult task because of the complex nature of the soil sorbing surfaces. Working with soil samples instead of pure mineral or organo-mineral phases provides a more realistic view of the interactions taking place between the different substrates. This study highlights the differences in PO4 and SO4 sorption between Bh and Bs horizons. Differences were mainly attributed to the different organic C contents of these horizons and also to the dissimilar affinities of the inorganic anions for reactive sites. All horizons studied were able to retain much higher amounts of PO4 than of SO4. Phosphate sorption was higher in the Bh and Bhs horizons than in the Bsh and Bs horizons, corresponding with their higher Alo and Feo contents. In contrast, SO4 sorption was always higher in the spodic subhorizon with the lower organic C content. Sorption of PO4 primarily displaced mineral-bound organic C and, to a lesser extent, Si into solution, whereas sorption of SO4 increased solution pH. The sum of Si + DOC + SO4 released generally followed a 1:1 ratio with PO4 sorbed for the Bsh, Bs, and C horizons, but not for the Bh and Bhs horizons. The organic-rich spodic horizons had a smaller DOC released/PO4 sorbed ratio than the Bsh and Bs horizons, in spite of their higher contents of Cp.


   ACKNOWLEDGMENTS
 
The authors thank R.M. Poch and J. Boixadera for providing the CER soil samples and information about this profile. Thanks are extended to the three anonymous reviewers for their helpful suggestions on an earlier version of this manuscript.

Received for publication January 2, 2001.


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




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