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Soil Science Society of America Journal 63:1906-1914 (1999)
© 1999 Soil Science Society of America

DIVISION S-7-FOREST & RANGE SOILS

Parent Material Influence on Sulfate Sorption in Forest Soils from Northwestern Spain

M.Camps Arbestaina, M.E. Barreala and F. Macíasa

a Departamento de Edafoloxía e Química Agrícola, Facultade de Bioloxía, Universidade de Santiago de Compostela, Santiago de Compostela-15706, Spain

edfmac{at}usc.es


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
Sulfate sorption by forest soils decreases the potential detrimental effect of S deposition on cation leaching. Twenty-four soils from the 3416-km2 area surrounding two lignite-fueled power-generating plants in NW Spain were studied to relate SO2-4 sorption to soil properties and ultimately to the parent material. The area contains a variety of parent materials, and has a wide range of acidic soils (Haplumbrepts, Fulvudands, and Kanhaplohumults). Total annual precipitation ranges from 1100 to 1900 mm yr-1 and mean annual temperature is 12°C. Sulfate sorption was studied by adding either 0.4 or 1.6 mM SO2-4 solutions to soils. Sorption decreased in the following order: amphibolite > biotitic schist > granite > phyllite and was positively related to Al extractable with sodium hydroxide, oxalate, and pyrophosphate, and to Fe extractable with dithionite-citrate and oxalate, which were high in soils derived from basic materials. Organic matter apparently had counteracting effects on SO2-4 sorption. First, sorption was important in surface horizons with pH-NaF >=9.7, mainly due to the presence of Al–humus complexes, which often give these soils an andic character. Second, sorption was specially low in surface horizons of soils derived from acidic materials with pH-NaF <8.0, which may be attributed to competition between SO2-4 and organic acids for sorption sites, in addition to the low content of reactive surfaces. Sorption was positively related to soil pH due to the positive relation between Fe and Al oxy-hydroxides and basic materials. These results suggest the need to take into account the influence of parent material on SO2-4 sorption in assessing the sensitivity of soils to S deposition.

Abbreviations: ECEC, effective cation-exchange capacity • BS, base saturation percentage • Fed, dithionite-citrate-extractable Fe • Feo and Alo, ammonium oxalate-extractable Fe and Al, respectively • Fep and Alp, sodium pyrophosphate-extractable Fe and Al, respectively • Aln+1/2Fed, sum of Aln fraction plus one-half of Fed fraction • Aln and Sin, sodium hydroxide-extractable Al and Si, respectively • Alo+1/2Feo, sum of Alo fraction plus one-half of Feo fraction


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
SULFATE ADSORPTION in soils is mainly associated with Al and Fe oxy-hydroxides and with allophanic constituents (Chao et al., 1964; Parfitt, 1978), compounds all characterized by having a pH-dependent surface charge. Anion adsorption has also been associated with the presence of Al–humus complexes and, to a lesser extent, with Fe–humus complexes (Wada and Gunjigake, 1979; Shoji and Fujiwara, 1984). On the other hand, when organic ligands are present, SO2-4 adsorption is often inhibited due to the competition of these anions with SO2-4 for adsorption surface sites (Inskeep, 1989; Evans and Anderson, 1990). Sulfate adsorption generally increases when the system is artificially acidified, which is related to the increased positive surface charge that variable-charged soils acquire as pH decreases (Chao et al., 1964; Zhang et al., 1987; Courchesne and Hendershot, 1989). Furthermore, SO2-4 adsorption is a concentration-dependent process, and as such, it is influenced by the concentration of SO2-4 in the incoming solution relative to the concentration at which the soil had previously equilibrated (Chao et al., 1962; Dahlgren et al., 1990). The mechanism of SO2-4 adsorption is not fully understood, but it is generally assumed that, in addition to the electrostratic retention by the positive sites on the surfaces of soil particles, a ligand exchange is involved in the process (Parfitt, 1978; Rajan, 1978; Zhang et al., 1987).

The parent material from which soils develop is a key factor that in many cases determines the kinds and contents of secondary minerals of soils. Formation of noncrystalline constituents and Al– and Fe–humus complexes takes place preferentially in soils derived from volcanic ejecta, giving these soils a typical andic character (variable surface charge, high water-holding capacity, high anion retention, low bulk density) (Shoji et al., 1996). However, andic properties can also be found in incipient weathered soils derived from nonvolcanic materials, such as basic or metabasic rocks developed under humid-temperate conditions, where the presence of abundant weatherable minerals gives rise to the release of significant amounts of Al, leading to the association of minerals of low crystallinity and Al–humus complexes (García-Rodeja et al., 1987). In highly weathered soils formed from base-rich materials, kaolinite and goethite are the only minerals remaining in subsurface horizons, and the significant presence of crystalline Fe, together with that of low activity clays, give a strong ferrallic character to these horizons (subsurface horizons with low ECEC, and low presence of weatherable minerals). On the other hand, dissolved organic acids predominate in surface horizons of forest soils developed from quartzitic rocks poor in weatherable minerals developed under cold humid climatic conditions. Thus, SO2-4 adsorption is expected to be higher in soils developed from base-rich materials having abundant Fe and Al oxy-hydroxides compared with that occurring in soils developed from more acidic materials, as already observed in previous studies (Scott, 1976; Barton et al., 1994; Merino and García-Rodeja, 1996).

The north of Galicia (NW Spain) is affected by S deposition episodes originating from emissions of two lignite-fueled power-generating plants, Meirama and As Pontes, located in the area. Soils are acidic and have low ECEC, low base saturation, and Al dominance on the exchange complex, and thus, are susceptible to Al mobilization. The area is endowed with a variety of parent materials (amphibolite, biotitic schist, granite, and phyllite) that have a strong influence on the properties that the different soils display. As SO2-4 adsorption by soils is currently regarded as a delaying mechanism for the detrimental effect of SO2-4 leaching in soils (base cation loss and Al release), the SO2-4 retention capacity of these soils must be known in order to predict the effect of S deposition on the ecosystems. Thus, the objectives of this research were (i) to determine the ability of soils located in the surroundings of the power-generating plants of As Pontes and Meirama (Galicia, NW Spain) to retain SO2-4, (ii) to compare SO2-4 adsorption potentials in soils developed from different parent materials, and (iii) to relate SO2-4 adsorption potentials to soil properties.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 Conclusions
 REFERENCES
 
Soils Studied
The study area is delimited by two circles each of 20-km radius, centered around the lignite-fueled power-generating plants of Meirama and As Pontes (Galicia, NW Spain), and by the parallel tangents common to both circles (Fig. 1) (total surface = 3416 km2). The mean annual temperature is 12°C and the total annual precipitation ranges between 1100 and 1900 mm yr-1. Sulfur atmospheric deposition is not uniformly distributed, tending to be higher along the SW–NE axis of both plants (Fig. 1). Maximum values of S wet deposition are around 40 kg S ha-1 yr-1 and mean values are around 20 kg S ha-1 yr-1, the latter falling within the range of those reported for Central Europe (Bredemeier et al., 1990; Matzner and Meiwes, 1994). Periodic rain samples have shown average pH values of between 4 and 6, and several observations of around pH 3.4 have been detected in the past years (data not presented). Twenty-four plots were selected from within the area (Fig. 1), and soil profiles were classified according to Soil Survey Staff (1997). Soil classification and site characteristics, such as vegetation and parent material, are reported in Table 1 . The area includes soils formed from different parent materials including schistic (Soils M1, M2, M3, M4, M5, M6, M7, M8, M9, M20, M21, P16), amphibolitic (Soil P11), granitic (Soils P15, PM10, PM14, PM13, P19, P22), and phyllitic (Soils P18, P27, P12, P17, F2) rocks. There is a clear difference in lithology between the areas around the Meirama and As Pontes plants (Fig. 1). Meirama is surrounded by schists, with an abundance of weatherable minerals (biotite, amphibole), and has stable geomorphologic surfaces that have allowed the development of thick soil profiles. As Pontes is situated on the west of a granitic belt, in an area dominated by metamorphic rocks (phyllite), rich in quartz and poor in weatherable minerals and characterized by a more pronounced topography. Metabasic rocks (amphibolite) can only be found to the north of As Pontes.



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  		Fig. 1 Location of the two lignite-fueled power-generating plants, sampling sites, and lithology of the study area (at the scale used here, the parent material of some of the soils studied could not be represented)

 

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  		Table 1 Site descriptions

 
Soil Characterization
Sampling sites were chosen taking into account (i) the proximity to precipitation monitoring stations, (ii) the presence of forest vegetation, (iii) accessibility, and (iv) representativeness of both soils and lithology in the area. Sites of precipitation monitoring stations were previously selected by personnel at the power-generating plants in order to optimize the monitoring of acid deposition produced by both plants. The coniferous forests of the area represent typical Galician reforested vegetation, while oak forests are native. Soil pits were excavated to the rooting depth and soil horizons were sampled. Soil samples were air dried and passed through a 2-mm sieve prior to analysis. Soil pH was measured in H2O and in 0.01 M KCl using a 1:2.5 soil/solution ratio. Soil pH in NaF at 2 min (Fieldes and Perrot, 1966) was also measured to identify the presence of "active" Al defined as (i) allophane, allophane-like constituents, or imogolite; (ii) exchangeable Al as well as interlayered hydroxy-Al ions in layer silicates; and (iii) Al–humus complexes in which hydroxy-Al ions are bound to carboxylate groups (Wada, 1980). Organic C content was analyzed by combustion with a LECO C 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). Separate ammonium oxalate (Blakemore, 1978) and sodium pyrophosphate (Bascomb, 1968) extractions for Fe and Al (Feo, Fep, Alo, and Alp) were performed for each sample. Dithionite-citrate-extractable Fe (Fed) (Holmgren, 1967) was also determined. The pyrophosphate reagent is an effective extractant of Al– and Fe–humus complexes. The oxalate reagent extracts Al– and Fe–humus complexes, noncrystalline oxy-hydroxides of Al and Fe, allophane, imogolite, and allophane-like constituents. Finally, the dithionite treatment extracts the same components as the oxalate extraction except allophane and imogolite, and in addition extracts crystalline oxy-hydroxides of Fe (Wada, 1989). All the Al forms described above, together with low crystalline Al components, such as cryptogibbsite, which are not extracted by the above treatments, were extracted with 0.5 M NaOH according to the methodology of Hashimoto and Jackson (1960) as modified by Borggaard (1985), and Al and Si were measured (Aln and Sin). Cations were determined by atomic absorption or emission spectrophotometry. Native SO2-4 was determined by a sequential extraction (1 h) of 3-g soil samples with 30 mL of distilled deionized water followed by 30 mL of 0.016 M KH2PO4 after the method of Fuller et al. (1985). Solutions were separated by centrifugation at 2100 g, filtered through 0.45-µm polycarbonate membranes, and analyzed for SO2-4 by ion chromatography (Model Dionex-4500i, Dionex Corp., Sunnyvale, CA). All samples were extracted and analyzed in duplicate.

Sulfate Adsorption Studies
Sulfate adsorption isotherms were produced for all horizons by adding 30 mL of 0 to 1.6 mM SO2-4 (K2SO4) solutions to 3 g of soil in a 50-mL centrifuge tube. Solutions were initially acidified to pH 3.0 and the ionic strength adjusted to 5.73 mM with KCl. This pH level was selected after having examined the pH values of wet deposition in the study area during recent years. The suspensions were shaken for 24 h. Solutions were then separated by centrifugation, filtered, and analyzed for SO2-4 as indicated above. Because no attempts were made in this study to distinguish between precipitation and surface reactions the term "sorption" will be used throughout the text to refer to SO2-4 removal from solution. Sorbed SO2-4 was calculated as the difference between initial and equilibrium concentrations. In this study, only SO2-4 sorption from both 0.4 and 1.6 mM solutions will be discussed. Potential SO2-4 sorption was calculated by adding the SO2-4 sorbed from either 0.4 or 1.6 mM solutions to total native SO2-4 (SO2-4 extracted with H2O + SO2-4 extracted with KH2PO4). Repeated measurements of 0.4 and 1.6 mM SO2-4 solutions prepared for each analytical run resulted in coefficients of variation of 3.6 and 1.6%, respectively.


   Results and discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
Conclusions
 REFERENCES
 
Soil Properties
The soils studied all have low pH, low ECEC, and low base saturation (Table 2) typical of soils located in a high-leaching environment. However, they differ in terms of the influence of parent material on the type of pedogenesis and of the degree of weathering.


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  		Table 2 Mean and ranges of selected soil properties of the soils studied grouped according to surface and subsurface horizons for each of the parent material groups. Surface horizons include all Ah horizons (Ah1 and Ah2). Soils P17 and P18 are described separately from their parent material group, as they have unique chemical properties

 
Those soils developed from base-rich materials (biotitic schist and amphibolite) are either fine- or loamy-textured and have a high Fed content mainly in a crystalline form as reflected by their Fec values (Fec = Fed - Feo) (Table 2). Crystalline Fe oxy-hydroxides (Fec) are the products of the weathering of Fe–Mg rich minerals found in these parent materials. Oxalate-extractable Al contents are high in surface horizons, with a dominance of Al–humus complexes according to their Alp/Alo ratios. All horizons were very reactive to the NaF test, with all values being above 9.4, indicating a dominance of active Al (Mizota and Wada, 1980). These characteristics lead to the identification of andic properties in most of these soils (except Soils M1 and M20) (Table 1). Andic properties are frequently found in surface horizons of soils from Galicia developed from basic or metabasic rocks, in which the association of minerals of low crystallinity (allophane, halloysite) and Al–humus complexes is common (García-Rodeja et al., 1987). In subsurface horizons of soils from these rocks displaying incipient weathering, the most frequent mineral association found is formed by halloysite, allophane, and gibbsite (Macías and Calvo de Anta, 1992), as reflected by the increase of the Aln minus Alp values with depth (Table 2). In contrast, in highly weathered soils developed from basic rocks, kaolinite and goethite are the only minerals remaining in the subsurface horizons, allowing the identification of a ferrallic character in some cases (Macías and Calvo de Anta, 1992). Soils developed from amphibolite differ from those developed from biotitic schist in that the degree of the andic character at the initial stages of weathering and that of the ferrallic character at the final stages are more accentuated.

Soils developed from 2-mica granite (with biotite and muscovite, the latter prevailing in most cases) and from granodiorite (biotite dominant) are coarse-textured, while those developed from gneiss are loamy-textured. These soils are heterogeneous in most of their characteristics, which is common in soils formed from these materials due to the different rates and directions that weathering processes take. The low Fe content of granite explains the low values of Fep, Feo, and Fed found in soils developed from these materials, while Fe values tended to be higher in soils derived from biotite-rich granodiorite and gneiss. Exchangeable Al in these soils was present in higher concentrations than in soils developed from base-rich materials, but the Al–humus complexes were less abundant (Table 2). In general, the highest response to the NaF test was observed where organic matter reached a certain degree of humification, the acidity of organic matter decreased, and the presence of Al–humus complexes became important, which generally occurred in the Ah2 horizons. The clay fraction of surface horizons of soils developed from granite is dominated by mica strongly weathered to vermiculite, with hydroxy-Al interlayers present in many cases (Macías and Calvo de Anta, 1992). In mineral horizons, gibbsite associated with halloysitic minerals is abundant within the clay fraction, as reflected by the Aln minus Alo values obtained (Table 2), and the ferrallic character is lacking.

This situation is similar in soils developed from phyllitic material when drainage is not impeded, although surface acidity (Table 2) as well as degree of weathering of 2:1 minerals both tend to be higher than in granitic soils. These soils are fine-textured, a trait inherited from the parent material, and subsurface horizons have, in general, a low permeability. This low permeability renders the formation of gibbsite more difficult than in soils developed from granite, as reflected by their Aln minus Alo values (Table 2), while association of kaolinite with 2:1 beidellitic clays is frequent. The various Fe and Al fractions measured were generally low (Table 2), there being a trend of increasing Fed with depth in some horizons affected by waterlogging conditions. An exception to this was the BA horizon of Soil P18 that had elevated extractable Fe contents, which may be attributed to an incipient podzolization process taking place in the profile (Table 2). The reactivity to the NaF test was frequently low or negative (<8.0), reflecting the scarcity of active Al in these soils. Only the Ah horizon of Soil P17 and the BA horizon of Soil P18 had a pH-NaF >=9.4 (Table 2) in agreement with their Alp and Alo values (Table 2).

Total Native Sulfate and Soil Properties
Native SO2-4 differed among soils developed from different parent materials, decreasing in the following order: amphibolite > biotitic schist > granite > phyllite (Table 3) . An exception to the group of soils developed from phyllite were Soils P17 and P18, which exhibited a high native SO2-4 content, in agreement with their andic and podzolic characteristics, respectively (Tables 2 and 3). Subsurface horizons tended to retain higher amounts of SO2-4 than did surface horizons, although those surface horizons with andic properties (most soils developed from biotitic schist, amphibolite, and Soil P17 from phyllite) retained considerable amounts of SO2-4 (Table 3). Two different regions with elevated total native SO2-4 can be distinguished in the study area: (i) one located southwest of the Meirama plant, which includes soils developed from biotitic schist all with an accentuated andic character (mainly Soils M5, M6, and M8) and (ii) another region located to the north and northeast of As Pontes plant, which includes Soil P11 developed from amphibolite and Soils P17 and P18 developed from phyllite.


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  		Table 3 Mean and ranges of native SO2-4, SO2-4 sorbed and potential SO2-4 sorbed in the soils studied grouped according to surface and subsurface horizons for each of the parent material groups. Surface horizons include all Ah horizons (Ah1 and Ah2). Soils P17 and P18 are described separately from their parent material group, as they have unique chemical properties

 
Total native SO2-4 correlated positively with pH-KCl, pH-NaF, and with the different Al and Fe fractions studied, except Fep (Table 4) . The correlation coefficient increased when the sum of Aln plus one-half Fed fractions (Aln+1/2Fed) was considered, reflecting the role of both Al and Fe compounds on SO2-4 sorption. Thus, native SO2-4 is clearly influenced by soil properties, which in turn are in most cases determined by the parent material from which the soils have developed. Our results reflect how inherited soil properties, characterized by remaining relatively unchanged by pollution deposition, may substantially alter the effect of this pollution (MacDonald et al., 1991), such as the mobility of SO2-4 through the soil profile. For this reason, knowledge of the relationships between the potential of these soils to sorb SO2-4, the soil properties, and ultimately the parent material is needed.


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  		Table 4 Values of correlation coefficients obtained between total native SO2-4 and soil properties

 
Sulfate Sorption Potential
Sulfate Sorption and Extractable Aluminum and Iron
Sulfate sorption potentials differed among soils developed from different parent material, decreasing in the same order as that described for native SO2-4: amphibolite > biotitic schist > granite > phyllite (Table 3). Again, an exception to the group of soils developed from phyllite were Soils P17 and P18 (Table 3). Subsurface horizons tended to sorb higher amounts of SO2-4 than surface horizons, although both surface horizons with andic properties and Ah2 horizons of soils developed from granite were able to retain considerable amounts of SO2-4. Significant positive relationships (at P < 0.01) between different Al and Fe forms (Aln, Alo, Alp, Fed, Feo, Fec, sum of Alo fraction plus one-half of Feo fraction [Alo + 1/2Feo], and Aln + 1/2Fed) and SO2-4 sorption potential were found when considering all soils together (Table 5) , and these generally agreed with results obtained in previous studies (Fuller et al., 1985; MacDonald and Hart, 1990; Merino et al., 1994). Organically complexed Fe (Fep) was poorly correlated with SO2-4 sorption potential (only significant at P < 0.05 for potential SO2-4 sorbed from 1.6 mM solution), in contrast with other Fe and Al forms.


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  		Table 5 Values of correlation coefficients obtained between potential sorbed from either 0.4 or 1.6 mM SO2-4 solution and soil properties

 
As SO2-4 retention behavior was often different in surface horizons compared with subsurface horizons (mainly because of different organic matter contents and degree of crystallinity of secondary minerals), a further attempt to study these variables was made by comparing surface (Ah) horizons with the subsurface horizons. Working only with surface horizons improved the r values between SO2-4 sorption potential and Al forms but not Fe forms (Table 5). For surface horizons, the r values for Alo vs. SO2-4 sorption potential were substantially higher than r values for Fec vs. SO2-4 sorption potential (Table 5). In contrast, for subsurface horizons, r values were similar. These results substantiate the more important role of active Al, in the form of Al–humus complexes and mineral associations of low crystallinity, on SO2-4 sorption in surface horizons compared with that of Fe. The distinct r values of pH-NaF (test used to identify presence of active Al) vs. SO2-4 sorption obtained for surface horizons (r >= 0.80) compared with those for subsurfaces horizons (r <= 0.47) (Table 5) corroborate this point. The effect of crystalline Fe oxy-hydroxides (Fec) on SO2-4 sorption was evident in the BA horizons of Soils P11 and P18 (Tables 2 and 3). Finally, the elevated r values obtained when considering the sum of Aln + 1/2Fed fractions (Table 5) reflect the need to take into account the amounts of both Al and Fe compounds when studying SO2-4 sorption in soils. This is evident in subsurface horizons (Table 5), probably due to the fact that the contents of Al and Fe compounds in these horizons are not correlated.

Sulfate Sorption and Soil Organic Matter
Organic C was not strongly correlated with SO2-4 sorption potential in the soils studied (except for granite, where r = -0.57, significant at P < 0.05). The effect of high organic matter content on SO2-4 sorption has been attributed to different counteracting mechanisms: (i) a negative mechanism, whereby dissolved organic C and SO2-4 directly compete for sorption sites thus decreasing SO2-4 sorption (Inskeep, 1989; Evans and Anderson, 1990) and (ii) positive mechanisms, whereby an increase in reactive surface is achieved by either stabilization of Al and Fe in Al– and Fe–humus complexes (Shoji and Fujiwara, 1984), or association of minerals of low crystallinity with organic matter thus delaying the crystallization of Al and Fe oxides (Huang and Violante, 1986; Schwertmann et al., 1986). An attempt was made to distinguish these opposite tendencies in the soils studied by plotting SO2-4 sorption potential in surface horizons vs. pH-NaF (Fig. 2) . The F- anion is supposed to react with active Al surfaces and release OH- to the solution (Mizota and Wada, 1980), causing an increase in the solution pH from its initial value of ~8. When organic acids are present, they might be able to neutralize part of the released OH- and even decrease the initial pH of the solution (Perrot et al., 1976; García-Rodeja et al., 1984). This negative reaction occurs when the organic matter is very acidic and has a low degree of humification; otherwise, in the presence of active Al, the organic matter response is masked by the high affinity of F- for Al (García-Rodeja et al., 1984).



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  		Fig. 2 Sulfate sorption potential from 0.4 mM SO2-4 solution for surface (Ah) horizons vs. pH-NaF

 
For surface horizons, the SO2-4 sorption potential from 0.4 mM solutions was negligible at pH-NaF <8, while it sharply increased at pH-NaF >=9.7 (Fig. 2). Similar results were obtained when working with SO2-4 sorption potential from 1.6 mM solution, and also with native KH2PO4-extractable SO2-4 (not shown). Most surface horizons of soils formed from phyllite had low pH-NaF values and low SO2-4 sorption. An exception was Soil P17, developed from phyllite, which exhibited high SO2-4 sorption in agreement with its andic properties. Surface horizons from granitic rocks extended from intermediate pH-NaF and low SO2-4 sorption to high pH-NaF and moderate SO2-4 sorption. These latter soils were Ah2 horizons, with a more stabilized organic matter. Finally, surface horizons of soils developed from biotitic schist and amphibolite showed high pH-NaF and high SO2-4 sorption. In soils developed from acidic parent materials, the dominance of organic acids as proton donors was also reflected by their relatively high levels of Al and H+ in the exchange complex (Table 2).

About 70% of the surface horizons with pH-NaF >=9.7 had values of Alp/Alo >=0.7, indicating that Al is largely complexed with humus. Shoji et al. (1982) suggested a predominance of organically complexed Al over Al minerals of low crystallinity in soils with pH-H2O <4.9, as is the case of most of the soils studied here. Ratios of Alp/Alo <= 0.6 were detected in only some surface horizons of soils developed from biotitic schists, and they had a pH-H2O >=4.9 (data not shown). The results obtained suggest an important contribution of the Al–humus complexes in SO2-4 sorption, in agreement with previous studies on PO3-4 (Bloom, 1981; Shoji and Fujiwara, 1984). However, further research on SO2-4 retention and the soils of our study is required to determine which mechanisms are taking place.

Sulfate Sorption and Soil pH
Sulfate sorption determinations were all made using solutions initially acidified to pH 3.0. Measurements of pH taken after solution equilibration (data not shown) showed similar values to those pH measurements made with a neutral salt (KCl) (Table 2), indicating that buffering reactions took place. Mechanisms of neutralization were not investigated here. However, results obtained in previous research on SO2-4 solutions with an initial pH of 3.0 suggest that in soils derived from acidic materials, acid neutralization occurred mainly through Al release from either organic or noncrystalline Al forms (Merino et al., 1994), while in soils derived from basic materials, SO2-4 sorption and Al mobilization were the main neutralization mechanisms (Merino et al., 1994; Camps and Macías, 1996). Thus, buffering reactions might have affected the reactive surfaces of soils developed from acid parent material to a greater degree than those of soils developed from basic parent material, possibly accentuating the differences regarding SO2-4 sorption responses and also the relationships as described below.

The significant positive relationship observed between SO2-4 sorption potential and either pH-KCl or pH-H2O (at P < 0.01) (Table 5) is, at least in part, due to the positive relationship existing between pH and different extractable Al and Fe forms (Alo, Alp, Aln, Fed, and Fec, at either P < 0.01 or P < 0.05), and it is ultimately related to the parent material. As has been described above, a trend toward an increase in SO2-4 sorption is observed from soils derived from acid materials to soils developed from basic parent materials (Table 3), and similar results have previously been described in studies on SO2-4 sorption in soils of different lithologies (Barton et al., 1994; Merino et al., 1994; Merino and García-Rodeja, 1996). When grouping data for each parent material (Table 6) , soil pH was not significantly correlated with SO2-4 sorption in soils developed from schistic rocks (at P < 0.05), but was positively correlated with soils developed from granitic rocks (P < 0.01) and phyllitic rocks (P < 0.05) (Table 6). Again, these results are probably masked by other factors such as the presence of organic matter; soils derived from these acidic materials had a considerably lower SO2-4 sorption in surface horizons than in subsurface horizons (Table 3), and these surface horizons also had a lower pH (Table 2).


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  		Table 6 Values of correlation coefficients obtained between potential SO2-4 sorbed from either 0.4 or 1.6 mM SO2-4 solution and pH-KCl for each parent material group (soils derived from amphibolite are not included as n = 2)

 
Another aspect that might have influenced the positive sign of the correlation coefficient obtained between pH and SO2-4 sorption (Table 5) is the range of pHs of the soils studied. A brief literature review shows that the inclusion of basic soils in correlation analyses gives rise to a negative relationship between the two variables (pH and SO2-4 sorption). The low or negligible positive surface charge present in basic soils would explain their poor SO2-4 sorption (Marsh et al., 1987). MacDonald and Hart (1990) found a significant negative relationship between SO2-4 sorption and soil pH when working with surface horizons of soils from the lower peninsula of Michigan with a wide range of soil pH-H2O (5.1–8.4). More recently, MacDonald et al. (1994) observed a significant negative relationship between SO2-4 sorption and soil pH when studying soils from a wider area in the Great Lakes region with a range of pH-CaCl2 from 3.6 to 8.0. On the other hand, Courchesne (1992) studied soils from Canada developed from glacial till, fluvial sand, and marine sand, with a pH-CaCl2 ranging from 3.2 to 5.2, and found a positive relationship between soil pH and the amount of SO2-4 sorbed. Finally, Harrison et al. (1989), working with soils from different sites in the USA, Canada, and Norway (pH range of 4.0–5.8), also observed a positive relationship.

The relationship between soil pH and SO2-4 sorption found for a range of soils cannot be compared with that obtained when a single soil is artificially acidified. In the latter case, there is an increase in sorption capacity related to the increased positive surface charge that variable-charged soils acquire as pH decreases (Chao et al., 1964; Zhang et al., 1987; Courchesne and Hendershot, 1989). On the other hand, when including soils from different sites, not only pH, but also other variables, such as organic matter or secondary minerals, will differ and may be interrelated, giving rise to different relationships, such as those described above.


   Conclusions
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
REFERENCES
 
Soils developed from base-rich parent material (amphibolite and biotitic schist) had, in general, more abundant extractable Fe and Al in contrast to soils developed from more acidic parent material (phyllite and granite), which translated into a higher SO2-4 sorption in soils developed from basic materials than in those developed from more acidic ones. Organic C apparently had two different and opposite effects on SO2-4 sorption. First, SO2-4 sorption in surface horizons, mostly developed from biotitic schist and amphibolite, with pH-NaF >= 9.7, was important mainly because of the presence of Al–humus complexes, which often gives these soils an andic character. Second, in surface horizons of soils developed from phyllite with a pH-NaF <8, a negative effect was detected which may be attributed to competition between SO2-4 and dissolved organic C for sorption sites, in addition to the low content in reactive surfaces. Finally, SO2-4 sorption potential was positively related to soil pH, which in turn was due to the positive relationship existing between content of Fe and Al oxy-hydroxides and basic parent material. To summarize, factors affecting SO2-4 retention can vary according to soil chemistries that depend to a great extent on regional differences in soil parent material characteristics. Thus, generalizations about effects of soil properties on SO2-4 sorption may differ between regions with different parent material.


   ACKNOWLEDGMENTS
 
The authors wish to express their gratitude to Dr. Eduardo García-Rodeja for his valuable suggestions; to Luis Rodríguez Lado, X.M. Lomba Martínez, and E. Luis Calvo for helping with the preparation of figures; to Teresa Martínez Iglesias for laboratory assistance; and to Dr. François Courchesne for his critical review on an early version of this paper. Finally, the authors thank the anonymous reviewers for their helpful comments.

Received for publication July 10, 1998.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 




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M. C. Arbestain, M. E. Barreal, and F. Macias
Phosphate and Sulfate Sorption in Spodosols with Albic Horizon from Northern Spain
Soil Sci. Soc. Am. J., March 1, 2002; 66(2): 464 - 473.
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