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Soil Science Society of America Journal 64:1078-1082 (2000)
© 2000 Soil Science Society of America

DIVISION S-7-FOREST & RANGE SOILS

Sulfate Pools in the Weathered Substrata of a Forested Catchment

B. Manderscheida, T. Schweissera, G. Lischeidb, C. Alewella and E. Matznera

a Dep. of Soil Ecology, Bayreuther Inst. for Terrestrial Ecosystem Research (BITÖK), Univ. of Bayreuth, D-95440 Bayreuth, Germany
b Department of Hydrogeology, Bayreuther Inst. for Terrestrial Ecosystem Research (BITÖK), Univ. of Bayreuth, D-95440 Bayreuth, Germany

egbert.matzner{at}bitoek.uni-bayreuth.de


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Methods
 Results
 Discussion
 REFERENCES
 
The mitigating effect of decreasing anthropogenic SO4 deposition on acidified soils and waters can be delayed by the release of previously stored soil SO4. We investigated SO4 pools and desorption in the weathered substrata (0.5–10 m depth) of a forested catchment on granite to quantify the importance of these layers to SO4 dynamics. Solid-phase materials from 10 boreholes to a maximum depth of 10 m were analyzed for water- and phosphate-extractable SO4, SO4 desorption, cation-exchange capacity (CEC), pH, and dithionite- and oxalate-extractable Fe (Fed and Feo) and Al (Ald and Alo). Seven of the investigated boreholes were used to monitor water table depth and to obtain samples for measurement of solution SO4 concentrations. The storage of phosphate-extractable SO4 in the weathered substrata was estimated at 90 kmol ha-1, of which {approx}50 kmol ha-1 were water soluble. Sulfate pools and their desorption behavior were highly variable, which could partly be explained by the variation of pH and extractable Fe and Al contents of the samples. Sulfate concentrations in groundwater were dependent on the depth of groundwater table and corresponded with the depth gradients of solid-phase SO4. The SO4 pools of the substrata were apparently regulating solution concentrations. Thus, groundwater acidification in such aquifers will not be easily reversed by decreasing SO4 deposition because of the release of previously stored SO4.

Abbreviations: Ald, diothionite-extractable Al • Alo, oxalate-extractable Al • CEC, cation-exchange capacity • Fed, dithionite-extractable Fe • Feo, oxalate-extractable Fe • IC, ion chromatography


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Methods
 Results
 Discussion
 REFERENCES
 
ANTHROPOGENIC SO2 emssions and the deposition of SO4 to forest soils have accelerated soil acidification and caused the acidification of surface and groundwater (Reuss and Johnson, 1986; Manderscheid et al., 1995; Alewell, 1998). In Germany, the acidification of groundwater is not restricted to shallow soils, but has been reported also for deeper aquifers in forested watersheds (Malessa, 1995; Landesamt für Wasserwirtschaft, 1997). The development of groundwater quality in these areas is of special importance, primarily because high Al concentrations and low pH threaten groundwater quality, and many of these watersheds are used for drinking water supplies. The major anion in acidified groundwaters is SO4.

Acid forest soils subjected to high deposition rates in the past have accumulated large amounts of SO4 (Erkenberg et al., 1996; Alewell, 1998). Sulfur deposition in central Europe and in northeastern America is currently much lower than during 1970 to 1990 (Driscoll et al., 1989; Manderscheid et al., 1995). Most of the formerly stored soil SO4 is considered to be reversibly bound (Alewell et al., 1997). The release of SO4 will counteract the mitigating effects of decreasing deposition rates on acidified soils (Mitchell et al., 1992; Alewell et al., 1995; Rustad et al., 1995; Giesler et al., 1996; Forsius et al., 1998; Kopacek et al., 1998). The response of groundwater and runoff quality to decreasing deposition largely depends on the soil SO4 pools, which are influenced by soil conditions, soil depth, and deposition history. Soil solution and runoff chemistry in shallow soils with low SO4 pools has been shown to respond rapidly to decreasing S deposition (Wright et al., 1988; Wright and Hauhs, 1991; Beier et al., 1995). Deeper soils with high SO4 contents are well buffered with respect to soil solution acidity and SO4 concentrations (Giesler et al., 1996; Alewell et al., 1997). In addition to the soil, weathered substrata may contain significant amounts of SO4. The term weathered substrata is defined here as being located between the mineral soil B horizon (in this study 0.50 m) and the unweathered bedrock. The chemistry of weathered substrata in forested catchments has been investigated in a few studies on different bedrock types (Böttcher et al., 1985; Meiwes et al., 1994; Malessa, 1995; Matzner and Davis, 1996). Acidification of the substrata to several meters in depth, as indicated by low pH and low base saturation of the CEC, was found in many cases. Sulfate contents of the weathered substrata were only investigated in a few studies and were often found to be at similar concentrations to those of upper soil horizons (Meiwes et al., 1994). Therefore, the SO4 storage in the weathered substrata might thus be high. In addition, little is known about the mobility of SO4 in deeper layers under conditions of decreasing deposition.

The objectives of our study were to quantify the SO4 pools in the weathered substrata of a granitic catchment, to describe the SO4 desorption behavior, and to evaluate the potential effect of these pools on the prediction of long-term groundwater quality.


   Methods
TOP
 ABSTRACT
 INTRODUCTION
 Methods
Results
 Discussion
 REFERENCES
 
The Lehstenbach watershed (4.2 km2) is covered by Norway Spruce (Picea abies Karst.) and located in the Fichtelgebirge, a mountainous area in Northern Bavaria, Germany close to the border of the Czech Republic (50°09' N, 11°52' E). The granitic bedrock weathered deeply during the Tertiary age down to the 30- to 40-m depth (Stettner, 1964). The weathered substrata are characterized by a mixture of sandy to loamy textures and coarse materials including rocks and boulders of different size and weathering degree.

The soils of the watershed are acidic and classified as dystric Cambisols and Podzols (FAO-classification, Inceptisols and Spodosols in U.S. soil taxonomy). The average precipitation in the area is {approx}1000 mm and the annual average temperature is 6°C. The Fichtelgebirge has a history of high S and N deposition. Sulfate deposition has significantly decreased in the last decade; however, the average deposition to the watershed from 1993 to 1996 (estimated from throughfall fluxes) was still 1 kmol SO4 ha-1 yr-1 and 1.5 kmol total N ha-1 yr-1. Recent results on the biogeochemistry of the watershed are summarized by Matzner et al. (2000). In 1996, 10 boreholes of 160-mm diam. were established down to a maximum depth of 10 m within the watershed. The boreholes were distributed throughout the upland soils of the watershed.

Seven of the 10 boreholes were used as groundwater wells following drilling. Groundwater sampling was conducted on 15 dates in 1997 and 1998, with the depth of the groundwater table recorded simultaneously. The samples were analyzed for SO4 by ion chromatography (IC) The solid-phase material of the 10 boreholes was classified by depth, texture, color, and degree of physical weathering. Samples were taken for further chemical and physical analysis such that the substrates of each borehole were represented. In total, 44 samples of the weathered substrata were analyzed after 5-mm sieving at field moisture content. The 5-mm wet sieving was applied to avoid drying effects on SO4 desorption (Comfort et al., 1991) and to include larger particles ranging from 2 to 5 mm in size. The coarse fraction of soils (>2 mm) has been shown to be chemically active and almost comparable with the <2-mm fraction (Ugolini et al., 1996). The CEC of the solid phase was determined by extracting 5 g of the material with 100 mL of 0.05 M NH4Cl according to Trüby and Aldinger (1989) and subsequent measurement of the cations Na, K, Ca, Mg, Mn, Al, and Fe by plasma emission spectroscopy (ICP-AES). The exchange capacity was calculated by summation of the extracted cations. Base saturation was calculated as (Na + K + Ca + Mg)/CEC. The pH was measured by a glass electrode in 0.01 M CaCl2 (pH CaCl2) and in distilled water (pH H2O) at a soil to solution ratio of 1:2.5. In addition, parameters potentially influencing SO4 sorption were evaluated using dithionite (Ald, Fed) and oxalate (Alo, Feo) extraction according to Schwertmann (1964), with subsequent measurement of Fe and Al by ICP-AES. Carbon content was determined as CO2 after combustion using a CHN-analyzer.

Water-soluble SO4 was extracted by five sequential batch extractions with distilled water using a soil/solution ratio of 1:5. Total SO4 was extracted by 0.02 M Na2HPO4 in two extractions with the same soil/solution ratio. Sulfate in the extracts was determined by IC. Desorption isotherms were established by extracting 20 g of soil with water in 10 steps. Equilibration time in each step was 18 h, and the soil/solution ratio changed from 2:1 at the first extractions to 1:10 at the last to fully extract the SO4 pool. The amount of desorbed SO4 was calculated from the SO4 in solution at each step and the data were adjusted to the Langmuir desorption isotherm:

(1)
where x is desorbed SO4 (µmol g-1), c is SO4 concentration in the equilibrium soil solution (mmol L-1), b is sorption maximum (µmol g-1), and 1/k = half maximum saturation point (mmol L-1), which is equal to the SO4 concentration at 1/2b.

During the extraction, only the desorbed SO4 amounts were measured and the absolute amount of sorbed SO4 (x) was not known. Therefore x was defined as:

(2)
where m is the amount of sorbed SO4 before desorption started (initial SO4 content) and x' is the desorbed SO4 per extraction step (measured).

The isotherm fitting was done with the Levenberg-Marquardt method (Press et al., 1986). The average isotherm and its standard deviation were computed for concentration steps of 0.03 mmol L-1 from 0 to 1 mmol L-1. The sorbed amounts were normally distributed at each point. This was tested with the W-statistic test (Shapiro and Wilk, 1965; SAS Institute, 1990).

Bulk densities of the samples were determined by coating subsamples with wax and subsequent volume measurement by water displacement (data not shown). The average bulk densities were 1.6 g cm-3 down to the 5-m depth and 1.8 g cm-3 from 5- to 10-m depth. Sulfate storage of the weathered substrata was calculated for each borehole separately using the measured densities and the SO4 concentrations of the <5-mm fraction. The fraction >5 mm (partially weathered soil skeleton) was smashed to pieces of {approx}5 mm. The SO4 concentration for this fraction was determined separately according to the description above. The SO4 pools were calculated as the sum of the >5 and <5 mm fractions. Sulfate storage of the mineral soil (0–0.5 m depth) was calculated using average SO4 concentrations (data by Jungnickel, 1996) and average bulk densities of the B horizons. The SO4 extraction from the B horizon samples followed the same method as for the samples from the weathered substrata.


   Results
TOP
 ABSTRACT
 INTRODUCTION
 Methods
 Results
Discussion
 REFERENCES
 
Within each borehole, the degree of physical weathering varied with depth in a nonsystematic way. Loamy layers rich in Fe oxides were followed by layers with only physically weathered sandy to gravely material, unweathered granite, and vice versa. The large spatial heterogeneity of the measured solid-phase parameters is indicated by the standard deviation and range of the solid-phase properties (Table 1) . The samples from 0.5 to 5 m had a higher degree of acidification (average base saturation of CEC: 36%; pH 5.1) as compared with samples from 5- to 10-m depth (average base saturation of CEC: 90%, pH 5.5), but there was no significant depth gradient for either of the extracted Al and Fe fractions (data not shown). Furthermore, there was no relationship between extractable SO4 to depth of the weathered substrata. The average concentration of Na2HPO4-extractable SO4 in the samples from the weathered substrata was about one-half of that found in the B horizons of the soils (Table 2) . The storage of Na2HPO4-extractable SO4 was estimated at 15 kmol ha-1 down to 0.5-m depth, which is about 15 times the actual annual deposition. The storage of SO4 bound in the weathered substrata down to 10-m depth was estimated at {approx}90 kmol ha-1 (Na2HPO4-extractable). The amount of Na2HPO4-extractable SO4 in the weathered substrata was about twice as high as the water-extractable SO4. The coefficient of variance of the SO4 contents and storages were large, reaching almost 100% for both types of extractions.


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  		Table 1 Mean solid phase properties of the weathered substrata, Lehstenbach watershed, Germany.{dagger}

 

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  		Table 2 Mean SO4 concentrations and estimated SO4 storage in soils and the weathered substrata of the Lehstenbach watershed, Germany.{dagger}

 
The SO4 desorption isotherms from the B horizon and the weathered substrata had a high degree of spatial variation (Fig. 1) . The slope of the isotherms for the B horizon and the weathered substrata, down to SO4 concentrations of {approx}0.2 mmol L-1, was found to be generally low. This indicates that the SO4 in solution is poorly buffered by sorption processes in the frequently occurring concentration range from 0.2 to 0.6 mmol L-1. The average parameters of the Langmuir SO4 desorption isotherms of the weathered substrata tend to have a higher sorption maximum (b), whereas the slope of the isotherm (indicated by k) was less, as compared with the mineral soil (Table 3) . The high spatial variation of the parameters of the isotherm can largely be explained by soil parameters influencing SO4 sorption in soils. The correlation of the isotherm parameter b with Alo and Ald was statistically significant. For the k parameter no significant correlation was found (Table 4) . The parameters b and k in Eq. [1] are related in a nonlinear manner and can, strictly speaking, not be treated as linear, additive influence factors. Therefore we introduce an integrating parameter {int}5–20, which considers the parameter b and k simultaneously. Parameter {int}5–20 is the amount of SO4 released at concentrations in soil solution between 0.16 and 0.62 mmol L-1, equivalent to 5 to 20 mg L-1. This parameter is best correlated with Ald. A similar strong correlation with Ald can be found for measured H2O- and NaH2PO4-extractable SO4 (Table 4).



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  		Fig. 1 Average and standard deviation of the SO4 desorption isotherms of (a) the B horizon and (b) the weathered substrata, Lehstenbach watershed, Germany. for the B horizon; for the weathered substrata. Desorption isotherms adjusted to the Langmuir model (Eq. [1])

 

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  		Table 3 Mean parameters of the Langmuir SO4 desorption isotherms for B horizons and weathered substrata

 

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  		Table 4 Pearson correlation coefficients of SO4 contents and isotherm parameters to pH and dithionite- and oxalate-extractable Fe and Al: weathered substrata.{dagger}

 
The groundwater chemistry of the different wells can be stratified into two groups: those with SO4 concentrations decreasing with depth of groundwater table (GW12, GW13, GW 15, GW16, GW17, GW20) and one with SO4 concentrations being independent of groundwater table (GW11) (Fig. 2) . The range of variation of the SO4 concentrations with depth in single wells was up to 0.2 mmol L-1. For those wells of the first group, the SO4 content at the solid phase at groundwater table depth was calculated using the corresponding SO4 desorption isotherm and the groundwater SO4 concentrations. There was a good agreement between measured and calculated SO4 concentrations ( , Fig. 3) . Thus, the groundwater SO4 seems to be in equilibrium with the solid-phase SO4 at groundwater table, and the observed changes of groundwater SO4 with depth might be attributed to spatial patterns of solid-phase SO4 pools.



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  		Fig. 2 Sulfate concentration vs. depth of groundwater table in seven boreholes sampled during 1997 and 1998, Lehstenbach watershed, Germany. (GW = number of groundwater well)

 


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  		Fig. 3 Calculated vs. measured SO4 concentrations of the solid phase in groundwater wells at the Lehstenbach watershed, Germany. The extractable SO4 of the solid phase at a specific depth of the groundwater table was calculated using the SO4 desorption isotherms of that depth and the corresponding groundwater SO4 concentration

 

   Discussion
TOP
 ABSTRACT
 INTRODUCTION
 Methods
 Results
 Discussion
REFERENCES
 
Acidification of deeper soil layers and weathered substrata is a widespread phenomenon in Central Europe. Although acidification is caused both by acidic deposition and by soil internal processes (Matzner and Davis, 1996), the accumulation of extractable SO4 in most cases results from the sorption of atmospherically deposited SO4. The S concentrations of many sedimentary rocks and of granite are low (Mückenhausen, 1993). Meiwes et al. (1994) reported SO4 concentrations in a sandstone profile of the German Solling area (depth 4–12 m) of 0.5 to 2.5 µmol g-1 soil. Sulfate concentrations increased with decreasing pH of the substrate. The calculated SO4 storage values of the sandstone profiles were 24 to 130 kmol ha-1. In a study on schist and gneiss in the German Harz area, Malessa (1995) found maximum SO4 concentrations in layers of 1- to 2-m depth reaching up to 5 to 8 µmol g-1. He calculated SO4 pools of up to 60 kmol ha-1 for a 2-m-deep profile. Both the concentrations and the storage of SO4 in deeper soil layers and the weathered substrata reported in the literature correspond with our data and emphasize the potential effect of this compartment on the turnover of SO4 in these watersheds.

The overall storage of SO4 in the weathered substrata can be explained by the adsorption of SO4 deposited across several decades. For example, in the Solling area, SO4 inputs of up to 3.5 kmol ha-1 yr-1 for the 1970s have been reported (Manderscheid et al., 1995), and it can be assumed that the Lehstenbach watershed received similar deposition rates in the past.

Several studies have investigated the soil properties affecting the sorption of SO4. In agreement with earlier studies (Johnson and Todd, 1983; Nodvin et al., 1986; Curtin and Syers, 1990; MacDonald and Hart, 1990; Stanko-Golden et al., 1994), we found that extractable Al and Fe and the pH explain the large spatial heterogeneity of SO4 content and desorption behavior in samples from weathered substrata. The reason for the variability of the extractable Al and Fe content is obviously the different degree of weathering found in the substrata. In the single boreholes, layers with highly weathered loamy material of high Al and Fe content varied in a nonsystematic way, with layers of coarse material and layers of only physically weathered rocks with low extractable Al and Fe. This heterogeneity of weathering might be caused by the heterogeneity of the bedrock itself, by the heterogeneity of weathering conditions and transport processes during the tertiary age, as well as by periglacial solifluction in the pleistocene. It was not the intention of this study to further investigate the reasons for the observed spatial heterogeneity, and we cannot conclude which of these factors is the major reason for the variation.

Water-extractable SO4 is considered to be readily released if SO4 deposition decreases, and {approx}60% of the total extractable SO4 in our study was found in this form. The adsorbed SO4 pools in the weathered substrata obviously influence the groundwater chemistry, as indicated by the comparison of the depth dependence of groundwater SO4 concentrations and solid-phase SO4 contents (Fig. 3). These findings suggest that the extractable SO4 in the weathered substrata is highly mobile and could have a significant effect on the biogeochemistry of the watershed. The slope of the SO4 desorption isotherms was generally low down to SO4 concentrations of 0.2 mmol L-1. Thus, decreasing input concentrations due to decreasing deposition rates might easily be reflected by changing solution concentrations. However, in the case of deeper groundwater, the large pools of SO4 bound in the weathered substrata of the deep aquifer will have a strong buffering effect despite the flat isotherms. Model simulations with MAGIC (Cosby et al., 1984) predicted the buffering of SO4 fluxes with seepage in the 5-m depth for more than 75 yr even if a drastic decrease of SO4 depositions to a preindustrial level from 1998 onwards was assumed (data not presented).

In summary, our findings suggest that the SO4 pools of the weathered substrata were substantially higher compared with the mineral soil. It might take several decades before changes in groundwater SO4 can be expected as a result of decreasing deposition. Groundwater acidification in such aquifers will not easily be reversed by decreasing SO4 deposition.


   ACKNOWLEDGMENTS
 
This project was financially supported by the German Ministry of Education, Science, Research and Technology, grant no. PT BEO 51-0339476B. The authors thank the staff of the Central Analytic Department of BITÖK for analyzing the water and soil samples and Uwe Hell and Andreas Kolb for field sampling.

Received for publication May 5, 1999.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Methods
 Results
 Discussion
 REFERENCES
 




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