Relationship between total dissolved organic carbon and SO4−2 in soil and waters

The Science of the Total Environment, 117/118 (1992) 449-461 Elsevier Science Publishers B.V., Amsterdam

449

Relationship between total dissolved organic carbon and 8 0 4 -2 in soil and waters George

R.

G o b r a n a n d Stephen Clegg

The Swedish University of Agricultural Science, Department of Ecology and Environmental Research, Box 70 72 EMC. 750 07 Uppsala, Sweden

ABSTRACT Soluble organic matter in forest soils appears to play a decisive role in the mobility of SO4-2. In this laboratory study, a forest Haplorthod soil was leached to investigate the relationship between total dissolved organic carbon (TOC) and SO4-2. The leaching solutions were distilled water (DW), sulfuric acid (S), sulphuric acid and nitric acid SN, (SN1, 1:1 M and SN2, 1:2 M). The pH of the acid solutions was 3.7. Pour chemical amendments were used. control (C), lime (L), calcium phosphate (P) and lime and calcium phoshpate (LP). After leaching, the soil columns (48 columns) were sampled and analyzed. Evaluation of the leachate data consistently showed that the TOC was negatively or positively correlated with the mobility of SO4-2 when the EC was > 100 or < 100 #S/cm, respectively. However, results of the extracted soil solution showed that at any given depth the TOC was positively correlated with SO4-2 (EC < 100 #S/cm). The positive relationship between TOC and SO4-2 can be attributed to competition between TOC and SO4-2 on the exchangeable positive charge sites. In contrast, the negative relationship between the TOC and SO4-2 was due to increasing salt content that caused the TOC to precipitate. Sulfate, then, becomes the dominant soluble anion and is present at a higher concentration than TOC. A negative correlation between TOC and SO4-2 in waters is often found in data from natural stream and lake waters. It is apparent that the salt effect and competition mechanism between TOC and SO4-2 provide an explanation for the increasing TOC levels in natural water, especially in areas known to be affected by with high sulfate deposition.

Key words." soil leaching; salt effect; competition mechanism; organic carbon; forest leaching; SO~2

INTRODUCTION A c i d i f i c a t i o n o f forest soils a n d waters c a u s e d by acid d e p o s i t i o n has led to m a n y investigations o f the different p a r a m e t e r s involved in the process. In Sweden, l o n g - t e r m field investigations o n soils d e m o n s t r a t e a p H decline (Hallbficken a n d T a m m , 1986; F a l k e g r e n - G r e r u p , 1987; J a c k s et al., 1988). T h e m o s t plausible e x p l a n a t i o n for this acidification a n d the c o n s e q u e n t

450

G.R. G O B R A N A N D S. C L E G G

losses of basic cations is an increased deposition of acids and acidifying S and N compounds (Andersson, 1989). Anions (inorganic and organic) associated with acidic inputs must be mobile in the soil if leaching is to occur. A restricted mobility of anions can effectively prevent base cation depletion. Since sulfate is one of the major anions in acidic deposition, its mobility in acid soils has received much attention (Johnson and Cole, 1980). In earlier work we hypothesized that the mobility of SO4 -2 in acid forest soils may be influenced by the presence of the total dissolved organic matter (TOC) released from the forest floor (Gobran and Nilsson, 1988). Since conifers produce more organic anions than do hardwoods, SO4 -2 concentration in percolating waters is greater than that from hardwood sites (Mollitor and Raynal, 1982). Sulfate and organic anions have similar impacts on the leaching of cations (Hultberg, 1985; Cronan and Aiken, 1985). Hence, soluble salt electrolytes in mineral soils may influence the soil solution pH (Richter et al., 1988), TOC (Evans et al., 1988) and even the type of organic acids in TOC (David et al., 1989). The contribution of organic matter to the acidity of humic water has been demonstrated by several investigators (Gorham et al., 1986; Brakke et al., 1987; Kortelainen et al., 1989). The role of naturally occurring organic acids in the acidification of surface waters has been widely debated in the literature (Krug and Frink, 1983; Reuss and Johnson, 1986; Driscoll et al.; 1989). The objective of this paper was to check the relationship between TOC and SO4 -2 in soil and water and how it is influenced by the salt content in a forest soil treated with different chemical amendments and leached with water and different acids. MATERIALS AND METHODS

Site description and soil sampling Soil samples were collected from Hassl6v in the coastal part of S.W. Sweden. The area contains Norway spruce growing on glacial till soil that is greater than 50 cm deep. The soil is a Leptic Podzol or Halorthod according to the FAO or U.S. Soil Taxonomy classification, respectively. The soil is shallow, has a silt loam texture and a depth of humus 3-5 cm. Selected soil chemical and physical characteristics are listed in Table 1. Humus samples were randomly taken after clearing sampling points of vegetation and litter. The humus (0 horizon) depth was 5 cm. Mineral soil samples were taken to a depth of 25 cm. The A horizon was from 5 to 10 cm and the B horizon was from 10 to 25 cm. The samples were separated into horizons and composited. Humus was sieved through a 3-mm mesh sieve and mineral soil through 2-mm mesh sieve.

451

T O T A L DISSOLVED O R G A N I C C A R B O N A N D SO4-21N SOIL A N D W A T E R S

t"q

o.~.

t'q

,q t.,i x~ ~1 oo

~ .,_.: -

ten

- ~ 5

~.o.c

¢:I

E

.o

452

G.R. GOBRAN AND S. CLEGG

Soil columns and treatments Soil columns were built by packing a specified weight of soil into Plexiglas rings (4.6 cm diameter and 5.0 cm long), equivalent to the bulk density of the soil in the field. Both ends of the column had locks with porous plastic filters which blocked the entry or exit of particles larger than 15 #m. Four chemical amendments (CA) were mixed with the top 5 mm of the humus used; control (C, no amendments), lime (CaCO3) at 5 t/ha, phosphate Ca(H2PO4)2.H20 at 322 kg/ha and a combination of lime and phosphate (at the same rates used when applied individually). Four leaching solutions (LS) were also used; distilled water (DW), 0.1 mM sulfuric acid (S), 0.067 mM sulfuric acid plus 0.067 mM nitric acid (SN) and 0.05 m M S plus 0.10 mM N (SN2). The pH and EC of the acid solutions were 3.7 and 140/~S/cm, respectively. The soil columns were leached in an inverted state from the bottom (FF) in an upward direction (A and B horizons) to ensure constant water contents in the soil columns. The columns were continuously leached at a constant rate of 1.2 crn/d. The total volume of leachates collected during the experiment was about 5-6 1, which was equivalent to about 2 years rainfall. Three replicate columns were used for each treatment. Leachate samples were taken at shorter time intervals at the beginning than towards the end of the experiment.

Analysis of leachates and soil samples The variables under this investigation were the pH, EC, TOC and 804 -2 in the leachate samples and water extract, and AI and Fe in the oxalate extract (Parfitt, 1989). Immediately after leachate collection the leachate volume, pH and EC were measured. The solution samples were then frozen until needed for further analysis. After leaching, the mineral soil was removed from the columns and split into four 5-cm sections and the humus into two 2.5-cm sections. The soil sections were kept wet at 4°C prior to extractions. The extraction consisted of adding 5 g of soil to 50 ml of distilled water and shaken for 2 h. In the water extract, the EC, pH, total organic carbon (TOC) and SO4 -2 were determined. Sulfate was measured by a Dionex 2000i HPLC using ion exchange columns. The TOC was analyzed on a Shimadzu total organic carbon analyser (TOC-500). Aluminum and Fe were extracted from 1 g soil samples containing 100 ml of 0.15 M oxalic acid, shaken for 4 h and filtered (Parfitt, 1989). Aluminum and Fe contents were determined with a Jobin Yvon 70 Plus ICP spectrophotometer.

453

TOTAL DISSOLVED ORGANIC CARBON AND SO4 21N SOIL AND WATERS

RESULTS

Soil chemical analysis Statistical analysis using the Pearson's correlation indicated that, regardless of the type of the treatments, the TOC was significantly correlated with SO4 -2 (R = 0.75). Factorial analysis also showed that TOC and SO4 -2 had a strongly underlying common factor. Multiple regression analysis (GLM) indicated that the TOC and SO4 -2 were significantly influenced by only the solution treatments (LS). The impact was significantly enhanced with soil depth. In order to illustrate the relationship between SO4 -2 and TOC in the leached soil, the data of all the soil columns were used. The data included the concentrations from the leached columns with water (DW-soil) and those leached with sulfuric acid (S-soil), including those containing all chemical amendments (CA). Figure 1 shows a straight line and significant correlation between water extract SO4 -2 ( D W S O 4 ) and TOC (DWTOC) in all soils. The distribution of SO4 -2, TOC, oxalate - A1 + Fe and pH with depth is depicted in Fig. 2, A, B, C and D, respectively.

[]

y = 7,0815e-2 + 4,

DWSO

• oO

ra

/

E]IU

0,5044

[]

a~r.~

y = 0,11005 + 6,6299e-3x R^2 = 0,293

20

40

60

80

TOC (mg/L) Fig. 1. The relationship between distilled water extractable (DWTOC) in leached soils for all chemical amendments.

S O 4 -2

(DWSO 4) and TOC

454

G.R, G O B R A N A N D S. C L E G G

A 1.0

70

0.8

~ 60

~0.6

E

E

/,

~50 +

0t ~ 0.4

O U. x.

0.2

"'""'"-~e

0.0 5

..........

x ..........

i

i

i

10

15

20

,/

4O

~<

30: 25

i

5

10

Depth (om)

i

i

15

20

25

DepZh (cm)

20

5.8

15

5.6

El0 O O

~s.4

I--

x.

"-.

.

5.2

,-"

l'

,/

0 5

1

i

l

10

15

20

D~

(cm)

5.0 25

i

5

10

15

L

20

25

De~h(cm)

Fig. 2. The distribution of SO4 -2 (A), oxalic acid extractable Fe + AI (B), TOC (C) and pH (D) soil depth. Solid lines represent distilled water leached columns. Broken lines represent sulfuric acid leached columns.

Leachate chemical analysis The GLM analysis indicated that neither the leaching solutions (LS = DW, S, SN and SN2) nor the chemical amendments (CA = C, L, P and LP) had any significant impact on the chemistry of leachate samples. The leaching volume, expressed as PV, significantly influenced the variable under investigation. The determination coefficient (R 2) was equal to 0.507, -0.728, -0.302 and -0.079 (P < 0.001) for the EC, pH, SO4 -2 and TOC, respectively. Two distinct phases of chemical composition of the leachates were noticeable (Fig. 3). Phase 1 lasted until the passage of 8-9 pore volumes (PV)

TOTAL DISSOLVED ORGANIC

C A R B O N A N D SO4-21N S O I L A N D W A T E R S

455

A 300 o

o ~,

~,200 <>

uJ 1 0 0

x

o

xo

/

o

D

-

D

0

i

0

10 Pore

o

o

r

i

.~

20

30

40

Volume

B 300

~

o

^ ~'

~

=

200

,~

x

D

~C~ 1 0 0

×

o

>¢~o

x× o

i

0

10

i

j

20

30

40

Pore Volume

30

~

20

¢~ 1 0

o

~,.~ , a ~ . . ~ , ~ 0

10

20 Pore

~ 30

40

Volume

Fig. 3. Electrical conductivity, EC (A), TOC (B) and SO4 -2 (C) in leachates sampled at differing pore volumes.

456

G.R. GOBRAN

A

A N D S.

B

300

300 A

o,ko

~ 200 E

o O ~- 100

CLEGG

o

&O¢< X ~

o

A

~-~200

ro o ~ 6 t, x~ox

o

.

o

E

^

o

~° o

o

-xX o o~ ~

o

o

x

o

o 100

o

x

×

×o

o

~

x

~

o &

100

150

200

25

50

EC (uS/L)

75

100

EC (uS/L)

D 30

2.0

'o

&

AI

Xx

L9

1.5

o

O D

o ox

~x

o

(

E 1.0

d"

co 10 °°

0

o

o&

o

Lx

oo

100

a

0.0

200

TOC (mg/l_)

o

A°

x

'

0

oxx &

0.5

x a ~

o

x

300

0

io

i

100

200

300

TOC (mg/l_)

Fig. 4. The effect of EC on TOC (A and B) and the relationship between TOC and SO4 -2 (C and D) in the leachates.

and phase 2 constituted the remaining leachate collection (PV 8 - 9 to 39). The first phase showed that as the leaching proceeded SO4 -2 and EC dropped from 27 to 4 mg/1 and from 270 to 100 #S/cm, respectively; whereas, TOC increased from 40 to 300 mg/1. The second phase showed that the concentration of SO4 -2, EC and TOC decreased to minimum values of 2 mg/l, 30 ~tS/cm and 40-50 mg/1, respectively. Discrepancies in the EC values existed just between the two phases, but they were few compared to the total number of observations. It is worth noting that the EC of the acid leaching solutions was 140 #S/cm but the EC of the first PV was 270 /~S/cm. This means that the soil contained considerable amounts of salt prior to leaching, very possibly since the soil was from a coastal area.

TOTAL DISSOLVED ORGANIC CARBON AND SO4-21N SOIL AND WATERS

457

DISCUSSION AND CONCLUSIONS Soil extraction

A positive relationship between TOC and SO4 -2 is illustrated in Fig. 1. These results support the hypothesis that organic ligands compete with SO4 -2 on the exchangeable positive charge sites (Gobran and Nilsson, 1988). A recent study by Inskeep (1989) on the adsorption of sulfate by kaolinite and amorphous iron oxides, in the presence of organic ligands, provides evidence that organic ligands will inhibit SO4 -2 adsorption by solid phases common in mineral soils. The possible role of organic matter and soluble organic carbon in the desorption of SO4 -2 by soils has been suggested by other researchers (Johnson and Todd, 1983; Fuller et al., 1985; Evans, 1986). Figure 1 also shows that S-soil released less SO4 -2 than DW-soil, indicating that the retention capacity of the soil increased due to the application of mineral acid solutions. This can also be seen in Fig. 3b which showed that SO4 -2 concentration decreased from 8.9 mg/l in the S leaching solutions to a maximum value of 4 mg/l in the leachate at the end of the leaching. This can clearly be seen when the water extracted SO4 -2 and TOC (Fig. 2, A and B, respectively) were plotted against depth for the DW-soil and S-soil treatments which had not received chemical amendments. The retention trend of SO4 -2 and TOC with depth (Fig. 2, A and B, respectively) significantly correlated with the AI and Fe concentrations extracted with oxalate (Fig. 2D), suggesting that at the end of the experiment the retention was mostly adsorption on the hydrous oxide A1 and Fe minerals. Due to the increased adsorption of both SO4 -2 and TOC, the soil pH should have increased (Chao et al., 1965). Figure 2D illustrates the expected trend; pH differences between treatments were, however, insignificant. It is also worth noting that the concentration of TOC in the A horizon of the original soil (before leaching) was 16.7 mg/1 and remained in the same order of magnitude after leaching in the DW-soil treatment, whereas it was drastically lowered after leaching in the S-soil (Fig. 2B). This should be due to the impact of the acidity, which hindered the transport of TOC in mineral soil horizons. The acidity added to the S-soil provided both H + and SO4 -2, resulting in increased SO4 -2 adsorption (Fig. 2A). Since the degree of dissociation of heavy organo-metals is proportional to the quantity of TOC that passes through the soil (Pohlman and McColl, 1988), organic acids (OA) would be less ionized and consequently less able to displace the strongly adsorbed SO4 -2 (Fig. 2A). The opposite happened in the DW-soil: more highly ionized OA released from the FF competed with SO4 -2 for the exchangeable ad-

458

G.R. GOBRAN AND S. CLEGG

sorption sites, resulting in higher concentrations of both 504 -2 and TOC in the water extract than in the water extract from the S-soil. This is possible since the adsorbate affinity of OA is higher than SO4 -2 for soil in the B horizon of a Spodosol (Nodvin et al., 1986). According to the foregoing considerations, it appears that the acid solution, S, of pH 3.7 did not acidify the water extract and generally decreased TOC leaching, a fact also hypothesized by Krug and Frink (1983). Driscoll et al. (1989) have also presented data that support this hypothesis demonstrating that inputs of mineral acids shift acidification of surface waters from organic acids to acidification by mineral acids.

Leachate analysis The insignificant effect of LS, CA and their combination on the chemistry of leachate samples could be due to the high buffer capacity contained in the entire soil column. The dominant buffering mechanisms may be AI dissolution and cation exchange at all measured pH values (Ulrich, 1983). Salt concentration in the soil solution appeared to be a more important parameter than anticipated in affecting the transport and interaction between TOC and inorganic acids. It is known that OA, like other organic colloids, can be precipitated from and dispersed in solutions of high and low electrolyte concentrations, respectively (van Olphen, 1977). Because of the high molecular weights, humic acids are more easily coagulated than fulvic acids (Stevenson, 1982). Moreover, soil solution pH of mineral soil could be depressed up to 1 pHunit by increasing salt concentration by as little as 0.3 mMc/1 (Richter et al., 1988). Accordingly, the quantity of water soluble electrolytes should play an important role in the leaching of TOC. Such impact was noticed during the first phase of leaching (Fig. 3A and Fig. 4A), a decreasing trend of EC coincided with an increased leaching trend of TOC (phase 1). This indicated that the TOC precipitated under high salt concentrations prior to leaching was resolubilized and leached out as the salt concentration decreased. In phase 2, when the soil solution was diluted by relatively low salt input (EC = 140 t~S/m), the TOC and EC of the leachate decreased. A positive trend between TOC and EC was observed when EC decreased to < 100/~S/m (Fig. 4B). This could be due to the prior precipitation of TOC by salt and to the dispersion of TOC under dilute conditions. 'Salt effect' seemed indeed to be an important driving force for the leaching of TOC. The relationship between TOC and SO4 -2 was also checked by using the leachate data (Fig. 4, C and D). In phase 1, a significant negative relationship (R 2 = 0.27) was obtained (Fig. 4C). In phase 2 (Fig. 4d) the relationship between TOC and SO4 -2 was reversed and became significantly positive

TOTAL DISSOLVED ORGANIC CARBON AND SO4-21N SOIL AND WATERS

459

(R 2 = 0.14, P < 0.0002). These results are in agreement with the results for the water extract (Fig. 1). The results of the first phase support our postulate that TOC was precipitated due to the salt effect. When the EC was between 270 and 100 t~S/cm (phase 1), TOC began to be leached due to dilution, SO4 2 was decreasing due to dilution and retention, resulting in a negative relationship between TOC and SO4 -2. As the soil solution electrolyte decreased from EC equal to 100 to 30 ~S/cm, the relationship became significantly positive (phase 2), supporting the results obtained from the soil water extract (Fig. 1). In such low EC soil solution, our hypothesis of a positive linear relationship between TOC and SO4 -2 (Gobran and Nilsson, 1988) seems to be supported. During the summer, field soils accumulate salt due to high evapotranspiration, biological activity and less frequent leaching, etc. If high leaching occurs (removal of the original salt) due to rainfall, a significant amount of TOC could be removed by leaching. The leaching of SO4 -2 is normally low during the summer and reaches a maximum during the fall for the same reason as stated above (Seip et al., 1985). This condition would produce a negative relationship between TOC and SO4 -2 as in phase 1. Our dynamic and continuous leaching during phase 1 probably simulates the natural conditions during summer time with the beginning of substantial rainfalls. Such a negative correlation between T O C - S O 4 -2 in waters is often found in data from natural stream and lake waters (Kerekes et al., 1986; Driscoll et al., 1989) Continuous and prolonged leaching during the fall (diluted conditions similar to phase 2) would eventually cause a depletion of TOC. The soil solution concentration of TOC and SO4 -2 will be determined by the competition mechanism. Since sulfate inputs are continuous, SO4 -2 leaching will increase relative to TOC. Leaching both TOC and SO4 -2 would cause an extensive depletion of base cations and accentuate soil and water acidification, a typical condition observed during the fall (Seip et al., 1985). Following this line of reasoning, soil solution electrolyte concentrations may have a more direct influence on the precipitation/release of organic substances than soil solution pH. It must be emphasized that the mechanisms proposed are consistent with the data, but do not necessarily give all possible explanations. Further research on sorting out some of those mechanisms is being conducted. Future research might point to humic substances as the most relevant of the components of TOC for the processes observed. It is worthwhile mentioning that Reuss (1983) indicated that acidic deposition of sulfate has increased electrolytes in soil solutions by 0.1-0.5 mMc/1 over large areas of eastern North America and Europe. Therefore, acidic deposition might be regarded as both acid and salt inputs to forest soils. These facts

460

G.R. G O B R A N A N D S. C L E G G

will have great ecological i m p o r t a n c e , because it will p r o v i d e m e a n s for assessing m o r e a c c u r a t e l y the i m p a c t o f acidic d e p o s i t i o n o n soil a n d water chemistry. ACKNOWLEDGMENTS This w o r k is p a r t o f a p r o j e c t s u p p o r t e d by the N a t i o n a l E n v i r o n m e n t a l P r o t e c t i o n B o a r d (SNV), Sweden. W e gratefully a c k n o w l e d g e P r o f e s s o r L.B. F e n n a n d P r o f e s s o r G. A g r e n for their c o n s t r u c t i v e criticism o f a draft o f this m a n u s c r i p t ; P r o f e s s o r F. A n d e r s s o n for c o n s t a n t help a n d e n c o u r a g e m e n t . T h a n k s to P r o f e s s o r E. G e o r g e for help with the statistical evaluation; T. L o h a m m a r a n d A. W i r e n for help in using SAS p r o g r a m s . REFERENCES Andersson, F., 1989. Air pollution impact on Swedish forests - - Present evidence and future development. Environ. Monit. Assess., 12: 29-38. Brakke, D.F., A. Henriksen and S.A. Norton, 1987. The relative importance of acidity sources for humic lakes in Norway. Nature, 329: 432-434. Chao, T.T., M.E. Harvard and S.C. Fang, 1965. Exchange reactions between hydroxyl and sulfate ions in soils. Soil Sci., 99: 104-108. Cronan, C.S. and G.R. Aiken, 1985. Chemistry and transport of soluble humic substances in forested watersheds of the Adirondack Park, New York. Geochim. Cosmochim. Acta, 49: 1697-1705. David, M.B., G.F. Vance, J.M. Rissing and F.J. Stevenson, 1989. Organic carbon fractions in extracts of o and b horizons from a New England spodosol: Effects of acid treatment. J. Environ. Qual., 18: 212-217. Driscoll, C.T., R.D. Fuller and W.D. Schecher, 1989. The Role of organic acids in the acidification of surface waters in the eastern U.S. Water Air Soil Pollut., 43: 21-40. Evans, A., Jr., 1986. Effects of dissolved organic carbon and sulfate on aluminum mobilization in forest soil columns. Soil Sci. Soc. Am. J., 50: 1576-1578. Evans, A., Jr., L.W. Zelazny and C.E. Zipper, 1988. Solution parameters influencing dissolved organic carbon levels in three forest soils. Soil Sci. Soc. Am. J., 52: 1789-1792. Falkengren-Grerup, U., 1987. Long-term changes in pH of forest soils in southern Sweden. Environ. Pollut., 43: 79-90. Fuller, R.D., M.B. David and C.T. Driscoll, 1985. Sulfate adsorption relationships in forested spodosols of the northeastern USA. Soil Sci. Soc. Am. J., 49: 1034-1040. Gobran, G.R. and S.I. Nilsson, 1988. Effects of forest floor leachate on sulfate retention in a Spodosol soil. J. Environ. Qual., 17: 235-239. Gorham, E., J.K. Undewood, F.B. Martin and J.G. Ogden III, 1986. Natural and anthropogenic causes of lake acidification in Nova Scotia. Nature, 324: 451-453. Hallb/icken, L. and C.O. Tamm, 1986. Changes in soil acidity from 1927 to 1982-1984 in forest area of southwest Sweden. Scan. J. For. Res., 1: 219-232. Hultberg, H., 1985. Budgets of base cations, chloride, nitrogen and sulphur in the acid lake G~rdsj6n catchment, S.W. Sweden. Ecol. Bull., Stockholm, 37: 133-157. Inskeep, W.P., 1989. Adsorption of sulfate by kaolinite and amorphous iron oxide in the presence of organic ligands. J. Environ. Qual., 18: 379-385.

TOTAL DISSOLVED ORGANIC CARBON AND SO4-21N SOIL AND WATERS

461

Jacks, G., S. Andersson and B. Stegman, 1988. pH-changes over 30-40 years along a depositional gradient. Presented at "Environmental threats to forests and other ecosystems". Symposium in Oulu, November 1988. Johnson, D.W. and D.W. Cole, 1980. Anion mobility in soils: Relevance to nutrient transport from terrestrial ecosystems. Environ. Int., 3: 79-90. Johnson, D.W. and D.E. Todd. 1983. Relationships among iron, aluminum, carbon and sulfate in a variety of forest soils. Soil Sci. Soc. Am. J., 47: 792-800. Kerekes, J., S. Beauchamp and R. Tordon, 1986. Sources of sulphate and acidity in wetlands lakes in Nova Scotia. Water Air Soil Pollut. 31: 207-214. Kortelainen, J., J. Mannio, M. Forsius, J. K/im/iri and M. Verta, 1989. Finnish lake survey: The role of organic and anthropogenic acidity. Water Air Soil Pollut., 46: 235-249. Krug, E.C. and C.R. Frink, 1983. Acid rain on acid soil: A new perspective. Science, 221: 520-525. Mollitor, A.V. and D.J. Raynal, 1982. Acid precipitation and ionic movements in Adirondack forest soils. Soil Sci. Am. J., 46: 137-141. Nodvin, S.C., C.T. Driscoll and G.E. Likens, 1986. Simple partitioning of anions and dissolved organic carbon in a forest soil. Soil Sci., 142: 27-35. van Olphen, H., 1977. An Introduction to Clay Colloid Chemistry for Clay Technologists, Geologists and Soil Scientists. Wiley-Interscience, New York. Parfitt, R.L., 1989. Optimum conditions for extraction of A1, Fe and Si from soils with acid oxalate. Commun. Soil Plant Anal., 20 (7,8) 801-806. Pohlman, A.A. and J.G. McColl, 1988. Soluble organic from forest litter and their role in metal dissolution. Soil Sci. Am. J., 52: 265-271. Reuss, J.O., 1983. Implications of the calcium-aluminum exchange system for the effect of acid precipitation on soils. J. Environ. Qual., 12: 591-595. Reuss, J.O. and D.W. Johnson, 1986. Acid Deposition and the Acidification of Soils and Waters. Ecological Studies 59, Springer Verlag, Berlin. Richter, D.D., P.J. Comer, K.S. King, H.S. Sawin and D.S. Wright, 1988. Effects of low ionic strength solutions on pH of acid forested soils. Soil Sci. Am. J., 55: 261-264. Seip, H.M., R. Seip, P.J. Dillon and E. de Grosbois, 1985. Model of sulphate concentration in a small stream in the Harp Lake catchment, Ontario. Can. J. Fish. Aquat. Sci., 42: 927-937. Stevenson, F.J., 1982. Humus Chemistry. Wiley Interscience, New York. Ulrich, B., 1983. Effects of accumulation of air pollutants in forest ecosystems. In: B. Ulrich and J. Pankrath (Eds), Effect of accumulation of air pollutants on forest ecosystems. Reidel, Boston.