Effects of simulated acid rain on the cation exchange capacities of two Podzolic soils, Canada

Effects of simulated acid rain on the cation exchange capacities of two Podzolic soils, Canada

Geoderma, 42 (1988) 105-114 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands 105 Effects of Simulated Acid Rain on the Cat...

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Geoderma, 42 (1988) 105-114 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

105

Effects of Simulated Acid Rain on the Cation E x c h a n g e Capacities of T w o Podzolic Soils, Canada J.A. HERN "1, G.K. RUTHERFORD and G.W. VAN LOON

Departments of Geography and Chemistry, Queen's University, Kingston, Ont. (Canada) (Received May 16, 1986; accepted after revision September 10, 1987 ) ABSTRACT Hern, J.A., Rutherford, G.K. and van Loon, G.W., 1988. Effects of simulated acid rain on the cation exchange capacities of two Podzolic soils, Canada. Geoderma, 42: 105-114. A laboratory column experiment was performed in which reconstructed profiles of a HumoFerric and a Ferro-Humic Podzolic soil were leached with simulated acid rain for two years. In order to assess the effects of treatment on the cation exchange capacity, the columns were sampled and negative charge on the sample was measured. Water- and phosphate-extractable sulphate, organic carbon content and non-silicate iron and aluminium were also determined. Negative charge was correlated with organic carbon in the organic horizons, but there was no significant effect due to treatment. A small decrease in negative charge with increasing acidity of treatment was observed in the B horizons of both columns, although the change was significant at the 95% confidence level only for the Humo-Ferric Podzol.

INTRODUCTION A l t h o u g h m a n y soils o f t h e t e m p e r a t e zone are d o m i n a t e d b y p e r m a n e n t ( p H - i n d e p e n d e n t ) c h a r g e m i n e r a l s , B h o r i z o n s of P o d z o l s c a n e x h i b i t p H d e p e n d e n t c h a r g e c h a r a c t e r i s t i c s due to a c c u m u l a t i o n of iron a n d a l u m i n i u m h y d r o u s oxides, organic m a t t e r a n d c e r t a i n silicate m i n e r a l s ( L a v e r d i e r e a n d W e a v e r , 1977 ). As a r e s u l t of acid p r e c i p i t a t i o n , t h e c h a r g e s in t h e s e h o r i z o n s c a n be a f f e c t e d in t w o o p p o s i n g b u t i n t e r r e l a t e d ways. T h e first is t h a t p r o t o n a t i o n of h y d r o u s oxide s u r f a c e sites leads to a d e c r e a s e in n e g a t i v e c h a r g e of t h e h o r i z o n ( v a n Raij a n d Peech, 1972; O k a m u r a a n d W a d a , 1983). A t t h e s a m e t i m e t h e specific a d s o r p t i o n o f s u l p h a t e ( M o t t , 1981 ) h a s b e e n s h o w n to c a u s e a n i n c r e a s e in n e g a t i v e c h a r g e ( R a j a n , 1978, 1979; W i k l a n d e r , 1980). T h e n e t effect of a n i n p u t of acid r a i n c o n t a i n i n g b o t h h y d r o g e n ion a n d sulp h a t e would d e p e n d on i n d i v i d u a l soil c h a r a c t e r i s t i c s as well as p r e c i p i t a t i o n composition. "~Present address: Dept. of Chemistry, Trent University, Peterborough, Ont. K9J 7B8 (Canada).

0016-7061/88/$03.50

© 1988 Elsevier Science Publishers B.V.

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Acid rain is a source of anions, as well as hydrogen ion, with nitrate and sulphate usually present in highest concentrations (Cronan et al., 1978). Nitrate is geochemically mobile but little is found in the soil solution where plants are growing (Hern et al., 1985) as most is taken up by actively growing roots. Sulphur is also a plant nutrient, but significant concentrations may remain in pore water because smaller amounts t h a n those of nitrogen are required by plants. Sulphate can be specifically adsorbed by the iron and aluminum hydrous oxides which are important components of m a n y soils. The capacity of a soil to adsorb sulphate and the effect this has on soil properties may, therefore, be of some importance in determining effects of acid rain on soil CEC. To assess the effects of acid rain on the charge properties of two soils under forest in the Canadian Shield, a laboratory experiment was conducted in which reconstructed soil profiles were leached with simulated rain over a two-year period. Measurements were made to determine the negative charges of the soil horizons after treatment, as well as sulphur and sulphate contents. No attempt was made to impose a particular pH on samples by adding acid or base during the measurement step. Rather, the charge measurements were made at the pH established by virtue of the nature of the sample, which was affected by the two-year simulated t r e a t m e n t with acid rain. Because hydrous iron and aluminum oxides are associated with anion adsorption, their nature and concentration were also measured, as was the organic carbon content. The objective of this study was to investigate factors affecting charge properties of a soil under forest. These factors include soil properties per se as well as acid inputs. EXPERIMENTAL

Soil samples were taken from two locations on the Canadian Shield, from M o n t m o r e n c y Forest (MF) north of Quebec City, Quebec and from the T u r k e y Lakes Watershed ( T L W ) north of Sault Ste. Marie, Ontario. The vegetative cover at MF is a boreal coniferous forest of the balsam fir-white birch climax zone, occurring over an Orthic Humo-Ferric Podzolic soil. At T L W the forest cover is mixed hardwood-softwood and is developed over an Orthic FerroHumic Podzolic soil of variable depth. Columns of reconstructed soil profiles 15.4 cm in diameter and 60 cm in length were prepared for studies in the laboratory. The top 17 cm were undisturbed cores taken with a steel sampler of the same internal diameter as the columns. Below the undisturbed cores the columns were completed by sampling layers in the field and reconstructing t h e m in the columns. The soil columns were leached with synthetic rain at three levels of acidity: pH 5.7, 3.5, and 2.0. Columns were watered with 360 ml of the appropriate solution (equivalent to 2 cm of rainfall) each week over a two-year period. Three columns subjected to each t r e a t m e n t were dismantled and sampled at

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several depths at the end of the watering period, for a total of nine columns for each of the two sampling sites. Sections of the full diameter of the columns were taken at 0-5, 15-20, 20-25 and 50-55 cm. These depths corresponded to organic, B1, B2 and C horizons, respectively. The site descriptions, synthetic rainfall composition and column construction have been reported in detail elsewhere (Hern et al., 1985). Negative charges on the soil samples were determined by a method modified from that of Greenland ( 1974 ) in which samples were equilibrated with cesium chloride solution. Adsorbed cesium was measured directly on the soil by X-ray fluorescence spectroscopy. This method has been used to measure charges both on pure clays and on whole soil samples (Greenland, 1975). One-half-gram samples of soil ( < 100 mesh) were shaken for 30 min in 80 ml of 0.005 M aqueous cesium chloride solution in a pre-weighed polypropylene centrifuge tube. The suspension was centrifuged and the supernatant decanted. This was repeated until the supernatant pH reached a constant value. Four equilibrations were usually sufficient, and final supernatant pH values ranged from 4.34 to 4.95. Immediately after the final supernatant was decanted the tubes were weighed to determine the amount of entrained solution. The samples were adjusted to pH 9 with sodium hydroxide to prevent loss of hydrochloric acid during heating and were oven-dried at 70 ° C. The entire sample was pressed into a 4.0-cm disk at 4500 kg/cm 2 in an aluminum SPEC-CAP (TM), backed by boric acid. Cesium was measured by XRF using a Siemens BRS vacuum Xray spectrograph and Siemens CR 50-42 X-ray tube operated at 50 kV and 40 mA. A gas flow detector and lithium fluoride crystal were employed for detection. The K-alpha fluorescence line was counted for 40 s at 91.91 ° and background was taken as the average counts at 90.00 ° and 94.00 °. Non-silicate iron and aluminum fractions were determined using dithonitecitrate-bicarbonate (Fed), acid ammonium oxalate (Feo and Alo), and pyrophosphate (Fep and Alp) extractants according to standard methods (Wang, 1978). The extraction results are interpreted as follows: Finely divided haematite and goethite: Fex = Fed-- Feo. Amorphous, inorganic metal oxide: Fea = F e o - Fep; Ala -- Alo- Alp. Organic complexed metal oxide: Fep; Alp. Total sulphur was determined by a combustion technique, followed by ion chromatographic analysis as described by Hern et al. (1983). In addition to total sulphur, both water- and phosphate-extractable sulphate were determined in the soil samples at the same depths as for charge and oxide measurements. The method described by Johnson and Todd (1983) was used except that all supernatants were analyzed for sulphate contents by ion chromatography. The sum of the two extractions is defined as total sulphate. Total organic carbon was measured using a LECO (TM) induction furnace and gas measuring buret according to standard procedures (McGill, 1978).

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Statistical analyses were performed using Statistical Analysis System (SAS Institute) software. RESULTS AND DISCUSSION

Negative charge The results of negative-charge measurement are given in Table I. To test for differences in negative charge between acid treatments, an analysis of variance was performed. The samples from a given depth and soil type (MF or TLW) were taken as a group, with treatment as levels. This was not done for C horizon samples due to a lack of replicates. A statistically significant difference between treatments at the 95% confidence level was found for MF Bhf2 horizon material - - a decrease in negative charge with increasing acidity of treatment. TLW Bfhl horizon exhibited a similar decrease, but the difference between treatments was not significant at 95%. No significant trends were observed in other horizons. Such an analysis of variance demonstrates statistically significant change in negative charge only when the changes are large relative to the inherent variability in the soils. Charge is correlated with a variety of soil components, and variance in amounts of these components account for some of the variability in charge between samples which is not due to treatment. These additional factors were thus incorporated in further data analysis to provide more information on the effects of treatment on charge. One important soil property having a major effect on negative charge is the organic matter content. This is shown by the correlation between negative charge and percent carbon in the organic horizons of both soils (Fig. 1). In TABLE I Charge on whole samples: means of three measurements in meq./100 g with standard deviation given in parentheses Horizon TLW negative: Organic Bfhl Bfh2 C MF negative charge: Organic Bhfl Bhf2 C

pH 5.7

pH 3.5

pH 2.0

5.78 ( 1.7 ) 3.11 (1.5) 2.22 (0.37) 0.68(-)

7.16 (2.8) 2.94 (0.28) 2.30 (0.07) 0.87(-)

5.71 ( 1.3 ) 2.36 (1.2) 1.92 {0.15) 0.81(-)

22.2 {9.6) 3.92 (0.74) 3.60 (0.22) 2.35 (0.08)

28.7 (20) 2.92 (0.71) 3.19 (0.05) 2.96 (0.26)

32.5 (18) 3.62 (0.86) 3.02 (0.19) 2.98 (0.04)

109 M F SOIL 60

E 40 < I

7

20.

Z •..'.:..

2O

3O

4"O

C A R B O N (%)

T L W SOIL 12

K

< I U



< °: Z

C A R B O N (%)

Fig. 1. Negative charge vs. carbon contents for organic horizons. R-squared equals 0.65 and 0.79 for MF and TLW soils, respectively

these soil horizons, however, analysis of variance of residuals from regression of charge against carbon content showed that treatment had no significant effect on negative charge. A correlation between charge and carbon content was not observed in the B horizons of the soils. In the B horizons, accumulated metal oxides, particularly of iron and aluminum, are a source of pH-dependent charge and can form coatings on soil particles. Non-silicate iron and aluminum contents in the horizons were, therefore, considered as a potential factor influencing negative-charge variability. To assess the contribution of these oxides as well as of the treatments,

110 TABLE II Correlation coefficients from multiple regression: negative charge on variables indicated (pH = treatmentpH, Fex= crystallineion oxides,Fep= organicallybound iron) Soil

Horizon

Variables in model

R2

TLW

Bfhl

Fex Fex,pH Fex,pH, Fep

0.72 0.81 0.81

MF

Bhf2

pH pH, Fex pH, Fe~,Fep

0.74 0.86 0.95

a stepwise multiple regression analysis relating these properties to negative charge was carried out (Table II). An effect of treatment on negative charge was seen in two cases, the MF Bhf2 horizon as noted previously, and the TLW Bfhl horizon. For MF Bhf2 horizon, treatment accounted for the largest proportion of total variance in negative charge, with crystalline iron oxide content and organically bound iron of secondary importance. The three factors together accounted for approximately 95% of the total variance in negative charge in that horizon. In that CEC decreased with increasing acidity and concentration of sulphate in input water, the effect of hydrogen ion on charge seems to have been dominant. The charge of Bfhl horizon of the TLW profile also depended on crystalline iron oxide content and treatment pH, but the order of importance of these factors was reversed compared with the Bhf2 horizon of the MF soil. None of the other measured properties made a significant contribution to charge variability. The observation that a smaller proportion of total charge variability in the TLW than the MF horizons is accounted for by treatment effects may be related to the relative capacities of the soils to adsorb sulphate.

Sulphate in soil columns The amount of water-extractable, phosphate-extractable and total sulphate are illustrated in Fig. 2. p H 5. 7 treatment. Total sulphate was approximately 5% of total sulphur in the organic horizons of both soils. The phosphate-extractable sulphate, as percent of total sulphur, was low: 0.7% and 2.0% for MF and TLW, respectively. The fraction of total sulphur as sulphate increased to about 8% in the B horizons of both soils. Phosphate-extractable sulphate was greater in the B horizon of the TLW soils indicating a greater capacity to adsorb sulphate. p H 3.5 treatment. In the B horizons, phosphate-extractable sulphate ranged from 1.25 to 2 times higher than in material leached at pH 5.7, and total sul-

111 SULPHATE Img/kg) 0

4(}0

300

200 i

100

I

I

MONTMORENCY FOREST

[:ff

p"-

pH 3.5

B1 B2 C

JZ'--~

ot'Z//,,I

pH 2.0

ul / f l / / J ~2r ~ / / / / / Z / , ' t

I

]

c t'////A

TURKEY LAKES WAEERSHED pH 5.7

pH 3.5

B1 B2 C

pH 2.0

B2 I ' z / / / / / / / / / / z / / / / l ' / / / / / / / J

o /,/////J B1~ / / / / / / I / / / / / / / / / / / / / i c Villi

]

~ ] T

I

Fig. 2. Extractable sulphate after treatments.White bars are water-extractablesulphate, crosshatchedbars are phosphate-extractablesulphate. phate accounted for 11 to 14% of the total sulphur in those horizons. No distinctions between columns leached at pH 5.7 and 3.5 were observed in the organic and C horizon materials. p H 2.0 t r e a t m e n t . Water-extractable sulphate in the organic layer was very high - - about 25% of total sulphur. This is consistent with inputs of the high (128 mg/1) sulphate concentrations in the simulated rain at pH 2.0. Even under these conditions phosphate-extractable sulphate was low relative to those of mineral soils: 1.6 and 5.4% of total sulphur for MF and TLW, respectively. In the B horizons of both soils, water-extractable sulphate was lower than in the organic horizons, but phosphate-extractable sulphate was much higher so that total sulphate ranged from 30 to 45% of total sulphur. The TLW B horizon contained 2 to 4 times as much phosphate-extractable sulphate as the equivalent MF B horizon material. In general, phosphate-extractable sulphate was higher in the TLW horizons for a given acid treatment, except for the organic horizon material, pH 3.5 and the C horizon, pH 2.0 treatment. Water-soluble sulphate was higher in MF horizons. These results are in keeping with pore water sulphate concentrations

1.24(.42) 1.34(.45) 1.17(.44)

Fep

Turkey Lakes watershed profile: 5.7 0.29(.20) 0.27(.06) 3.5 0.38(.08) 0.21(.09) 2.0 0.32(.08) 0.25(.04)

Fea 1.33(.52) 0.97(.41) 1.28(.18)

Bhfl Fex

Montmorency forest profile: 5.7 0.07(.11) 0.39(.32) 3.5 0.05(.04) 0.11(.10) 2.0 0.11(.01) 0.16(.08)

pH treatment

1.82(.76) 1.56(.51) 1.66(.18)

1.44(.78) 0.78{.33) 1.06(.01)

Ala

1.21(.32) 1.02(.30) 1.42(.21)

0.92(.33) 0.64(.20) 0.69(.06)

Alp

0.28(.02) 0.33(.02) 0.24{.05)

0.11(.08) 0.12(.02) 0.09(.08)

Bhf2 Fex

0.40(.04) 0.36(.05) 0.28(.07)

0.02(.06) 0.28(.07) 0.29(.06)

Fe.

Non-silicate Fe and A1 contents of B horizon materials in per cent with standard deviation in parentheses

TABLE III

1.04(.02) 1.08(.02) 1.10(.02)

1.40{.13) 1.32(.06) 1.33(.06)

Fep

2.34(.32) 2.29(.08) 2.36(.34)

1.38(.16) 1.51(.18) 1.46(.02)

Ala

1.59 (.03)

1.55 (.05) 1.57 (.02)

0.98(.14) 1.02(.07) 1.04(.05)

Alp

t~

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monitored during the column leaching period, and reported elsewhere (Hern et al., 1985). They indicate that T L W horizons have a greater capacity to adsorb sulphate than MF horizons. This difference was not due to anion exchange capacity, as measured AEC was greater for MF than for T L W horizons and was less than 1.0 meq./100 g in all cases. The difference may be due to iron and aluminum oxide contents, which are higher in TLW B horizon by a factor of two to three (Table III). CONCLUSIONS

The negative charge was correlated with organic carbon content in the organic horizons, but no effect of treatment was observed. Acid leaching caused a small reduction in the negative charge in the B horizons, of both soils, although the change was statistically significant only for the MF B horizon. Several factors may contribute to the small magnitude of observed changes. The effects of hydrogen ion and sulphate input on charge are opposites, so small net changes may be expected over a short period of time, particularly if a soil has a large sulphate sorptive capacity. In this experiment, no plants were growing in any columns. Therefore, nitrification continued in the soils without corresponding plant uptake of nitrate generated (Hern et al., 1985). This resulted in a net in-situ production of hydrogen ion. We calculated that externally added hydrogen ion was equivalent to approximately 10% of that generated internally for the pH 3.5 treatment and 300% for the pH 2.0 treatment. For this reason, acidification effects might be reduced, especially between the pH 5.7 and 3.5 treatments, when compared to results in the field where plant uptake of nitrogen occurs. In terms of evaluating the susceptibility of soils to acid rain, knowledge of the anion sorptive capacity and factors influencing its magnitude are of importance. Significant adsorption of sulphate would tend to offset decreases in negative charge due to acidification. This study has shown that the decrease in negative charge in the B horizon was of lesser magnitude in the soil with higher sulphate sorptive capacity. This soil (TLW) also had higher crystalline iron oxide and amorphous aluminum oxide contents in the B horizon. Stepwise multiple regression showed a relationship between charge and both treatment and oxide contents in the B horizons for the two soils. The observed changes highlight the need for further information on the effects of acid rain on long-term changes in cation exchange capacities. REFERENCES Cronan, C.S., Reiners, W.A., Reynolds, R.C., Jr. and Lang, G.E., 1978. Forest floor leaching: contributions from mineral, organic and carbonic acids in New Hampshire subalpine forests. Science, 200: 309-311.

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Greenland,D.J., 1974. Determination of pH-dependent charges of clays using caesium chloride and X-ray fluorescence spectrography. Trans. 10th Int. Congr. Soil Sci.,Moscow, 2: 278-285. Greenland, D.J., 1975. Charge characteristicsof some kaolinite-ironhydroxide complexes. Clay Miner., 10: 407-416. Hern, J.A., Rutherford, G.K. and van Loon, G.W., 1983. Determination of chloride,nitrate,sulphate and totalsulphur in environmental samples by single-column ion chromatography. Talanta, 30: 677-682. Hern, J.A., Rutherford, G.K. and van Loon, G.W., 1985. Chemical and pedogenetic effects of simulated acid precipitation on two eastern Canadian forest soils, I. Nonmetals. Can. J. For. Res., 15: 839-847. Johnson, D.W. and Todd, D.E., 1983. Relationships among iron, aluminum, carbon and sulfate in a variety of forest soils. Soil Sci. Soc. Am. J., 47: 792-800. Laverdiere, M.R. and Weaver, R.M., 1977. Charge characteristics of spodic horizons. Soil Sci. Soc. Am. J., 41: 505-510. McGill, W.B., 1978. Total C, N, S, and P. In: J.A. McKeague (Editor), Manual on Soil Sampling and Methods of Analysis. Soil Research Institute, Ottawa, Ont., pp. 109-112. Mott, C.J.B., 1981. Anion and ligand exchange. In: D.J. Greenland and M.H.B. Hayes (Editors), The Chemistry of Soil Processes. Wiley, New York, N.Y., 714 pp. Okamura, Y. and Wada, K., 1983. Electric charge characteristics of horizons of Ando (B) and RedYellow B soils and weathered pumices. J. Soil Sci., 34: 287-295. Overrein, L.N., Seip, H.M. and Tollan, A., 1980. Acid Precipitation - - Effects on Forest and Fish. Final report on the SNSF project, Aas/NLH, Norway. Rajan, S.S.S., 1978. Sulfate adsorbed on hydrous alumina, ligands displaced, and changes in surface charge. Soil Sci. Soc. Am. J., 42: 39-44. Rajan, S.S.S., 1979. Adsorption and desorption of sulfate and charge relationships in aliophanic clays. Soil Sci. Soc. Am. J., 43: 65-69. Van Raij, B. and Peech, M., 1972. Electrochemical properties of some oxisols and alfisols of the tropics. Soil Sci. Soc. Am. Proc., 36: 587-593. Wang, C., 1978. Extractable A1, Fe and Mn. In: J.A. McKeague (Editor), Manual on Soil Sample and Methods of Analysis. Soil Research Institute, Ottawa, Ont., pp. 98-102. Wiklander, L., 1980. Interaction between cations and anions influencing adsorption and leaching. In: T.C. Hutchinson and M. Havas (Editors), Effects of Acid.Precipitation on Terrestrial Ecosystems. Plenum Press, London, pp. 239-354.