Equilibrium models of aluminium and iron complexation with different organic acids in soil solution

Geoderma 94 Ž2000. 201–221

Equilibrium models of aluminium and iron complexation with different organic acids in soil solution P.A.W. van Hees ) , U.S. Lundstrom ¨

1

Department of Chemistry and Process Technology, Mid Sweden UniÕersity, S-85170 SundsÕall, Sweden Received 5 February 1998; received in revised form 4 November 1998; accepted 6 November 1998

Abstract The percentage of Al and Fe bound to identified low molecular weight ŽLMW. organic acids and phosphate in soil solution was calculated using a chemical equilibrium model. The highest fractions were obtained for the O1 horizon solutions with median values between 38–49% for Al and 18–29% for Fe. Generally the percentage declined in the deeper horizons Acceptable agreement with experimental values using ultrafiltration Ž- 1000 D. was found especially for the spring and summer samplings. In the autumn samples, larger deviations between the modelled and ultrafiltered fractions were found. The major part of the remaining Al and Fe in solution was calculated to be bound to undefined organic acids most likely of higher molecular weight. Speciation studies of some individual organic acids are presented. Citric acid proved to be the most important complex former of the LMW acids in the O and E horizons while oxalic acid was dominant in the B horizon solutions. The total level of organic complexation of Al, Fe, Ca and Mg was also studied. It was found that ) 85% of the Al and ) 95% of the Fe were organically bound. The opposite was seen for Ca and Mg for which ) 85% was modelled to occur as inorganic ions. The modelling results support the theory that LMW organic acids contribute to the translocation of Al and Fe in the podzolization process. q 2000 Elsevier Science B.V. All rights reserved. Keywords: equilibrium models; organic acids; aluminium; iron; soil solution; podzols

) 1

Corresponding author. E-mail: [email protected] E-mail: [email protected]

0016-7061r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 6 - 7 0 6 1 Ž 9 8 . 0 0 1 3 9 - 6

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1. Introduction The importance of low molecular weight ŽLMW. organic acids in the complexation and transport of Al and Fe in forest soils has been emphasized in a number of papers ŽBuurman and Van Reeuwijk, 1984; Lundstrom ¨ and Giesler, 1995; van Hees et al., 1996.. The presence of identified LMW organic acids in soil solution of podzols has been shown Ž Hue et al., 1986; Fox and Comerford, 1990; Krzyszowska et al., 1996; van Hees et al., 1996, 2000a. . Many Al and Fe complexes with these acids, for example citric and oxalic acids, have high stability constants ŽHue et al., 1986; McColl and Pohlman, 1986.. It has been shown that about 1r3 of the Al in soil solutions of the O horizon is bound to these kinds of organic solutes Ž van Hees et al., 1996, 2000a. . Due to their complexing forming ability and readiness for microbial decomposition Ž Boudot et al., 1989. they may play an important role in the podzolization process. The objective of this study was to compare the fraction of Al and Fe bound to LMW organic acids using a chemical equilibrium model, and experimental values. In addition, the level of organic complexation of Al, Fe, Ca and Mg and the speciation of four different acids were studied.

2. Sites and material 2.1. Sites Hyytiala ¨ ¨ ŽHy. Forestry Field Station in Finland, Ž61848X N, 24819X E.: The site was forested with Norway spruce Ž Picea abies . and Scots pine Ž Pinus sylÕestris. about 100 years old. The field layer consisted of Vaccinium myrtillus and moss. The soil was podzolized and developed on sorted glacial–fluvial coarse sand material. The soil horizons had thicknesses of about: O:10 cm, E:15 cm and B: 40 cm. Ž Ny., Svarberget research park, 70 km NW of Umea, Nyanget ¨ ˚ Sweden Ž64814X N, 10846X E. : The site was mainly forested with Scots pine Ž P. sylÕestris. associated with Norway spruce Ž P. abies . about 70 years old. The field layer consisted of dwarf V. myrtillus, Vitis-idaea and moss. The soil was podzolized and developed on a till material. The soil horizons had thicknesses of about: O: 10–15 cm, E: 10–15 cm and B: 40 cm. Heden ŽHe., Svarberget research park, 70 km NW of Umea, ˚ Sweden Ž64814X N, X 10846 E.: The site was mainly forested with Scots pine Ž P. sylÕestris. associated with Norway spruce Ž P. abies . about 70 years old. The field layer consisted of V. myrtillus and moss. The soil was podzolized and developed on fluvial sediment of 93% fine sand. The soil horizons had thicknesses of about: O:5 cm, E:5 cm and B:30 cm.

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For a more detailed description of the three sites the reader is referred to Ilvesniemi et al. Ž2000.. 2.2. Sampling To obtain soil samples a 1 m3 Ž 1 = 1 = 1 m. pit was dug. Individual soil samples were taken by driving 18 cylindrical sampling devices 7 cm horizontally into one side of the pit. Each sample core contained approximately 115 cm3 of soil. Seven different horizons were sampled, namely O1, O2, E, B1, B2, B3, C6 and C14. The B1, B2 and B3 samples were taken at 0–5, 5–10 and 10–15 cm depth of the illuvial horizon, respectively. The C6 and C14 samples were sampled at 25–30 and 60–65 cm depth beneath the border between the E and B layers. At the Nyanget site two E horizon samples were taken denoted E1 and ¨ E2. All sites were sampled six times over a growing season MayrJune to October Žsee additional information in van Hees et al., 2000a. .

3. Experimental method 3.1. Soil solution The centrifugation drainage technique described by Giesler and Lundstrom ¨ Ž1993. was used to obtain soil solutions. Soil samples were collected and centrifuged field-moist for 30 min at a speed of 14 000 rpm. The collected centrifugates were filtered through a 0.45 mm filter ŽMillex-HV, Millipore. . The pH determination and Al speciation were made as soon as possible after centrifugation. Samples for DOC and HPLC determination were deep frozen. A Beckman u32 pH-meter with a Beckman 39847 combination electrode was used for pH determination. A flow injection analysis Ž FIA. method according to Clarke et al. Ž 1992. was employed for the determination of quickly reacting Ž QR. Ž mainly inorganic. Al. For the O horizon no determination was performed due to the interference by the high levels of coloured substances in these samples. Due to the large content of organic acids and relatively low concentration of Al in this horizon it can be assumed that almost all the Al is organically bound Ž e.g., Lundstrom, ¨ 1993.. Furthermore, the values of inorganic Al in ultrafiltered Ž- 1000 D. from the O horizon was normally below the limit of detection Ž 0.025 ppm. . Size fractionation of organic Al complexes was performed using ultrafiltration of the soil solution of the O1–B3 horizons. The ultrafiltration was carried out using a stirred cell Žmodel 8050, Amicon, Beverly, MA, USA. . The filter

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employed ŽDiaflo YM-1, Amicon. had a cut-off size of 1000 Daltons, i.e., 90% of solutes with MW s 1000 is retained by the filter. Inorganic ŽQR. Al was determined in the filtrates and was subtracted from the total Al passing the filter giving the concentration of organically bound Al Ždenoted organic Al UF . . Due to the reduction in colour due to the ultrafiltration it was possible to determine inorganic Al also in the O horizon solutions. Determination of the total concentrations and the concentrations in ultrafiltered samples of Al, Fe, Ca, Mg, Mn, Si and a number of other cations was carried out on acidified samples employing ICP-AES Ž Perkin-Elmer Plasma 400, Norwalk, CO, USA. . Sulphate was determined using ion exchange chromatography Ž Dionex 4000i, Sunnyvale, CA, USA.. Phosphate was analysed employing FIA Ž E to C horizons. or ion exchange chromatography ŽO horizon.. LMW organic acids were determined using the HPLC method of van Hees et al., 1999. The method is based on the use of a Supelcogel C610-H ion exclusion column. The mobile phase consisted of 0.2 vol% Ž85%. H 3 PO4 ŽpH 2.01. and the column was operated at 608C for detection of citric, lactic and shikimic acids and at 308C for the other acids. All together 12 acids were calibrated. The acids were detected at 210 nm. The chromatograph Ž Shimadzu LC-10, Shimadzu, Osaka, Japan. featured a diode array detector and the spectrum between 200–300 nm was monitored simultaneously. In order to remove interfering compounds, the sample Ž10 ml. was ultra filtered twice through a 1000 D filter to ensure sufficient removal using the equipment described above. The sample was then pumped through a cation exchanger Ž0.4 ml Bio-Rex 70, 200–400 mesh. in the Hq form. The sample was immediately run on the HPLC column. Titration of acidified ŽpH ; 2.5. and de-gassed samples was performed employing an ABU900 Autoburette system controlled by a TIM900 Titration Manager ŽRadiometer, Copenhagen, Denmark. . The samples were titrated by means of 0.01 M NaOH to pH 11.3. The evaluation was based on the Gran extrapolation giving the concentrations of strong and weak acids. The sum of strong and weak acids is referred to as total acidity Ž TA. . The obtained values were corrected for titrated silicic acid, but not for phosphate. Cation exchange did not significantly change the titration values Ž not shown. , and it was assumed that Al and Fe organic complexes re-formed during the titration. This assumption was supported by MINEQLq calculations performed at pH 11.3 using the approach named MINEQLq Žsee Section 3.3., which indicated that the concentrations of hydroxy Al species could be neglected. 3.2. Computer programs The chemical equilibrium program MINEQLq ŽEnvironmental research software, Edgewater, MD, USA. was used for calculation of Al 3q activity and

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complexation of Al, Fe, Ca and Mg to LMW organic acids Že.g., citric and oxalic acids. . Excel 7.0 Ž Microsoft. was used for data treatment and additional calculations. 3.3. Modelling of Al, Fe, Ca and Mg bound to low molecular weight organic acids Modelling of the fraction of Al and Fe bound to LMW organic acids and phosphate was performed for the O1–B2 horizon using the chemical equilibrium program MINEQLq . Besides Al and Fe, Mg and Ca were included in the calculations. As input values the pH, and concentrations of SO42y, PO43y, LMW organic acids Žfrom HPLC determinations. , and total Al, Fe, Mg and Ca were used. The organic acidity less LMW acids, denoted Org HMW , was estimated from the titrations of the soil solution according to: Org HMW s HOrg HMW q Orgy HMW s TA y LMWacids y PO43y

tot

Ž mol c ly1 .

Ž1.

where HOrg HMW q Orgy HMW is the acidity of HMW acids and TA is the total acidity obtained from the titrations of the soil solution. TA was preferred instead of the weak acidity since it appeared that the first proton of, e.g., citric and oxalic acids was included in the strong acidity fraction. H 2 PO4y was the only phosphate species that was assumed to be titrated in the pH range of the titrations. In eight samples for which TA values were not available or very extreme, TA was estimated using the DOC Ž8.8 mmol c gy1 DOC; Bergelin et al., 2000.. The HMW organic acidity was treated as monoprotic acids according to Lundstrom ¨ Ž1993., and p K a and log K AlOrg were obtained from this paper. The stability constant with Al Ž K AlOrg . for these acids was on average lower in the autumn than in the summer and spring. The compounds and complexes considered are summarised in Table 1. All complexes of Al, Fe, Mg and Ca with the LMW acids and Org HMW were taken into account. All constants were extrapolated to zero ionic strength using the extended Debye–Huckel equation ¨ with the correction factor of Guggenheim and Bates. The stability constants derived at the lowest ionic strength of the ones reported were with very few exceptions used. In case a stability constant of a complex between one of the metals and an acid was missing, an approximate value of a 1:1 complex was estimated using the procedure outlined by Martell et al. Ž 1988. . The stability constants of malonic and propionic acids were used for the estimations. The MINEQLq runs were performed at 258C because no D H values were available. Ionic strength correction was employed. This model is subsequently called MINEQLq . For the samples where ‘free’ Al 3q activity was available from determination of inorganic ŽQR. Al, mainly E and B1 samples, Al bound to LMW organic

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Table 1 Stability constants and complexation reactions for the HMW acidity ŽOrgy . and the LMW organic acids with Al 3q, Fe 3q, Ca2q, Mg 2q, and Hq . HMW-acidity ŽOrgy HMW Ion

Reaction

Season

Log K

Reference

xq² 5 : Orgy HMWqMe Ž xy1.q MeOrg HMW

springrsummer autumn springrsummer autumn

q² 5 : Orgy HMWqH HOrg HMW

springrsummer autumn

5.42 4.87 8.00 7.45 2.50 2.50 4.30 4.50

Lundstrom ¨ Ž1993. Lundstrom ¨ Ž1993. estimated estimated estimated estimated Lundstrom ¨ Ž1993. Lundstrom ¨ Ž1993.

3q

Al Al 3q Fe 3q Fe 3q Ca2q Mg 2q Hq Hq

LMW organic acids and sulphateqphosphate Acid

Complexing ion

Al 3q, Fe 3q, Ca2q, Mg 2q, Hq Al 3q Fe 3q, Ca2q, Mg 2q, Hq formic Al 3q, Fe 3q, Ca2q, Mg 2q, Hq fumaric Ca2q, Hq Al 3q, Fe 3q, Mg 2q lactic Mg 2q, Hq Al 3q, Ca2q Fe 3q malic Al 3q Fe 3q, Ca2q, Mg 2q, Hq malonic Al 3q, Fe 3q Ca2q, Mg 2q, Hq oxalic Al 3q Fe 3q, Ca2q, Mg 2q, Hq t-aconitic Ca2q Hq Al 3q, Fe 3q, Mg 2q sulphate Al 3q, Hq phosphate Al 3q, Fe 3q, Ca2q, Mg 2q, Hq

acetic citric

Martell and Smith Ž1977. Gregor and Powell Ž1986. Martell and Smith Ž1977. Martell and Smith Ž1977. Martell and Smith Ž1977. estimated Martell and Smith Ž1977. Perrin Ž1983. estimated Perrin Ž1983. Martell and Smith Ž1977. Dutt et al. Ž1976. Martell and Smith Ž1977. Bilinski et al. Ž1986. Martell and Smith Ž1977. Martell and Smith Ž1977. Martell Ž1964. estimated Ball et al. Ž1980. Stumm and Morgan Ž1981.

acids was also calculated using this activity. The procedure for calculation of Al 3q activity is presented in van Hees et al. Ž 2000b. . In the MINEQLq runs the Al 3q activity was fixed, and the concentration of Al required to maintain the equilibrium was calculated employing Al 3q, pH and LMW organic acids as input values. The obtained Al concentration corresponds in other words to the concentration of Al bound to LMW organic acids ‘needed’ to obtain the observed ‘free’ Al 3q activity. This approach was named MINEQLq Ž Al 3q . . Al phosphate complexes were not considered in this approach.

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4. Results 4.1. Al and Fe bound to LMW organic acids— comparison with experimental Õalues The percentage of four metals bound to LMW organic acids and phosphate in the O1–B2 horizons were calculated using the chemical equilibrium program MINEQLq . The species and constants included are summarised in Table 1. The springrsummer and autumn values were applied to sampling occasions 1–3 q Ny 97 and 4–6, respectively Žsee van Hees et al., 2000a for dates. . The runs for the model named MINEQLq are summarised in Tables 2 and 3. For the horizons where inorganic Al was detected, the amount of Al bound to LMW organic acids was also calculated using the Al 3q activity, named MINEQLq ŽAl 3q ., computed from the FIA determinations.

Table 2 Maximum, minimum and median values of the fraction of Al and Fe bound to LMW organic acids and phosphate calculated using MINEQLq Site

Horizon

Al

Fe

%LMW Al-MINEQLq Maximum

%LMW Fe-MINEQLq

Minimum

Median

Maximum

Minimum

Median

Nyanget ¨ Ž ns6. a

O1 O2 E1 E2 B1 B2

71 Ž76. 59 Ž52. 30 Ž41. 17 Ž12. 66 Ž29. 98 Žy.

31 Ž36. 15 Ž27. 15 Ž19. 0 Ž12. 0 Ž9. 5 Žy.

49 Ž41. 29 Ž29. 22 Ž24. 11 Ž12. 1 Ž17. 34 Žy.

47 Ž60. 35 Ž36. 15 Ž35. 7 Ž8. 73 Žy. 100 Žy.

19 Ž18. 9 Ž15. 5 Ž6. 0 Ž9. 0 Žy. 0 Žy.

29 Ž20. 12 Ž17. 10 Ž19. 4 Ž11. 4 Ž7. 58 Žy.

Heden Ž ns6. b

O1 O2 E B1 B2

50 Ž32. 27 Ž28. 38 Ž21. 100 Ž36. 32 Ž38.

22 Ž24. 12 Ž21. 5 Ž25. 2 Ž24. 0 Ž17.

38 Ž38. 22 Ž28. 23 Ž19. 16 Ž24. 15 Ž6.

26 Ž16. 8 Ž19. 19 Ž18. 99 Ž31. 53 Žy.

11 Ž8. 4 Ž12. 2 Ž29. 0 Ž28. 0 Žy.

18 Ž17. 7 Ž19. 9 Ž18. 12 Ž22. 37 Žy.

Hyytiala ¨¨ Ž ns 3.

O1 O2 E B1 B2

58 Ž46. 41 Ž35. 38 Ž30. 57 Ž62. 33 Ž41.

39 Ž36. 16 Ž23. 21 Ž17. 5 Ž26. 2 Žy.

42 Ž36. 30 Ž29. 29 Ž17. 22 Ž26. 2 Žy.

48 Ž26. 21 Ž21. 20 Ž27. 69 Ž71. 31 Žy.

17 Ž19. 6 Ž13. 9 Ž13. 0 Ž10. 0 Žy.

21 Ž19. 13 Ž18. 14 Ž23. 27 Ž5. 0 Žy.

The number in brackets is the fraction determined using ultrafiltration Ž -1000 D. for the same sample Žy denotes values below the detection limit.. a For the E2 horizon ns 5. b For the O2 horizon ns 5.

208

Horizon

Species

Acid Citric acid

O1

O2

Fumaric acid

Oxalic acid

n

Average

Standard deviation

n

Average

Standard deviation

concentration ŽmM. AlA Ž%. FeA Ž%. MgA Ž%. CaA Ž%. A Ž%. HA Ž%. H 2 A Ž%. H 3 A Ž%.

13

103.3 17.4 0.8 0.4 1.9 - 0.1 5.4 55.3 18.5

56.4 8.3 0.4 0.2 1.3

13

4.0 0.3 - 0.1 - 0.1 - 0.1 11.0 68.7 19.7

2.9 0.2

concentration ŽmM. AlA Ž%. FeA Ž%. MgA Ž%. CaA Ž%. A Ž%. HA Ž%. H 2 A Ž%. H 3 A Ž%.

16

53.6 20.8 1.4 0.1 0.4 - 0.1 2.6 46.9 26.1

42.9 11.6 1.6 0.1 0.3

2.3 7.8 5.8

1.2 9.9 7.6

15

2.0 0.6 - 0.1 - 0.1 - 0.1 5.8 63.5 30.0

n

2.2 5.7 7.6

Standard deviation

n

Average

Standard dev.

7

7.0 37.5 16.5 2.2 1.3 9.4 34.5

3.1 7.7 2.9 1.2 0.8 1.9 5.3

13

4490 0.6 0.2 0.5 1.4 16.5 82.1

2000 0.2 0.1 0.3 0.9 7.2 6.5

8

6.2 48.0 17.3 0.8 0.4 4.9 28.2

4.1 9.6 5.3 0.5 0.3 2.3 9.1

16

3560 1.3 0.6 0.2 0.4 10.1 88.3

2778 0.9 1.2 0.1 0.3 4.2 4.8

4.2 3.1 7.1 1.5 0.5

Average

Org HMW

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Table 3 Average concentration and speciation of citric, fumaric, oxalic and Org HMW for the O1–B2 horizons

E

B2

17

concentration ŽmM. AlA Ž%. FeA Ž%. MgA Ž%. CaA Ž%. A Ž%. HA Ž%. H 2 A Ž%. H 3 A Ž%.

3

concentration ŽmM. AlA Ž%. FeA Ž%. MgA Ž%. CaA Ž%. A Ž%. HA Ž%. H 2 A Ž%. H 3 A Ž%.

0

23.4 71.3 3.5 - 0.1 - 0.1 - 0.1 1.8 15.2 4.4 23.3 70.0 3.7 1.5 0.8 - 0.1 6.2 5.0 0.2

7.3 9.6 1.7

16

1.1 7.6 3.8 5.3 19.6 1.8 2.3 1.3

1

5.0 8.1 0.4 0

2.2 4.0 - 0.1 - 0.1 - 0.1 14.1 67.0 14.9 1.1 0.6 - 0.1 - 0.1 - 0.1 70.4 3.7 33.0

1.4 3.8

10

6.3 3.4 6.7 9

8

4.7 79.6 12.4 - 0.1 2.4 5.5

4.7 7.6 4.5

19

1303 7.2 2.2 - 0.1 0.1 19.9 70.4

856 6.8 3.8

16

441 5.5 0.9 0.3 0.3 67.1 25.9

225 5.2 1.0 0.1 0.2 22.0 23.6

15

419 1.9 9.8 0.2 0.3 76.4 11.2

407 3.2 20.5 0.2 0.2 25.2 12.3

1.8 2.8

3.5 68.6 14.9 0.4 - 0.1 11.9 3.9

0.9 15.3 8.7 0.2

3.4 45.9 7.1 1.1 0.3 39.3 5.4

0.3 24.7 4.4 1.0 0.3 24.2 5.4

5.8 3.6

0.1 8.6 13.7

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B1

concentration ŽmM. AlA Ž%. FeA Ž%. MgA Ž%. CaA Ž%. A Ž%. HA Ž%. H 2 A Ž%. H 3 A Ž%.

209

210

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The result of the MINEQLq run when using the monoprotic model for Org HMW ŽMINEQLq . was compared to the total Al and Fe in solution and the concentration of ultrafiltered Ž- 1000 D. Fe ŽFe UF . and ultrafiltered organic-Al Žorganic Al UF . . The comparison is shown for the six sampling occasions at Nyanget in Fig. 1 for Al and in Fig. 2 for Fe. Acceptable agreement was found ¨ between the modelled concentrations and organic Al UF and Fe UF especially for the spring and summer samplings Ž Figs. 1 and 2. . With respect to the E horizon the fraction of Fe bound to LMW organic acids and phosphate commonly under-estimated the Fe UF concentration ŽFig. 2. which also was observed at the two other sites Ž not shown. . In the autumn the same was frequently noted for Al ŽFig. 1.. As a whole, the results for the autumn samples were more scattered for all three sites. The declining level of LMW organic acids in deeper horizons Žvan Hees et al., 2000a. was reflected in the decreasing amount of Al and Fe bound to these compounds. The fraction of Al phosphate of the modelled concentration of Al bound to LMW organic acids and phosphate in the O–E horizons was on average 5.1% for the springrsummer and 11.5% for the autumn samples. AlHPO4 was the dominant Al phosphate complex. It is not certain to what extent the Al phosphate complexes were subtracted as inorganic Al Ž Clarke, 1994. in the ultrafiltered samples; if not subtracted, these complexes are included in the organic Al UF . In some B horizon samples the percentage of Al phosphate complexes were sometimes higher, most likely as a result of the small total concentrations of Al. The values of LMW Org-Al obtained employing the Al 3q Ž MINEQLq ŽAl 3q .. was on average 27% higher than the ones yielded using Org HMW as input value. The Al 3q approach seemed to explain better the autumn values for the E horizon when comparing with the ultrafiltration, while for the spring and summer samples concentrations higher than the ultrafiltration were seen Ž Fig. 1. . The maximum, minimum and median values of the fraction of Al and Fe bound to LMW organic acids and phosphate using the monoprotic Ž MINEQLq . model are summarised in Table 2 together with the corresponding Fe UF and organic Al UF values. Average values, including all sites and sampling occasions, of the Fe UF and organic Al UF concentrations are shown in Fig. 3c–d. From Table 2 it can be seen that generally acceptable agreement was obtained considering the number of variables and the lack of optimisation. Some extreme values, in particular for the B2 horizon, were observed. This is most likely a result of the very small concentrations of the analytes in the lower horizons introducing large relative errors. The minimum value for Al and Fe in the E horizon at Heden was caused by the absence of citric acid in this sample. The observation that the fraction of Fe UF and organic Al UF frequently increased in the B1 horizon compared to the E layer Žvan Hees et al., 2000a. could in most cases not be elucidated by the model. No obvious trend concerning seasonal variation of maximum or minimum values was apparent.

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Fig. 1. Plot of Al bound to LMW organic acids modelled using MINEQLq Žincluding Al-phosphate. with Org HMW or MINEQLq with Al 3q activity as input value for the six sampling occasions at the site Nyanget. Comparison to organic Al Ž -1000 D. and total Al in solution. ¨

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Fig. 2. Plot of Fe bound to LMW organic acids modelled using MINEQLq with Org HMW as input value for the six sampling occasions at the site Nyanget. Comparison to Fe Ž -1000 D. and ¨ total Fe in solution.

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Fig. 3. Average values for the distribution of inorganicrorganic forms of Ža. Ca, Žb. Mg, Žc. Al and Žd. Fe for the O1–B2 calculated by MINEQLq. The grey areas denotes free metal ion, the white areas metal ion bound to Org HMW and the black areas metal ion bound to LMW organic acids and phosphate.

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4.2. Speciation of Al, Fe, Mg and Ca in soil solution The level of organic complexation with regards to Org HMW and LMW acids for Ca, Mg, Al and Fe was obtained from the MINEQLq runs. The results are shown in Fig. 3. The concentration of Ca and Mg were always highest in the O horizon and declined rapidly in the lower layers. The runs showed that little Ca and Mg were organically complexed. The highest level of organic complexation was obtained for the O1 horizon 13.8 and 13.1% for Ca and Mg, respectively. For the other horizon ŽO2–B2. the percentage of organic-metal complexes of the total metal concentration varied between 6.0–8.9% for Ca and 5.9–9.0% for Mg. The fraction of the two metals bound to LMW organic acids and phosphate was generally insignificant. The highest level was recorded for Ca in the O1 horizon, 1.1% of the total Ca concentration, and for Mg in the B2 horizon corresponding to 0.6% of the total Mg in solution. The opposite pattern was seen for Al and Fe. The highest concentration was generally observed in the E horizon. In the O1 horizon, 98.8 and ) 99.9% of the Al and Fe, respectively, were complexed to organic acids or phosphate. The fraction of inorganic Al increased deeper down in the profile and was 4.0, 9.6 and 15.2% for the E, B1 and B2 horizons, respectively. The figure for inorganic Al includes free Al 3q, AlŽOH. 2q and AlSO4q. Al phosphate complexes were not included in this fraction for the same reasons as discussed in Section 4.1. Also the fraction of inorganic Fe 3q increased in the deeper horizons but was much smaller than for Al. 4.3. Speciation of citric, oxalic and fumaric acids and Org HM W The HPLC determinations revealed that the major LMW organic acid in the O and E horizon solutions was citric acid. Fumaric acid was also identified in most samples from these horizons. In the B horizon solutions oxalic acid was dominant but the concentrations were generally - 4 mM. For further information on the LMW organic acids see van Hees et al. Ž 2000a. . The speciation and average concentrations of citric, oxalic and fumaric acids together with Org HMW is shown in Table 3. The data show the percentage of the acids bound to different metal ions or protons. With regard to citric acid, a large fraction was bound to Al and Fe and the greatest fraction was observed in the E horizon in which 71% of the citrate was complexed with Al. The percentage bound to Fe was always much smaller. The same pattern was seen for oxalic acid. In the E horizon, 80% of the oxalate was associated with Al. The fraction bound to Fe Ž7–17%. was larger than for citrate and this was the case for all horizons. Fumaric acid, which was estimated to form much weaker Al and Fe complexes than citric and oxalic acids, appeared mainly as HAy in the horizons where it was most common Ž O1–E. . In all cases the fraction of Org HMW complexed with Al and Fe was - 10%. The highest value for Al was obtained in the E horizon

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Ž7.2%. and for Fe in the B2 horizon Ž 9.8%.. The lowest fractions was computed for the O1 horizon. For all horizons the percentage of Org HMW bound to Ca and Mg was low Ž- 2.2%.. The major fraction of Org HMW was in the form of HOrg HMW in the O1–E horizons and in the dissociated form in the B horizon. The pH in the B horizon was generally much higher than in the O and E horizons Žvan Hees et al., 2000a. . Regarding the Al and Fe bound to LMW organic acids and phosphate, it was observed that in samples containing citric acid, this acid was the by far most important complex former. This acid has the highest stability constant for Al and Fe of all the identified acids. Al complexed to citrate, mostly in a 1:1 complex, commonly made up about 80% of the total amount bound to LMW organic acids and phosphate Ž not shown. . In the B horizon where citric acid was very seldom observed, oxalate proved to be the most important acid binding c. 85% of the LMW organic Al and Al phosphate. Concerning Fe, oxalic acid proved to be of great importance in the samples of the O and E horizons where it was identified. The fraction of Fe oxalate commonly corresponded to 25–40% of the total amount of bound to LMW organic acids and phosphate in these samples Žnot shown.. As in the case of Al, oxalic acid was dominant for Fe in the B horizon complexing about 90% of the LMW fraction.

5. Discussion 5.1. Modelling parameters In the MINEQLq runs the acidity not explained by the LMW organic acids Žcalled Org HMW . was treated according to the monoprotic model of Lundstrom ¨ Ž1993., and the stability constant with Al was also taken from that study. The constants were derived from studies of soil solutions from the Heden site. Lundstrom ¨ Ž1993. observed that the average of the apparent stability constant, log K AlOrg , was lower in the autumn than in the spring and summer Ž log K AlOrg s 4.87 and 5.42, respectively. . The reason for this observation might be a pH dependence of the stability constant. This was the case in the work of Lundstrom ¨ Ž1993., and this observation has also been made in other studies ŽBizri et al., 1984; Filella et al., 1995. . The pH was generally somewhat lower in the autumn also in this study. The apparent stability constant can also be affected by the metal to organic ligand ratio Ž Filella et al., 1995. . This ratio did, however, not vary with season to any greater extent in the samples analysed. Another possible cause for the seasonal variation of log K AlOrg is a different composition of the acids in the autumn than in the springrsummer Ž Lundstrom, ¨ 1993.. The use of seasonal stability constants improved the fit of the autumn samples, although larger deviations to experimental values were still seen compared to the springrsummer samples. The apparent p K a values varied

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between 4.0 and 4.7 ŽLundstrom, ¨ 1993., and a value of 4.3 and 4.5 was set for the summerrspring and autumn samplings, respectively. However, these values were not optimised. All the Fe in solution was treated as trivalent. This assumption is based on the observation that Fe in soils forms stronger complexes with fulvic acid than Al does ŽSchnitzer and Hansen, 1970.. In another investigation of soil solution it was found that the percentage of inorganic Fe was lower than that for Al Ž Evans and Manley, 1983. . These findings indicate that most Fe would be in the ferric state since the trivalent ion forms much stronger complexes with most organic acids than the divalent one does Ž e.g., Martell and Smith, 1977. . However, the presence of ferrous iron in soils is normally thermodynamically possible Že.g., O’Neill, 1993.. Although inorganic Fe was not determined in this work, it was assumed that the major part of the metal was organically bound, and that this was the case for both the total and ultrafiltered concentrations. The stability constant Žlog K Fe . for Fe with Org HMW was set to 8.00 and 7.45 for the springrsummer and autumn samples, respectively. The difference between log K Al and log K Fe was of the same order of magnitude as reported by Schnitzer and Hansen Ž1970. . Also the difference between 1:1 complexes of strong complex forming LMW organic acids, e.g., citrate show a difference of about 2–3 powers ŽMartell and Smith, 1977; Gregor and Powell, 1986. . No fitting experiments regarding the stability constants were carried out. Regarding the stability constants for Ca and Mg, these were estimated to log K s 2.50. This value is somewhat lower but comparable to those reported by Schnitzer and Hansen Ž1970.. No adjustment of the log K values for the autumn samplings was carried out. A drawback of using a monoprotic model for Org HMW is that there is a great shift in the level of dissociated acid once the pH becomes higher than the p K a value. This was demonstrated for the E and B1 horizons Ž Table 3. . Considering the complexity of fulvic and humic acids such shifts are less likely. In order to overcome such problems, more advanced models have to be applied, e.g., WHAM ŽTipping, 1994. . 5.2. Complexation of Al and Fe to LMW organic acids The results Ž Figs. 1–3 and Tables 2 and 3. show that a significant fraction of Al and Fe is bound to LMW organic acids and phosphate. The modelling results were compared to those from the ultrafiltration Žorganic Al UF and Fe UF . . The applied stability constants for Org HMW also seemed to explain the finding from the ultrafiltration experiments that the fraction of LMW-Fe Ž- 1000 D. was less than for Al Žvan Hees et al., 2000a.. This was achieved, although the stability constants for all LMW acids with Fe were higher than for Al. A fairly good agreement was found between the ultrafiltered and computed concentrations for the springrsummer samplings. The autumn values were more

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disperse. It was found that the DOC levels were generally higher in the autumn ŽRiise et al., 2000., and it was also observed that a slightly higher fraction of the DOC was of molecular weight - 1000 D Žnot shown. . An explanation for the greater deviation, in particular for the E horizon, for the autumn samplings might be that the impact of fulvic acids of relatively low molecular weight is greater during this season. The low modelled values in the E horizon were even more pronounced for Fe. This finding could be explained by the lack of optimisation or the above mentioned fulvic acids. Another possibility is a significant impact of siderophores which are of relatively low MW. This kind of strongly complexing Fe ligand has been found in aqueous extracts of a coniferous forest soil Ž Reid et al., 1984. . However, the ‘total’ fraction of Fe UF and organic Al UF does not appear to be much altered over the growing season Ž van Hees et al., 2000a.. The fraction of Al bound to LMW organic acids was also determined experimentally for the O1 and O2 horizons using size exclusion chromatography ŽSEC. Ž van Hees et al., 2000a.. The fraction of LMW-Al employing this technique was somewhat lower than for the ultrafiltration runs, but this could partly be accounted for by the incomplete recovery of the SEC runs. Al phosphate complexes were not determined in the SEC experiments. As a whole, the two methods showed a similar pattern. Citric acid, which is the acid present at the highest concentration in the O–E horizons Ž van Hees et al., 2000a. , is also the acid with the highest complexing ability of the studied acids Že.g., Martell and Smith, 1977. . Of the Al bound to LMW organic acids and phosphate, the fraction bound to citrate was generally about 80% in the samples where this acid was identified. In contrast about 20% of the citrate in the O layer and 70% in the E horizon were bound to Al. The higher figure for the E horizon is most likely due to the lower concentrations of citric acid and higher levels of Al than in the O horizon. In the B horizon, oxalic acid was in most cases the dominant acid, and was also the major complex former. Shikimic acid, which was identified in all horizons in the range 1–35 mM Žvan Hees et al., 2000a. , was not included in the calculations since no stability constants for Al nor Fe were found. However, the exclusion of this acid should not have affected the calculations much. The concentration of this acid was generally low and it is less likely that it should have a high stability constant ¨ considering its chemical structure ŽOhman and Hogberg, personal communica¨ tion.. The role of LMW organic acids as carriers of Al and Fe in the podzolization process have been stressed in a number of papers Ž Aritovskaya and Zykina, 1977; Buurman and Van Reeuwijk, 1984; Lundstrom ¨ and Giesler, 1995.. At subsequent biodegradation of the organic complexes, inorganic Al and Fe could be released into solution. These free metal ions could then be immobilised in secondary mineral phases or the exchangeable pool. The equilibrium calcula-

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tions together with the experimental results Ž van Hees et al., 2000a. show that this hypothesis is possible. Parallel processes are likely, however, since most Al and Fe appeared to be bound to organic acids of higher molecular weight. 5.3. Organic complexation of Ca, Mg, Al and Fe The level of organically bound Ca, Mg, Al and Fe was calculated using the MINEQLq runs. The results are presented in Fig. 3a–d. Concerning Al, the fraction of calculated inorganic Al Ž excluding Al phosphate. can be compared to the experimental ones obtained using FIA Žvan Hees et al., 2000a.. For the E and B1 horizons, in which inorganic Al normally could be detected by FIA, median values of 10–20% ŽE. and 8–17% ŽB1. of the total Al were observed. Corresponding MINEQLq values were 4% for E and 10% for B1. The B1 figure is within the experimental range, but the reasons why the E figure is much lower are not obvious. One possible explanation is the lack of parameters that can be varied to improve the fit of the data. If the Al phosphate complexes were included in the calculated inorganic fraction this would still be much too low for the E horizon. However, the magnitude of the concentration of inorganic Al ŽAl 3q activity. is better predicted from equilibrium calculations including total soluble Al and other species in solution than, for example, mineral equilibria only Žvan Hees et al., 2000b.. Since inorganic Fe was not experimentally determined, the modelling results can not be compared, but as mentioned above it is reasonable to expect that the fraction of inorganic Fe is small. The level of organically bound Ca and Mg was also calculated. Since the fraction of Ca and Mg bound to LMW organic acids in most cases was very small the amount of these metals passing the ultrafilter Ž- 1000 D. could be estimated to be inorganic. However, very scattered values of the fraction - 1000 D, in the range 30–90%, were observed. The ultrafiltered fraction was generally smaller than the calculated one, but due to the disperse experimental values no precise conclusions could be made.

6. Conclusions The fraction of Al and Fe bound to LMW organic acids and phosphate was calculated using a chemical equilibrium model. Acceptable agreement with experimental values using ultrafiltration Ž- 1000 D. was found especially for the spring and summer samplings indicating that significant amounts of Al and Fe in the O1–B2 horizons were complexes of these compounds. The largest fractions were found for the O1 horizon, 38–49% for Al and 18–29% for Fe. Generally the fraction declined in the deeper horizons. In the autumn, larger deviations between the modelled and ultrafiltered fractions were seen implying that fulvic acids of relatively low MW might be more important during this

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season. Citric acid was found to be the most important complex former in the O1–E horizons and oxalic acid in the B horizon. The modelling results support the theory that LMW organic acids contribute to the translocation of Al and Fe in the podzolization process. Calculations of the fraction of inorganic Al indicated that 1–15% of the element was in this form. The fraction increased downwards in the profile. Acceptable agreement with experimental values was found for the B1 horizon while the model underestimated inorganic Al in the E horizon. Modelling of the fraction of organically bound Ca and Mg showed that 90–95% appeared as inorganic ions in the O2–B2 layers. In the O1 horizon the figure was about 86%. Complexation to LMW organic acids was generally negligible. Acknowledgements The authors would like to thank L. Dunas-Karlsson, A.-M. van Hees, C. Kave ˚ ˚ and a number of project workers for skilfully performing all the analyses, and L. Backman for performing soil sampling at the Hyytiala ¨ ¨ site. A.-M. van Hees is also acknowledged for data compilation. This work was financially supported by the Swedish Council for Forestry and Agricultural Research Ž SJFR. and the Swedish Natural Science Research Council Ž NFR. . References Aritovskaya, T.V., Zykina, L.V., Biological factors of aluminium migration and accumulation in soils and weathering crusts. In: Problems of Soil Science. NAUKA, Moscow, 1977, pp. 175–182. Ball, J.W., Nordstrom, D.K., Jenne, E.A., 1980. Additional and revised thermochemical data for WATEQ2, computerized model for trace and element speciation in mineral equilibria of natural waters. USGS Water Resour. Invest., Menlo Park, CA. Bergelin, A., van Hees, P.A.W., Wahlberg, O., Lundstrom, ¨ U.S., 2000. The acid-base properties of high and low molecular weight organic acids in soil solutions of podzolic soils. Geoderma 94, 221–233, Žthis issue.. Bilinski, H., Horvath, L., Ingri, N., Sjoberg, S., 1986. Equilibrium aluminium hydroxo-oxalate ¨ phases during initial clay formation; Hq –Al 3q-oxalic acid–Naq system. Geochim. Cosmochim. Acta 50, 1911–1922. Bizri, Y., Cromer, M., Scharff, J., Guillet, Rouiller, J., 1984. Constantes de stabilite´ de complexes organo-mineraux. Interactions des ions plombeux avec les composes ´ organiques hydrosolubles des eaux gravitaires de podzol. Geochim. Cosmochim. Acta 48, 227–234. Boudot, J.-P., Bel Hadji Brahim, A., Steiman, R., Seigle-Murandi, F., 1989. Biodegradation of synthetic organo-metallic complexes of iron and aluminium with selected metal to carbon ratios. Soil Biol. Biochem. 21, 961–966. Buurman, P., Van Reeuwijk, L.P., 1984. Proto-imogolite and the process of podzol formation: a critical note. J. Soil Sci. 35, 447–452. Clarke, N., 1994. Speciation of aluminium and iron in natural fresh waters. PhD thesis, The Royal Institute of Technology, Stockholm.

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