- Email: [email protected]
Mobility of different size fractions of organic carbon, Al, Fe, Mn and Si in podzols
Geoderma 94 Ž2000. 237–247
Mobility of different size fractions of organic carbon, Al, Fe, Mn and Si in podzols Gunnhild Riise
a,)
, Patrick Van Hees b, Ulla Lundstrom ¨ b, Line Tau Strand a
a
Department of Soil and Water Sciences, Agricultural UniÕersity of Norway, P.O. Box 5028, ˚ Norway N-1432 As, b Department of Chemistry and Process Technology, Mid Sweden UniÕersity, S-85170 SundsÕall, Sweden Received 10 November 1998; received in revised form 27 May 1999; accepted 2 June 1999
Abstract The influence of size distribution on the translocation of organic carbon, Fe, Al, Mn and Si in soil waters, has been studied in Scandinavian podzol profiles from three different locations. Soil solutions from different horizons have been filtered through a 0.45 mm membrane filter prior to further size fractionation by ultramembrane techniques. Organic carbon, Fe, Al, Mn and Si were determined in: ŽI. a colloidal fraction–nominal molecular weight cutoff ) 3 kDa, ŽII. a medium size fraction–nominal molecular weight cutoff 1–3 kDa, and ŽIII. a low molecular size fraction– nominal molecular weight cutoff - 1 kDa. The colloidal fraction comprise large proportions of organic carbon in the O and the E horizons, while low molecular size organic carbon fractions increase with depth in the B horizon on a percentage basis. A major part of Fe and Al seems to be associated with the colloidal fraction of organic carbon in the O horizon, and results suggest that colloidal organic matter contribute to the vertical translocation of metals through the E horizon. Maximum concentrations of Si, Al and Fe are found in the E horizon. Concentration of colloidal Si is significantly higher in the E than the other horizons. Although the total concentration of Si is four times the concentration of Al, colloidal Si and Al are present in more or less similar concentrations. This suggest that Si and Al are associated with each other in colloids in the E horizon. Equilibrium calculations, however, indicate that proto-imogolite sols are not very likely, and correlation coefficients rather suggest interaction of Al with colloidal organic carbon. The major part of Fe is in the colloidal fraction in all horizons. In contrast, Mn decreases sharply with
)
Corresponding author. Tel.: q47-64-94-83-54; Fax: q47-64-94-82-11. E-mail address: [email protected] ŽG. Riise..
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 9 . 0 0 0 4 4 - 0
238
G. Riise et al.r Geoderma 94 (2000) 237–247
depth, and is generally in a low molecular weight form. This indicates that Mn is related to the litter turnover process and that Mn takes little part in the podzolization process. q 2000 Elsevier Science B.V. All rights reserved. Keywords: podzols; organic matter; soil water; size distribution; Fe; Mn; Al; Si
1. Introduction The behaviour of organic carbon in soil water is essential for the migration of metals such as Fe and Al which play important roles in the podzolization process. Earlier results have shown that the size distribution of organic carbon in soil water is of importance for its vertical migration ŽRiise et al., 1994. . As soil waters pass through different horizons, parameters such as pH, ionic strength and concentrations of polyvalent ions will change, all of which may influence the size distribution of organic carbon. Characterization of organic matter in soils is often based on analyses of material extracted by chemical reagents. In order to study dynamic processes in soils, however, non-disturbing separation techniques are needed, such as centrifugation and filtration methods. To obtain new insight into translocation processes that occur in podzol profiles, soil waters have been collected from three locations ŽNyanget, Heden and Hyytiala ¨ ¨ . by centrifugation. The soil waters have been fractionated according to size by means of ultramembrane techniques. Major emphasis has been placed on the colloidal size fraction, as colloidal organic matter makes up a large part of organic carbon in soil waters. In addition, colloids can represent transition states between solid and aqueous phases and can be important transporting agents for associated metals. It is therefore suggested that the mobilization and immobilization of organic colloids can play a major role in the development of podzol profiles.
2. Experimental Ž 64814X N, 10846X E. , The investigated podzol profiles were located at Nyanget ¨ X X X X Heden Ž64814 N, 10846 E. and Hyytiala ¨ Ž61848 N, 24819 E.. For further description of the profiles see Ilvesniemi et al. Ž2000. . Soil waters were separated from fresh soil samples according to a centrifugation method given in Giesler and Lundstrom ¨ Ž1993. and filtered through a 0.45 mm membrane filter ŽMillex-HV, Millipore.. To minimize storage effects, the freshly collected soil water samples were transported by express mail within a couple of days to the individual laboratories for size fractionation. The smallest size fraction Ž nominal molecular weight cutoff 1 kDa. was separated by means of a stirred cell method Ž Amicon model 8050., where water samples passed a Diaflow YM-1 membrane under a
G. Riise et al.r Geoderma 94 (2000) 237–247
239
N2 pressure of 3–3.5 bar. Prior to use, the prewashed Diaflow YM-1 membranes were conditioned with 2 ml sample solutions which were discarded from the rest of the sample. The next size fraction Žnominal molecular weight cutoff 3 kDa. was separated by means of a Centriprep concentrator method Ž Amicon Centriprep 3.. Prior to use, the YM membranes of the Centriprep concentraters were washed with 0.01 M NaOH, 0.01 M HNO 3 and Milli-Q water. In the prewashed Centriprep concentrators, water samples were filled into an outer container and centrifuged for 3 h at 3000= g. Clogging of the membrane was reduced as the ultrafiltrate was collected in an inner collector, while the high molecular weight fraction Ž) 3 kDa. was concentrated in an outer container during the centrifugation. The fractions - 0.45 mm, - 3 kDa and - 1 kDa was analysed for total organic carbon by a Shimadzu TOC-5000 instrument and for Fe, Al, Mn and Si by ICP. The following size fractions were then obtained: Ž I. a colloidal fraction: nominal molecular weight ) 3 kDa, ŽII. a medium size fraction: nominal molecular weight 1–3 kDa, and Ž III. a low molecular weight fraction: nominal molecular weight - 1 kDa. The linear correlation analyses ŽPearsons correlations. based on data from all three sites were calculated using procedures given in the Statistical Analyses System Ž SAS Institute, 1987. . The correlations are treated as significant if p - 0,05.
3. Results and discussion The investigated period started with a rainy season in the spring and early summer, followed by an exceptionally long dry period lasting from the end of July to nearly the end of September 1996. Limited precipitation and high evaporation contributed to a substantial increase in the concentrations of organic carbon, Si, Al, Fe and Mn in the upper organic horizon during the dry period, with a peak in September. During the dry period soil solution samples could not always be obtained due to lack of water. Thus, the average values of organic carbon, given in Table 1, are not based on the same number of samples in all horizons. In spite of these limitations, a more or less similar size distribution pattern of organic carbon was generally observed for the three different locations Ž Table 1. . The correlation coefficients given in the Hyytiala, ¨ Heden and Nyanget ¨ text are based on results from all three sites. In the figures, however, results only from the Nyanget profile are shown. ¨ 3.1. Organic carbon (- 0.45 m m) Below the O1 horizon, there is an overall decrease in the concentration of organic carbon with depth ŽFig. 1.. Considering the size distribution, the colloidal fraction Ž) 3 kDa. makes up a large part of organic carbon in the upper horizons, varying from 45–75% in the two upper O horizons, with an
G. Riise et al.r Geoderma 94 (2000) 237–247
240
Table 1 Average values of organic carbon in different size fractions of the Hyttiala ¨ ŽHy., Heden ŽHe. and Nyanget profiles. The numbers are given as concentration ŽmM. and relative values Ž%. ¨ Horizon ) 3 kDa Hy
-1 kDa
1–3 kDa He
Ny
Hy
He
Ny
Hy
He
Ny
mM Ž%. mM Ž%. mM Ž%. mM Ž%. mM Ž%. mM Ž%. mM Ž%. mM Ž%. mM Ž%. O1 O2 E1 E2 B1 B2 B3
18 12 2
Ž59. 20 Ž67. 13 Ž41. 2
5
Ž18. 7 2 2
Ž65. 30 Ž61. 19 Ž51. 4 3 Ž34. 1 Ž19. 4 Ž35. 2
Ž58. Ž60. Ž49. Ž58. Ž58. Ž33. Ž40.
7 1 7
Ž21. 3 Ž8. 3 Ž16. 5
8
Ž28. 5 2
Ž11. 7 Ž12. 3 Ž12. 1 7 Ž24. 4 Ž21. 1
Ž13. Ž8. Ž17. Ž12. Ž15. Ž11.
6 4 2
Ž20. 7 Ž24. 6 Ž44. 2
2
Ž54. 9 5 4
Ž24. 14 Ž26. 10 Ž36. 3 2 Ž43. 1 Ž59. 6 Ž61. 3
Ž29. Ž32. Ž34. Ž31. Ž49. Ž56. Ž56.
average value close to 60% Ž Figs. 1 and 2. . This proportion corresponds well with earlier results reported from organic horizons from American forest soils ŽHomann and Gringal, 1992.. At the end of the dry period in September, there was an increase in the colloidal concentration in the O1 horizon, reaching 58 mM in the Nyanget profile. In spite of increased ionic concentrations during this ¨ period, the colloidal proportion was not much influenced as colloidal organic carbon was close to the average value of 60%. Organic matter is generally not precipitated in the E horizon, so colloids present in the E horizon are probably being transported from the O horizon through the E horizon. Below the E horizon, the proportion of colloidal organic carbon decreases ŽFig. 2. , while the proportion of the low molecular weight form of organic carbon Ž- 1 kDa. increases with depth. This may be due to Ž I. preferential filtering of larger sized colloidal organic carbon and Ž II. a selective sorption of the more hydrophobic high molecular weight forms of organic matter ŽHerbert and Bertsch, 1995.. Low pH values in the upper organic layers ŽFig. 1. suggest high concentrations of organic acids causing weathering and dissolution of metals such as Si, Fe, Al and Mn. In addition, organic matter is an important chelating agent for Al and Fe in the upper organic horizon of podzols. A large part of these chelates seems to belong to the colloidal fraction, as the colloidal forms of Fe and Al are positively correlated with the colloidal form of organic carbon in the O1 and O2 horizons, where the correlation coefficients are r s 0.677 and r s 0.907, respectively. Also in the E horizon the correlations of colloidal organic carbon with colloidal Al Ž0.832. were high, supporting the hypothesis of colloidal organic matter as a transporting agent for Al through the E horizon. The total concentration of organic carbon Ž- 0.45 mm. decreases and the proportion of the low molecular organic carbon increases with depth in the B
G. Riise et al.r Geoderma 94 (2000) 237–247 Fig. 1. pH and the size distribution of organic carbon in absolute ŽmM. and relative terms Ž%. in the Nyanget profile during the growing season 1996. ¨
241
242
G. Riise et al.r Geoderma 94 (2000) 237–247
Fig. 2. Distribution of the colloidal fraction Ž ) 3 kDa. of organic carbon ŽmM. in the Nyanget ¨ profile during the growing season 1996.
horizon. This was observed also in podzol profiles from the western part of Norway, where soil water from lysimeters was ultrafiltered by means of hollow fibres ŽRiise et al., 1994. . The atomic ratios between AlrC and FerC in the colloidal fraction increase with depth below the soil surface to reach their maximum in the B1 horizon Ž Fig. 3. . This suggests an increased neutralization of negatively charged colloidal surfaces by polyvalent cations. In experiments
Fig. 3. Element ratios between AlrSi Žleft. and AlrC and FerC Žright. in the Nyanget profile ¨ during the growing season 1996.
G. Riise et al.r Geoderma 94 (2000) 237–247
243
with additions of Al and Fe to soil extracts it has been demonstrated that precipitation will occur when the atomic ratio is less than 17 for CrAl and 12 for CrFe ŽMcKeague et al., 1971; Petersen, 1976. . In this investigation corresponding values were CrAl 40 and CrFe 125 in soil solutions colloids Ž) 3 kDa., indicating that the stage of precipitation is not yet reached. Comparison between different studies might, however, be difficult as variations in pH, properties of organic carbon, etc., can influence the precipitation ratio. In this study, positive correlations between the colloidal fraction of organic carbon and the colloidal fraction of Fe Ž r s 0.772. and Al Ž r s 0.735. in the B1 horizon suggest that organic matter may be associated with colloidal Fe and Al in the soil solutions. Upon increased aggregation these colloids may coagulate and precipitate, and thereby be immobilized. In addition, direct interaction between organic carbon and the solid phase in the B horizons may take place, as proposed by Jardine et al. Ž1989. and Qualls and Haines Ž 1992. , who consider adsorption reactions between organic carbon and the mineral soil, particularly oxides, to be responsible for low organic carbon concentrations in deeper soil layers. 3.2. Silicon and aluminium Concentrations of both Al and Si peak in the E horizon, where maximum weathering intensity occurs Ž Fig. 4. . In the E horizon, maximum concentrations of Si and Al were reached at the end of the summer, in August ŽFig. 5.. A similar seasonal pattern has been observed in the E horizon of a subalpine
Fig. 4. Size distribution of Si ŽmM. and Al ŽmM. in the Nyanget profile during the growing season ¨ 1996.
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G. Riise et al.r Geoderma 94 (2000) 237–247
Fig. 5. Distribution of the colloidal fraction Ž ) 3 kDa. of Si ŽmM. and Al ŽmM. in the Nyanget ¨ profile during the growing season 1996.
Spodosol, with lowest concentrations of Al and Si in spring and highest concentrations in fall at the time of highest soil temperature ŽZabowski and Ugolini, 1992. . In all horizons, the major part of Si is in a low molecular weight form, presumably H 4 SiO4 , while Al in soil solutions is mainly colloidal Ž Fig. 4. . In the E horizon, the concentration of Si is about four times the concentration of Al. In the colloidal fraction, however, Al and Si occur in more or less similar concentrations Ž Fig. 3., from 40 to 60 mM ŽFig. 5.. Colloidal Si was positively correlated with colloidal Al Ž r s 0.735. and Fe Ž r s 0.659. in the E horizon, which might indicate interactions between these elements. Farmer Ž 1987. has suggested that Si and Al, liberated by weathering in the podzol E horizon, migrate downward in inorganic forms and once the soil pH is sufficiently high ŽpH s 4.5., they form proto-imogolite, sols that may precipitate as imogolite-type materials Ž ITM.. However, results from many investigations have shown that the main part of Al in unacidified podzol E horizons is organically bound as determined by the method of Driscoll Ž 1984. Ž Ugolini and Dahlgren, 1987; Lundstrom, ¨ 1993; Giesler et al., 1996.. Correlation between colloidal Al and colloidal organic carbon Ž r s 0.832. also supports that Al is primary bound to organic matter in the E horizon. Based on equilibrium calculations ŽVan Hees et al., 2000. proto-imogolite sols could form only in the B horizons, where ITM in fact also was found ŽKarltun et al., 2000.. Thus, the possibility of proto-imogolite formation in the E horizon is not very likely. Instead of separate inorganic colloids, however, organo-mineral associations may form. The interac-
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245
Fig. 6. Size distribution of Mn ŽmM. and Fe ŽmM. in the Nyanget profile during the growing ¨ season 1996.
tions between inorganic ŽAl, Si, Fe. and organic colloids can, however, be rather complex and needs further study. 3.3. Manganese and iron Manganese reached quite high concentrations in the O1 horizon, more than 100 mM, but decreases sharply with depth Ž Fig. 6.. Mn seems, therefore, to take
Fig. 7. Distribution of the colloidal fraction Ž ) 3 kDa. of Mn ŽmM. and Fe ŽmM. in the Nyanget ¨ profile during the growing season 1996.
246
G. Riise et al.r Geoderma 94 (2000) 237–247
little part in the soil forming processes in the mineral soil Ž E and B horizons. . The major part of Mn was in a low molecular weight form, in agreement with its mobile character under acidic conditions. Much lower concentration of Mn further down the profile may indicate that the Mn in the upper horizons is related to the litter turnover process and is relatively quickly taken up by the vegetation or other soil organisms. Mn was positively correlated with organic carbon in the O1 Ž r s 0.742. and O2 Ž r s 0.909. horizons. The concentration of Fe is generally four to five times lower than of Al, and in all horizons the major part of Fe is colloidal Ž Figs. 6 and 7. . Generally, colloidal Fe is significantly correlated with Al in all horizons down to the B1 horizon.
4. Summary Ultrafiltration of soil waters reveals that a major proportion of organic carbon is colloidal in the upper horizons. During the growing season colloidal organic matter varied from 45 to 75% , with an average of 60% in the two upper organic horizons. Systematic seasonal changes in the colloidal proportions were not observed, as colloidal organic carbon was close to average at the end of the summer, at the time of highest ionic concentration and highest concentration of organic carbon. There seems to be a selective retention of colloidal organic matter, causing an increase in the proportion of the low molecular weight forms with depth. Correlations between colloidal organic carbon and colloidal Al and Fe suggest vertical translocation of metals in the form of organic colloids. While the major part of Fe was associated with the colloidal size fraction, Mn was generally in a low molecular weight form and consequently not much influenced by the vertical translocation of colloidal organic matter. In the E horizon, increased concentrations of colloidal Si and Al were correlated, which might suggest that Si and Al are associated with each other in colloids in these horizon. Equilibria calculations, however, imply that proto-imogolite sols are not very likely ŽVan Hees et al., 2000. and correlation between colloidal Al and colloidal organic carbon rather support that Al is primary bound to organic matter in the E horizon. The interaction between inorganic and organic colloids can, however, be rather complex and needs further study.
References Driscoll, C.T., 1984. The determination of specific forms of aluminium in natural waters. Chem. Geol. 15, 177–191. Farmer, V.C., 1987. The role of inorganic species in the transport of aluminium in podzols. In: Righi, D., Chauvel, A. ŽEds.., Podzols and Podzolization. Assoc. Franc. Etude Sol Plaisir, pp. 187–194.
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Giesler, R., Lundstrom, ¨ U., 1993. Soil solution chemistry: effects of bulking soil samples. Soil Sci. Soc. Am. J. 57, 128. Giesler, R., Moldan, F., Lundstrom, ¨ U.S., Hultberg, H., 1996. Reversing acidification in a forested catchment in southwestern Sweden: effects on soil solution chemsitry. J. Environ. Qual. 25, 110–119. Herbert, B.E., Bertsch, P.M., 1995. Characterization of dissolved and collodial organic matter in soil solution. In: McFee, W.W., Kelly, J.M. ŽEds.., Carbon Forms and Functions in Forest Soils. Soil Sci. Soc. Am., Madison, USA, pp. 63–88, Chap. 5. Homann, P.S., Grigal, D.F., 1992. Molecular weight distribution of soluble organics from laboratory-manipulated surface soils. Soil Sci. Soc. Am. J. 56, 1305–1310. Ilvesiemi, H., Giesler, R., Magnusson, T., Melkerud, P.A., 2000. General description of three investigated podzolic sites and sampling techniques. Geoderma 94, 109–122, Žthis issue.. Jardine, P.M., Weber, N.L., McCarthy, J.F., 1989. Mechanisms of dissolved organic carbon adsorption on soils. Soil Sci. Soc. 53, 1378–1385. Karltun, E., Bain, D., Gustafsson, J.P., Mannerkoski, H., Murad, E., Wagner, U., Fraser, T., Mc Hardy, B., Starr, M., 2000. Surface reactivity of poorly ordered minerals in podzol B horizons. Geoderma 94, 236–286, Žthis issue.. Lundstrom, ¨ U.S., 1993. The role of organic acids in soil solution chemistry in a podzolized soil. J. Soil Sci. 44, 121–133. McKeague, J.A., Brydon, J.E., Miles, N.M., 1971. Differentiation of forms of extractable iron and aluminium in soils. Soil Sci. Soc. Am. Proc. 35, 33–38. Petersen, L., 1976. Podzols and podzolization. PhD thesis. Royal Veterinary and Agricultural University, Copenhagen. Qualls, R.G., Haines, B.L., 1992. Biodegradability of dissolved organic matter in forest throughfall, soil solution, and stream water. Soil Sci. Soc. Am. J. 56, 578–586. Riise, G., Salbu, B., Vogt, R.D., Ranneklev, S.B., Mykkelbost, T.C., 1994. Mobility of humic substances, major and minor elements in lake Skjervatjern and its catchment area. Environ. Int. 20, 287–298. SAS Institute, 1987, SASrSTATe. Guide for Personal Computers, Version 6 Edition. SAS Institute, Cary, NC, 1028 pp. Ugolini, F.C., Dahlgren, R., 1987. The mechanism of podzolization as revealed by soil solutions studies. In: Righi, D., Chauvel, A. ŽEds.., Podzols and Podzolisation. INRA, Assoc. Franc. Etude Sol. Plaisir et Paris. Van Hees, P., Lundstrom, ¨ U., Starr, M., Giesler, R., 2000. Factors influencing aluminium in soil solution of podzolic soils. Geoderma 94, 287–308, Žthis issue.. Zabowski, D., Ugolini, F.C., 1992. Seasonality in the mineral stability of a subalpine Spodosol. Soil Sci. 154, 497–507.
Mobility of different size fractions of organic carbon, Al, Fe, Mn and Si in podzols Gunnhild Riise
a,)
, Patrick Van Hees b, Ulla Lundstrom ¨ b, Line Tau Strand a
a
Department of Soil and Water Sciences, Agricultural UniÕersity of Norway, P.O. Box 5028, ˚ Norway N-1432 As, b Department of Chemistry and Process Technology, Mid Sweden UniÕersity, S-85170 SundsÕall, Sweden Received 10 November 1998; received in revised form 27 May 1999; accepted 2 June 1999
Abstract The influence of size distribution on the translocation of organic carbon, Fe, Al, Mn and Si in soil waters, has been studied in Scandinavian podzol profiles from three different locations. Soil solutions from different horizons have been filtered through a 0.45 mm membrane filter prior to further size fractionation by ultramembrane techniques. Organic carbon, Fe, Al, Mn and Si were determined in: ŽI. a colloidal fraction–nominal molecular weight cutoff ) 3 kDa, ŽII. a medium size fraction–nominal molecular weight cutoff 1–3 kDa, and ŽIII. a low molecular size fraction– nominal molecular weight cutoff - 1 kDa. The colloidal fraction comprise large proportions of organic carbon in the O and the E horizons, while low molecular size organic carbon fractions increase with depth in the B horizon on a percentage basis. A major part of Fe and Al seems to be associated with the colloidal fraction of organic carbon in the O horizon, and results suggest that colloidal organic matter contribute to the vertical translocation of metals through the E horizon. Maximum concentrations of Si, Al and Fe are found in the E horizon. Concentration of colloidal Si is significantly higher in the E than the other horizons. Although the total concentration of Si is four times the concentration of Al, colloidal Si and Al are present in more or less similar concentrations. This suggest that Si and Al are associated with each other in colloids in the E horizon. Equilibrium calculations, however, indicate that proto-imogolite sols are not very likely, and correlation coefficients rather suggest interaction of Al with colloidal organic carbon. The major part of Fe is in the colloidal fraction in all horizons. In contrast, Mn decreases sharply with
)
Corresponding author. Tel.: q47-64-94-83-54; Fax: q47-64-94-82-11. E-mail address: [email protected] ŽG. Riise..
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 9 . 0 0 0 4 4 - 0
238
G. Riise et al.r Geoderma 94 (2000) 237–247
depth, and is generally in a low molecular weight form. This indicates that Mn is related to the litter turnover process and that Mn takes little part in the podzolization process. q 2000 Elsevier Science B.V. All rights reserved. Keywords: podzols; organic matter; soil water; size distribution; Fe; Mn; Al; Si
1. Introduction The behaviour of organic carbon in soil water is essential for the migration of metals such as Fe and Al which play important roles in the podzolization process. Earlier results have shown that the size distribution of organic carbon in soil water is of importance for its vertical migration ŽRiise et al., 1994. . As soil waters pass through different horizons, parameters such as pH, ionic strength and concentrations of polyvalent ions will change, all of which may influence the size distribution of organic carbon. Characterization of organic matter in soils is often based on analyses of material extracted by chemical reagents. In order to study dynamic processes in soils, however, non-disturbing separation techniques are needed, such as centrifugation and filtration methods. To obtain new insight into translocation processes that occur in podzol profiles, soil waters have been collected from three locations ŽNyanget, Heden and Hyytiala ¨ ¨ . by centrifugation. The soil waters have been fractionated according to size by means of ultramembrane techniques. Major emphasis has been placed on the colloidal size fraction, as colloidal organic matter makes up a large part of organic carbon in soil waters. In addition, colloids can represent transition states between solid and aqueous phases and can be important transporting agents for associated metals. It is therefore suggested that the mobilization and immobilization of organic colloids can play a major role in the development of podzol profiles.
2. Experimental Ž 64814X N, 10846X E. , The investigated podzol profiles were located at Nyanget ¨ X X X X Heden Ž64814 N, 10846 E. and Hyytiala ¨ Ž61848 N, 24819 E.. For further description of the profiles see Ilvesniemi et al. Ž2000. . Soil waters were separated from fresh soil samples according to a centrifugation method given in Giesler and Lundstrom ¨ Ž1993. and filtered through a 0.45 mm membrane filter ŽMillex-HV, Millipore.. To minimize storage effects, the freshly collected soil water samples were transported by express mail within a couple of days to the individual laboratories for size fractionation. The smallest size fraction Ž nominal molecular weight cutoff 1 kDa. was separated by means of a stirred cell method Ž Amicon model 8050., where water samples passed a Diaflow YM-1 membrane under a
G. Riise et al.r Geoderma 94 (2000) 237–247
239
N2 pressure of 3–3.5 bar. Prior to use, the prewashed Diaflow YM-1 membranes were conditioned with 2 ml sample solutions which were discarded from the rest of the sample. The next size fraction Žnominal molecular weight cutoff 3 kDa. was separated by means of a Centriprep concentrator method Ž Amicon Centriprep 3.. Prior to use, the YM membranes of the Centriprep concentraters were washed with 0.01 M NaOH, 0.01 M HNO 3 and Milli-Q water. In the prewashed Centriprep concentrators, water samples were filled into an outer container and centrifuged for 3 h at 3000= g. Clogging of the membrane was reduced as the ultrafiltrate was collected in an inner collector, while the high molecular weight fraction Ž) 3 kDa. was concentrated in an outer container during the centrifugation. The fractions - 0.45 mm, - 3 kDa and - 1 kDa was analysed for total organic carbon by a Shimadzu TOC-5000 instrument and for Fe, Al, Mn and Si by ICP. The following size fractions were then obtained: Ž I. a colloidal fraction: nominal molecular weight ) 3 kDa, ŽII. a medium size fraction: nominal molecular weight 1–3 kDa, and Ž III. a low molecular weight fraction: nominal molecular weight - 1 kDa. The linear correlation analyses ŽPearsons correlations. based on data from all three sites were calculated using procedures given in the Statistical Analyses System Ž SAS Institute, 1987. . The correlations are treated as significant if p - 0,05.
3. Results and discussion The investigated period started with a rainy season in the spring and early summer, followed by an exceptionally long dry period lasting from the end of July to nearly the end of September 1996. Limited precipitation and high evaporation contributed to a substantial increase in the concentrations of organic carbon, Si, Al, Fe and Mn in the upper organic horizon during the dry period, with a peak in September. During the dry period soil solution samples could not always be obtained due to lack of water. Thus, the average values of organic carbon, given in Table 1, are not based on the same number of samples in all horizons. In spite of these limitations, a more or less similar size distribution pattern of organic carbon was generally observed for the three different locations Ž Table 1. . The correlation coefficients given in the Hyytiala, ¨ Heden and Nyanget ¨ text are based on results from all three sites. In the figures, however, results only from the Nyanget profile are shown. ¨ 3.1. Organic carbon (- 0.45 m m) Below the O1 horizon, there is an overall decrease in the concentration of organic carbon with depth ŽFig. 1.. Considering the size distribution, the colloidal fraction Ž) 3 kDa. makes up a large part of organic carbon in the upper horizons, varying from 45–75% in the two upper O horizons, with an
G. Riise et al.r Geoderma 94 (2000) 237–247
240
Table 1 Average values of organic carbon in different size fractions of the Hyttiala ¨ ŽHy., Heden ŽHe. and Nyanget profiles. The numbers are given as concentration ŽmM. and relative values Ž%. ¨ Horizon ) 3 kDa Hy
-1 kDa
1–3 kDa He
Ny
Hy
He
Ny
Hy
He
Ny
mM Ž%. mM Ž%. mM Ž%. mM Ž%. mM Ž%. mM Ž%. mM Ž%. mM Ž%. mM Ž%. O1 O2 E1 E2 B1 B2 B3
18 12 2
Ž59. 20 Ž67. 13 Ž41. 2
5
Ž18. 7 2 2
Ž65. 30 Ž61. 19 Ž51. 4 3 Ž34. 1 Ž19. 4 Ž35. 2
Ž58. Ž60. Ž49. Ž58. Ž58. Ž33. Ž40.
7 1 7
Ž21. 3 Ž8. 3 Ž16. 5
8
Ž28. 5 2
Ž11. 7 Ž12. 3 Ž12. 1 7 Ž24. 4 Ž21. 1
Ž13. Ž8. Ž17. Ž12. Ž15. Ž11.
6 4 2
Ž20. 7 Ž24. 6 Ž44. 2
2
Ž54. 9 5 4
Ž24. 14 Ž26. 10 Ž36. 3 2 Ž43. 1 Ž59. 6 Ž61. 3
Ž29. Ž32. Ž34. Ž31. Ž49. Ž56. Ž56.
average value close to 60% Ž Figs. 1 and 2. . This proportion corresponds well with earlier results reported from organic horizons from American forest soils ŽHomann and Gringal, 1992.. At the end of the dry period in September, there was an increase in the colloidal concentration in the O1 horizon, reaching 58 mM in the Nyanget profile. In spite of increased ionic concentrations during this ¨ period, the colloidal proportion was not much influenced as colloidal organic carbon was close to the average value of 60%. Organic matter is generally not precipitated in the E horizon, so colloids present in the E horizon are probably being transported from the O horizon through the E horizon. Below the E horizon, the proportion of colloidal organic carbon decreases ŽFig. 2. , while the proportion of the low molecular weight form of organic carbon Ž- 1 kDa. increases with depth. This may be due to Ž I. preferential filtering of larger sized colloidal organic carbon and Ž II. a selective sorption of the more hydrophobic high molecular weight forms of organic matter ŽHerbert and Bertsch, 1995.. Low pH values in the upper organic layers ŽFig. 1. suggest high concentrations of organic acids causing weathering and dissolution of metals such as Si, Fe, Al and Mn. In addition, organic matter is an important chelating agent for Al and Fe in the upper organic horizon of podzols. A large part of these chelates seems to belong to the colloidal fraction, as the colloidal forms of Fe and Al are positively correlated with the colloidal form of organic carbon in the O1 and O2 horizons, where the correlation coefficients are r s 0.677 and r s 0.907, respectively. Also in the E horizon the correlations of colloidal organic carbon with colloidal Al Ž0.832. were high, supporting the hypothesis of colloidal organic matter as a transporting agent for Al through the E horizon. The total concentration of organic carbon Ž- 0.45 mm. decreases and the proportion of the low molecular organic carbon increases with depth in the B
G. Riise et al.r Geoderma 94 (2000) 237–247 Fig. 1. pH and the size distribution of organic carbon in absolute ŽmM. and relative terms Ž%. in the Nyanget profile during the growing season 1996. ¨
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Fig. 2. Distribution of the colloidal fraction Ž ) 3 kDa. of organic carbon ŽmM. in the Nyanget ¨ profile during the growing season 1996.
horizon. This was observed also in podzol profiles from the western part of Norway, where soil water from lysimeters was ultrafiltered by means of hollow fibres ŽRiise et al., 1994. . The atomic ratios between AlrC and FerC in the colloidal fraction increase with depth below the soil surface to reach their maximum in the B1 horizon Ž Fig. 3. . This suggests an increased neutralization of negatively charged colloidal surfaces by polyvalent cations. In experiments
Fig. 3. Element ratios between AlrSi Žleft. and AlrC and FerC Žright. in the Nyanget profile ¨ during the growing season 1996.
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with additions of Al and Fe to soil extracts it has been demonstrated that precipitation will occur when the atomic ratio is less than 17 for CrAl and 12 for CrFe ŽMcKeague et al., 1971; Petersen, 1976. . In this investigation corresponding values were CrAl 40 and CrFe 125 in soil solutions colloids Ž) 3 kDa., indicating that the stage of precipitation is not yet reached. Comparison between different studies might, however, be difficult as variations in pH, properties of organic carbon, etc., can influence the precipitation ratio. In this study, positive correlations between the colloidal fraction of organic carbon and the colloidal fraction of Fe Ž r s 0.772. and Al Ž r s 0.735. in the B1 horizon suggest that organic matter may be associated with colloidal Fe and Al in the soil solutions. Upon increased aggregation these colloids may coagulate and precipitate, and thereby be immobilized. In addition, direct interaction between organic carbon and the solid phase in the B horizons may take place, as proposed by Jardine et al. Ž1989. and Qualls and Haines Ž 1992. , who consider adsorption reactions between organic carbon and the mineral soil, particularly oxides, to be responsible for low organic carbon concentrations in deeper soil layers. 3.2. Silicon and aluminium Concentrations of both Al and Si peak in the E horizon, where maximum weathering intensity occurs Ž Fig. 4. . In the E horizon, maximum concentrations of Si and Al were reached at the end of the summer, in August ŽFig. 5.. A similar seasonal pattern has been observed in the E horizon of a subalpine
Fig. 4. Size distribution of Si ŽmM. and Al ŽmM. in the Nyanget profile during the growing season ¨ 1996.
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Fig. 5. Distribution of the colloidal fraction Ž ) 3 kDa. of Si ŽmM. and Al ŽmM. in the Nyanget ¨ profile during the growing season 1996.
Spodosol, with lowest concentrations of Al and Si in spring and highest concentrations in fall at the time of highest soil temperature ŽZabowski and Ugolini, 1992. . In all horizons, the major part of Si is in a low molecular weight form, presumably H 4 SiO4 , while Al in soil solutions is mainly colloidal Ž Fig. 4. . In the E horizon, the concentration of Si is about four times the concentration of Al. In the colloidal fraction, however, Al and Si occur in more or less similar concentrations Ž Fig. 3., from 40 to 60 mM ŽFig. 5.. Colloidal Si was positively correlated with colloidal Al Ž r s 0.735. and Fe Ž r s 0.659. in the E horizon, which might indicate interactions between these elements. Farmer Ž 1987. has suggested that Si and Al, liberated by weathering in the podzol E horizon, migrate downward in inorganic forms and once the soil pH is sufficiently high ŽpH s 4.5., they form proto-imogolite, sols that may precipitate as imogolite-type materials Ž ITM.. However, results from many investigations have shown that the main part of Al in unacidified podzol E horizons is organically bound as determined by the method of Driscoll Ž 1984. Ž Ugolini and Dahlgren, 1987; Lundstrom, ¨ 1993; Giesler et al., 1996.. Correlation between colloidal Al and colloidal organic carbon Ž r s 0.832. also supports that Al is primary bound to organic matter in the E horizon. Based on equilibrium calculations ŽVan Hees et al., 2000. proto-imogolite sols could form only in the B horizons, where ITM in fact also was found ŽKarltun et al., 2000.. Thus, the possibility of proto-imogolite formation in the E horizon is not very likely. Instead of separate inorganic colloids, however, organo-mineral associations may form. The interac-
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Fig. 6. Size distribution of Mn ŽmM. and Fe ŽmM. in the Nyanget profile during the growing ¨ season 1996.
tions between inorganic ŽAl, Si, Fe. and organic colloids can, however, be rather complex and needs further study. 3.3. Manganese and iron Manganese reached quite high concentrations in the O1 horizon, more than 100 mM, but decreases sharply with depth Ž Fig. 6.. Mn seems, therefore, to take
Fig. 7. Distribution of the colloidal fraction Ž ) 3 kDa. of Mn ŽmM. and Fe ŽmM. in the Nyanget ¨ profile during the growing season 1996.
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little part in the soil forming processes in the mineral soil Ž E and B horizons. . The major part of Mn was in a low molecular weight form, in agreement with its mobile character under acidic conditions. Much lower concentration of Mn further down the profile may indicate that the Mn in the upper horizons is related to the litter turnover process and is relatively quickly taken up by the vegetation or other soil organisms. Mn was positively correlated with organic carbon in the O1 Ž r s 0.742. and O2 Ž r s 0.909. horizons. The concentration of Fe is generally four to five times lower than of Al, and in all horizons the major part of Fe is colloidal Ž Figs. 6 and 7. . Generally, colloidal Fe is significantly correlated with Al in all horizons down to the B1 horizon.
4. Summary Ultrafiltration of soil waters reveals that a major proportion of organic carbon is colloidal in the upper horizons. During the growing season colloidal organic matter varied from 45 to 75% , with an average of 60% in the two upper organic horizons. Systematic seasonal changes in the colloidal proportions were not observed, as colloidal organic carbon was close to average at the end of the summer, at the time of highest ionic concentration and highest concentration of organic carbon. There seems to be a selective retention of colloidal organic matter, causing an increase in the proportion of the low molecular weight forms with depth. Correlations between colloidal organic carbon and colloidal Al and Fe suggest vertical translocation of metals in the form of organic colloids. While the major part of Fe was associated with the colloidal size fraction, Mn was generally in a low molecular weight form and consequently not much influenced by the vertical translocation of colloidal organic matter. In the E horizon, increased concentrations of colloidal Si and Al were correlated, which might suggest that Si and Al are associated with each other in colloids in these horizon. Equilibria calculations, however, imply that proto-imogolite sols are not very likely ŽVan Hees et al., 2000. and correlation between colloidal Al and colloidal organic carbon rather support that Al is primary bound to organic matter in the E horizon. The interaction between inorganic and organic colloids can, however, be rather complex and needs further study.
References Driscoll, C.T., 1984. The determination of specific forms of aluminium in natural waters. Chem. Geol. 15, 177–191. Farmer, V.C., 1987. The role of inorganic species in the transport of aluminium in podzols. In: Righi, D., Chauvel, A. ŽEds.., Podzols and Podzolization. Assoc. Franc. Etude Sol Plaisir, pp. 187–194.
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Giesler, R., Lundstrom, ¨ U., 1993. Soil solution chemistry: effects of bulking soil samples. Soil Sci. Soc. Am. J. 57, 128. Giesler, R., Moldan, F., Lundstrom, ¨ U.S., Hultberg, H., 1996. Reversing acidification in a forested catchment in southwestern Sweden: effects on soil solution chemsitry. J. Environ. Qual. 25, 110–119. Herbert, B.E., Bertsch, P.M., 1995. Characterization of dissolved and collodial organic matter in soil solution. In: McFee, W.W., Kelly, J.M. ŽEds.., Carbon Forms and Functions in Forest Soils. Soil Sci. Soc. Am., Madison, USA, pp. 63–88, Chap. 5. Homann, P.S., Grigal, D.F., 1992. Molecular weight distribution of soluble organics from laboratory-manipulated surface soils. Soil Sci. Soc. Am. J. 56, 1305–1310. Ilvesiemi, H., Giesler, R., Magnusson, T., Melkerud, P.A., 2000. General description of three investigated podzolic sites and sampling techniques. Geoderma 94, 109–122, Žthis issue.. Jardine, P.M., Weber, N.L., McCarthy, J.F., 1989. Mechanisms of dissolved organic carbon adsorption on soils. Soil Sci. Soc. 53, 1378–1385. Karltun, E., Bain, D., Gustafsson, J.P., Mannerkoski, H., Murad, E., Wagner, U., Fraser, T., Mc Hardy, B., Starr, M., 2000. Surface reactivity of poorly ordered minerals in podzol B horizons. Geoderma 94, 236–286, Žthis issue.. Lundstrom, ¨ U.S., 1993. The role of organic acids in soil solution chemistry in a podzolized soil. J. Soil Sci. 44, 121–133. McKeague, J.A., Brydon, J.E., Miles, N.M., 1971. Differentiation of forms of extractable iron and aluminium in soils. Soil Sci. Soc. Am. Proc. 35, 33–38. Petersen, L., 1976. Podzols and podzolization. PhD thesis. Royal Veterinary and Agricultural University, Copenhagen. Qualls, R.G., Haines, B.L., 1992. Biodegradability of dissolved organic matter in forest throughfall, soil solution, and stream water. Soil Sci. Soc. Am. J. 56, 578–586. Riise, G., Salbu, B., Vogt, R.D., Ranneklev, S.B., Mykkelbost, T.C., 1994. Mobility of humic substances, major and minor elements in lake Skjervatjern and its catchment area. Environ. Int. 20, 287–298. SAS Institute, 1987, SASrSTATe. Guide for Personal Computers, Version 6 Edition. SAS Institute, Cary, NC, 1028 pp. Ugolini, F.C., Dahlgren, R., 1987. The mechanism of podzolization as revealed by soil solutions studies. In: Righi, D., Chauvel, A. ŽEds.., Podzols and Podzolisation. INRA, Assoc. Franc. Etude Sol. Plaisir et Paris. Van Hees, P., Lundstrom, ¨ U., Starr, M., Giesler, R., 2000. Factors influencing aluminium in soil solution of podzolic soils. Geoderma 94, 287–308, Žthis issue.. Zabowski, D., Ugolini, F.C., 1992. Seasonality in the mineral stability of a subalpine Spodosol. Soil Sci. 154, 497–507.