Geoderma

Volume 94, Issues 2–4, February 2000, Pages 335-353

Advances in understanding the podzolization process resulting from a multidisciplinary study of three coniferous forest soils in the Nordic Countries

https://doi.org/10.1016/S0016-7061(99)00077-4 Get rights and content

Abstract

Geochemical, mineralogical, micromorphological, microbiological, hydrochemical and hydrological joint investigations were performed at two coniferous podzolic sites in the north of Sweden and at one in the south of Finland. Mycorrhizal fungi were found to create numerous pores (3–10-μm diameter) in many weatherable mineral grains in the eluvial (E) horizon. During the growing season, identified low molecular weight (LMW) organic acids such as citric, shikimic, oxalic and fumaric acids comprised 0.5–5% of the DOC and 0.5–15% of the total acidity in soil solutions. Between 20% and 40% of the dissolved Al was bound to the identified LMW organic acids. Mineral dissolution via complexing LMW acids, probably exuded in part by the mycorrhiza hyphae, is likely to be a major weathering process in podzols . We found no evidence for a decreasing C/metal ratio of the migrating organo-metal complexes that could explain the precipitation of secondary Fe and Al in the illuvial (B) horizon. Instead, microbial degradation of organic ligands resulting in the release of ionic Al and Fe to the soil solution may be an important process facilitating the formation of solid Al–Si–OH and Fe–OH phases in the podzol B horizon. However, within the B horizon transport as proto-imogilite (PI) sols might be possible. In the B horizon, the extractable Al and Fe was predominantly inorganic. The large specific surface area (SSA) removable by oxalate extraction, the high point of zero charge salt effect (PZSE), the low cation exchange capacity (CEC) and the high sulphate exchange capacity (SEC), pointed to the presence of short-range ordered variable charge phases. Imogolite type material (ITM) was indeed identified in all B horizons by IR spectroscopy and crystalline imogolite was found in the deep B horizon of one profile. Mössbauer spectroscopy indicated that Fe in the form of ferrihydrite was formed by intergrowth with an Al–Si–OH phase. The high amounts of Fe and Al transported from the O to the E horizon indicate that there could be an upward transport of these elements before they are leached to the B horizon. We hypothesize that the LMW Al complexes are transported by hyphae to the mor (O) layer, partly released and subsequently complexed by high molecular weight (HMW) acids.

Introduction

Podzols are the dominant soils in boreal forest zones with a humid climate. Podzols occur worldwide, but are most common in the northern hemisphere. Podzols are characterized by their organic surface horizon (O), underlain first by a weathered grey eluvial (E) horizon enriched in minerals with a slow weathering rate, e.g., quartz, and next by a dark brownish–reddish illuvial (B) horizon, which is enriched by Al and Fe, followed by the relatively unaffected C horizon.
Podzols have been intensively studied over a long period of time and different theories for their formation have been put forward, as reviewed by Lundström et al. (2000).
Two main theories for the mechanisms of podzolization are now prevalent, differing in their explanation for the mechanism of immobilization of Al and Fe in the B horizon. It is, however, generally accepted that Al and Fe migrate downwards from the E to the B horizon in the form of organic complexes.
In the adsorption/precipitation theory, relatively high molecular weight (HMW) organic (fulvic) acids leached from the O horizon, form complexes with Al and Fe thus creating the eluvial E horizon. These precipitate due to continuing addition of metals during the downward migration until a certain C/metal ratio has been reached, causing the complex to precipitate, thus creating the B horizon McKeague et al., 1971, Petersen, 1976.
In the biodegradation theory the organic ligands, mainly low molecular weight (LMW) organic acids, are microbially decomposed during downward migration, releasing ionic Al and Fe, which precipitate to form Al–Si–OH and Fe–OH solid phases as imogolite type material (ITM) and ferrihydrite in the B horizon Aristovskaya and Zykina, 1977, Lundström, 1993, Lundström et al., 1995. HMW organics may be adsorbed on the surfaces of the amorphous material in the B horizon.
Farmer (1979) and Anderson et al. (1982) proposed that ITM precipitates in the B horizon from Al–Si–OH sols formed at shallower depth.
The on-going acidification of podzolic soils in central and northwestern Europe caused by anthropogenic atmospheric deposition Giesler et al., 1996a, Giesler et al., 1996b, Lundström et al., 1998 and its impacts has stressed the necessity for a better understanding of the fundamental mechanisms involved in podzolization, especially as proposed models, mostly have failed to explain the consequences of the acidification (Løkke et al., 1996). Understanding of podzolization has many practical consequences, not least for forest management. Treatments with lime and wood ash of forest soils have been discussed.
This paper summarizes the results of a joint multidisciplinary project aimed at testing existing theories on the formation of podzols and throw more light on the fundamental mechanisms of the process. We studied three podzolic sites in northern Scandinavia assumed to be largely unaffected by anthropogenic acidification, with a wide array of methods and approaches.

Section snippets

Sites

The study sites are Nyänget and Heden 64°15′N, 19°45′E (Svartberget Forest Research Station) both situated 55 km NW of Umeå in northern Sweden and Hyytiälä 61°48′N, 24°19′E (Forestry Field Station of the University of Helsinki), 190 km NW of Helsinki in the south of Finland. At all sites the forest consists of 60–100-year-old Norway spruce (Picea abies) and Scots pine (Pinus silvestris). Nyänget is a glacial till, Heden and Hyytiälä are post-glacial fluvial sediments. Nyänget and Hyytiälä are

Weathering

The kinetics of weathering were slow enough to maintain ionic concentrations far below equilibrium with the primary minerals. The main minerals in the three soils were quartz, plagioclase and K-feldspar (Melkerud et al., 2000). The effects of weathering were most extreme in the E horizon, but some evidence of weathering was also observed in the B horizon. Effects of weathering included (i) higher amounts of quartz and less plagioclase in the E horizons than in the deeper soil horizons, (ii)

Conclusions

Mycorrhizal fungi create numerous pores in the weatherable mineral grains in the E horizon by their hyphae exuding organic acids. This might be the main weathering process in these soils. Low molecular weight acids such as citric, oxalic, shikimic, fumaric comprised 0.5–5% of the DOC and 0.5–15% of the total dissolved acidity. In the O and E horizons about 15% of the total Al was inorganic, 20–40% of the total Al was bound to LMW and the rest was bound to HMW organics. The large proportion of

Acknowledgments

The authors would like to thank all the people, who have been involved in the field, laboratory and data work. This work was financed by the Swedish Council of Forestry and Agricultural Research, SJFR.

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