Factors of Soil Formation

The Podzol has been elected “Soil of the year 2007” in Germany. This article reviews the present knowledge on the development, functions, and threats of Podzols. The main theories on mobilization and transport of organic matter, Fe, Al, and Si are (1) metal-organic migration, (2) metal reduction, and (3) inorganic sol migration. Immobilization theories include precipitation or polymerization due to increasing pH/abundance of base cations with depth, mechanical filtering in soil pores, oxidation of metal-organic complexes, biodegradation of the organic part, decreasing C-to-metal ratios during translocation, adsorption to soil particles, and flocculation at the point of zero charge. Podzolization is discussed also on the catena scale, where vertical and lateral translocation processes (across pedon boundaries) need to be considered to understand Podzol patterns in landscapes. Chronosequence studies show that incipient podzolization usually becomes visible between 100 and 500 y and mature Podzols develop in 1,000–6,000 y. The occurrence of Podzols worldwide is concentrated mainly on the boreal zone and mountain regions within the humid temperate zone. Smaller Podzol areas are found in some perhumid tropical and subtropical regions. In Germany, Podzols occur in the Alps, in the glaciofluvial valleys and heathlands of N Germany, and in the mountain ranges. They fulfil several ecological functions, especially for groundwater recharge. Main threats for these mostly sandy soils are wind erosion and surface mining of sand. Two pedons which were chosen to represent the “Soil of the year 2007” are presented. Finally, some conclusions about podzolization processes are drawn, which may explain the diverse observations reported in the literature.

Podzol: Soil of the Year 2007. A review on its genesis, occurrence, and functions

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Alteration, formation, and occurrence of minerals in soils

Historically, attention on soil organic matter (SOM) has focused on the central role that it plays in ecosystem fertility and soil properties, but in the past two decades the role of soil organic carbon in moderating atmospheric CO{sub 2} concentrations has emerged as a critical research area. This chapter will focus on the storage and turnover of natural organic matter in soil (SOM), in the context of the global carbon cycle. Organic matter in soils is the largest carbon reservoir in rapid exchange with atmospheric CO{sub 2}, and is thus important as a potential source and sink of greenhouse gases over time scales of human concern (Fischlin and Gyalistras 1997). SOM is also an important human resource under active management in agricultural and range lands worldwide. Questions driving present research on the soil C cycle include: Are soils now acting as a net source or sink of carbon to the atmosphere? What role will soils play as a natural modulator or amplifier of climatic warming? How is C stabilized and sequestered, and what are effective management techniques to foster these processes? Answering these questions will require a mechanistic understanding of how and where C is stored in soils. The quantitymore » and composition of organic matter in soil reflect the long-term balance between plant carbon inputs and microbial decomposition, as well as other loss processes such as fire, erosion, and leaching. The processes driving soil carbon storage and turnover are complex and involve influences at molecular to global scales. Moreover, the relative importance of these processes varies according to the temporal and spatial scales being considered; a process that is important at the regional scale may not be critical at the pedon scale. At the regional scale, SOM cycling is influenced by factors such as climate and parent material, which affect plant productivity and soil development. More locally, factors such as plant tissue quality and soil mineralogy affect decomposition pathways and stabilization. These factors influence the stability of SOM in part by shaping its molecular characteristics, which play a fundamental role in nearly all processes governing SOM stability but are not the focus of this chapter. We review here the most important controls on the distribution and dynamics of SOM at plot to global scales, and methods used to study them. We also explore the concepts of controls, processes, and mechanisms, and how they operate across scales. The concept of SOM turnover, or mean residence time, is central to this chapter and so it is described in some detail. The Appendix details the use of radiocarbon ({sup 14}C), a powerful isotopic tool for studying SOM dynamics. Much of the material here was originally presented at a NATO Advanced Study Institute on 'Soils and Global Change: Carbon Cycle, Trace Gas Exchange and Hydrology', held June 16-27, 1997, at the Chateau de Bonas, France.« less

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Storage and Turnover of Organic Matter in Soil

Pedogenesis of Chernozems in Central Europe — A review

A comparison was made between two soil climosequences on north- and south-facing slopes in northern Italy to determine the influence of slope aspect on soil processes. The climosequences span an elevational gradient ranging from moderate (1200 m a.s.l.) to high alpine (2420 m a.s.l.) climate zones on surfaces having an age of about 15 000 years. The soils were investigated with respect to organic C, oxalate and dithionite extractable Fe, Al and Si, elemental losses (Ca, Mg, K, Na, Fe, Al, Si, Mn) and clay minerals. The stocks of soil org. C as well as of oxalate-extractable Fe and Al was greatest in the subalpine zone near the timberline. There are no clear differences in organic C content between the soils on north- and south-facing sites. Fe-oxalate and to a lesser extent Alo-stocks were, however, greater on north-facing sites, indicating that weathering is greater there. Eluviation and illuviation of Al and Fe within the soil profile, typical for podzolisation, was more distinctly expressed on the N slopes. The probability of ITM (Imogolite-type-material) formation in the soil seemed to be greater on south-facing sites. On the north-facing sites, element leaching was most intense in the subalpine zone close to the timberline while on the south-facing sites this was only the case for the base cations. The N slopes exhibited higher leaching of elements which generally indicates a higher weathering intensity. On south-facing sites, typical podzolisation processes were measurable only above 2000 m a.s.l. The development of smectites is also a reflection of the weathering intensity; smectite was discernible in the surface horizon at all sites on N slopes but the highest amount was detected in the sub-alpine climate zone. For the south-facing sites only in the alpine climate zone could smectite be detected. Higher temperatures and an increased number of freeze-thaw cycles on south-facing slopes should theoretically enhance rates of chemical weathering. This could, however, not be confirmed with our measurements. The degree of chemical weathering increases from the south- to the north-facing sites that are characterised by lower temperatures, lower evapotranspiration and consequently by a higher humidity. Although precipitation in Alpine regions is abundant, the availability and flux of water through the soil is the prime factor in weathering intensity.

Effect of north and south exposure on weathering rates and clay mineral formation in Alpine soils

Vegetation effects on pedogenetic forms of Fe, Al and Si and on clay minerals in soils in southern Switzerland and northern Italy

Two soil profile sequences on paragneiss debris in the Val di Rabbi (Northern Italy) along an altitude gradient ranging from 1200 to 2400 m a.s.l. were studied to evaluate the effect of aspect on the weathering of clay minerals. All the soils had a coarse structure, a sandy texture and a low pH. Greater weathering intensities of clay-sized phyllosilicates (greater content of smectites) were observed in soils on the north-facing slope. On the south-facing slope, smectite was found only in the surface horizon of the soil profile at the highest altitude. Hot citrate treatment of north-facing soils revealed the presence of low-charged 2:1 clay minerals, the expansion of which was hindered in the untreated state by interlayered polymers. However, the hot citrate treatment encountered some problems with the samples of the south-facing soils: as confirmed by Fourier transform infrared spectroscopy, the hot citrate treatment was unable to remove all interlayer Al polymers. The 2:1 phyllosilicates were not expanded by ethylene glycol solvation in several samples, although thermogravimetric analyses indicated the presence of clay minerals with interlayer H 2 O. At the same time, the collapse of clay minerals to 1.0 nm following K-saturation was evident. Theoretically, this should indicate that 2:1 phyllosilicates had no evident substitution of trioctahedral cations (Mg 2+ , Fe 2+ ) by dioctahedral cations (Al 3+ and Fe 3+ ). X-ray diffraction analysis of the d 060 region and determination of the layer charge of clay minerals by the long-chain (C 18 ) alkylammonium ion, however, did not confirm this. A transformation from trioctahedral to dioctahedral species was observed and low-charge clay minerals (ξ ~0.30) were identified in the surface horizons of the south-facing sites. In the south-facing soils, the podzolization process was less pronounced because of a lower water flux through the soil and probably less complexing organic molecules that would remove the interlayer polymers. Besides the eluviation process, clay minerals underwent a process of ionic substitutions in the octahedral sheet that led to the reduction of the layer charge. This process was more obvious in the north-facing sites.

https://www.zora.uzh.ch/id/eprint/76303/1/2007_EgliM_CM-Egli_07_Kopie_.pdf

Effect of slope aspect on transformation of clay minerals in Alpine soils

Two soils, a haplic Podzol and a dystric Cambisol, developed from post-glacial tills, were studied with respect to their soil chemistry and clay mineralogy. Although the state factors (age, geology, topography, climate) of soil formation were almost identical, two different types developed. The E horizon of the Podzol contained more smectite, characterized by a montmorillonite-beidellite-mixed phase. The neoformation of smectite could be traced back to the weathering of mica and chlorite. The Podzol had less hydroxy-interlayered smectite (HIS) in the surface horizons than the Cambisol. A larger amount of chelating compounds in the E horizon of the Podzol either transformed HIS into smectites or inhibited the formation of HIS and favoured the formation of smectites. The physical structure of the soil material is believed to be the most important factor in the different modes of soil evolution. The greater abundance of coarse pores in the topsoil at the Podzol site probably led to a faster eluviation of base cations, a different vegetation at ground level, and, consequently, to a faster soil evolution with the formation of spodic horizons.

Soil evolution and development of the clay mineral assemblages of a Podzol and a Cambisol in ‘Meggerwald’, Switzerland

Many soils in southern Switzerland have a black color, contain a large amount of soil organic matter (SOM) and seem to have some andic properties although they did not develop on volcanic parent material. We investigated three typical 'black' soils to determine the mechanisms of (clay) mineral formation and transformation. We measured total element pools as well as the dithionite-, pyrophosphate- and oxalate-extractable fractions (Fe, Al, Si). The clay fraction (<2 μm) was analyzed using X-ray diffraction and FTIR spectroscopy. Iron speciation in the solid phase was determined by Mossbauer spectroscopy. With increasing weathering conditions, the plagioclase (albite) content decreases, trioctahedral species in the clay fraction such as biotite, chlorite or trioctahedral vermiculite either decompose or transform into a dioctahedral mineral such as dioctahedral vermiculite or hydroxy interlayered smectite (HIS). Typical weathering products were hydroxy interlayered vermiculite (HIV), HIS, interstratified minerals and kaolinite. The oxidation of Fe(II) into Fe(III) was evident and contributes to the transformation of trioctahedral mineral species into dioctahedral ones. In one soil, a large part of the Fe (up to 41%) was found in the form of Fe oxides. In the surface horizon, the poorly crystalline mineral ferrihydrite was dominant, while in the subsoil goethite prevailed. Maghemite (or maghemite/hematite mixture) was, furthermore, found in distinct concentrations down to a depth of ∼50 cm. The formation of this mineral requires high temperatures which means that a forest fire can influence soil mineralogy down to a considerable depth. The specific climatic conditions with periods of strong humidity alternating with periods of winter droughts, sporadic fire events and the relatively large content of poorly crystalline fractions of Fe and Al contributed to the stabilization of SOM.

Remo Zanelli

Papers

Effect of north and south exposure on weathering rates and clay mineral formation in Alpine soils

Vegetation effects on pedogenetic forms of Fe, Al and Si and on clay minerals in soils in southern Switzerland and northern Italy

Effect of slope aspect on transformation of clay minerals in Alpine soils

Soil evolution and development of the clay mineral assemblages of a Podzol and a Cambisol in ‘Meggerwald’, Switzerland

'Black' soils in the southern Alps: clay mineral formation and transformation, X-ray amorphous Al phases and Fe forms