Chemical Geology 363 (2014) 13–21

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Chemical Geology

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Characterisation of Andosols from Laacher See tephra by wet-chemicaland spectroscopic techniques (FTIR, 27Al-, 29Si-NMR)

Thilo Rennert a,⁎, Karin Eusterhues b, Syuntaro Hiradate c, Hergen Breitzke d, Gerd Buntkowsky d,Kai U. Totsche b, Tim Mansfeldt e

a Fachgebiet Bodenchemie mit Pedologie, Institut für Bodenkunde und Standortslehre, Universität Hohenheim, 70593 Stuttgart, Germanyb Lehrstuhl für Hydrogeologie, Institut für Geowissenschaften, Friedrich-Schiller-Universität Jena, Burgweg 11, 07749 Jena, Germanyc Biodiversity Division, National Institute for Agro-Environmental Sciences (NIAES), 3-1-3 Kan-nondai, Tsukuba, Ibaraki 305-8604, Japand Eduard-Zintl-Institut für Anorganische und Physikalische Chemie, Technische Universität Darmstadt, Petersenstr. 20, 64287 Darmstadt, Germanye Department für Geowissenschaften, Bodengeographie/Bodenkunde, Universität zu Köln, Albertus-Magnus-Platz, 50923 Köln, Germany

⁎ Corresponding author. Tel.: +49 711 45922325; fax:E-mail address: t.rennert@uni-hohenheim.de (T. Renn

0009-2541/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.chemgeo.2013.10.029

a b s t r a c t

a r t i c l e i n f o

Article history:Received 12 July 2013Received in revised form 21 October 2013Accepted 27 October 2013Available online 4 November 2013

Editor: Carla M. Koretsky

Keywords:AndosolsAllophane29Si- and 27Al-NMR spectroscopyExtraction

At 12,900 a BP, the eruptionof the Laacher See volcano generated a newparentmaterial forHolocene soil formationin parts of Western Germany. Weathering of these ashes commonly includes the formation of poorly crystallineminerals such as allophane, imogolite and ferrihydrite. Detection of these minerals in soil is difficult, yet animportant task, because they may govern soil functions and processes, e.g., stabilisation of organic matter andnutrient availability. Therefore, we characterised three forested Andosols by a combination of wet-chemicaland spectroscopic techniques including infrared and (27Al, 29Si) nuclearmagnetic resonance (NMR) spectroscopytogetherwith X-ray diffractometry. Deconvoluting the 29Si-NMR spectra revealed that 1.6 to 10.4% of total Si waspresent as allophanic compounds, which coincided with the amounts of oxalate-extractable Si. Since extractionmethods are not completely selective,we observed a slight overestimation of allophanic Si estimated fromoxalateextraction. Although the sites under study are located close to each other in similar relief positions andwith similarvegetation, the combination of our results revealed varying amounts of loess in the parent materials and varyingweathering intensity. High weathering intensities correlate with the amounts of allophane.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

About 6.3 km3 of phonolitic magmawere erupted by the Laacher Seevolcano (40 kmSof Bonn, Rhineland–Palatinate, Germany) 12,900 yearsbefore present (Schmincke et al., 1999). Fallout deposits make up thevast majority of the tephra volume (approx. 88%), pyroclastic flowdeposits approx. 10%, and base surge deposits a further b2% (van denBogaard et al., 1990). The bulk of the Laacher See tephra (LST) wasdeposited relatively close to the volcano in the Neuwied basin. Thethickness of the LST decreases from N50 m close to the vent to b1 mat a distance of about 50 km to the northeast (Schmincke et al., 1999).However, long-range aeolian transport has led to thin layers of LST inScandinavian soils and sediments several 100 kmaway. The LST,mainlyconsisting of ashes and pumice lapilli (Schmincke et al., 1999), is an im-portant parent material for Holocene soil development in the Neuwiedbasin. The material was rearranged by solifluction and slope-wash dur-ing the Younger Dryas, and loess and other foreign materials may havebeen incorporated (Stöhr, 1963; Gebhardt et al., 1969). In fact, LST is theyoungest sediment in the area, but erosion, subsequent deposition and

+49 711 45923117.ert).

ghts reserved.

mixingwith othermaterialsmay even have led, depending on the relief,to an inverse stratification of the soil-forming substrateswith LST in thesubsoil (Stöhr, 1963).

Soil formation on such volcanic tephras typically includes the forma-tion of characteristic poorly crystalline minerals such as allophane,imogolite and ferrihydrite (Dahlgren et al., 2004). Allophane is a short-range ordered aluminosilicate and forms hollow spherical particleswith diameters of 3–5 nm and a molar Si:Al ratio typically rangingfrom 1:2 to 1:1 (Wada, 1989). The spherules are made up of agibbsite-like Al-octahedral sheet and an inner surface consisting ofSi tetrahedra (Lowe, 1995). Imogolite is made up of the same structuralelements (molar Si:Al ratio 1:2), but forms hollow tubes with an outerdiameter of approx. 2 nm (Wada, 1989). Allophanic compounds resultfrom rapid weathering of volcanic glasses. Generally, allophane isformed at pH N 4.7 in drained soils with an udic moisture regime(Parfitt and Kimble, 1989). Depending on the activities of silicic acidand the availability of Al3+, Al-rich (or imogolite-like) allophane (Si:Al ≥ 1:2) or Si-rich (halloysite-like) allophane (Si:Al = 1) may form(Lowe, 1995).

Weathering of volcanic ashes, including the formation of allophaniccompounds, typically leads to the formation of Andosols (IUSSWorkingGroup WRB, 2006). Andic properties, the key to the reference soil typeof Andosols, are the combination of Alox + ½Feox N 20 g kg−1 soil

14 T. Rennert et al. / Chemical Geology 363 (2014) 13–21

(ox = extractable by acid oxalate solution), bulk density ≤ 0.9 kg dm−3,phosphate retention ≥ 85% and organic C ≤ 25%. These propertiesmust be present in at least one horizon within 100 cm below the soilsurface with a thickness of at least 30 cm, starting within the upper25 cm of the soil profile; or the sum of the horizons' thickness withthese properties must amount to N60% of the soil profile. Dependingon Al speciation, Andosols may be further differentiated as sil-andic(dominance of allophanic compounds) or alu-andic, where Al complex-ation with soil organic matter (SOM) dominates.

The formation of allophanic compounds is inhibited in surface hori-zons when Al released from volcanic glass by acid weathering is com-plexed by SOM. Then, Al is lacking for the precipitation with Si to formallophanic compounds, but on the other hand, complexation of SOMwith Al causes stabilisation of SOM against microbial degradation(Wada, 1987). This is one of the explanations for another striking prop-erty of Andosols: the accumulation of SOM, which is more pronouncedthan in any other mineral soil type globally, when relating soil organiccarbon (SOC) contents to the areas covered by the respective soil type(Eswaran et al., 1993). Several further explanations have been pro-posed. Burial of humic topsoils by repeated sedimentation of volcanicejecta leads to accumulation of SOM in the subsoil (Dahlgren et al.,2004). Other explanations include interactions of minerals with SOMlike adsorption on the surfaces of poorly crystalline minerals (Tornet al., 1997), formation of phases stabilising SOMwith intermediate de-grees of polymerisation between allophane and Al complexed bySOM (Basile-Doelsch et al., 2007), and both adsorption and co-precipitation on/with ferrihydrite (Eusterhues et al., 2011). Another as-pect of SOM stabilisation may be the fractal structure of the soil pores'network, which results in restricted accessibility for microorganismsin both allophanic (Chevallier et al., 2010) and non-allophanicAndosols (Filimonova et al., 2011).

These studies collectively suggest that the interactions of SOM withsoil minerals formed in Andosols are the key for SOM stabilisation.However, the mineralogical characterisation, especially of allophanicAndosols, with wide-spread techniques such as X-ray diffractometry(XRD) and Fourier-transform infrared (FTIR) spectroscopy is limited.The presence of short-range ordered minerals such as allophane andferrihydrite and amorphous volcanic glass broadens absorption bandsin FTIR spectra, which complicates phase identification. Their detectionby XRD is also hampered because of their broad diffuse instead of sharpreflections. Furthermore, allophane prevents the orientation of clayminerals. They are only partially detected byXRD (Gebhardt et al., 1969).

Alternatively, minerals in allophanic soils and sediments have beencharacterised by electron-microscopic techniques (e.g., Kawano et al.,1997; Malucelli et al., 1999; Kleber et al., 2004; Kaufhold et al., 2009)or nuclear magnetic resonance (NMR) spectroscopy. Analyses basedon 27Al and 29Si give valuable information on the molecular environ-ment of these atoms, which allows for a differentiation of octahedraland tetrahedral Al and a variety of Si-containing phases such asallophane, imogolite, volcanic glass and opaline silica, which cannot beunambiguously detected by XRD and FTIR spectroscopy (e.g., Shimizuet al., 1988; MacKenzie et al., 1991; Petrini et al., 1999; Hiradate, 2004,2005; Hiradate and Wada, 2005; Hiradate et al., 2006; Basile-Doelschet al., 2007). The NMR techniques have helped to enlighten the contri-bution of soil minerals to SOM stabilisation under tropical weatheringconditions (Basile-Doelsch et al., 2007), but to the best of our knowl-edge, they have not been applied to soils developed from volcanicashes in the temperate climate of Central Europe, where weatheringof primary minerals also leads to a varying composition of secondarysoil minerals. Identification of allophane is further complicated by thepresence of foreign, non-volcanic material like loess, as is frequentlyfound in LST. We therefore aimed at studying Andosols formed fromLST by a combination of wet-chemical and spectroscopic techniques.Their applications help to characterise soil minerals in terms of short-range ordered aluminosilicates, which are hard to detect owing totheir poor crystallinity and small particle size. We consider this

intensive characterisation necessary because the large surface areaand reactivity of theseminerals accounts for several soil-chemical prop-erties and functions of Andosols such as nutrient availability (e.g., phos-phate), filter and buffer function, aggregation, SOM sequestration andstabilisation.

2. Materials and methods

2.1. Study site and soil sampling

The study area is located in the “Märkerwald” in the Neuwied basin,NE Rhineland–Palatinate, Germany, about 20 km N of the city of Ko-blenz and about 3 km W of the city of Dierdorf. The “Märkerwald” is adeciduous forest covering approximately 35 km2, mainly stocked withbeech. We studied three soil profiles (Soil1, 50°31′52.24″ N, 7°37′15.47″ E; Soil2, 50°31′54.78″ N, 7°36′17.0″ E; Soil3, 50°32′21.30″ N,7°37′9.55″ E) at 310 m (Soil1, Soil3) and 340 m(Soil2) absolute altitudein almost flat terrain. The annual mean temperature is 8 °C, and themean annual precipitation 965 mm. According to Schmincke et al.(1999), the thickness of tephra deposited in this area is about 1 mand consists of the phonolitic feldspar-rich middle Laacher See tephraC (MLST C). Samples used for the determination of the bulk densitywere taken with steel cylinders. Samples used for all other analyseswere taken with a spattle. These samples were air-dried and sievedto b2 mm. All samples were taken in excavated pits at three depthsaccording to their genetic horizons.

2.2. General characterisation

Soil pHwasmeasured potentiometrically in de-ionised H2O at a soil-to-solution ratio of 1:2.5. Clay fractions were separated from the bulksoil by sedimentation after sonication at 400 J ml−1 after removal ofthe sand fraction by sieving. The bulk soils were extracted in duplicatewith dithionite–citrate-bicarbonate (DCB; Mehra and Jackson, 1960),oxalate (ox; Schwertmann, 1964) and pyrophosphate (py; McKeague,1968). The DCB extracts were analysed for Fe, the oxalate extracts forFe, Mn, Al and Si, and the pyrophosphate extracts for Al by atomicabsorption spectrometry (SIMAA 6000, PerkinElmer, Rodgau, Germany).Phosphate retention was determined after Blakemore et al. (1987) byspectrophotometrical detection (PerkinElmer Lambda 25).

For the quantification of allophane, we used the methods of Mizotaand van Reeuwijk (1989),

%allophane ¼ 100

−5:1Alox−Alpy

� �

Sioxþ 23:4

�%Siox;

and by Parfitt (1990), where the allophane content is calculated bymul-tiplying Siox by a factor K derived from the ratio (Alox − Alpy):Siox. Theoriginal publication by Parfitt (1990) gives factors ranging from 5 to16 for (Alox − Alpy):Siox ratios ranging from 1 to 3.5 in steps of 0.5. Toaccount for the exact ratios of our samples, we fitted an empiricalthird-order polynomial to the factors K and Al:Si ratios (expressing theratio (Alox − Alpy):Siox) given by Parfitt (1990):

K ¼ 4:81−0:73Al : Siþ 0:86Al : Si2 þ 0:07Al : Si3; r2 ¼ 0:99;n ¼ 6:

Total C and N in the bulk soils and the clay fractions were quantifiedby elemental analysis (Vario EL, Elementar Analysensysteme, Hanau,Germany) in duplicate. As the soils were free of carbonates, total Cequals organic C. X-ray fluorescence analysis (XRF; Philips PW 2400(PANanalytical, Almelo, The Netherlands)) was used to quantify majorelements in bulk soils on Li2B4O7 fusion disks. Bulk density was deter-mined in triplicate with 100 ml steel cylinders according to IUSSWorking Group WRB (2006).

15T. Rennert et al. / Chemical Geology 363 (2014) 13–21

2.3. Characterisation of soil minerals

The clay fractions were characterised by Fourier-transform infrared(FTIR) spectroscopy in the mid-IR range (4000–400 cm−1). About1 mg of each sample was gently mortared and mixed with KBr. Pellets(1.3 cm diameter) were pressed at 31 MPa for 3 min. Spectra wererecorded in transmission mode using a Nicolet iS10 FTIR spectrometer(Thermo Fisher Scientific, Dreieich, Germany) with 16 scans per spec-trum and a resolution of 4 cm−1. The spectra were baseline correctedby subtracting a straight line running between the two minima ofeach spectrum and normalized by dividing each data point by thespectrum's maximum. A sample containing 76% allophane by weightas determined by the Rietveld XRD technique (PM4-6; Kaufholdet al., 2009, 2010) was also analysed.

Powder X-ray diffraction of the bulk soil and the clay fractions wasperformedwith a Bruker D8 Advance (Billerica, USA) using Cu Kα radi-ation at 40 kV and 40 mA, step-scanning from 3 to 70 °2θ with incre-ments of 0.02 °2θ and a counting time of 1 s per step. Slurries of theclay fractions (separated from the b2 mm soil samples after pyrophos-phate treatment by sieving and sedimentation) were deposited onporous ceramic tiles to produce oriented specimens. X-ray diffrac-tion patterns were measured on untreated samples, after ethyleneglycol solvation and after heating to 550 °C.

Solid-state NMR spectra of the b2 mm soil samples were recordedwith an FT-NMR system (Alpha 300, JEOL, Tokyo, Japan). Soils werefinely ground with a mortar, and approximately 200 mg was tightlypacked into a high-speed spinning NMR tube (rotor, zirconia; cap,Kel-F; 6 mm I.D.; JEOL). Signals of 27Al were recorded at 78.2 MHz in asingle-pulse experiment without decoupling and with a flip angle ofπ/2 for 27Al (0.9 μs as a pulse width), an observation band of 80 kHz,an observation point of 4096 (resolution 19.5 Hz), an acquisition timeof 0.013 s, a pulse interval of 2 s, a number of scans of 4000–5000(2.5–3.5 h) and 8 kHz of magic angle spinning (MAS). The standardchemical shift (0 ppm) was adjusted externally using 1 M AlCl3 solu-tion. Signals of 29Si were recorded at 59.6 MHz in a single-pulse exper-imentwithout decoupling, andwith a flip angle of π/2 for 29Si (5.0 μs asa pulse width), an observation band of 50 kHz, an observation point of4096 (resolution; 12.2 Hz), an acquisition time of 0.082 s, a pulse inter-val of 10 s, a number of scans of 8000–10,000 (22–28 h) and 6 kHz ofMAS. Chemical shifts were quoted with respect to tetramethylsilane(TMS) butwere determined by referring to an external sample of siliconrubber (−22 ppm). A broadening factor of 100 Hzwas employed in theFourier-transform procedure for all NMR experiments.

After removal of the sand fraction by sieving, the clay and siltfractions (b63 μm) of the CBw horizons of the profiles were addi-tionally characterised by 29Si-NMR spectroscopy on a 400 MHz pro-ton frequency Bruker Avance II+ spectrometer, employing a Bruker4 mm H/X MAS probe. Spectra were recorded in single pulse experi-ments without proton decoupling. The excitation pulse length was setto 5 μs and the spinning speed to 12 kHz. Between 2000 and 10,000 sin-gle scans were accumulated with relaxation delays of 10 s. Chemicalshifts were referenced with respect to TMS by utilizing pure kaolin asexternal standard. Line broadening of 150 Hz was applied for thespectra.

We used the 29Si-NMR measurements of the bulk soils as a furthermethod to quantify Si present in allophanic compounds. The spectral re-gion from −160 to −40 ppm was baseline corrected and normalisedby height. The partial spectrum was deconvoluted with Lorentzianfunctions using OMNIC (Thermo Fisher Scientific) to obtain the entirearea in the spectral region from −160 to −40 ppm. We did not fixthe number of peaks, nor their position, nor their full width at halfheight prior to the calculation. The area of the peak calculated with acentre at −79 ± 1 ppm, which is characteristic of allophanic com-pounds (Hiradate et al., 2006), was related to the entire area of the par-tial spectrum to obtain the percentage of total Si present in allophaniccompounds. This percentage and the SiO2 content analysed by XRF

were then used to calculate the proportion of allophanic Si per massof soil.

The relative amounts of tetrahedral and octahedral Al (IVAl and VIAl)were calculated using Origin 7.5 (OriginLab, Northampton, USA) byintegrating the signals at 50 ppm (IVAl) and 0 ppm (VIAl) and dividingthe respective signal by the sum of the signals.

3. Results and discussion

3.1. General characterisation

According to the World Reference Base for Soil Resources (IUSSWorking Group WRB, 2006), the three soils under study are classifiedas Andosols, because they fulfil the preconditions of andic propertiesmentioned in the Introduction (Table 1). Although the bulk densitiesof our samples are very low, Wada (1987) reported even lower values(0.25 kg dm−3) for subsoil horizons of Japanese Andosols. High phos-phate retention is caused by AlOH and AlOH2 groups at defect siteson the allophane surface that strongly adsorb phosphate by ligandexchange (Parfitt, 1990). Thus, phosphate retention increases exponen-tially with the Alox and Siox contents as indicators of allophane (PO4

ret. = 104.5 − 98.5 × exp(−Alox / 13.8), r2 = 0.94; PO4 ret. =96.4 − 90.2 × exp(−Siox / 4.2), r2 = 0.88). The horizons withandic properties can be designated in further detail as sil-andic. Thecriteria of Siox N6 g kg−1 and Alpy:Alox ratio b0.5 are fulfilled, and thusthey are allophanic Andosols. In all samples, the amounts of pedo-genic Fe (as derived from FeDCB) prove weathering and formationof secondary minerals. The mostly high Feox:FeDCB ratios point tothe dominating formation of poorly crystalline and/or small Fe oxides,except for the CBw horizon of Soil1. The CBw horizon of Soil3 containsmore Alox than the overlying horizons, and it is in the range of the Ahand Bw horizons of the other profiles. Except for the CBw horizon ofSoil1, where only little Alox has formed, the amounts of organic Al(Alpy) were distinctly smaller than those of Alox, resulting in Alpy:Aloxratios between 0.02 and 0.07.

According to bothmethods to quantify allophanewith extraction, allhorizons except for the CBw horizon of Soil1 contain allophane. Theyields obtained with the method of Parfitt (1990) exceed those by themethod of Mizota and van Reeuwijk (1989). The data of bothapproaches were positively correlated, with r2 = 0.92. They bothdepend on extraction procedures, which are not completely selec-tive. For instance, pyrophosphate is commonly used for the quantifi-cation of organic Al, and its content is subtracted from Alox in bothmethods to account for Al in short-range ordered secondary silicates.However, pyrophosphate was also supposed to dissolve Al not asso-ciated with SOM in spodic B horizons (Higashi et al., 1981). To someextent, it also dissolves poorly crystalline Al hydroxides and gibbsite(Kaiser and Zech, 1996), the presence of which we cannot exclude inour samples. Further, Al dissolved by oxalate does not exclusivelyoriginate from allophane, but may also be originally incorporatedin oxalate-extractable Fe oxides or adsorbed on their surfaces. The lattercannot be excluded because of the acidic pH in all soils (Table 1) thatpromotes Al mobility. It is not possible to correct for this Al based onwet-chemical extractions. On the other hand, oxalatemay dissolve allo-phane incompletely (Hiradate et al., 2006) so that allophane quantifica-tion using non-destructive techniques such as the Rietveld-XRDtechnique (Kaufhold et al., 2010) or 29Si-NMR spectroscopy (Hiradateet al., 2006) may be recommended as an alternative.

The soil profiles under study are characterised by C-rich surfacehorizons and a decrease in the Corg contents with depth (Table 1). Thenarrow C:N ratios are typical of mull and are consistent with our fieldobservations. Soil OM is enriched in the clay fraction as compared tothe bulk soil, and enrichment increases up to 3.7 fold in the subsoil ofSoil3. The C contents of the bulk soils correlate slightly positively withthe contents of Feox (r = 0.52) and Alpy (r = 0.48) indicating the

Table 1Chemical and physical characteristics of three Andosols developed from Laacher See tephra.

Site Horizon Depth pH (H2O) dBa Sand Silt Clay Cbulk Nbulk C:Nbulk Cclay Nclay C:Nclay Feox FeDCB Feox:FeDCB Alox Alpy Siox PO4 ret.b

(cm) (g cm−3) (%) (g kg−1) (g kg−1) (g kg−1) (%)

Soil1 Ah 0–15 4.5 0.61 46 46 8 90.5 6.3 14 139.3 13.3 10.5 8.0 12.1 0.67 31.6 1.8 15.5 96Bw 20 4.4 0.54 72 25 3 25.8 2.2 12 50.3 5.1 9.8 9.1 13.9 0.66 34.6 0.7 33.3 99CBw 50 6.4 0.75 2 75 23 2.2 0.3 7 5.3 0.7 7.1 1.8 10.1 0.17 0.7 0.9 0.3 11

Soil2 Ah 0–15 4.5 0.63 64 26 10 41.4 3.0 14 74.5 6.3 11.8 7.6 11.7 0.65 34.8 1.1 18.7 98Bw 20 4.8 0.60 57 26 17 13.7 1.0 14 28.5 3.0 9.4 6.3 10.4 0.61 46.0 1.8 32.7 98CBw 50 7.1 0.77 68 26 6 4.2 0.4 10 9.3 0.8 11.4 2.9 6.8 0.43 17.7 0.6 8.9 61

Soil3 Ah 0–15 4.8 0.77 44 42 14 60.5 4.7 13 101.3 14.7 6.9 6.2 10.3 0.61 18.7 1.3 5.3 84Bw 20 4.8 0.76 45 37 18 31.5 2.6 12 77.0 6.7 11.6 7.7 12.0 0.64 19.2 1.0 8.9 91CBw 60 5.4 0.83 64 26 10 13.9 1.4 10 51.9 8.7 6.0 7.8 12.1 0.64 33.6 1.5 19.9 91

a Bulk density.b Phosphate retention.

16 T. Rennert et al. / Chemical Geology 363 (2014) 13–21

possible association of SOM with short-range ordered pedogenic Feoxides and the formation of Al humates.

The chemical composition of bulk soils differs between the profilesand the horizons (Table 2). Generally, larger amounts of Fe2O3 than innon-weathered LST (Frechen, 1953; Gebhardt et al., 1969) indicatethat foreign Fe-rich materials may be present in the soil horizonsunder study. The SiO2 and Al2O3 data point to differences in soil devel-opment and/or parentmaterials. In most of the horizons, the Al2O3 con-tent is larger and the content of K2O, CaO andNa2O is smaller than in theLST (Soil1 (Ah, Bw), Soil2 (Ah, Bw, CBw), Soil3 (CBw)). This indicatesfixation of Al by the formation of secondary clayminerals and allophaneduring soil development, while released alkali metals were leached. TheAh and Bw horizons of Soil3 contain almost as much as Al2O3 as thereferences, but clearly contain more SiO2, which may be a first indica-tion of a larger contribution of quartz or volcanic glass. As with thewet-chemical analyses and the XRD measurements (see below), theCBw horizon of Soil1 stands out owing to the striking dominance ofSiO2, the low Al2O3 content and the high content of quartz, silt andwell crystalline Fe oxides (FeDCB–FeOX). Weathering indices (Table 2)were calculated after Parker (1970;WIP (weathering index of Parker)),Nesbitt and Young (1982: CIA (chemical index of alteration)), Harnois(1988; CIW (chemical index of weathering)) and Fedo et al. (1995;PIA (plagioclase index of alteration)). They all indicate that the LSTreference material is unaltered whereas the degree of weathering in-creases from Soil3 Ah, Bw, CBw, Soil2 CBw to Soil2 Ah, Bw, Soil1 CBwto Soil1 Bw, Ah. All horizons of Soil3 show a very similar degree ofweathering, but the CBw horizons of Soil2 and Soil3 are less weathered

Table 2Allophane contents calculated from extraction data, X-rayfluorescence analysis of horizons fromindices.

Site Horizon Allophanea Allophaneb XRF results

SiO2 Al2O3 Fe2O3 MnO(%)

Soil1 Ah 6.6 11.4 61.4 24.5 5.8 0.3Bw 14.2 16.9 54.9 30.5 6.3 0.5CBw 0.1 0.2 76.8 12.3 4.1 0.1

Soil2 Ah 8.0 13.0 58.4 26.2 5.5 0.4Bw 14.0 18.6 58.2 26.6 4.9 0.3CBw 3.8 6.5 60.6 22.8 4.5 0.3

Soil3 Ah 2.3 8.0 62.9 19.9 5.4 0.3Bw 3.8 7.0 62.3 20.8 5.7 0.3CBw 8.5 12.6 59.1 23.4 6.1 0.2

Gebhardt et al. (1969) 57.2 20.4 2.7 0.214 Averaged samples (Frechen, 1953) 57.7 19.8 3.7 0.4

a After Mizota and van Reeuwijk (1989).b After Parfitt (1990).c Molar ratio from XRF data.d WIP (Parker, 1970): 100 × (2Na2O / 0.35 + MgO / 0.9 + 2K2O / 0.25 + CaO / 0.7); CIA (

1988): 100 × [Al2O3 / (Al2O3 + CaO + Na2O)]; PIA (Fedo et al., 1995): 100 × [(Al2O3–K2O) /

than their Ah and Bw horizons. The only sample for which not allweathering indices indicate the same degree of weathering is the CBwhorizon of Soil3. Here, the WIP gives the highest degree of weatheringand all other indices point to a moderate degree.

3.2. Characterisation of soil minerals: X-ray diffractometry and FTIRspectroscopy

X-ray diffractograms of the bulk soils (Fig. 1) are dominated byquartz, feldspars and micaceous minerals. This is in accordance withthe assumed mixture of LST (rich in sanidine plagioclase and X-rayamorphous glass, accidental quartz (van den Bogaard and Schmincke,1985)) and loess (rich in quartz, feldspars, micas and clay minerals) inthe substrate. Quartz-rich soil horizons (Soil1 Ah, Bw, CBw) are there-fore consistent with a large contribution of loess, whereas feldspar-rich horizons may be explained by a larger contribution of non-weathered LST (Soil2 Bw, CBw; Soil3 CBw). Most X-ray diffractogramsof the clay fractions (data not shown) do not show reflexes indicativeof clay minerals (Soil1 Ah, Bw; Soil2 Ah, Bw; Soil3 Ah, Bw, CBw). Thismay be caused by a high abundance of poorly crystalline minerals orclay-sized glass shards, but is also a special feature of allophanic soils.Allophane cements phyllosilicates, thus preventing their orientationand the detection of their (001) lines by XRD (Gebhardt et al., 1969).In the CBw horizon of Soil1, we identified smectite (expansion of the1.4 nm peak to 1.6 nm after glycolation, followed by a collapse to1.0 nm after heating to 550 °C), micaceous minerals (presence of a1.0 nm-peak after all treatments) and kaolinite (peak at 0.7 nm, which

threeAndosol profiles togetherwith published data of Laacher See tephra andweathering

Weathering indicesd

MgO CaO K2O Na2O TiO2 Si:Alc WIP CIA CIW PIA–

1.0 1.4 2.7 1.6 1.0 2.13 44 75 83 811.1 1.4 2.4 1.3 1.0 1.53 39 81 87 860.9 0.7 2.5 1.1 0.8 5.32 36 68 80 761.1 1.8 3.1 1.7 1.0 1.90 50 74 81 791.2 1.8 3.6 2.2 0.8 1.86 59 71 79 771.1 1.9 3.9 3.1 0.8 2.26 70 64 73 681.1 2.0 3.9 2.1 1.1 2.69 61 64 74 691.2 2.2 4.0 2.1 1.1 2.54 62 64 74 691.5 2.5 4.1 2.4 1.1 2.14 67 64 73 690.3 2.4 7.4 7.9 0.4 2.38 143 45 54 421.5 3.5 5.7 7.2 0.6 2.47 128 45 52 43

Nesbitt and Young, 1982): 100 × [Al2O3 / (Al2O3 + CaO + Na2O + K2O)]; CIW (Harnois,(Al2O3 + CaO + Na2O − K2O)].

Fig. 1. X-ray diffractograms of bulk soils from three Andosol profiles.

Fig. 2. Fourier-transform infrared spectra of clay fractions from three Andosol profiles.

17T. Rennert et al. / Chemical Geology 363 (2014) 13–21

collapses after 550 °C; data not shown). Theuntreated CBwsample fromSoil3 shows only one peak at 1.0 nm,which slightly expands to 1.1 afterethylene glycol solvation and which disappears after heating to 550 °C.It can therefore be attributed to hydrated halloysite.

The FTIR analyses of the clay fractions gave some more detailedinformation on the varying composition of the samples. Roughly, theyare divided into two groups (Fig. 2). The spectra within the first group(Fig. 2a) indicate the presence of allophane. Apart from the region at3500–3400 cm−1 that reflects adsorbedH2O, the spectra of the samplesSoil1 Bw, Soil2 Ah, Bwand Soil3 CBw feature theirmaximumabsorptionat 1000–984 cm−1 and secondary maxima at 569–558 cm−1. For com-parison, we also recorded a spectrum of the allophane-rich (76%) sam-ple PM4-6 (Fig. 2a), which was intensively characterised by Kaufholdet al. (2009, 2010). It shows similar absorption maxima at 993 and567 cm−1, but the absorption bands are broader than in the spectra ofthe clay fractions. Therefore, a contribution of allophane to the spectraof our Andosol samples shown in Fig. 2a is very likely. We attributethe sharper absorption bands in the clay spectra to the additional pres-ence of more crystalline minerals and/or smaller particle size. We didnot detect absorption bands or shoulders at 3700–3600 cm−1, whichpoints to the absence of clay minerals and is again consistent with theXRD results.

Compared to the spectra of the samples discussed before, the spectraof the samples Soil1 CBw, Soil2 CBw, Soil3 Ah and Bw show a shift of thewave number of maximum absorption to 1036–1028 cm−1 (Fig. 2b),and this maximum absorption band is sharper than those in Fig. 2a.Absorption at 1030, 1009, 3695, 3920, 915, 540 and 470 cm−1 (Soil1CBw, Soil2 CBw, Soil3 Ah, Bw) retraces the characteristic bands of kao-linite (Martinez et al., 2010) and halloysite. We cannot differentiatethese similar 1:1 clay minerals by FTIR spectroscopy in the presence offurther silicates. Both have been identified as typical weathering prod-ucts from volcanic glass and allophane (e.g., Gebhardt et al., 1969;Kawano et al., 1997; Ndayiragije and Delvaux, 2003). At least expressedas a shoulder, we also detected absorption at 1080 cm−1, indicative ofthe main absorption band of quartz. The doublet at 800 and 780 cm−1

also indicative of quartzwasweak in all spectra. Thus, FTIR spectroscopy

points to two distinctly different groups of soil horizons, one moreallophanic, the other dominated by phyllosilicates (esp. kaolinite/halloysite) and quartz. The spectrum of the Ah horizon of Soil1 showsfeatures of both groups. Apart from compositional differences of theoriginal soil-forming parent materials, the intensity of weathering bydesilification, which is strongly affected by the availability of H2O, mayalso have contributed to some extent to the two groups. In fact, themolar Si:Al ratios (calculated from XRF data, Table 2) of the samples ofthe allophanic group, are smaller than those of the quartz/phyllosilicategroup.

In summary, the results obtained by wet extractions, XRF, XRD andFTIR spectroscopy point to compositional and pedogenic differencesbetween the profiles. High contents of bulk SiO2, quartz and silt pointto a relatively high contribution of loess within Soil1 Ah, Bw, CBw andSoil3 Ah and Bw. The degree of weathering is highest in Soil1 Ah andBw, moderate in Soil1 CBw, Soil2 Ah and Bw and lowest in Soil2 CBwand Soil3 Ah, Bw and CBw. The CBw horizon of Soil1 differs from allother horizons by a high content of quartz, SiO2 and well crystalline Feoxides, but low contents of Al2O3, alkali metals, Siox and Alox.

3.3. Characterisation of soil minerals: 27Al- and 29Si-NMR spectroscopy

We applied 29Si- and 27Al-NMR spectroscopy to better understandthe distribution of quartz, clay minerals and allophane-like phases.

18 T. Rennert et al. / Chemical Geology 363 (2014) 13–21

Especially 29Si-NMR spectroscopy has been proven a powerful tool toanalyse substrates derived from volcanic ashes (e.g., Shimizu et al.,1988; Petrini et al., 1999; Hiradate, 2004; Hiradate, 2005). Threemajor mineral groups can be distinguished by signals in 29Si-NMRspectral regions, which are important for the interpretation of Andosolspectra (Hiradate et al., 2006): allophanic constituents includingimogolite (at −78 ppm), phyllosilicates including clay minerals andmicas (at approx. −90 ppm), and SiO2 including volcanic glass, quartzand silica-gel-like constituents at approx. −105 ppm. Supplementaryinformation is given by 27Al-NMR spectroscopy, because it allows for adifferentiation between tetrahedral Al (IVAl; at approx. 50 ppm) andoctahedral Al (VIAl; at approx. 0 ppm). In soil affected by volcanic ejecta,IVAl may be present in volcanic glasses, feldspars or as isomorphic sub-stitution in phyllosilicates, while VIAl may be incorporated in Al-humuscomplexes, allophanic compounds, gibbsite and Al-octahedral sheets ofphyllosilicates (Hiradate et al., 2006).

Broadmaxima at−99 to−97 ppm in 29Si-NMR spectra indicate thedominance of primary and secondary silicates such as 1:1 and 2:1 clayminerals, micas and feldspars. In a complexmatrix like soil, it seems im-possible to distinguish theseminerals as individual species by 29Si-NMRspectroscopy, because these primary and secondary soil minerals causebroad peaks with typical chemical shifts between−100 and−85 ppm(Hiradate, 2004), and the overlapping signals form featureless spectra.The continuum of these minerals is obvious in all spectra shown inFig. 3a–c, where broad maxima, partially with distinct shoulders, char-acterise the entire spectra. The soils fulfil the conditions required to be

Fig. 3. 29Si-NMR spectra of bulk soil samples b2 mm of three Andosol profiles (a, Soil1

classified as Andosols. Nonetheless, peaks of allophanic constituentstypical of Andosols do not dominate. Instead, the spectra are dominatedby peaks associated with primary silicates and phyllosilicates. This is incontrast to previously published studies applying 29Si-NMR spec-troscopy to Andosols or weathered pumice (e.g., Shimizu et al., 1988;MacKenzie et al., 1991; Hiradate andWada, 2005; Hiradate et al., 2006).

The 29Si-NMR spectra of the Ah and Bw horizons from the profilesSoil1 and Soil2 are very similar (Fig. 3a, b). A sharp signal at−79 ppmindicates the presence of allophanic constituents. In both profiles, thesignal intensity is stronger in Bw than in Ah horizons, which may becaused by an inhibition of allophane formation by SOM (Inoue andHuang, 1986), the contents of which are larger in the Ah topsoil hori-zons. We detected a signal at −107 ppm in all samples from Soil1.Both non-weathered volcanic glass and quartz may have caused thissignal. Volcanic glass in pumice and small pumice fragments may bepresent owing to the known deposition of LST in the area, to the lowsoil bulk density and to the possibility that it may not have weatheredin the subsoil yet and thus was still present. However, the larger SiO2

content as compared to the non-weathered LST references (Table 2)and the X-ray diffractograms of the bulk soils also indicate quartz. Fur-thermore, an FTIR spectrum of pure volcanic glass from LST reveals abroad maximum absorption band (data not shown), because the glassis almost non-crystalline. However, the actual spectrum of the bulksoil of the CBwhorizon of Soil1 (data not shown) is only slightly broaderthan that of the clay fraction (Fig. 2a). This result and the large quartzpeaks in the XRD points to a substantial contribution of quartz to the

; b, Soil2; c, Soil3) and of fractions b63 μm of CBw horizons from the profiles (d).

Fig. 4. Oxalate-soluble Si (Siox) and Si calculated from the signal at −79 ± 1 ppm in29Si-NMR spectra (Si−79 ppm).

19T. Rennert et al. / Chemical Geology 363 (2014) 13–21

29Si-NMR signal of this sample at −107 ppm, maybe with some addi-tional non-weathered volcanic glass.

A weak and broad shoulder in the region of allophanic compoundswas present in the spectra of the CBw horizons of Soil1 and Soil2, andit was more pronounced in the spectrum of the CBw horizon of Soil3.Distinctly clearer signals at −80 ppm became obvious after removingthe sand fractions in the spectra of the samples from Soil2 and Soil3(Fig. 3d) proving the presence of allophanic compounds in the CBwhorizons of Soil2 and Soil3. Such an enrichment of allophanic compoundsin the fraction b63 μm accords with 29Si-NMR data for the fractions2–20 μm and b0.2 μm of Japanese Andosols (Hiradate and Wada,2005). In the spectrum of the b63 μm fraction from Soil1 (Fig. 3d),the signal of clay minerals and other phyllosilicates slightly exceedsthat at−107 ppm, which contrasts with the spectrum of the bulk sam-ple from this horizon (Fig. 3a). This relative enrichment of phyllosilicatesis caused by the removal of the sand fraction N63 μm, which typicallycontains quartz grains.

Based on the results of 29Si-NMR spectroscopy, we aimed at quanti-fying allophanic compounds by evaluating spectral regions characteris-tic of Si in different molecular environments. Hiradate et al. (2006)deconvoluted 29Si-NMR spectra by fitting three Lorentzian functionscentred at −78, −90 and −105 ppm to spectra of horizons fromallophanic Andosols, which clearly showed sharp signals in the spectralregion of allophanic compounds. However, in case of ill-defined SiOx

structures, the spectra will be composed of overlapping and broadenedpeaks as obvious in our measured spectra. Therefore, using threeLorentzian functions, according to the three main chemical shifts maybe inaccurate to deconvolute the measured spectra. Consequently, wefitted Lorentzian functions without previous fixing of their numberand positions as explained in 2.3. The number of individual bands todeconvolute the measured spectra ranged from 12 to 20. Themeasuredand fitted intensities were strongly linearly correlated with r2 ≥ 0.994and slopes ranging from 0.93 to 1.00.

The contents of oxalate-soluble Si (Siox) are commonly used forthe quantification of allophanic compounds (Table 2; Mizota and vanReeuwijk (1989), Parfitt (1990)). The amounts of allophanic Si calcu-lated from 29Si-NMR spectra were similar to those of Siox (Table 3).Most of the data pairswere below the 1:1 line (Fig. 4), and consequentlythe slope of the regression line was b1: Si−79 ppm = 1.43 + 0.76Siox;r2 = 0.85. The regression equation shows that by trend, smaller valuesof Si−79 ppm are correlated with larger values of Siox. In poorly crystal-line, i.e., oxalate-soluble, Fe oxides, Si may be structurally incorporatedand substitute for Fe (Carlson and Schwertmann, 1981). This additionalportion of Siox, which does not originate from allophanic compounds,may explain the tendency expressed by the regression equation.

The quantification of 29Si-NMR spectra confirms our previous quali-tative finding from FTIR spectroscopy. The Ah and Bw horizons of Soil1and Soil2 reflect a soil environment, in which allophanic compoundsform. Smaller contents of allophane were found in the CBw horizons

Table 3Speciation of Al and Si in bulk soils calculated from 27Al- and 29Si-NMR spectroscopy andoxalate-soluble Si (Siox) for comparison. The Si−79 ppmdatawerederived fromdeconvolutingthe spectra with Lorentzian functions, IVAl and VIAl from integrating the signals at 50 and0 ppm, respectively.

Site Horizon IVAl VIAl Siox Si−79 ppm Si−79 ppm

(% of total Al) (g kg−1) (% of total Si)

Soil1 Ah 39.2 60.8 15.49 22.49 7.7Bw 22.4 77.6 33.28 26.98 10.4CBw 74.5 25.5 0.31 1.90 0.5

Soil2 Ah 45.7 54.3 18.65 14.15 5.1Bw 45.1 54.9 32.74 25.37 9.2CBw 64.5 35.5 8.86 4.69 1.6

Soil3 Ah 68.7 31.3 5.34 5.50 1.8Bw 66.0 34.0 8.87 7.10 2.4CBw 58.2 41.8 19.87 12.96 4.6

of Soil2 and Soil3, where allophane was detectable after removing thesand fraction.

The amounts of allophane increase with increasing degree ofweathering, but generally the contents of Si−79 ppm are relatively low(2–26 g kg−1). Furthermore, similar amounts of allophane wereformed in Soil1 from a loess-rich substrate and in Soil2 from an LST-rich substrate. This indicated that allophane formationmay not be com-plete in the LST-rich substrate. Kaolinite and halloysite have very likelyformed from weathering of metastable allophanic compounds, feld-spars or volcanic glass (Gebhardt et al., 1969).

As pointed out in the Introduction, allophanic minerals have beenconsidered important for SOM storage. However, the correlation be-tween Si−79 ppm as an indicator of allophanic minerals and parametersconcerning SOM quality and quantity (contents of C and N in bulk soiland clay fraction, C:N ratio) was weak (r b 0.46). This maximum valuewas found for the correlation with Nbulk, but that with Nclay was 0.03.Thus, we cannot prove preferential interactions of allophanic mineralswith SOM. However, the lack of correlation may also be caused by thepresence of notable amounts of particulate OM,whichwe did not quan-tify. For the same reason, correlations indicating interactions of SOMwith poorly crystalline Fe oxides and complexation of Al by SOM (seeSection 3.1) remainweak. However, two findings on SOM in the profilesunder study are noteworthy. First, the C:N ratio of the clay fraction ofthe Ah horizon of Soil3 is distinctly smaller than the C:N ratios of theclay fractions of the other two Ah horizons (6.9 versus 10.5 and 11.8;Table 1). The sites do not differ in vegetation, and the “Märkerwald”has been stocked with deciduous trees at least since the Middle ages.Therefore, the mineral composition of the Ah horizon of Soil3 with theleast contribution of allophanic compounds of the Ah horizons (as de-rived from Si−79 ppm and Siox (Table 3)) may be the key to this finding.Second, the CBw horizon of Soil3 is the CBw horizon with the largestamount of allophanic compounds and poorly crystalline Fe oxides(Table 3). In contrast to the other CBw horizons, it is also characterisedby accumulation of Corg in both the bulk soil and in the clay fraction(13.9 and 51.9 g kg−1; Table 1). As therewere no subsequent eruptionsof the Laacher See volcano during soil development, the large amount ofsubsoil SOM cannot derive from burial of surface horizons with freshtephra. Contribution of particulate OM is usually small in C horizonsand therefore less likely to blur relations between minerals and totalorganic C like in surface horizons. Accumulation of SOM in this subsoilhorizon may be in line with Basile-Doelsch et al. (2007), who foundthat SOM in a 3Bw horizon of a tropical volcanic soil was preferentiallyassociated with imogolite-type minerals, which bound six times moreSOM than feldspars and 3.5 times more than Fe oxides.

The 27Al-NMR spectra of weathered pumice or allophane-rich sub-strates are commonly characterised by a dominating signal at approx.0 ppm representing octahedral Al (VIAl) from allophanic compounds(e.g., Hiradate et al., 2006). It is also present in octahedral sheets of

Fig. 5. 27Al-NMR spectra of bulk soil samples b2 mm of three Andosol profiles.

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phyllosilicates. Table 3 and Fig. 5 show that VIAl dominates only in sam-ples inwhichwehave identified large amounts of allophanic compounds(Ah and Bw horizons of Soil1 and Soil2). The other spectra reveal thedominance of IVAl, whichmay be present in feldspars or due to isomor-phic substitution of Al in the tetrahedral layers of phyllosilicates (Basile-Doelsch et al., 2007). The very small signal for IVAl in Soil1 Bw is inaccordance with a smaller contribution of feldspars as observed for allSoil1 horizons and a very small contribution of clay minerals (clay frac-tion amounts to only 3%).

3.4. General discussion

The soils under study are classified as Andosols with a dominance ofclayminerals and additional primaryminerals such as quartz,micas andfeldspars. Nonetheless, they are classified as allophanic Andosols. ThepHwas proposed as a master parameter for the formation of allophanicor non-allophanic Andosols (Dahlgren et al., 2004). In the presence ofSOM and at pH b 5, Al3+ is complexed by SOM, and non-allophanicAndosols form. At pH N 5, Al3+ is available for co-precipitationwith silica, thus forming the allophanic compounds that characteriseallophanic Andosols. In the soils under study, the pH is low (4.5–4.8for Ah horizons, 4.4–4.8 for Bw horizons, 5.4–7.1 for CBw horizons),and Al3+ complexation with SOM seems important, but allophanicminerals have also formed in all horizons. Further conditions favouringthe formation of non-allophanic Andosols at pH b 5 are precipitationN1000 mm a−1 and large inputs of organic acids. The mean annualprecipitation in this region is close to 1000 mm a−1, with some dryerand wetter periods since the deposition of the parent material. TheLST has been rearranged and mixed with non-volcanic materials(loess) under periglacial conditions during the Younger Dryas (Stöhr,1963; Gebhardt et al., 1969; van den Bogaard and Schmincke, 1985).Originally, the loess contained carbonates, whichwere dissolved duringthe Holocene soil formation (Pécsi and Richter, 1996). Therefore,pH N 5 will have prevailed for a certain time of soil developmentallowing for the formation of allophanic minerals. Recent formation ofallophane from ongoing weathering of primary minerals and glassmay not occur in the present acidic geochemical milieu of the Ah andBw horizons, but the present soil pH should not be misinterpreted toexclude the previous formation of allophanic compounds.

Similarly, the “locus typicus” of soils developed fromLST (Stöhr, 1971)and characterised by formation of short-range ordered minerals andvery low bulk density has been described by Kleber and Jahn (2007) asformed from “weathered tephra with loess over coarse and fine grainedLST over remnants of weathered schist with loess”. Although LST is

doubtlessly a material contributing to the formation of this soil, it is notclassified as Andosol, but as Haplic Cambisol according to theWRB classi-fication 2007, because within the upper 25 cm, the sum of Alox and0.5Feox did not exceed 16 g kg−1, nor did the phosphate retentionexceed 74% (Kleber and Jahn, 2007). However, the formation ofallophanic compounds in that soil cannot be excluded, and our studysuggests that the presence of these minerals cannot be proven exclu-sively by extractions, but additional 29Si-NMR spectroscopy could bevery helpful. The combination of spectroscopic methods and wet-chemical analyses allows for a qualitative reconstruction of the soildevelopment from amixture of parentmaterials as shown in our study.

Apart from the qualitative detection of allophanic compounds, their“correct” quantification is also desirable. As pointed out before, pyro-phosphate is not selective for organic Al, nor is oxalate selective for Aland Si in allophanic compounds. Thus, quantification of allophanic com-pounds in soils based on these extractions is potentially uncertain. Con-sequently, we recommend applying 29Si-NMR spectroscopy or, withvery allophane-rich soils, XRD combined with Rietveld quantification(Kaufhold et al., 2010) to circumvent the described problems withchemical extractions. Especially in tropical soils or soils formed frommafic volcanic ejecta, 29Si-NMR spectroscopy may be a powerful toolfor the identification and quantification of allophanic compounds,because species typically forming from mafic materials (poorly crys-talline Fe oxides with considerable amounts of Al and Si, Al hydrox-ides, Al-organic complexes) or in tropical soils may also be susceptibleto extraction with pyrophosphate and oxalate.

4. Conclusions

The combination of chemical and spectroscopic techniques (FTIR,29Si-NMR spectroscopy) has shown that soils developed from amixtureof tephra and loess contain allophanic compounds. Further, it allowedfor the detection and differentiation of volcanic glass and quartz. Soilcharacterisation with these techniques may be an additional criterionto differentiate (allophanic/non-allophanic) Andosols from other soiltypes, and may thus be a tool for soil classification. Another futuretask is the comparison of quantification of allophanic compounds by29Si-NMR spectroscopy and by the Rietveld-XRD technique. Our resultsmay also indicate differences in SOM as affected by the composition ofthe soil minerals, but verification of these effects of soil-mineral compo-sition on SOM quantity and stabilisation in Andosols of temperatelatitude requires further approaches, for instance the chemical andphysical characterisation of SOM together with that of the soil minerals.

Acknowledgements

We would like to thank Dr. Stephan Kaufhold (BGR Hannover)for providing the reference sample PM 4–6 and Karin Greef andMaria Greiner for help in the laboratory. The help of Helmut Rieger(Forstverwaltung Dierdorf) during the field work is gratefullyacknowledged.

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