Fjölrit RALA - 15.06.2004, Page 126

Characterization of reactive components using selective dissolution
methods, and their relation to soil properties in European Volcanic Soils
E. García-Rodeja1, J.C. Nóvoa1, X. Pontevedra1, A. Martínez1 and P. Buurman2
'Dept. de Edafología y Química Agrícola. Facultad de Biología. Universidad de Santiago de Compostela.
2Laboratory ofSoil Science and Geology. Wageningen University.
Soils developed from pyroclastic materials are characterized by a mixture of short range order
components such as allophane, imogohte, ferrihydrite, opaline silica and/or Al(Fe)-humus
complexes, in addition to the crystalline minerals halloysite, gibbsite, goethite, and
aluminised 2:1 phyllosilicates. The accumulation of highly reactive components formed upon
rapid weathering of volcanic rocks is the main cause for the unique physical and chemical
properties of Andosols. Different selective dissolution methods (SDM) are used to quantify
and characterize some of these components, but it’s the specificity of such methods is limited
by the existence of a continuum between very short-range ordered to crystalline components.
After a previous study on the fractionation of A1 and Si in some volcanic soils (García-Rodeja
et al., 2004), we evaluate several selective dissolution methods (SDM) for Al, Fe, Si and Mn,
their use in the characterization of soil components and their relation to some of the
characteristic properties of these soils.
The soils, formed under different environmental conditions and with contrasting properties,
were 20 COST Action 622 European reference volcanic soils from Italy (EUR01-04),
Portugal (Azores, EUR05, 06), Iceland (EUR07-09), Spain (Tenerife, EURlO-12), Greece
(EUR13-15), France (EUR16, 17) and Hungary (EUR18-20). The SDM and the elements
measured were: i) 0.5 M NaOH (Aln, Sin): (Borggaard, 1985); ii) 0.2 M NH4 oxalate-oxalic
acid at pH 3 (Al0, Fe0, Si0, Mn0) (Buurman et al., 1996), iii) Na citrate-dithionite (Fed, Mnd,
Ald, Sid) (Holmgren, 1967; iv) 0.1 M Na-pyrophosphate (pH 10) (Alp, Fep) (Buurman et al.,
1996) and v) 0.5 M CuCU (pH 2.8) (Alcu) (Juo and Kamprath, 1979). Total contents of these
elements were determined by XRF.
NaOH dissolved less than 5% of total A1 (Alt) from the soils from Greece, Hungary and
EUR01 and EUR02 from Italy. In the remaining soils, Aln represented >10% and frequently
>20% of Alt (in soils from France and Azores the dissolved fraction was 25-55% of the total).
Since oxalate and NaOH extracted approximately the same quantity of A1 (r2=0.96, slope
0.96), the origin of the extracted A1 is allophane, imogolite and/or Al-humus complexes. Main
exceptions are some horizons from the soil EUR2 where the relatively higher Aln values are
attributed to the dissolution of gibbsite and possibly some halloysite.
The extractable fraction of Si was very small: <10% was extracted by NaOH or oxalate
extracted <10% of total Si from most soils, and even less (<4%) from the soils from Greece,
Hungary and EUR01 and EUR02 from Italy. Higher amounts were found in some horizons of
EUR16 from France, EUR10 from Tenerife and EUR5 and 6 from Azores. NaOH and oxalate
exctracted similar amounts of Si: r2=0.82, slope 1.12.
Free iron (Fed) represented a small fraction of total Fe content (Fet) in the soils from Greece
(<10% of Fet), followed by the soils EUR09 and EURll (11-24%). In the other soils Fed
represented between 25-70% of Fet, with the highest relative amounts in EUR06 (>70% in all
horizons). Crystalline Fe oxides are dominant (Feo/Fed<0.5) in the soils from Hungary,
Greece, EUR01-03from Italy, EUR10 and especially in EUR12 (Feo/Fed<0.2) from Tenerife,
together with some horizons of EUR05 and 06 from Azores, and EUR07 and 08 from Iceland.
In the other soils Feo/Fed is >0.5 indicating the dominance of non-crystalline Fe components,
which predominate in the soil EUR17 from France (Feo/Fed>0.8). The effect of oxalate
extraction on minerals as magnetite and the need of more than one extraction to dissolve all
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