Fjölrit RALA - 15.06.2004, Síða 25

Non-volcanic Andosols in Europe
B. Delvaux, T. Delfosse and A.J. Herbillon
Soil Science Unit, Université catholique de Louvain, Belgium
In Europe, reported occurrences of non-volcanic Andosols are linked with Hercynian rocks in
smoothened highlands with a cool and humid climate. These occurrences have been reported
in British islands, Northwest Portugal and Spain, France, Switzerland and Austria. Most of
these non-volcanic Andosols are non-allophanic and thus exhibit an aluandic diagnostic
horizon in the WRB-system. Their most prominent feamres are strong acidity, large
accumulation of organic carbon and incorporation of iron and aluminum into metal-humus
complexes.
The regional distribution of non-volcanic Andosols matches very well with the one
described for the new “Umbrisol” Reference Soil Group, since the latter occur in “cool and
humid regions, mostly mountainous and with little or no soil moisture deficit”. In these
regions, developed soils associated with Umbrisols are either non-volcanic Andosols or
Podzols or both.
The genesis of Umbrisols promotes the accumulation of organic carbon as well as strong
acidity and base desaturation despite of limited profile development. These conditions make
Fe and A1 available for the formation of metal-humus complexes, since weatherable minerals
act as metal sources during soil genesis. These complexes are major components of non-
allophanic non-volcanic Andosols. In these soils, the relative dominance of Fe or A1 in metal-
humus complexes -as estimated by selective extraction- is linked with the nature of soil
parent material, which encompasses a large diversity: oxide-rich clay regoliths, basic and
metabasic rocks, as well as more siliceous rocks such as micaschist and granite. In several
cases, the mobilization and complexation of iron contributes to the formation of the andic
horizon, which is in fact part of an umbric horizon in non-allophanic non-volcanic Andosols.
The occurrence of Umbrisols, Andosols and Podzols in the ecological conditions of cool
and humid European mountains thus strongly suggest common soil processes involved in the
formation of these three reference soil groups. We believe that these groups actually form an
evolutive sequence Umbrisol—>Andosol—>Podzol. In this sequence, non-allophanic Andosols
are likely Umbrisols further developed by increasing Fe and A1 complexation (acido-
complexolysis). They may also be regarded as poorly developed Podzols -‘abortive Podzols’-
, wherein the mobility of organic matter has been hampered by carbon stabilization into
metal-humus complexes. We believe that the Umbrisol-Andosol-Podzol association or
sequence requires further studies to better understand C storage and fate in mountain soils,
and to better know the distribution and occurrence of European Andosols.
14