Fjölrit RALA - 15.06.2004, Síða 93

Soil development processes in non-volcanic Andosols.
R. Báumler
Institute ofSoil Science, Department of Ecology, Technical University ofMunich, Germany
In the literature an increasing number of sites is described having andic and partly podzolic
characteristics, that have developed in non-volcanic and commonly non-allophanic materials,
and lacking typical Podzol eluvial and illuvial horizons. They cover a wide range of parent
materials under different temperature and moisture regimes. They have been regarded as
restricted to small areas, so that they were of minor interest. They were either assigned to
Andisols/Andosols, Podzols/Spodosols or andic Inceptisols in WRB and Soil Taxonomy,
sometimes also named Cryptopodzols (Garcia-Rodeja et al., 1987; Hunter et al., 1987;
Alexander et al., 1993; Báumler and Zech, 1994; Blaser et al., 1997; Aran et al., 1998; Caner
et al, 2000). Recent soil survey in Bhutan showed that these soils are widespread all over the
country between 2200 - 3500 m asl covering several biochmatic zones (Baillie et al., 2004).
The aim of this paper is to give an overview about the specific properties and processes of soil
formation described so far to enhance the discussion about their position in the world of soils.
In general these soils are characterised by high contents of organic carbon even at greater
depth, pHmo values <5, extremely low bulk densities (105°C) partly <0.5 g cm'3 despite clay
contents of >50 %, P retention of >85%, low CEC at soil pH, and in many cases a dominance
of Al-hydroxy-interlayered 2:1 clay minerals. The results further indicate advanced soil
development with high amounts of both poorly and well-crystalhsed oxidic Fe and A1
compounds. Thixotropic features of subsoil B horizons are common as well. Recent REM
studies of their sand fractions indicate microaggregates of clay and fine silt particles, which
were highly resistent to any dispersion procedure. In consequence specific surface areas of the
<2 mm fractions are comparably high partly exceeding 50 m2 g"1. Colum experiments indicate
podzolisation with mobilisation and translocation of a cocktail of DOC, Fe and A1 from
topsoil to subsoil being responsible of the high SOM contents in the subsoil. In addition, the
mobilisation of an inorganic A1 compound could be shown in the subsoil horizons.
Radiocarbon ages of the organic matter in B horizons are high with respect to subsoils (up to
16 ka BP; KI-4987), which might not be fossil A horizons and which are subject to recent
biogenic processes. 13C solid state NMR spectra of the soil organic matter commonly provide
a dominance of aryl- and carbonyl-C compounds (Caner et al., 2003; Báumler et al., 2004). It
confirms the comparably high radiocarbon ages, as such organic compounds are generally
known as partly resistant to biodegradation. The results are clearly different from Podzols
having strong signals from O-alkyl and alkyl C (Wilcken et al., 1997). NMR spectroscopy,
14C ages, and colum experiments therefore indicate re-stabilisation of DOM against biodecay
despite recent rooting, continuous biodegradation, and rejuvenation processes in a leaching
environment. Applying soil classification criteria these soils appear to have andic and
podzolic characteristics, but fail sole diagnostic features of Andosols/Andisols and
Podzols/Spodosols, i.e. no visible E horizon or Al0+V2Fe0.
Summarizing the results upon these soils almost all sites appear to merge soil forming
conditions favourable to andosolisation and/or podzolisation, i.e. moderate or cooler
temperatures and high humidity, high input of organic material, good drainage, and
weathering conditions or weatherabihty of the parent materials providing a fast release of
metal cations, forming metal-organic compounds and most probably also acting as binding
cations to form pseudosand-like microaggragates partly causing thixotropy. Commonly these
soils appear not to dry out at all despite the fact that some of the studied sites have monsoon
climate with a dry season.
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