Geoscience Reference
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the supply of organic acids were to increase
(Gustafsson
et al
. 1995).
In fact, we see the Bh as a natural form of evolution, then of
degradation of the Bs by ageing and polymerization of complexes
followed by acidification and/or oxidation. The passing of these
horizons from the one to the other is also morphologically
progressive.
At the top of the Bh, at the boundary with the E, the organic
substances can be remobilized. This was experimentally
demonstrated by Jansen
et al
. (2005). There are several reasons
for this. Firstly, the C/metal ratio rises there because of the
continual additions of carbon originating from the litter or roots.
Secondly, the permanent washing of the top of the B by H
+
-ions
tends to separate the iron and aluminium because the ability
to complex metals is reduced by acidification (Buurman 1986).
Lastly, in an oxygen-rich medium, Sideropsaceae bacteria destroy
the [Fe(OH)
3
]
n
-fulvic acid complexes and appear to favour for-
mation of ferrihydrite (Vodyanitskii 2003). Bardy
et al
. (2008)
wrote: '
In the Bhs, the illuviated Al-OM complexes are most probably
degraded progressively, which can lead to the remobilization of Al by
fresh inputs of dissolved organic compounds
'. Also: '
There must be
a mechanism that removes previously accumulated organic matter in
the upper horizons to form an E horizon that is depleted of organic
matter
' (Buurman and Jongmans 2005). Rid of their iron and
their aluminium, some humic substances could then set out on
a new voyage downwards or even be destroyed more easily by
microorganisms (Bel Hadj Brahim 1987). Some authors identify
in the Bh horizon an 'eluvial' structure (Anderson
et al
. 1982). In
our opinion 'residual' is more appropriate!
Overall, progressive descent of the horizons appears very probable
and a new scheme of podzolization can be proposed (Fig. 11.10).
Formation and degradation of complexes constitute a sort of cycle
but open at the top (partial mineralization) and at the bottom (limited
displacement or export); the overall result is thickening of the E and
descent of the B horizons.
The system works because the Bh is saturated with organic acids and
can be traversed by new organic acids that continue to arrive in excess
from the top layer. In addition, the fulvic acids and short-chain acids are
the most difficult to be precipitated by metal ions (Schmitt
et al
. 1996).
Thus, these acids directly reach the Bs through the Bh. We are back to
square one! In this cycle, and in spite of the arrows pointing upwards,
