Geoscience Reference
In-Depth Information
arthropods (environment with a dry season) or can even contain
worm casts (calcareous milieu),
these layers rest on others that are not purely organic and do not
belong to humus proper; they can be an organo-mineral horizon
(A) or completely mineral horizon (E).
Forest humus
. The classification into mull, moder and mor (according
to the layers mentioned above) was developed by German scientists,
then revived by Ph. Duchaufour in his
Précis de Pédologie
of 1960 and
further refined later (RP 1995; Fig. 2.22). It is valid for the temperate
region where excess water is not present. The leaves, needles and diverse
plant debris fall on the surface of the soil. Incorporation into the soil
thus presumes (i) involvement of small burrowing animals that perform
a mixing action, or (ii) biochemical transformation resulting in the
appearance of soluble products that can be transported downward by
water. The first pathway is predominant in slightly acid humus and the
second in other forms. The accumulation of carbon remains essentially
near the surface of the soil.
Humus of grasslands
. The extreme case is the mown grassland. There
is almost no replenishment of organic matter from above and therefore
no transfer of carbon across the atmosphere/soil interface. Most of the
carbon is restored to the soil by the roots in two ways: decomposition
in situ
and root exudates. The latter are considerable and can even
reach 40 per cent of the carbon absorbed annually by the plant. They
correspond to three types of constituents: residues of external root
layers, mucilages (polysaccharides and polygalacturonic acid) and soluble
organic compounds with low molecular weight (sugars, organic acids,
amino acids, phenols). The exudates constitute the adaptive strategy of
the roots to protect themselves from desiccation, improve their contact
with the solid phase and create a very favourable environment for
microorganisms, thereby facilitating or restricting the absorption of such
or such ion (Marschner and Römheld 1996).
Humus of very wet equatorial forests
. This is a special case, but important
(Fölster
et al
. 2001). The soil is acid, saturated with water and has a
cation exchange capacity dominated by aluminium. It constitutes an
environment hostile to roots which, for more than 50 per cent of their
mass, do not push into the soil, but toward its surface, into the humus.
Under these conditions the forest is fragile and sensitive to summer
droughts that seem to be a natural cause of degradation towards savanna.
Classification of humus
