Geoscience Reference
In-Depth Information
restoration of health of the trees reduced the supply of litter. Finally, the
hot summers of 1989-1992 stimulated the mineralization of the H layer
resulting in loss through leaching. Thus, there is a time lag between
climatic phenomena and modification of humus. In every way, it is
useful that humus releases its reserves slowly.
The presence of superposed L, F and H shows that the transformations
(humification and mineralization) occur slowly in succession and that
the organic matter sometimes has time to accumulate (
mor
). This is
what happens on acid soil in the mountains, under coniferous forest.
On the contrary, a humus reduced to a few leaves or needles indicates
that the turnover of organic matter is rapid (
mull
). This is the case in the
plains, under broad-leaved forests on neutral or calcareous soil.
Moder
is intermediate between the two. But at equilibrium, the return of bases
to humus by plants is equivalent, on average, to the restoration of bases
to the soil by humus. Let us look at another picture. In a river, at the
level of a reservoir, the quantity of water that flows in is equal over
time to the quantity of water that flows out; the storage may rise or not
and represent a long or short stability, and there might be several fine
modalities of filling and emptying. Here evaporation is ignored, which
is acceptable because our topic concerns non-volatile elements. In other
words, an accumulation of humus is not the sign of cessation or even a
long-term slowing down of the cycle of bases, contrary to what is often
repeated. Morphology does not actually give an account of functioning.
After all, conifers grow faster than broad-leaved trees! This would not
have been the case if the biological functioning of their environment
were as poor as one likes to say.
2.5.4 The Cycle of Bases
The functioning of the 'plant/humus/soil/parent material' system is
examined here for the temperate forests in montane climate and from
the viewpoint of the cycle of basic cations. Nitrogen, which is prone to
be mineralized to gaseous form, is not considered.
The bases extracted by roots represent the annual
withdrawals
. A small
portion of them is finally
fixed
in the wood formed. But they, for the
most part, return to the soil as components of leaves and needles, and
diverse debris. These are
restitutions
. Added to them is
excretion
, which
supplies elements dissolved in water or organic liquids.
Mineralization
of
humus permits the cycle to be completed (at least for the non-gaseous
elements) and ensures return to the soil of bases drawn but not fixed.
Establishment of the balance (Ranger et al. 1995; Fig. 2.23)
