Geoscience Reference
In-Depth Information
60-year-old forest
Deforested
Recovery
120
Calcium Ca 2+
60
0
Potassium K +
20
0
Nitrate NO 3 -
400
0
Time
Figure 21.12 Effects of forest clearance on the leakage of nutrients in the Hubbard's Brook experiment.
and oxides of iron and aluminium or sesquioxides.
These colloids are formed under the influence of high
temperatures and high leaching rates. Unlike large
lattice clay minerals, they can hold few nutrients by
ionic bonding.
nitrogen, sulphur and phosphorus, which occur mainly
in organic forms.
Nutrient cycling in tropical rain forests
The higher inputs of solar energy over tropical rain forests,
when compared with middle latitudes, leads to faster,
more dynamic systems, owing to the greater amount
available for photosynthesis. The vast bulk of nutrients are
stored within the living biomass (the biota), and there is
an absence of nutrient reserves outside the biota. However,
there are several exceptions to this, as in forests on young
volcanic soils (e.g. in Zaire or in the Pacific), where the
nutrient input from weathering can be large. Also the
flood plains of tropical rivers are similar, where annual
floods supply large volumes of nutrient-rich sediments to
the system. Generally, however, nutrient reserves in the soil
component of the ecosystem are low. There are five main
reasons for this:
2
Decomposers like termites and ants flourish within the
continually maintained organic debris of the forest;
they quickly decompose litter on and in the soil.
3
Micro-organisms (fungi and bacteria) thrive in the
hot and humid conditions at the soil surface, and are
capable of completely removing nutrients from the soil
surface.
4
Trees have a high capacity for the uptake of nutrients
through their symbiotic relationships with a root
fungus. This relationship gives mycorrhizae (particu-
larly the type VAM, vesicular arbuscular mycorrhizae),
and is an association of a fungus with the root of a
higher plant. They are present in most latitudes, but are
particularly ubiquitous in the tropical zone. VAM are
fungi which penetrate the root in order to feed on the
cell contents. The benefit to the tree is that it is able to
absorb nutrients from the fungi, which in turn are able
1
The cation exchange capacity of the soil is small, owing
to the presence of less reactive kaolinite clay minerals
 
 
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