Environmental Engineering Reference
In-Depth Information
have seen how inorganic nitrogen ions ammonium (NH
4
+
) and nitrate (NO
3
−
) are
produced by the mineralization of organic matter in the nitrogen cycle. Most natural plant
species and agricultural crops can use both these ions as a nitrogen source. However,
NH
4
+
ions are toxic to some plants, which must therefore take up their nitrogen as NO
3
−
ions. These are called
nitrophilous
plants, and they tend to grow only on neutral and
alkaline soils where most of the ionic nitrogen will occur as NO
3
−
. In vegetation
communities such as heaths and coniferous forests, where soils are acidic, ionic nitrogen
occurs as NH
4
+
, with virtually no NO
3
−
, as there are not enough nitrifying bacteria to
convert the NH
4
+
to NO
3
−
. This presents no problem to the heather and conifers, since
they have evolved to tolerate high NH
4
+
levels, partially due to mycorrhizae fungi on
their roots. Difficulties can occur when clear-cut harvesting removes all the trees, and the
ground is replanted with coniferous seedlings. On cleared sites, many plant species
invade rapidly, soil pH rises and a new population of soil micro-organisms appears,
including nitrify ing bacteria which convert the NH
4
+
to NO
3
−
. The planted conifers are
poor competitors for inorganic NO
3
−
nitrogen and the site becomes dominated by
nitrophilous vegetation such as ferns and herbs in the ground flora, and deciduous tree
seedlings of birch and aspen. Coniferous seedlings die from nitrogen starvation, and this
is the main reason why in the forest industry of North America the failure of replanted
conifers to establish themselves is a considerable problem.
considering nutrients such as nitrogen, sulphur and phosphorus, which occur mainly in
organic forms. The situation is in marked contrast with that in tropical forests, where the
bulk of the nutrients are in the living biomass, with only small proportions in the soil or
the litter. The mechanisms by which tropical rain forests are able to be sustained, despite
the low content of soil and litter, are discussed in the next section.
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:
1 The cation exchange capacity of the soil is small, owing to the presence
of less reactive kaolinite clay minerals and oxides of iron and aluminium
('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.
