Geoscience Reference
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of calcium, magnesium and potassium is much lower in
acid soils than in near-neutral or alkaline soils.
In their studies of the calcium cycle in the natural
forest of Hubbard Brook, Bormann and Likens discovered
that the forest ecosystem is extremely conservative in its
nutrient cycling. The precipitation input of calcium of
2·6 kg ha
-1
yr
-1
is matched by a loss in stream output of
only 12 kg ha
-1
yr
-1
of calcium. This is a small rate of loss
considering the large amounts of calcium in the calcium
stores of the watershed, and is probably balanced by 9·1
kg ha
-1
yr
-1
released by weathering. The main stores and
flows of calcium in Hubbard Brook are illustrated in
The conclusion is that natural ecosystems have many
nutrient-conserving mechanisms - in the living biomass,
in the soil and in the micro-organism population. They
result in the nutrients being recycled in a very efficient and
tight manner. One effect of human interference is to break
such conservational cycles and to cause serious depletion
of nutrients from the ecosystem. As part of their
experimental work in the Hubbard Brook catchment
Bormann and Likens experimentally clear-cut several
small watersheds and monitored dissolved nutrients in the
stream water. The results are shown in
Figure 21.12
.
The
low figures in the sixty-year-old forest are increased
enormously upon deforestation. This is due to the
increased mineralization of litter and plant debris, the
elimination of plant uptake, and the destruction of
the buffering power of soil humus colloids. With time,
recovery will be brought about by the reinvasion of the
cleared sites by ground vegetation, shrubs, seedlings and
ultimately trees. The vegetation will eventually re-establish
the nutrient cycles and lead once more to nutrient
conservation. Temperate forests such as those of Hubbard
Brook have the main store of nutrients in the litter layer,
slightly less in the living biomass of trees, shrubs and
ground vegetation, and even less on the soil colloids. This
is particularly so when considering nutrients such as
Calcium cycle
kg ha
-1
yr
-1
METEOROLOGICAL INPUT
2.6
VEGETATION BIOMASS
ORGANIC DEBRIS
570
1740
NEW
MINERALS
PRIMARY AND
SECONDARY
MINERALS
AVAILABLE
NUTRIENTS
WEATHERING
9.1
28550
690
BIOLOGICAL OUTPUT
11.7 DISSOLVED
0.3 PARTICULATE
