Geography Reference
In-Depth Information
For a human to eat 1 kilogram of wild salmon would require 1,000
kilograms of phytoplankton to have been produced. In addition to
energy transfers, material is transferred through an ecosystem. The
trophic system means that certain toxins that might be present
originally in low concentrations can become concentrated in the
high trophic level consumers under certain conditions (see Box 5.2).
Two further important examples of material transfers are through
the nitrogen and phosphorus cycles which are described below.
Phosphorus
Rock weathering enables phosphorus to be released in a soluble
form in water solution. It then becomes available within soils or
water bodies for uptake by plants before being consumed by organ-
isms at higher trophic levels. Excretion or death means that phos-
phorus can be taken up by decomposers and the phosphorus
becomes part of the soil of water solution again. This cycle can
continue over long periods. On a larger scale phosphorus may be
washed off the land and into the deep ocean sediments. Over time
this may form a sedimentary rock which then later reaches the
surface, weathers and becomes part of the water solution in soil.
Nitrogen
Nitrogen is essential for life. Nitrogen exists in large quantities in
the atmosphere. Very small amounts of nitrogen gas react with
oxygen during lightning to form nitric oxide, which eventually
reaches the ground as nitrate. More importantly, soil bacteria fix
nitrogen
to make reactive forms of ammonium and nitrate which
can be used by plants. Some of these nitrogen-ixing bacteria form
close relationships with plants such as legumes (e.g. clover). When
used in crop rotations these nitrogen-ixing plants can help fertilise
the soil from the atmosphere, ready for the next crop. Ammonium
and nitrate are taken up by plants to make protein and other parts
of the plant matter. Herbivores then eat plant material to obtain
their protein and therefore their useable nitrogen. Once passed
through the trophic system, dead plants and animals decompose
and some of the nitrogen is acted upon by denitrifying bacteria.
Nitrogen is thereby returned to the atmosphere.
