Geoscience Reference
In-Depth Information
in the tropical seas, but in temperate and polar waters,
where mixing of ocean currents brings sedimentary
particles to the surface to feed phytoplankton.
Soil micro-organisms play a key role in nutrient cycling
in terrestrial ecosystems. The largest store of nutrients in
most ecosystems is the organic matter, whether living in
biomass or dead in litter and humus. Nutrient elements
contained in those stores are mineralized by microbial
decomposition, releasing cations and anions which can
again be absorbed by plants. These microbial processes are
mostly carried out by a wide range of general-purpose soil
micro-organisms, but in the case of nitrogen and sulphur
many reactions are carried out by highly specialized
autotrophic bacteria (
Plate 21.6
).
Plate 21.6
The surface of a red lateritic soil (FAO: Rhodic
roots and microbes are concentrated in the Ah to 'catch'
nutrients released by mineralization.
Photo: Ken Atkinson
KEY POINTS
1
Gross primary productivity (GPP) is the sum total of energy fixed by autotrophic organisms through
photosynthesis. Energy which is not used by the autotrophs themselves for respiration is termed net
primary productivity (NPP). Some of the NPP will be grazed each year by herbivores, and some organisms
will die and become decomposed. The remainder will cause an increase in biomass.
2
Production in ecosystems depends on an assured supply of nutrients, including water, in addition to light
and heat energy from the sun. Two types of nutrient cycle provide the many nutrients necessary to plants.
Gaseous cycles provide carbon, oxygen, nitrogen and sulphur to the biosphere through fixation.
Sedimentary cycles provide elements such as potassium, phosphorus and calcium through the weathering
of rock minerals.
3
Living plant material (biomass) and dead organic matter (litter and humus) contain a great reservoir of
nutrients. This reservoir is released through decomposition by micro-organisms. Individual species (e.g.
coniferous trees) and ecosystems (e.g. tropical rain forests) have evolved many mechanisms for cycling
nutrients efficiently, with a minimum of loss from the system. The efficiency of nutrient cycling is perhaps
the hallmark of climax vegetation. Understanding of nutrient cycles is vital in the management of renewable
resources such as agriculture, forestry and water systems.
4
Biogeographers continue to investigate how cycles of energy and materials are reacting to changes in
atmospheric carbon dioxide contents and atmospheric nitrogen deposition. Fundamental research into
changing carbon fluxes comes from eddy covariance experiments on undisturbed ecosystems, forest
inventory data and mathematical modelling studies. The conclusion is emerging that there is currently a
significant terrestrial sink for anthropogenically released carbon dioxide. However, there is still much
controversy about the spatial distribution of the sink, and its variations over time.
