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Phosphorus Cycle
•Phosphorus is essential to the energetics, genetics and structure of
living systems. Phosphorus forms part of the ATP, RNA, DNA and
phospholipid molecules.
•The phosphorus cycle is crucial because phosphorus is usually the
limiting nutrient in ecosystem. In contrast to C and N, there are no
common stable gaseous forms of phosphorus, so the phosphorus
cycle is endogenic.
[endogenic cycles: predominantly involve subsurface rocks of
various kind. Exogenic cycles: occur largely on Earth’s surface and
usually have an atmospheric component]
•The main reservoirs of phosphorus are rock, natural phosphate
deposits and marine sediments. Phosphorus is release from the rocks
and minerals by weathering, leaching, erosion, and mining for use as
agricultural fertilizers.
•Soil may contain substantial quantities of phosphorus. However,
much of the phosphorus in soils occurs in chemical forms not
directly available to plants.
•Phosphorus, like nitrogen, must be present in a simple inorganic
form before it can be taken up by plants.
•Phosphorus is slowly released to terrestrial and aquatic ecosystems
through the weathering of rocks. As phosphorus is released from
mineral deposits, it is absorbed by plants and recycled within
ecosystems.
•The phosphates (inorganic phosphates, mainly orthophosphate
ions; PO43-, HPO42-, H2PO4-, H3PO4) are utilized by plants in
metabolism and then passed on to heterotrophic organism through
food chain. The decomposition of organic matter of plant and animal
by microorganisms releases the phosphates in the soil and makes
them available to plants once again.
•Soluble phosphorus from phosphate minerals and other sources
such as fertilizers is taken up by plants and incorporated into nucleic
acids, which make up the genetic material of organisms.
•In the pH range that is present in most soils, H2PO4- and HPO42- are
the predominant orthophosphate species.
•Orthophosphate is most available to plants at pH values near
neutrality. In relatively acidic soil, orthophosphate ions precipitated
or sorbed by species of Al(III) and Fe(III).
In alkaline soils, orthophosphate may react with calcium carbonate
to form relatively insoluble hydroxyapatite:
3HPO42- + 5CaCO3 + 2H2O
Ca5(PO4)3(OH) + 5HCO3- + OH-
•In marine and freshwater ecosystem, the phosphorus cycle has 3
states:
- particulate organic phosphates, dissolved organic phosphates and
inorganic phosphates.
•Organic phosphates are taken up quickly by all forms of
phytoplankton, which are eaten in turn by zooplankton and detritusfeeding organisms.
•Zooplankton excretes as much phosphorus (more than half is
inorganic phosphate) daily as it stores in its biomass, returning it to
the cycle. The inorganic phosphate is taken up by phytoplankton.
•The phosphorus in aquatic ecosystem in organic compounds is
utilized by bacteria, which fail to regenerate much dissolved
inorganic phosphates. Bacteria are consumed by the microbial
grazers, which then excrete the phosphate they digest. Part of
phosphate is deposited in shallow sediments and in deep water.
•In recent years the excessive use of phosphate fertilizers to increase
primary production, and the use of detergents in household and
elsewhere, has created pollution problems and the loss of
phosphorus to oceans and other fresh water bodies. This is also one
cause of the eutrophication of fresh water bodies.
•The global phosphorus cycle is unique among the major
biogeochemical cycles in having no significant atmospheric
components, However, it may be found in air as tiny solid particles.
The Phosphorus Cycle:
Soluble inorganic phosphate,
As HPO42-, H2PO4-,
and polyphosohates
Assimilation by
organisms
Biodegradation
Fertilizer runoff, wastewater,
detergent wastes
Xenobiotic
organophosphates
Biological phosphorus,
predominantly nucleic
acids, ADP, ATP
Precipitation
Dissolution
Insoluble inorganic phosphate,
Such as Ca5(OH)(PO4)3
or iron phosphate
Biological organic and inorganic
phosphates in sediments
The phosphorus cycle in aquatic and terrestrial ecosystems