Geoscience Reference
In-Depth Information
ATMOSPHERE
Phosphorus cycle
CONTINENTAL
BIOSPHERE
10
PO
4
3-
EARTH
Volcano
1,2
HYDROSPHERE
(Ocean)
PO
4
3-
1
8
9
Rock
(Apatite)
1
MARINE
BIOSPHERE
2
PO
4
3-
5
4
PO
4
3-
3
7
POP
SEDIMENT
6
Rock (Apatite)
Continental crust
LITHOSPHERE
Oceanic crust
Figure 4.7.
The phosphorus cycle does not involve the atmosphere. Large reservoirs
of inorganic P include the lithosphere, soil and marine sediments, the hydrosphere
and the biosphere. The short P cycle is essentially organic and the long cycle is
inorganic, the first being interconnected with the second. Full arrows represent
natural flows and dotted arrows represent flows associated with human activity; the
numbers next to the arrows refer to the flow descriptions in the text
The duration of the short phosphorus cycle ranges between a few
days (sometimes less) and a few years. In the soil or water,
phosphorus is available to living organisms as the macronutrient
phosphate (PO
4
3-
, of which there are many chemical forms in the
natural environment). However, there is usually little PO
4
3-
in water or
soil, and hence, the little bioavailable P is rapidly taken up by
photosynthetic organisms [
1
]. Moreover, the latter must often compete
for PO
4
3-
with heterotrophic bacteria. The P that is incorporated in
organic matter rapidly circulates in the food web and is finally
excreted by metabolic activity or is recirculated by bacterial
decomposition of organic matter [
2
]. In the ocean, part of the
particulate organic phosphorus (POP) that has sunk to depth (marine
snow) [
3
] is regenerated in deep waters to PO
4
3-
[
4
], which returns to
the surface during episodes of vertical mixing or in upwelling areas
[
5
], thereby compensating for the losses from surface waters. Another
part of the POP that sinks to depth [
3
] reaches the ocean floor, where
it is buried in marine sediments. In soils and marine sediments
,
some
phosphorus of biological origin combines with inorganic or organic
