Visualizing Environmental Science

PROCESS DIAGRAM

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Phosphate

rocks

Soil phosphates

200,000 Phosphate rocks

Marine

sediments

4×10^9

Marine

organisms

Phosphates

released by

excretion and

decomposition

Burial and

compaction to

form rock

Internal

cycling

1000

Internal

cycling

60

Erosion

Dissolved

phosphates

90,000

Plants take up soil

phosphates;

animals obtain

phosphates from

food

Phosphate

mining

Phosphate rocks

10,000 (mineable) Erosion of

phosphate minerals

Phosphates

deposited on

seafloor

Phosphates

released in animal

waste and

by decomposition

Rocks containing

phosphorus are

weathered,

becoming soil

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ddddd

Fertilizer

containing

phosphates

Geologic processes

(e.g., uplift)

expose seafloor

sediments as

new land

Phosphorus moves from the land through aquatic and terrestrial communities, between

organisms in these communities, and back to the land in a process known as the

phosphorus cycle. Unlike other biogeochemical cycles, the phosphorus cycle does not

involve the atmosphere. The values shown in the figure for the global phosphorus budget

(expect that for marine sediments) are expressed as 10^12 g of phosphorus per year. For

example, each year an estimated 60 × 10^12 g of phosphorus cycles from the soil to

terrestrial organisms and back again.

Values are from Schlesinger, W. H.

Biogeochemistry: An Analysis of Global Change

, 2nd edition.

Academic Press, San Diego (1997) and based on several sources.

from one organism to another, and finally back to the land

(Figure 5.14). The erosion of phosphorus-containing min-

erals releases phosphorus into the soil, where plant roots

absorb it in the form of inorganic phosphates. Phosphates

are used in biological molecules such as nucleic acids and

ATP, a compound that is important in energy transfer reac-

tions in cells. Like carbon and nitrogen, phosphorus moves

through the food web as one organism consumes another.

Phosphorus cycles through aquatic communities

in much the same way that it does through terrestrial

communities. Dissolved phosphorus enters aquatic

communities as algae and plants absorb and assimilate

it; plankton and larger organisms obtain phosphorus

when they consume the algae and plants. A variety of

fishes and molluscs eat plankton in turn. Ultimately,

decomposers release inorganic phosphorus into the

112 CHAPTER 5 How Ecosystems Work

If the phosphates present in fertilizer

on a field are too abundant to be taken up by plants and

animals, how are those excess phosphates likely to move

in the phosphorus cycle?

Think Critically