Page 146 - Environment: The Science Behind the Stories

human activities have affected the nitrogen cycle in diverse in sediments. Because most phosphorus is bound up in rock
and often far-reaching ways. and only slowly released, environmental concentrations of
phosphorus available to organisms tend to be very low. This
The phosphorus cycle circulates scarcity explains why phosphorus is frequently a limiting fac-
a limited nutrient tor for plant growth and why an influx of phosphorus can pro-
duce immediate and dramatic effects.
The element phosphorus (P) is a key component of cell mem-
branes and of several molecules vital for life, including DNA, Food webs Aquatic producers take up phosphates from
RNA, ATP, and ADP (pp. 57, 50). Although phosphorus is surrounding waters, whereas terrestrial producers take up
indispensable for life, the amount of phosphorus in organisms phosphorus from soil water through their roots. Primary con-
is dwarfed by the vast amounts in rocks, soil, sediments, and sumers acquire phosphorus from plant tissues and pass it on
the oceans. Unlike the carbon and nitrogen cycles, the phos- to secondary and tertiary consumers (Chapter 4). Consum-
phorus cycle (Figure 5.20) has no appreciable atmospheric ers also pass phosphorus to the soil through the excretion of
component besides the transport of tiny amounts in wind- waste. Decomposers break down phosphorus-rich organisms
blown dust and sea spray. and their wastes and, in so doing, return phosphorus to the soil.

Geology and phosphorus availability The vast We affect the phosphorus cycle
majority of Earth’s phosphorus is contained within rocks
and is released only by weathering (p. 237), which releases People increase phosphorus concentrations in surface waters
phosphate ions (PO ) into water. Phosphates dissolved in through runoff of the phosphorus-rich fertilizers we apply to
3−
4
lakes or in the oceans precipitate into solid form, settle to the lawns and farmlands. A 2008 study determined that an aver-
bottom, and reenter the lithosphere’s phosphorus reservoir age hectare of land in the Chesapeake Bay region received a

Biotic cycling
Biotic cycling
1150
1150
Atmosphere
Consumers Producers Transport of dust and seaspray 1
Transport of dust and seaspray 1

Decomposers
Uptake 2 Mineable rock
Uptake 2
12,800
Weathering
Weathering
Mining 25
Mining 25
Rivers
Runoff Erosion Land plants
Runoff
Erosion
21 500 Consumers
21
Pollution CHAPTER 5 • Envi R onm E n TA l S y STE m S A nd E C o S y STE m E C ology
Pollution
Fertilizers
Uptake
and detergents GeologicGeologic Uptake
85
Burial uplift 85
Burial
uplift
19
19 Decomposers
Oceans
Burial 2
90,000 Burial 2 Soils
66,000
Sediment and sedimentary rock
4,000,000,000
Figure 5.20 The phosphorus cycle summarizes the many routes that phosphorus atoms take as they
move through the environment. Gray arrows represent fluxes among reservoirs, or pools, for phosphorus.
Most phosphorus resides underground in rock and sediment. Rocks containing phosphorus are uplifted geolog-
ically and slowly weathered away. Small amounts of phosphorus cycle through food webs, where this nutrient is
often a limiting factor for plant growth. In the figure, pool names are printed in black type, and numbers in black
type represent pool sizes expressed in teragrams (units of 10 g) of phosphorus. Processes, printed in italic red
12
type, give rise to fluxes, printed in italic red type and expressed in teragrams of phosphorus per year. Data from
Schlesinger, W.H., 2013. Biogeochemistry: An analysis of global change, 3rd ed. Academic Press, London. 145

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