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Phosphorus and Sulfur Cycles
Facts about the Phosphorus cycle
 Cycle most affected by humans!
 Phosphorus is mostly found in the form a phosphate
 In living things it is in phospholipids, DNA and RNA
More facts
 Very SLOW cycle. Phosphorus can be trapped for millions of year.
 The only cycle that does not have a gas phase and does not enter the
atmosphere.
 The main stores of phosphorus in the biosphere are in the Earth’s crust.
 Bacteria not a major player
 Washes from the land into streams, then the sea.
 Can be deposited as sediment and remain for millions of years.
 Often a limiting factor for plant growth on land.
 Also limits growth in lakes and streams because phosphate salts are only slightly
soluble in water.
Weathering
With weathering
rocks release
phosphate . The
phosphate enters
the soil where
plants take it up by
the roots.
When animals eat
plants the
phosphorus becomes
incorporated into
their systems. When
plants or animals
release wastes or die
the decomposers
break down the
phosphorus and
release it to the soil
Plants,
Animals and
Decomposers
Phosphorus enters
rivers and oceans by
runoff. It then can
enter the marine
food chains.
Phosphorus can cycle
through animals for
100,00o years before
settling to the
bottom to become
sediments which
become rocks.
Runoff
How Humans affect the Phosphorus Cycle
 Humans have altered the cycle by mining and
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fertilizing
Humans reduce the available phosphate in tropical
soils by clearing tropical forests.
Humans disrupt aquatic systems with phosphates
from runoff of animal wastes and fertilizers.
Sewage treatment also adds phosphorus to lakes,
rivers and oceans
Extra phosphorus can put some organisms, such as
algae, at an advantage which can disrupt ecosystems.
Facts about the Sulfur Cycle
 Sulfur is an essential component of proteins
(primarily two amino acids: cysteine and
methionine), therefore essential for life.
 An “average” cell may have approximately 1% S by
dry weight.
 The largest physical reservoir is the Earth's crust
where sulfur is found in gypsum (CaSO4) and pyrite
(FeS2).
 The largest reservoir of biologically useful sulfur is
found in the ocean as sulfate anions (2.6 g/L),
dissolved hydrogen sulfide gas, and elemental sulfur.
The sulfur cycle
contains both
atmospheric and
terrestrial
processes. Within
the terrestrial
portion, the cycle
begins with the
weathering of
rocks, releasing
the stored sulfur.
The sulfur then
comes into
contact with air
where it is
converted into
sulfate (SO4).
The sulfate is
taken up by plants
and
microorganisms
and is converted
into organic
forms; animals
then consume
these organic
forms through
foods they eat,
thereby moving
the sulfur through
the food chain. As
organisms die and
decompose, some
of the sulfur is
again released as
a sulfate and
some enters the
tissues of
microorganisms.
There are also a
variety of natural
sources that emit
sulfur directly into
the atmosphere,
including volcanic
eruptions, the
breakdown of
organic matter in
swamps and tidal
flats, and the
evaporation of
water.
Human Impacts on the Sulfur Cycle
 Human impact on the sulfur cycle is primarily in the
production of sulfur dioxide (SO2) from industry (e.g.
burning coal) and the internal combustion engine.
 Sulfur dioxide can precipitate onto surfaces where the
following can happen:
 it can be oxidized to sulfate in the soil (it is also toxic to
some plants)
 oxidized to sulfate in the atmosphere as sulfuric acid, a
principal component of acid rain.
 Lower the pH in lakes and rivers
 At pHs lower than 5 most fish eggs will not hatch and
lower pHs can kill adult fish.
Effect of acid rain on a forest, Jizera Mountains, Czech Republic