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Unit A: 3.3
THE NITROGEN AND PHOSPHORUS
CYCLES
NITROGEN CYCLE
Required so that cells can make proteins
 Required to synthesize deoxyribonucleic acid
(DNA)

NITROGEN FIXATION
Nitrogen gas makes up nearly 79% of Earth’s
atmosphere
 However, nitrogen must be converted into
nitrate (NO3) in order to be useful to plants

NITROGEN FIXATION (CONTINUED)
Two ways atmospheric nitrogen can be converted
into nitrate:
1) Lightning – a small amount of nitrogen gas in
the atmosphere is converted, using the energy
from lightning to react with oxygen to form
nitrate.
2.



Nitrogen Fixing Bacteria – produce the
majority of nitrates found in ecosystems
Found mostly in soil
Can be found in nodules on the roots of
legumes (clover, soybeans, peas and alfalfa)
Aboriginal peoples used the practice of
planting the “three sisters” corn, beans and
squash to capitalize on the properties of each
plant to improve growing conditions of the soil
NITROGEN FIXING BACTERIA
AMMONIFICATION

Fungi and bacteria (decomposers) convert
nitrogen in organic molecules from
decomposed organic wastes and dead
organisms into ammonia (NH3)
NITRIFICATION
Bacteria convert ammonia into nitrites and still
other bacteria convert nitrites into nitrates.
 These bacteria require oxygen to function
 The nitrates are then absorbed by plant roots,
converted into plant proteins and then
consumed by animals, beginning the cycle
again

DENITRIFICATION
During the decay process, bacteria can break
down nitrates into nitrites and then into
nitrogen gas that is released back into the
atmosphere
 These bacteria do not require oxygen

PROCESS OF AERATING
Exposing denitrifying bacteria to oxygen reduces
the breakdown of nitrates into nitrogen gas,
causing more nitrates to remain in the soil for
plant use.
FERTILIZER
Industrial fertilizers convert nitrogen into
ammonia
 Can double crop yields

NEGATIVE EFFECTS OF FERTILIZER
Too much fertilizer can affect the acidity of soil
 Accumulation of nitrogen and phosphate
fertilizers can increase algae growth, increase
bacteria that feed on dead algae and cause a
decrease in the oxygen levels in lakes and
ponds. This can lead to the death of fish and
other aquatic animals

NITROGEN CYCLE
THE PHOSPHORUS CYCLE
Phosphorus is essential for the formation of
cell membranes, DNA, and the calcium
phosphate of bones.
 The phosphorus cycle has a long-term cycle
that involves the rocks of the Earth’s crust and
can take millions of years.

LONG-TERM CYCLE
Phosphorus is found in bedrock
 Erodes from rock and dissolves in water
 Absorbed in photosynthetic organisms
 Passed into food chains
 Phosphates eroded from rocks are carried into
rivers and then the ocean
 Phosphates in the ocean are absorbed by
algae, plants and enter the marine food chain

LONG-TERM CYCLE (CONTINUED)
Marine animals use phosphates to make bones
and shells
 Dead marine animals form deposits on the
ocean floor and eventually form rock
 Geological upthrusts bring the phosphorus
back to Earth’s surface to begin erosion and
starts the long-term cycle again

PHOSPHORUS SHORT CYCLE
Photosynthetic organisms take in phosphates
through their roots
 Phosphates enter the food chain
 Wastes and dead tissue are recycled by
decomposers and release the phosphates
which are carried back into the water supply
where they are again absorbed by
photosynthetic organisms

PHOSPHORUS CYCLE