Download Section 2.6 The Nitrogen Cycle

Section 2.6
The Nitrogen Cycle
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The nitrogen cycle refers to the cycling of nitrogen though the ecosystems,
soil and atmosphere.
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Nitrogen gas makes up almost 79% of the Earth’s atmosphere yet it can only
be used by living organisms in the form of nitrate ions NO3-.
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Nitrogen in the atmosphere can be converted into nitrates through a
process called nitrogen fixation.
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Nitrogen fixation occurs through two process:
1.
lightning
2.
bacteria in the soil
Nitrogen Fixation Through Lightning
Energy from lightning causes nitrogen gas (N2) to react with oxygen (O2)
producing nitrates. The nitrates dissolve in rain or surface water, enter the soil,
and move into plants through their roots.
Plants cells use the nitrates to make DNA and amino acids, which they than string
together to make proteins.
Nitrogen Fixation by Bacteria
Bacteria in the soil provide the vast majority of nitrates found in the ecosystem.
Some bacteria is also found in small nodules on the roots of legumes (clover,
soybean and peas).
The bacteria provide the plant with useable nitrogen while the plants supply the
nitrogen-fixing bacteria with the sugar they need to make the nitrates. The
plant-bacteria combination usually makes more nitrates than they need, therefore
the excess is released into the soil.
Denitrification
Denitrification - the decay process in which denitrifying bacteria break down
nitrates, into nitrites and then into nitrogen gas. This process ensures the balance
between soil nitrates, nitrites and atmospheric nitrogen which complete the
nitrogen cycle.
Denitrifying bacteria grow well in areas of low oxygen, such as your lawn. By
aerating your lawn, the breakdown of nitrates into nitrogen gas is reduced and
your lawn will stay green longer (nitrates stay in your lawn).
When soil is very acidic, the denitrification slows down.
< Example: bogs.
The Phosphorus Cycle
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Phosphorus is a key element in cell membranes, molecules that help release
chemical energy, making molecules of DNA, and making bones.
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Phosphorus cycles in two ways:
1.
Long cycle - rocks of the Earth’s crust
2.
Short cycle - living organisms
Long cycle - phosphates eroded from rocks are carried to the ocean by rivers,
streams, and lakes. They enter the food chain and are used to make
bones and shells. When these animals die they end up at the bottom of
the ocean where over time they may become rock again.
Short cycle
- decomposers recycle dead organisms which releases the
phosphates back into the ecosystem. Now phosphate can enter
the food-chain and begin the cycle again.
Nutrient Cycling
Nutrients are chemical which are essential to living things. The rate at which they
cycle through the ecosystem depends on the rate of decomposition. In warm
moist areas this process is rapid, while in cold, dry areas this process is slow. Soil
chemistry and frequency of fire can also affect the rate of cycling.