Transcript nitrogen_cycle

The nitrogen cycle
Learning objectives:
• To be able to describe how the element
nitrogen cycles through the environment
• To be able to explain the central importance
of soil nitrates in the process and the
importance of microbes and bacteria in the
process.
• To be able to identify the form in which
nitrogen is found at different stages.
The Nitrogen Cycle
Why is nitrogen important?
• It is part of proteins, DNA, RNA and
chlorophyll. It is necessary for life.
Where is most nitrogen contained?
• Most of the nitrogen in the world is nitrogen
gas (N2) 78% of the air
• It is inert (unreactive) and most organisms
cannot use it.
• Organisms use nitrogen when it has been
fixed (made into a more reactive, compound)
The Nitrogen Cycle
Nitrogen-fixation
Biological
• Mutualistic nitrogen fixing bacteria in root
nodules produce ammonia
• Free-living nitrogen-fixing bacteria in soil
Industrial
• the Haber process produces ammonium ions
(NH4+) in fertiliser
Natural
Lightening bolts produce enough energy to
react nitrogen and oxygen to form nitrates
• NITROGEN-FIXING BACTERIA
are found in the soil. They can
convert nitrogen from the air
into nitrates.
• Plants are able to take up the
nitrates with their roots.
• These bacteria are commonly
found in the roots of legume
plants like peas, beans and
clover.
• They make lumps on the roots
called root nodules. They
change the nitrogen into
nitrates that the plant can use.
• N2 + 6H
2NH3 using the
enzyme nitrogenase
Nitrogen gas is changed into
nitrates in the following ways
• LIGHTENING
• The energy in a lightning bolt can split
the nitrogen molecule in the air
allowing each nitrogen atom to react
with oxygen to form nitrogen oxides.
• These oxides are washed to the ground
by the rain where they form nitrates.
• DECOMPOSERS
• Decay bacteria break down the dead
remains and animal wastes releasing
ammonia compounds into the soil
• Ammonification.
• Microbial saprophytes break down
proteins in detritus to form ammonia in two
stages:
• first they digest proteins to amino acids
using extracellular protease enzymes,
• then they remove the amino groups from
amino acids using deaminase enzymes.
The Nitrogen Cycle
NITRIFICATION
• Nitrifying bacteria are autotrophic
organisms that obtain their energy from
ammonium or nitrite ions.
• They oxidise ammonia to nitrate and
use the energy released to live instead
of respiration
• These bacteria are useful to humans
because they increase the fertility of
the soil.
The Nitrogen Cycle
• Green plants absorb nitrogen in the
form of ions:
• ammonium (NH4+) or nitrate (NO3-) ions
via the roots.
• Green plants use these ions to make
amino acids and proteins, which are
passed to consumers by feeding.
The Nitrogen Cycle
• Nitrogen leaves organisms by excretion
(urine), death and decomposition.
• Saprophytes breakdown dead organic
matter ( detritus) and release
ammonium ions.
The Nitrogen Cycle
• DENITRIFICATION
• In anaerobic soils denitrifying bacteria
convert nitrate ions (NO3-) into nitrogen
gas (N2)
• This reduces soil fertility.
• These only thrive in waterlogged soils
(e.g. bogs). This is why carnivorous
plants live there (no nitrates available
to them in the soil).
The Nitrogen Cycle
The Nitrogen Cycle
• MAN'S EFFECT ON THE NITROGEN CYCLE
• Growing crops reduces the nitrogen content
of soil
• Adding nitrate fertilisers increases plant
productivity
• Crops with root nodules do not need
fertilisers and can increase soil fertility if
they are ploughed back into the ground
• Excess nitrates are washed into rivers and
lakes where they can cause
EUTROPHICATION