Ecosystems
Topic 4.5 Continuned: Biogeochemical
Cycles
Part of the Local Ecosystems Module
Spotlight Biology Preliminary Text Chapter 4
Authors: D. Heffernan, J. Bastina, B. Grieve, K.
Humphreys, A. Sartor
Science Press 2002
The Nitrogen Cycle
The nitrogen cycle is another important biogeochemical cycle.
Primary producers absorb nitrogen in a simple form, usually as
either ammonia (NH₃), ammonium ions (NH₄), or nitrate ions
(NO₃).
mypowerplants.blogspot.com
The Nitrogen Cycle
Plants use energy from photosynthesis to concentrate nitrogen
in their tissues and then use more solar energy to enable the
combining of nitrogen, hydrogen, carbon, oxygen and other
elements into proteins and amino acids (these are the complex
molecules used as the building blocks in living systems)
mypowerplants.blogspot.com
The Nitrogen Cycle
When animals eat plants or other animals, they digest proteins,
breaking them down into their component nitrogen-containing
amino acids. Some are re-formed into the proteins of the
animal that has eaten them while others are broken down to
release energy they contain releasing nitrogen in the form of
ammonia.
funnypictures24.com
The Nitrogen Cycle
Because ammonia is toxic to many cells, it must be eliminated.
Most aquatic animals excrete it into the water around them
while terrestrial animals often convert nitrogen wastes into
urea and concentrate it in urine before eliminating.
kimballstock.com
The Nitrogen Cycle
Once the nitrogen is back into the environment, it does not last
long. A group of bacteria called Nitrosomonas combine
ammonia with oxygen and convert it into nitrite ions (NO₂).
This is then changed into nitrate (NO₃) ions by bacteria called
Nitrobacter. These two processes are called nitrification.
attila.sdsu.edu
The Nitrogen Cycle
Although they rarely appear in food-web diagrams, the bacteria
and fungi of decay are vital to the nitrogen cycle. Nitrogen in
dead animals and plants is useless to primary producers.
Therefore these decomposers break up the organic molecules
into simpler forms which plants are then able to use.
filebox.vt.edu
The Nitrogen Cycle
When nitrate ions, nitrite ions and ammonia are released into
the soil of a healthy forest or water filled with algae, they may
quickly be reabsorbed by the primary producers. Many
terrestrial plants absorb nitrate ions best which is why most
fertilisers contain a lot of nitrogen as a nitrate compound. And
the cycle starts over
brenlissaonlinenursery.com.au
The Nitrogen Cycle
In the open sea, dead marine organisms and their nutrient rich
solid wastes sink rapidly out of the surface layers. Large
quantities of nutrients end up on the bottom of the ocean
where primary producers cannot reach them.
ronalee.org
The Nitrogen Cycle
Some areas however have ocean currents which force this
nutrient laden water back to the surface into a zone where
photosynthesis can occur. This is called upwelling. In areas
where this is steady, phytoplankton grow at rapid rates. Up to
50% of fish caught world wide come from upwelling. Why do
you think this may be?
cce.lternet.edu
Earth’s Nitrogen Reserve
While aquatic and terrestrial primary producers compete for
available nitrogen, an enormous reservoir of gaseous nitrogen
exists out of reach in the atmosphere. The paired atoms (N₂) in
nitrogen gas are held together by a covalent bond which cannot
be broken down by most organisms.
examiner.com
Earth’s Nitrogen Reserve
Only a few bacteria can break this bond and incorporate
atmospheric nitrogen into living tissue. This process is known
as nitrogen fixation (taking nitrogen gas from the air and turning
it into living tissue). The most familiar are the nitrogen-fixing
bacteria that live on the roots of plants (peas and soybeans).
The most important nitrogen fixers in the sea are
photosynthetic cyanobacteria (blue green algae).
en.wikipedia.org
Earth’s Nitrogen Reserve
Denitrifying bacteria do the opposite. They convert
organic nitrogen into nitrogen gas. The constant addition of
nitrogen fertilisers to farm soil can encourage their growth.
Why would this be a bad thing? (pumping nitrogen out of the soil
into the atmosphere)
meredithseed.com.au
Review/Overview
Plants and animals need nitrogen to
make proteins. But they cannot get
nitrogen directly from the air because,
as a gas, nitrogen is fairly unreactive.
Plants are able to take up nitrogen
compounds such as nitrates and
ammonium salts from the soil.
Making nitrogen compounds from
nitrogen in the air is called nitrogen
fixation.
en.wikipedia.org
Earth’s Nitrogen Reserve
Nitrogen fixation happens in three ways:
 The energy in lightning splits nitrogen molecules into
individual nitrogen atoms. These react with oxygen to form
nitrogen oxides. Nitrogen oxides are washed to the ground
by rain, where they form nitrates in the soil.
 Nitrogen-fixing bacteria found in the soil and in the root
nodules of leguminous plants, such as peas, beans and clover,
fix nitrogen gas into nitrogen compounds.
 The Haber process is used by industry to produce ammonia
from nitrogen and hydrogen. Ammonia is used to make
nitrogen compounds that are used as fertiliser by farmers.
Homework
1.
2.
3.
4.
5.
Outline what happens during nitrogen fixation
How does nitrification differ from denitrification
Where is most of the worlds nitrogen stored?
How are you part of the nitrogen cycle today?
Is the flow of nitrogen in balance?