nitrogen cycle is dominated by the N2
in the atmosphere. Nitrous oxide, N2O is the second common
form. N20 (the gas commonly known as laughing gas) is a greenhouse
gas. Seventy-nine percent of the atmosphere is nitrogen in the form of
N2 gas. Because N2 has low reactivity, it offsets
the high reactivity of oxygen, O2, the other major constituents
of the atmosphere. For example when we light a match, the nitrogen does
not burn with the oxygen. It does not react with any other element or
common compound under ordinary conditions. This property of nitrogen has
been called the "fire insurance" of our atmosphere. If the nitrogen
was not "diluting" the flammability of 02, every
spark from a match could lead to a large fire!
to its different valences (3,4,5,), nitrogen can form a multiplicity of
compounds into the same element. For example, it can combine with oxygen
to form N2O, NO, NO2, or N2O5!
a group, these oxides are (except for N2O5) denoted
compounds form an important category of air pollutants, for example, as
a result of the nitrogen and oxygen combining in the extremely hot environment
of an automobile engine. Nitrogen oxides and hydrocarbons, in the presence
of sunlight, give rise to the photochemical smog and tropospheric ozone
problems, described in the Atmospheric
System. Natural and anthropogenic nitrogen oxides also contribute
to acid rain.
- Essential for Life
is an essential element for life. Amino acids, which are the building
blocks of proteins, contain nitrogen as NH2, the "amino"
part of the molecule. The four building blocks of DNA [Adenine (A), Cytosine
(C), Guanine (G), and Thymine (T)] consist of single or double rings of
carbon and nitrogen atoms, with various side chains. Nitric oxide is a
all living organisms require large amounts of nitrogen. However, in the
form of N2, nitrogen is unusable by all organisms except for a few primitive
bacteria that are capable of converting N2 gas to ammonia (NH3).
This process of conversion is called nitrogen fixation, and makes the
nitrogen available for use by organisms. In the atmosphere, nitrogen is
fixed (i.e. N2 is converted to NH3) in three ways:
(1) bacteria, (2) by humans through a manufacturing process called Haber
process used in fertilizer production, and (3) through a chemical process
initiated by lightning.
bacteria are diazotrophs (or more simply, nitrogen-fixers). These bacteria
possess an enzyme which converts N2 gas into NH3
or "fixes" the nitrogen.
on soybean roots
may be symbiotic, living as nodules in roots of plants such as legumes.
A type of bacterial called cyanobacteria live on lichens, mosses, and
ferns. Some cyanobacteria are free-living and capable of photosynthesis.
nitrogen fixation is an important process for biological functioning.
Legumes such as peas, clover, and beans have nitrogen-fixing bacteria
in their roots. This enables them to grow in nitrogen-poor soil. Plants
take up nitrates through their roots, and convert them into proteins and
other compounds. Animals get their nitrogen from plants. Wastes and remains
of animals and plants contain organic nitrogen compounds which are then
broken down by bacteria and converted into compounds such as ammonia (NH3).
Other bacteria (denitrifying bacteria), found especially in waterlogged
soils, convert nitrates back into nitrogen gas and make it unavailable
again. Plants can not use N2, and the N2 can therefore
escape into the atmosphere. Farmers normally try to prevent the soil from
becoming waterlogged. This is the problem with over-watering houseplants
is an electrical discharge through the air, and can cause N2
and O2 molecules to change into the atomic form, combine with
water to form weak nitric acid (HNO3), and precipitate atmospheric
nitrogen to the earth, adding nitrogen to the soil in a usable form (nitrate,
NO3). Inside plants and other organisms, the nitrates are converted
into amino-acids and other vital compounds.
agriculture uses artificial fertilizers such as ammonium nitrate (NH4NO3)
to capture nitrogen. For example, if you examine the box of "plant
food" that is a fertilizer designed for "acid-loving" plants
such as azaleas or rhododendrons, you see the numbers 30-10-10, where
the 30 stands for N, the first 10 for phosphorus, and the second 10 for
potassium. This fertilizer then contains 30% of a nitrogen compound, mostly
ammonium nitrate (NH4NO3) with some urea. While
this improves yield, it upsets the natural balance of nitrogen in the
ecosystem. Too much of nitrogen added to the soil through fertilizers
washes out into ponds and rivers and causes overgrowth of algae in large
patches. These algae blooms prevent light from entering the water and
smother other aquatic life.
Nitrogen Cycle is shown in Figure N1.
and lightning fix nitrogen in the atmosphere. When plant matter (biomass)
is burned, the organic fixed nitrogen is converted into nitrogen oxides
and released. The clearing of forests by fire and burning of leftover
debris from farmland creates large emissions of nitrogen oxide.
oceans and sediments also contain large amounts of nitrogen as nitrates.
Ammonia (NH3) is another form of fixed nitrogen. Ammonia is
produced by bacteria after they consume organic matter. This accounts
for the ammonia smell from the cat's litter-box resulting from the bacterial
emissions. Before chlorofluorocarbons were invented, ammonia was the most
common refrigerant. While the figure shows the main global routes of cycling
nitrogen, in some locations (for example the Los Angeles basin, Mexico
City, and in other industrial cities), nitrogen oxides (NOx)
and nitric acid (HNO3) form a significant fraction of the local