Nitrogen is needed to make proteins, which are vital to
life. Plants and animals recycle nitrogen through the air and soil in a process
called the NITROGEN CYCLE. As a gas, nitrogen makes up 78 per cent
of air. At everyday temperatures it is very unreactive. It is used in place of
air in crisp packets, for example, so the contents do not go stale. Nitrogen is
also used to make industrial chemicals such as fertilizers and
explosives.
When nitrogen gas is cooled to -196ºC (-320ºF), it
turns to a liquid. Liquid nitrogen is so cold that it can freeze a substance in
seconds. In hospitals, it is used to preserve blood and body parts for
transplant. The material to be preserved is placed in a special, sealed
container that is filled with liquid nitrogen. Because nitrogen is so
unreactive, it does not alter the preserved materials in any way.
The heat produced by lightning forces nitrogen molecules in the air
to split. Nitrogen atoms bond with oxygen to form nitrogen oxides, which
dissolve in water to create nitric acid. Weak nitric acid falls to the soil,
where it splits apart to form the compounds nitrates and nitrites. These
compounds are essential to life for plants and micro-organisms.
Farmers often use fertilizers to help their crops grow well. Many
fertilizers contain nitrogen in the form of nitrates, because this is the form
that plants can use. Natural fertilizers are made from compost and manure.
Synthetic fertilizers are made by combining nitrogen from the air with hydrogen
from natural gas.
Nitrogen compounds are used to make explosives. These compounds
contain chemicals that break apart easily to release huge volumes of gases
extremely quickly. They can be used in a controlled way to demolish a building
without harming other buildings nearby. The explosive TNT (trinitrotoluene)
releases hydrogen, carbon monoxide, and nitrogen, and carbon powder, which
produces black smoke.
All living things need nitrogen, but most cannot use
nitrogen gas directly from the air. The nitrogen has to be fixed (combined)
with other elements to form nitrites and nitrates. This is done by lightning
and by nitrogen-fixing bacteria. The nitrates are taken up by plants, which are
eaten by animals. This starts the continual cycle of nitrogen called the
nitrogen cycle.
Nitrogen from the air is fixed to make nitrates in the soil by
nitrifying bacteria. The nitrates are taken up by plants to build plant
protein. When an animal eats a plant, it turns the plant protein into animal
protein. Denitrifying bacteria convert the nitrogen contained in animal waste
and in decaying plant and animal material back into nitrogen gas again.
NITRIFYING BACTERIA IN ROOT NODULES
Nitrifying bacteria are a key part of the nitrogen cycle. Some
live in the root nodules of legumes (peas and beans), like this nodule from the
root of a pea plant. Others live free in the soil. Bacteria in the soil make
nitrates from nitrites and other nitrogen molecules. Bacteria in legume root
nodules take up nitrates from the soil.
