The Nitrogen Cycle
 Earths atmosphere, which is 79% nitrogen gas (N2), is
the major reservoir of nitrogen.
 Nitrogen is essential for many biological processes; and
is crucial for life here on Earth.
 Nitrogen is needed by both plant and animals to build
cellular materials like amino acids, proteins, and DNA.
 Living organisms CANNOT use nitrogen gas (N2) directly
because it is a stable molecule.
 It must be broken apart so that the individual nitrogen atoms can combine with other
atoms to form compounds that can be used by living things.
 Nitrogen fixation (nitrification), is the process that converts nitrogen gas (N2) into
compounds that can be use by living organisms. This process requires tremendous
energy. There three main ways that nitrogen fixation occurs are:
1. atmospheric nitrogen fixation occurs naturally when the enormous energy of
lightning splits is able to break the nitrogen molecules apart in to two individual
nitrogen atoms
 the nitrogen atoms then able to combine with oxygen atoms to form the nitrate
ion (NO3-1)
 this nitrate solution is carried to the earth dissolved in rain water were plant
uptake it from there roots.
2. biological nitrogen fixation occurs naturally by certain nitrogen-fixing bacteria that
live free in the soil. (needs oxygen)
3. industrial nitrogen fixation occurs at high temperature. With the use of a catalyst,
atmospheric nitrogen and hydrogen can be combined to form ammonia (NH3).
Ammonia can be used directly on the land in fertilizer.
 Plants obtain the nitrogen they need as nitrate from the soil
 Animals receive the required nitrogen they need for metabolism, growth, and
reproduction by the consumption of living or dead organic matter containing
molecules composed partially of nitrogen.
 The proteins made by plants enter and pass through food webs just as
carbohydrates do. At each trophic level, their metabolism produces organic nitrogen
compounds that return to the environment, chiefly in urine and feces.
.
Denitrification reduces nitrates to nitrogen gas, thus replenishing the atmosphere.
Once again, bacteria are the agents. They live deep in soil and in aquatic sediments where conditions are anaerobic.
 Despite its abundance in the atmosphere, nitrogen is often the most limiting nutrient
for plant growth.
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