Name _________________________
Nitrogen cycle
Put the steps in order of occurrence.
1. Lightning converts (nitrogen fixation) small amounts of N2
(nitrogen gas) into NO3- in the atmosphere.
____ Animals eat plants making (assimilation) the organic nitrogen
compounds part of their body.
_____ Legumes plants with specialized roots containing nitrifying
bacteria) change (nitrogen fixation) N2 (nitrogen gas) into NH4+
(ammonium) in the soil.
____ Denitrifying bacteria in the soil changes some of the NO2- (nitrite)
and NO3- (nitrate) into N2 (nitrogen gas) that is then released into the
atmosphere. This is called denitrification.
___ Nitrate bacteria in the soils change NH3 (ammonia) from the soil into
NO2- (nitrite). This is another form of nitrification.
____ Plants take NO3- (nitrate) and NH4+ (ammonium) from the soil in
through their roots in order to make (assimilation) organic nitrogen
compounds needed for all living things.
____ Rain carries NO3- (nitrate) to the earth.
____ Plants and animals die.
____ Nitrate bacteria in the soil changes NO2- (nitrite) into NO3- (nitrate).
This is a continuation of the nitrification process.
___ Decomposing bacteria found in the soil breaks down
(decomposition) dead plant and animal compounds in NH3 (ammonia)
The Chemical Components
The Nitrogen Cycle
N2 (nitrogen gas) – This is found in the atmosphere and soil. It cannot be directly
used by plants and animals to become compounds in their bodies.
NH3 (ammonia) – This is found in the soils from decomposing compounds but
cannot be used by plants and animals.
NO2- (nitrite) - This is found in the soil but cannot be used by plants or animals.
NO3- (nitrate) and NH4+ (ammonium) – These are the only nitrogen compounds that
plants and animals can convert to other compounds in their bodies.
Various organic nitrogen compounds such as amino acids – Theses are the many
complex organic compounds with nitrogen as a key element as part of the
compounds.
Processes
Assimilation – This is the process in which plants and animals convert nitrate and
ammonium into complex organic nitrogen compounds as part of their biomass.
Decomposition – This is the process of breaking down the complex organic nitrogen
compounds from dead plants and animals into simple nitrogen compounds such as
ammonia.
Denitrification – This is the process of changing nitrogen compounds such as nitrite
or nitrate into nitrogen gas.
Nitrification – This is a process that occurs in a few steps in which unusable nitrogen
compounds (nitrogen gas, ammonia, and nitrite) are changed into compounds
(nitrate and ammonium) used by plants and animals to become part of their
biomass.
Nitrogen Fixation – This is the process in which either lightning or specialized plants
(legumes) converts nitrogen gas into either ammonium or nitrate that can be
directly used by plants.