Lesson 32 Mineral Cycling - Alaska Geobotany Center

Another gaseous cycle: the nitrogen cycle
5
N 2
1
4
7
6
Nitrogen
fixation
3
2
1. Nitrogen is taken up from the soil by plants in
theformofNH 4 + or NO 3 - , where it is used to
form organic compounds (amino acids,
proteins, nucleic acids).
2. It returns to the soil via litter, which is
decomposed to form ammonia. This process
of decomposition is also called nitrogen
mineralization.
3,4. The ammonia is oxidized in a two step
nitrification process to form nitrates
(NO 3 - ),which are an available form of N for
plant uptake.
5. NO 3 - -N can also be converted back to the
gaseous NO, N 2 O, and N 2 , through the
process of denitrification. The atmospheric
store of N is about 1 million times larger than
the total N in living organisms.
6. Nitrogen-fixation transforms atmospheric
N 2 into ammonia (NH 3 ) , which is also an
available form of N for plant uptake.
7. NO 3 - is also easily leached from the soil.
Courtesy of Michael Pidwirny and Jim Deacon web sites
http://helios.bto.ed.ac.uk/bto/microbes/nitrogen.htm
http://www.physicalgeography.net/fundamentals/9s.html