Nitrogen Cycle
Definition and meaning of Nitrogen Cycle in chemistry.
Nitrogen cycle is the series of biochemical processes by which nitrogen from the atmosphere is converted into various chemical forms used by organisms, then returned to the atmosphere. It involves nitrogen fixation, nitrification, denitrification, and ammonification.
In more detail
Nitrogen gas (N2) comprises 78% of the atmosphere but most organisms cannot use it directly because of its extremely strong triple bond. Specialized nitrogen-fixing bacteria in soil and root nodules convert N2 into ammonia (NH3) and ammonium ions (NH4+), which plants absorb and use to synthesize proteins and nucleic acids. Other soil bacteria oxidize ammonium through nitrification to nitrite (NO2-) and then nitrate (NO3-). When organisms die, decomposer bacteria break down nitrogen-containing compounds through ammonification, releasing ammonia. Finally, denitrifying bacteria convert nitrate back to N2, completing the cycle. This cycle is essential because nitrogen is a limiting nutrient for most ecosystems.
Key facts
| Field | Biochemistry |
|---|---|
| Key chemical forms | N2 (atmospheric nitrogen), NH4+ (ammonium), NO3- (nitrate) |
| Essential organisms | Nitrogen-fixing bacteria (Rhizobium, Azotobacter), nitrifying bacteria (Nitrosomonas, Nitrobacter), denitrifying bacteria |
| Biological role | Provides bioavailable nitrogen for synthesis of proteins, DNA, and RNA |
Legume plants like soybeans form symbiotic relationships with Rhizobium bacteria in their root nodules, where these bacteria fix atmospheric N2 into NH4+ that the plant uses for growth; when the plant dies, decomposition releases this nitrogen back into the soil and atmosphere through successive microbial transformations.
Frequently asked questions
Why can organisms not use atmospheric nitrogen directly?
Atmospheric N2 has an extremely strong triple bond between nitrogen atoms. Only bacteria carrying the nitrogenase enzyme can break this bond and fix nitrogen into usable chemical forms.
How do legume crops improve soil nitrogen?
Legumes host nitrogen-fixing bacteria in root nodules that convert atmospheric N2 into bioavailable NH4+ and NH3. This enriches the soil for subsequent crops, which is why legumes are used in crop rotation.