Nitrogen Cycle
Definition and meaning of Nitrogen Cycle in chemistry.
Nitrogen cycle is the set of natural processes that move nitrogen between the atmosphere, soil, and living things. Nitrogen gas makes up 78% of the air, but most organisms cannot use it directly. The cycle converts this gas into forms that plants and animals can use, then eventually returns it to the atmosphere.
In more detail
The cycle has four main steps. In nitrogen fixation, specialized soil bacteria break the strong triple bond in N2 gas and convert it into ammonia, NH3, and ammonium ions, NH4+. Plants absorb these forms and use them to build proteins and nucleic acids.
Nitrogenase, the enzyme that performs fixation, works only in bacteria and a few related microbes, never in plants or animals directly. In nitrification, other soil bacteria oxidize ammonium first into nitrite, NO2-, and then into nitrate, NO3-, which plants can also absorb.
When plants and animals die or release waste, decomposer bacteria break down nitrogen-containing molecules in a process called ammonification, releasing ammonia back into the soil. Finally, denitrifying bacteria convert nitrate back into N2 gas, completing the loop. This cycle matters because nitrogen is often the nutrient that limits plant growth the most, even though nitrogen gas is everywhere in the air.
Human activity, especially the industrial production of ammonia through the Haber process for fertilizer, now fixes roughly as much nitrogen as all natural processes combined, which has significantly altered the natural nitrogen cycle.
Key facts
| Field | Biochemistry |
|---|---|
| Key chemical forms | N2 (atmospheric), NH4+ (ammonium), NO3- (nitrate) |
| Essential organisms | Nitrogen-fixing bacteria (Rhizobium, Azotobacter), nitrifying bacteria (Nitrosomonas, Nitrobacter) |
| Four main steps | Fixation, nitrification, ammonification, denitrification |
| Biological role | Supplies nitrogen for proteins, DNA, and RNA |
Soybean plants form a partnership with Rhizobium bacteria that live in small swellings on their roots called nodules. The bacteria fix atmospheric N2 into NH4+, which the soybean plant uses to grow. When the plant later dies and decomposes, this nitrogen returns to the soil and atmosphere through ammonification, nitrification, and denitrification. Excess fertilizer nitrogen that is not absorbed by crops can also run off into rivers and lakes, contributing to algal blooms.
Frequently asked questions
Why can organisms not use atmospheric nitrogen directly?
Atmospheric N2 has an extremely strong triple bond between its two nitrogen atoms. Only bacteria carrying the enzyme nitrogenase 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 crops planted afterward, which is why legumes are used in crop rotation.
What is nitrification?
Nitrification is the process in which soil bacteria oxidize ammonium into nitrite and then nitrate, forms that plants can absorb through their roots.