Nitrogen Cycle
The nitrogen cycle is the biogeochemical process by which nitrogen is converted between its various chemical forms as it circulates through terrestrial and aquatic ecosystems, including the atmosphere. Because nitrogen is a key component of amino acids, proteins, and nucleic acids, all life depends on it, yet atmospheric nitrogen (N₂) is largely inaccessible to most organisms without fixation. The cycle includes key processes: nitrogen fixation (N₂ → NH₃), nitrification (NH₃ → NO₂⁻ → NO₃⁻), assimilation, ammonification, and denitrification (NO₃⁻ → N₂).
Key Formula
N2 + 8H+ + 8e- → (nitrogenase) → 2NH3 + H2
LaTeX: \text{N}_2 + 8\text{H}^+ + 8e^- \xrightarrow{\text{nitrogenase}} 2\text{NH}_3 + \text{H}_2
| Symbol | Meaning | Unit |
|---|---|---|
| N₂ | Atmospheric dinitrogen (substrate) | mol |
| NH₃ | Ammonia produced by nitrogen fixation | mol |
| nitrogenase | Enzyme complex in nitrogen-fixing bacteria | catalyst |
| H₂ | Hydrogen gas released as byproduct | mol |
Key Processes in the Nitrogen Cycle
| Process | Transformation | Organisms Involved | Ecological Role |
|---|---|---|---|
| Nitrogen Fixation | N₂ → NH₃ | Rhizobium, Azotobacter, Cyanobacteria | Makes N available to plants |
| Nitrification | NH₃ → NO₂⁻ → NO₃⁻ | Nitrosomonas, Nitrobacter | Converts ammonia to usable nitrate |
| Assimilation | NO₃⁻ → Organic N | Plants, algae | Incorporates N into biomolecules |
| Ammonification | Organic N → NH₃ | Decomposer bacteria, fungi | Releases N from dead organisms |
| Denitrification | NO₃⁻ → N₂O → N₂ | Pseudomonas, Thiobacillus | Returns N to atmosphere |
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Related Terms
Carbon Cycle
The carbon cycle is the biogeochemical process by which carbon atoms continuously move through the atmosphere, hydrosphere, lithosphere, and biosphere through processes such as photosynthesis, respiration, decomposition, combustion, and ocean absorption. Carbon exists in various forms — as CO₂ in the atmosphere, as carbonate in rocks, as organic molecules in living organisms, and dissolved in water. Human activities such as burning fossil fuels and deforestation have significantly accelerated the movement of carbon into the atmosphere, driving climate change.
Decomposer
Decomposers are organisms — primarily fungi and bacteria — that break down dead organic matter (detritus) into simpler inorganic compounds, releasing nutrients back into the soil, water, and atmosphere. This process of decomposition is essential for nutrient cycling, making elements like carbon, nitrogen, and phosphorus available again for producers such as plants and algae. Without decomposers, ecosystems would quickly become buried in dead material and nutrient reservoirs would be permanently locked away.
Water Cycle
The water cycle, also known as the hydrological cycle, is the continuous movement of water through Earth's systems — from the oceans to the atmosphere through evaporation, to the land through precipitation, and back to the oceans via runoff and groundwater flow. Driven primarily by solar energy and gravity, the water cycle regulates climate, freshwater availability, and supports all life on Earth. Key processes include evaporation, transpiration (evapotranspiration from plants), condensation, precipitation, infiltration, and surface runoff.
Nitrogen from Greek nitron ("native soda") + genes ("forming"), reflecting its early association with nitre (potassium nitrate). Identified as an element by Daniel Rutherford in 1772. The word "cycle" from Greek kyklos, "circle".