Ecology
Nitrogen Cycle
Movement of nitrogen through biosphere — bacteria-mediated transformations
The nitrogen cycle is the movement of nitrogen between atmosphere (78% N₂), soil, water, and organisms. Mostly bacteria-mediated. Key processes: (1) Nitrogen fixation — N₂ → NH₃/NH₄⁺ by bacteria (nitrogen-fixing) or industrial Haber-Bosch. (2) Nitrification — NH₄⁺ → NO₂⁻ → NO₃⁻ by bacteria. (3) Assimilation — plants take up NH₄⁺/NO₃⁻ → proteins, nucleic acids. (4) Decomposition — organic N → NH₄⁺. (5) Denitrification — NO₃⁻ → N₂ back to atmosphere. Plus: humans add ~150 Mt N/year via fertilizers. Critical for: agriculture, water quality, ecosystem function.
- Atmospheric N₂78% of atmosphere
- Nitrogen fixationN₂ → NH₃ (legumes' rhizobia, free-living bacteria, lightning)
- Industrial fixationHaber-Bosch (~150 Mt N/year)
- NitrificationNH₄⁺ → NO₂⁻ → NO₃⁻ by bacteria
- DenitrificationNO₃⁻ → N₂ by bacteria
- Plant uptakeAs NH₄⁺ or NO₃⁻ from soil
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Why nitrogen cycle matters
- Agriculture. N is limiting; fertilizers critical.
- Water quality. Eutrophication.
- Climate. N₂O is greenhouse gas.
- Food security. Haber-Bosch enables current population.
- Ecosystems. Major nutrient cycle.
- Pollution. Acid rain (HNO₃).
- Microbiology. Bacterial roles fundamental.
Common misconceptions
- Plants take N₂ from air. Most can't; need fixed forms.
- Nitrogen rare. 78% of atmosphere; just inert form.
- Cycle balanced now. Humans doubled fluxes.
- One bacterial type for all transformations. Many specialized groups.
- NH₄⁺ same as NO₃⁻. Different mobility, plant preference.
- Lightning fixes most N. Bacteria do most.
Frequently asked questions
Why is nitrogen so important?
Component of: amino acids (proteins), nucleic acids (DNA, RNA), chlorophyll, ATP. Limiting nutrient in many ecosystems. Plants especially: need N for proteins and chlorophyll. Why fertilizers full of N (urea, ammonium nitrate). Animals get N from plants (or animals that ate plants). Critical for life.
What's nitrogen fixation?
Converting unreactive N₂ to bioavailable form. Three pathways. (1) Biological: bacteria with nitrogenase enzyme convert N₂ to NH₃. Examples: rhizobium in legume root nodules, free-living bacteria, cyanobacteria. (2) Atmospheric: lightning oxidizes N₂ → NO_x → falls as nitric acid. Small contribution. (3) Industrial: Haber-Bosch process. Provides most synthetic N for fertilizers. Bacteria: most natural fixation.
What's the rhizobium-legume symbiosis?
Legumes (peas, beans, clover, alfalfa) form root nodules with rhizobium bacteria. Bacteria fix N₂ → NH₃ for plant. Plant provides bacteria with sugars, oxygen-controlled environment. Critical: legumes can grow without N fertilizer; enrich soil for next crop (rotation). Important: agricultural practice; foundational for sustainable farming.
What's nitrification?
NH₄⁺ → NO₂⁻ → NO₃⁻ by soil bacteria. Two-step process. (1) Nitrosomonas: NH₄⁺ + 1.5 O₂ → NO₂⁻ + H₂O + 2H⁺. (2) Nitrobacter: NO₂⁻ + 0.5 O₂ → NO₃⁻. Aerobic process; needs O₂. Plants prefer NO₃⁻ (more mobile in soil). Inhibitors: low pH, waterlogged conditions.
What's denitrification?
NO₃⁻ → N₂ + N₂O by anaerobic bacteria. Returns N to atmosphere. Occurs in: waterlogged soils, sediments, GI tracts. Loses N from ecosystem. Industrial concern: N₂O is greenhouse gas (~300× CO₂); produced via incomplete denitrification. Wetlands: significant N₂O sources. Reduces fertilizer effectiveness in waterlogged soil.
What's eutrophication?
Excess nutrients (especially N and P) stimulate algal blooms. From: agricultural runoff, sewage, atmospheric deposition. Algae grow rapidly; die; decomposed by bacteria → consume oxygen → dead zones. Examples: Gulf of Mexico (drained from Mississippi River), Chesapeake Bay, Baltic Sea. Major water quality problem; reduces fisheries, biodiversity.
How do humans affect the cycle?
Major changes. (1) Industrial fixation (Haber-Bosch): ~150 Mt N/year — exceeds natural fixation. (2) Fossil fuels: burning releases NO_x. (3) Deforestation: changes terrestrial N cycling. (4) Manure runoff: agricultural pollution. Result: doubled global N flux. Consequences: water pollution, eutrophication, atmospheric N₂O (greenhouse gas), ozone destruction, biodiversity loss.