Earth as a System: Energy, Matter, and Life · Lesson 11 of 13
Nitrogen Cycle and Oxygen Cycle
“The air is full of nitrogen and oxygen, but atoms still insist on taking the scenic route.”
• Explain why organisms need nitrogen. • Describe nitrogen fixation, assimilation, ammonification, nitrification and denitrification. • Identify the roles of key microorganisms in the nitrogen cycle. • Explain the benefit and cost of industrial nitrogen fixation. • Trace oxygen through photosynthesis, respiration and combustion. • Relate the nitrogen, carbon and oxygen cycles.
A plant stands in air containing abundant nitrogen, yet it cannot usually use nitrogen gas directly. At the same time, it releases oxygen that animals use in respiration. Microorganisms, plants, animals and chemical reactions cooperate to change these elements into usable forms and return them to major reservoirs.
Nitrogen Cycle
Nitrogen fixation is the conversion of atmospheric nitrogen into nitrogen compounds that can enter biological systems.
| Process | Conversion or movement | Important agents |
|---|---|---|
| Fixation | Atmospheric nitrogen to usable nitrogen compounds | Rhizobium, free-living bacteria such as Azotobacter, and lightning |
| Assimilation | Soil nitrogen compounds into plant molecules | Plants, followed by feeding in animals |
| Ammonification | Organic nitrogen in wastes and remains to ammonia or ammonium compounds | Decomposers |
| Nitrification | Ammonia compounds to nitrites and then nitrates | Nitrosomonas and Nitrobacter |
| Denitrification | Nitrates to atmospheric nitrogen | Denitrifying bacteria such as Pseudomonas |
Nitrogen is needed to build proteins and nucleic acids. Nitrogen gas is stable because its two atoms are strongly bonded. Nitrogen-fixing bacteria provide usable compounds. Plants assimilate nitrate or ammonium compounds, animals obtain nitrogen by feeding, and decomposers return organic nitrogen to soil. Nitrifying and denitrifying bacteria complete further conversions.
Problem
How does a legume gain nitrogen?
- 1.Rhizobium bacteria live in nodules on the roots.
- 2.The bacteria convert atmospheric nitrogen into usable nitrogen compounds.
- 3.The plant uses the compounds to build amino acids and proteins.
- 4.The bacteria receive food and a protected habitat from the plant.
- 5.Nitrogen may later enter animals or soil through feeding, waste and decay.
Ready to Go Beyond
The Haber-Bosch process combines atmospheric nitrogen and hydrogen under controlled high temperature and pressure to make ammonia. This “bread from air” technology supplies most modern nitrogen fertiliser and played an important role in the Green Revolution. More than half of the nitrogen atoms in the human body are estimated to have passed through this industrial route. The process also uses roughly 1–2 per cent of global energy, and excessive fertiliser can degrade soil and water. A useful technology must therefore be managed carefully.
Problem
Why does adding more fertiliser not always mean more useful plant growth?
- 1.Plants can absorb nitrogen only at a limited rate.
- 2.Rain may carry unused nitrate into groundwater, streams or lakes.
- 3.Microorganisms may transform some compounds into gases.
- 4.Excess nutrients in water can stimulate algal blooms.
- 5.Correct quantity, timing and placement reduce losses.
Oxygen Cycle
Oxygen makes up about 21 per cent of the atmosphere. It is also present in water, carbon dioxide, metal oxides, minerals and biological molecules. Photosynthesis splits water and releases oxygen. Respiration uses oxygen to release usable energy from food, producing carbon dioxide and water. Combustion also consumes oxygen and commonly produces carbon dioxide and water. Oxidation during decomposition and the formation of mineral oxides provide additional pathways from free oxygen into combined forms.
Pause and Ponder
The carbon and oxygen cycles are linked through opposite-looking reactions. Photosynthesis uses carbon dioxide and releases oxygen, while respiration uses oxygen and releases carbon dioxide. They are not perfect reversals in every detail, but together they exchange matter between organisms and the atmosphere.
Roots absorb nitrogen mainly as dissolved compounds. Fixation is the chemical conversion that makes atmospheric nitrogen available to the biological cycle.
Quiz
Which description best matches Nitrogen Fixation?
Which term matches this description: Nitrogen fixation is the conversion of atmospheric nitrogen into nitrogen compounds that can enter biological systems.
Which statement is a key takeaway from this lesson?
Which additional statement is also a key takeaway from this lesson?
Which further statement is also a key takeaway from this lesson?
Practice Problems
- Place the five major nitrogen processes in a connected sequence.
- Explain the roles of Rhizobium, Nitrosomonas and denitrifying bacteria.
- Give one benefit and one environmental cost of industrial fertiliser.
- Compare the roles of photosynthesis and respiration in the oxygen cycle.
Key Takeaways
• Nitrogen must be converted before most organisms can use it. • Microorganisms drive crucial nitrogen transformations. • Industrial fixation supports agriculture but can disturb natural cycles. • Photosynthesis, respiration and combustion connect oxygen with carbon.