14.1 Essential ideas

14.1.6 Nitrogen and phosphorus cycles (HL)

Nitrogen and phosphorus are two important nutrient components of soil. 

The nitrogen cycle

the nitrogen cycleFigure 14.1.6a – The nitrogen cycle

  • Atmospheric nitrogen exists as a diatomic gas, N2. Elemental nitrogen gas cannot be assimilated by either animals or plants. It must first be converted into ammonium and nitrate ions. This involves many different kinds of bacteria.
  • Nitrogen-fixing bacteria – only certain bacteria have the enzymes necessary to convert nitrogen gas into ammonia and ammonium ions.
  • Some nitrogen-fixing bacteria are free-living and contribute to the overall nutrient structure of soils. Some examples are Azotobacter and Clostridium.
  • Rhizobia are species of nitrogen-fixing bacteria that attach to root nodules in leguminous plants. These bacteria establish mutual relationships with their hosts by providing ammonium ions in exchange for carbon compounds produced by the plant. Rhizobium does not exist as a free-living bacterium.
  • Nitrifying bacteria reduce ammonium ions into nitrites and nitrates. Nitrosomonas and Nitrobacter are examples.
  • Plants assimilate ammonium and nitrates into proteins which are transferred through the food chain.
  • Decomposers – when plants and animals die, their remains are decomposed by aerobic and anaerobic bacteria which return ammonium ions to the soil.
  • Denitrifying bacteria – in the absence of oxygen, these anaerobic bacteria convert nitrates in the soil into elemental nitrogen gas and release it into the atmosphere. An example is Pseudomonas denitrificans.


Skill – draw and label a diagram of the nitrogen cycle 
Use the information above to draw a nitrogen cycle, being sure to include all of the relevant processes and organisms involved. Remember: processes are represented by arrows and storages are represented by boxes. Check your answer >

The phosphorus cycle

phosphorus cycleFigure 14.1.6b – The phosphorus cycle

  • Inorganic phosphates (PO43-) are added to soil by geomorphological processes, especially weathering and erosion.
  • Phosphates are incorporated into living organisms as complex organic molecules such as DNA, RNA, ATP and phospholipids, and are transferred through food chains.
  • Decomposers (bacteria, saprophytes) return inorganic phosphates to the soil.

Turnover rate and availability of phosphorus

Unlike the nitrogen cycle, the phosphorus cycle has no atmospheric component. Instead, phosphates are added to soil through very slow geomorphological processes. This means that phosphorus is transferred slowly through the cycle. The amount of phosphorus that is released in a given amount of time – the turnover rate – is very low compared with movement in the nitrogen cycle. 

Phosphorus is removed from the cycle when crops are harvested, and is added when fertilisers are used in agriculture. It is likely that phosphorus availability may limit agricultural production in the future because of slow turnover rates.

Eutrophication and increased biochemical oxygen demand (BOD)

  • Biochemical oxygen demand (BOD) is a measurement of how much oxygen is used by aerobic microbes to break down organic molecules in a certain volume of water in a certain amount of time.
  • BOD can also act as an indication of pollution levels because a high BOD indicates there is less dissolved oxygen available in the water.

biochemical oxygen demand

Figure 14.1.6c – How Biochemical Oxygen Demand (BOD) indicates pollution level

  • In unpolluted water, there are high levels of dissolved oxygen, and little organic matter (See Figure 14.1.6c - left). Bacterial respiration rates are low, so BOD is also low.
  • In polluted water, bacterial respiration rates are higher because there is more organic pollution (See Figure 14.1.6c - right), so BOD is higher.
  • One of the main causes of increased BOD is eutrophication due to fertiliser use. When mineral nutrients leach into streams and lakes, this encourages algal blooms and disrupts ecosystem function.

Essential idea

Soil cycles are subject to disruption.

Concept help

  • All nitrogen-fixing bacteria, whether free-living or associated with plant roots, fix nitrogen in the following way:

N2 + 8H+ + 8e → 2NH3 + H2 → 2NH3 + 2H+ → 2NH4+

  • Lightning also provides enough energy to break apart atmospheric nitrogen molecules, and synthesise ammonia or nitrates, but this accounts for only 10% of global nitrogen fixation. Bacteria are doing 90% of the work!

BOD tableFigure 14.1.6d – BOD table
High biochemical oxygen demand is associated with high levels of organic pollution.

Did you know?

Different agricultural crop groups have different nutritional needs. Crop rotation allows the renewal of soil nutrients, especially nitrogen, by providing a fallow period. Suggested crop rotations are shown in Figure 14.1.6e.

crop rotation

Figure 14.1.6e – Crop rotation

Further reading

Commercial fertilisers produced using the Haber process are ecologically unsustainable. For example, they require fossil fuels to generate energy. Read more about the unsustainability of commercial food production in:

Pollan, M. (2006) The Omnivore’s Dilemma: A Natural History of Four Meals. New York: Penguin.