Nutrient cycles

Biology · Relationships of organisms with one another and with the environment. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Energy flows through an ecosystem once, but chemical elements such as carbon and nitrogen are used again and again. They move in cycles between living things and the environment.

In the carbon cycle, photosynthesis takes carbon dioxide out of the air and builds it into plants. Feeding passes the carbon to animals. Respiration, decomposition and combustion return carbon dioxide to the air.

In the nitrogen cycle, plants cannot use nitrogen gas. They absorb nitrate ions from the soil and use them to make amino acids and proteins. Animals get nitrogen by eating and digesting protein. Bacteria carry out the steps that make nitrate available again. Decomposers (fungi and bacteria) recycle the nutrients in dead material.

Rules to remember

Photosynthesis removes carbon dioxide from the air; respiration (in plants, animals and decomposers) and combustion add it.
Fossil fuels form over millions of years from dead organisms that did not fully decompose; burning them releases the stored carbon as carbon dioxide.
Decomposition: decomposers break down protein in dead organisms and waste into ammonium ions.
Nitrification: bacteria change ammonium ions into nitrate ions. It needs oxygen.
Nitrogen fixation: nitrogen gas is changed into nitrogen compounds by lightning and by bacteria (in soil and in root nodules).
Denitrification: bacteria change nitrate ions back into nitrogen gas, mainly in waterlogged soil with little oxygen.
Plants absorb nitrate ions → amino acids → protein → eaten and digested by animals.

Common mistake

Nitrification and nitrogen fixation are often confused. Fixation starts from nitrogen gas in the air; nitrification starts from ammonium ions in the soil.

Worked example

A farmer grows clover, which has nitrogen-fixing bacteria in its root nodules, and then ploughs the whole crop into the soil. Why does the next crop grow better?

Bacteria in the root nodules change nitrogen gas into nitrogen compounds, which the clover uses to make protein.
After ploughing, decomposers break down the clover protein into ammonium ions.
Nitrifying bacteria change the ammonium ions into nitrate ions.
The next crop absorbs the nitrate ions and uses them to make amino acids and protein for growth.

Answer: The soil contains more nitrate ions, so the next crop can make more protein and grow faster.

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