Biogeochemical Cycles

1. What are biogeochemical cycles?

Biogeochemical cycles are representations of the circulation and recycling of matter in nature.

The main biogeochemical cycles studied in Ecology are the water cycle, the carbon cycle and the nitrogen cycle.

2. What is the respective importance of water, carbon and nitrogen for living beings?

Water is the main solvent of living beings and it is necessary practically for all biochemical reactions, including as reagent of photosynthesis. Many properties of water are very important for life.

Carbon is the main chemical element of organic molecules; carbon dioxide is also reagent of photosynthesis and product of the energetic metabolism of living beings.

Nitrogen is a fundamental chemical element of amino acids, the building blocks of proteins that in their turn are the main functional molecules of living beings; nitrogen is also part of the nucleic acid molecules, the basis of reproduction, heredity and protein synthesis.

3. What is the water cycle?

The water cycle represents the circulation and recycling of water in nature.

Liquid water on the planet surface is heated by the sun and turns into water vapor that gains the atmosphere. In the atmosphere large volumes of water vapor form clouds that when cooled precipitate liquid water as rain. Therefore water comes back to the planet surface and the cycle is completed. As possible steps of the cycle, water may still be stored in subterranean reserves or in the form of ice in mountains and oceans and it may also be used in the metabolism of living beings, incorporated into the body of the individuals or excreted through urine, feces and transpiration.

 4. Why is the sun the “motor” of the water cycle?

The sun can be considered the motor of the water cycle because upon its energy the transformation of liquid water into water vapor depends. So the sun is the energy source that causes water to circulate in nature.

5. What is the carbon cycle?

The carbon cycle represents the circulation and recycling of the chemical element carbon in nature as a result of the action of living beings.

Photosynthetic beings absorb carbon as carbon dioxide available in the atmosphere and the carbon atoms become part of glucose molecules. During the cellular respiration of these beings part of this organic material is consumed to generate ATP and in this process carbon dioxide is returned to the atmosphere. The other part is incorporated by the photosynthetic organisms into the molecules that compose their structure. The carbon atoms incorporated into the producers are transferred to the next trophic level and again part is liberated by the cellular respiration of the consumers, part becomes a constituent of the consumer body and part is excreted as uric acid or urea (excretes later recycled by decomposer bacteria). Therefore carbon absorbed by the producers in photosynthesis returns to the atmosphere through cellular respiration along the food chain until the decomposers that also liberate carbon dioxide in their energetic metabolism. Under special conditions in a process that takes millions of years carbon incorporated into organisms may also constitute fossil fuels stored in deposits under the surface of the planet; as fossil fuels burn the carbon atoms return to the atmosphere as carbon dioxide or carbon monoxide. The burning of vegetable fuels, like wood, also returns carbon to the atmosphere.

6. What is the main biological process that consumes carbon dioxide?

The main biological process that consumes carbon dioxide is photosynthesis. 

7. How is carbon dioxide made by producers and consumers?

Carbon dioxide is made by producers and consumers through cellular respiration.

8. What are fossil fuels?

Fossil fuels, like oil, gas and coal, form when organic material is preserved from the complete action of decomposers, generally buried deep and under pressure over millions of years. Under such conditions the organic material transforms into hydrocarbon fuels.

Fossil fuels are a natural reservoir of carbon. When oxygen is present these fuels can be burned and carbon dioxide and carbon monoxide are released into the atmosphere.

9. What is the most abundant form under which nitrogen is found in nature?

The most abundant nitrogen-containing molecule found in nature is molecular nitrogen (N2). The air is 80% constituted of molecular nitrogen. 

10. Under which form is nitrogen fixed by living beings?

Most living beings cannot use molecular nitrogen to obtain nitrogen atoms. Producers fix nitrogen mainly from nitrate (NO3-). Some plants also fix nitrogen from ammonia. Consumers and decomposers acquire nitrogen through digestion of mainly proteins and nucleic acids from the body of other living beings.

11. What is the nitrogen cycle?

The nitrogen cycle represents the circulation and recycling of the chemical element nitrogen in nature.

The nitrogen cycle basically depends on the action of some specialized bacteria. Bacteria of the soil called nitrogen-fixing bacteria present in plant roots absorb molecular nitrogen from the air and liberate nitrogen in the form of ammonia. The decomposition of organic material also produces ammonia. In the soil and roots (mainly of leguminous plants), a first group of chemosynthetic bacteria called nitrifying bacteria, the nitrosomonas, produces energy consuming ammonia and releasing nitrite (NO2). The second group of nitrifying bacteria, the nitrobacteria, uses nitrite in chemosynthesis releasing nitrate (NO3). In the form of nitrate, nitrogen is then incorporated by plants to be used as constituent of proteins and nucleic acids and the element then follows along the food chain. Nitrogen returns to the atmosphere by the action of denitrifying bacteria that use nitrogen-containing compounds from the soil and release nitrogen gas (molecular nitrogen).

12. Why is a leguminous crop rotation used in agriculture?

Leguminous crop rotation and other crop rotations are used in agriculture because in these plants many bacteria important for the nitrogen cycle live. The leguminous crop rotation (or cojointly with the main crop) helps the soil to become rich in nitrates that are then absorbed by the plants.

Green manure, the covering of the soil with grass and leguminous plants, is also a way to improve the fixation of nitrogen and it is an option in avoiding chemical fertilizers.

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