Nutrient Cycling in Ecosystems

The following cycles are discussed here:

• Nitrogen Cycle
• Carbon Cycle
• Water Cycle
• Phosphorus Cycle
• Calcium Cycle

All organisms are involved in the nitrogen cycle, which is the changing of nitrogen between an inorganic and organic state. Nitrogen is an essential part of amino acids, proteins, nucleotides and their nucleic acids to name a few of the very import molecules requiring nitrogen.

Visit Fundamentals of Physical Geography for a through treatment of the nitrogen cycle. For more about the nitrogen cycle visit:

• Mark Whitelaw's Nitrogen Cycle article
• Dartmouth's map of the Nitrogen Cycle
• The importance of the Nitrogen Cycle in the home aquarium
• The Marine Institute's Diagram of the Nitrogen Cycle
• Essay on the Nitrogen Cycle from Kimball's Biology Pages
• Microbial World's Nitrogen Cycle and Nitrogen Fixation

The cycle has 2 parts or subcycles.

1. In the first, atmospheric nitrogen's triple covalent bond is broken. Hydrogen combines with a nitrogen atom to form ammonia. This is known as nitrogen fixation. This highly endergonic reaction is accomplished by the nitrogen fixing bacteria Rhizobium.. Another group of bacteria convert the ammonium ion (NH₄⁺) into nitrates (NO₃^-) to from nitric acid, making nitrogen available to green plants. Certain soil bacteria can put nitrogen gas back into the atmosphere through a process called denitrification. In this step, the bacteria break down urea, uric acid, and other nitrogen compounds produced by decaying organic matter to reform N₂ gas.

1. In the second part of the nitrogen cycle plants combine the nitrates with products of photosynthesis to form amino acids, the basic building block of proteins. Herbivores then digest these plants and break down plant proteins and use the amino acids to make their own proteins. Finally decomposers help recycle the nitrogen by converting it to ammonia or other soluble inorganic form.

The diagram below outlines the important steps in the nitrogen cycle.

 

A recent study indicates that we may be producing too much nitrogen and disrupting ecosystems around the world.

The atmosphere and ocean are the earth's greatest reservoirs of carbon in the form of the gas carbon dioxide. However, a sizable amount is present in carbonate rock and fossil fuel.

The cycling of carbon begins with carbon dioxide entering plants or algae during photosynthesis and becoming incorporated into glucose and other carbohydrates. Plants then carry on respiration, releasing some carbon back into the atmosphere and soil as carbon dioxide. Some of the plant carbohydrates enter animals and other heterotrophs, where carbon dioxide is returned to the atmosphere through respiration, excretion, fermentation, and decay.

The cycle includes one more significant source of carbon dioxide -- the burning of fossil fuels by humans.

The diagram below illustrates the relationship among the steps in the carbon cycle.

 For more information about the carbon cycle visit the following sites.

• The global carbon cycle - Oak Ridge National Lab
• The carbon cycle and global warming - Kyoto climate conference

Visit the web sites listed below for additional information about the water cycle.

• Marietta College - Biology 102 Lecture
• EPA

Visit the following sites for more information about the phosphorus cycle:

• Marietta College - Biology 102 Lecure
• Alien Earth

What is calcium? It is a chemical element, an alkaline earth metal, number twenty in the periodic table.

• Calcium Cycle - Geology 202
• Calcium Cycle - Yale

Modified Nov. 7, 2002

---