GED Science: Nitrogen Cycle

Nitrogen is another important nutrient for plants and animals.  Nitrogen can affect the rate at which producers turn carbon dioxide into energy through photosynthesis, and can affect the rate of decomposition of plant and animal material.  Furthermore, nitrogen is a component of amino acids, which are the building blocks for proteins. The nitrogen in the proteins of many animals can be traced back to the ingestion of plant materials.  Thus, shortages of nitrogen can have large effects on food chains and communities of organisms.  Remember from the first ecology lesson that plants are the base of the food chain.  If the growth of plants is stunted by the absence of available nitrogen, all further levels of the food chain (first-order consumers, etc.) will be affected.

In order to understand the nitrogen cycle, you need to understand the different forms in which nitrogen can be found.  The largest store of available nitrogen is in the atmosphere.  78% of our atmosphere is made up of nitrogen.  However, nitrogen is this form is not useful for plant growth or for decomposition.  So ecologists are concerned with how this atmospheric nitrogen can be fixed, or made available for use by plants.  There are two main processes that fix nitrogen.  One is through metabolic processes of bacteria to convert the atmospheric nitrogen to a different form.  Another is through lightning chemically converting the atmospheric nitrogen to a different form.

While all plants need nitrogen, the fixation of atmospheric nitrogen is only one way that plants can obtain that nitrogen.  Additionally, when organisms die, bacteria convert the nitrogen in those organisms to ammonia, which can then be converted to a form that is useful for plants.  Additionally, humans can artificially introduce nitrogen in a form that is useful for plants through the addition of fertilizers to the soil.  The main component of fertilizers is nitrogen.  The following diagram shows some of these processes in the nitrogen cycle:


While ecologists are concerned with a lack of available nitrogen for plants, they are also concerned with an overabundance of nitrogen in the form of plant and animal wastes.  This overabundance is of special concern in marine ecosystems.  An overabundance of nitrogen can lead to reduced oxygen levels in the water, which affect a number of plant, and subsequently animal, communities.  Additionally, an overabundance of nitrogen is toxic to fish species.  Hobby aquarium keepers are well-versed in the nitrogen cycle, including the various forms of nitrogen and their effects on fish.


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