We've seen how carbon is cycled from unusable to usable forms as it moves through an ecosystem. The atmospheric form of carbon, carbon dioxide, is unusable by heterotrophs, and it must be converted to a usable form by autotrophs. Another important element, nitrogen, has an unusable atmospheric form that must be converted. But as we'll see in a moment, primary producers aren't responsible for the conversion.

Nitrogen is found in all amino acids, and therefore in all proteins. It is also found in the nitrogenous bases that make up DNA. Most nitrogen in the atmosphere is in the form of nitrogen gas, N2. Nitrogen gas is not a usable form of nitrogen for plants. Plants can only use nitrogen when it's in the form of either ammonium or nitrate.

A small amount of atmospheric nitrogen is made up of ammonium and nitrate, which are deposited in the soil by rain. But this only makes up 5 to 10% of the nitrogen used by plants. Let's see how nitrogen gas is converted to ammonium and nitrate.

The process of converting nitrogen in the atmosphere to usable forms is a complex process called nitrogen fixation. Certain prokaryotes convert atmospheric nitrogen to ammonia, NH3. The bacteria use some ammonia for their own metabolism, and release the excess. Most soil is acidic, so the ammonia picks up a hydrogen ion to form ammonium, NH4.

Some of the nitrogen-fixing bacteria have a symbiotic relationship with a specific plant. For example, some legumes have root nodules that contain the nitrogen-fixing bacterium Rhizobium. Other nitrogen-fixing bacteria are free-living in the soil. In aquatic environments, some cyanobacteria carry out nitrogen fixation.

Ammonium is also produced by the decomposition of organic material, a process called ammonification. Ammonification is carried out primarily by bacterial and fungal decomposers. Ammonium in the soil can be used directly by plants, but most of it is converted to nitrate in a process called nitrification. Nitrifying bacteria in the soil convert ammonium to nitrites and then to nitrates. The nitrates can then be used by plants. Nitrates are removed from the soil as they are incorporated into plants.

Nitrates are also removed in a process called denitrification. In anaerobic conditions, some bacteria get their oxygen from nitrate. In the process, nitrate is converted to nitrogen gas, and released back into the atmosphere.

While the nitrogen cycle includes atmospheric nitrogen, most cycling actually occurs between the soil and organisms. Nitrogen fixation accounts for only a small amount of the nitrogen incorporated by plants. But many plants still have symbiotic relationships with nitrogen-fixing bacteria. Most nitrogen is recycled locally through decomposition, ammonification, nitrification, and incorporation.

Just as humans affect the carbon cycle by burning fossil fuels, we are also having an impact on the nitrogen cycle. To make fertilizer, factories convert large quantities of atmospheric nitrogen to nitrates. Industrial fixation has doubled the nitrogen fixation rate. The excess nitrates found in fertilizers end up in runoff and eventually make their way to lakes and rivers. The excess nitrates can lead to areas of overgrowth of algae and aquatic plants.

The extra growth of aquatic plants and algae may not seem like a big problem. However, when the algae and plants die, decomposers use up all the oxygen in the water for cellular respiration. As a result, all of the aerobic life in the water, including fish, doesn't have enough oxygen, and they die. This is an example of how human involvement can have far-reaching repercussions, and it demonstrates why we must be careful in the way we manipulate our ecosystems for our own use.

To see how well you understand the nitrogen cycle, drag the labels to the appropriate place on the diagram. You may have to use a label more than once. Click Submit to see if you're correct.

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Nitrogen gas deposited in the soil is converted to ammonium through nitrogen fixation. Ammonium is also produced through decomposition, a process called ammonification. The ammonium is converted to nitrite and then nitrate though nitrification. Nitrates can be used by plants. Nitrates can also be converted to nitrogen gas through denitrification.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education