Nitrogen Cycle

All organisms need nitrogen to make proteins and nucleic acids. The complex pathway that nitrogen follows in an ecosystem is called the nitrogen cycle, as shown in Figure 18-14. Nitrogen gas, N2, makes up about 78 percent of the atmosphere, so it might seem that it would be readily available for living things. However, most plants can use nitrogen only in the form of nitrate. The process of converting N2 gas to nitrate is called nitrogen fixation.

Most organisms rely on nitrogen-fixing bacteria to transform nitrogen gas into a usable form. These bacteria live in the soil and inside swellings on the roots of some kinds of plants, such as beans, peas, clover, and alfalfa. These plants supply carbohydrates for the bacteria, and the bacteria produce usable nitrogen for the plant. Additional nitrogen is released into the soil.

Atmospheric figure 18-14

This figure shows the cycling of nitrogen within a terrestrial ecosystem. Bacteria are responsible for many of the steps in the nitrogen cycle, including the conversion of atmospheric nitrogen into ammonium. Nitrogen-fixing bacteria live in the soil or in the roots of plants. These bacteria convert nitrogen gas into ammonium. Other bacteria convert the ammonium into nitrates. Plants take up the nitrates produced by the bacteria. Animals get nitrogen by eating plants or other animals.

Atmospheric

Nitrogen Nitrogen-fixing L fixation bacteria in soil

Ammonia (NH3) and Ammonium (NH4+)

Nitrogen Nitrogen-fixing L fixation bacteria in soil

Ammonia (NH3) and Ammonium (NH4+)

Recycling Nitrogen

The bodies of dead organisms contain nitrogen, mainly in proteins and nucleic acids. Urine and dung also contain nitrogen. Decomposers break down these materials and release the nitrogen they contain as ammonia, NH3, which in soil becomes ammonium, NH4+. This process is known as ammonification. Through this process, nitrogen is again made available to other organisms.

Soil bacteria take up ammonium and oxidize it into nitrites, NO2~, and nitrates, NO3~, in a process called nitrification. The erosion of nitrate-rich rocks also releases nitrates into an ecosystem. Plants use nitrates to form amino acids. Nitrogen is returned to the atmosphere through denitrification. Denitrification occurs when anaerobic bacteria break down nitrates and release nitrogen gas into the atmosphere. Plants can absorb nitrates from the soil, but animals cannot. Animals obtain nitrogen in the same way they obtain energy—by eating plants and other organisms and then digesting the proteins and nucleic acids.

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