Figure 3–18 The Nitrogen Cycle The atmosphere is the largest reservoir of nitrogen in the biosphere. Nitrogen also cycles through the soil and through the tissues of living organisms. Interpret Visuals Through which two processes does nitrogen gas get converted into usable forms for organisms?

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The Nitrogen Cycle All organisms require nitrogen to make amino acids, which combine to form proteins, and nucleic acids, which combine to form DNA and RNA. Many different forms of nitrogen occur naturally in the biosphere. Nitrogen gas (N₂) makes up 78 percent of Earth's atmosphere. Nitrogen-containing substances such as ammonia (NH₃), nitrate ions (NO₃⁻), and nitrite ions (NO₂⁻) are found in soil, in the wastes produced by many organisms, and in dead and decaying organic matter. Dissolved nitrogen also exists in several forms in the ocean and other large water bodies. Figure 3–18 shows how different forms of nitrogen cycle through the biosphere.

Although nitrogen gas is the most abundant form of nitrogen on Earth, only certain types of bacteria can use this form directly. These bacteria convert nitrogen gas into ammonia, a process known as nitrogen fixation. Some of these nitrogen-fixing bacteria live in the soil and on the roots of certain plants, such as peanuts and peas, called legumes. Other bacteria convert that fixed nitrogen into nitrates and nitrites. Once these forms of nitrogen are available, primary producers can use them to make proteins and nucleic acids. Consumers eat the producers and reuse nitrogen to make their own nitrogen-containing compounds. Decomposers release nitrogen from waste and dead organisms as ammonia, nitrates, and nitrites that producers may take up again. Other bacteria obtain energy by converting nitrates into nitrogen gas, which is released into the atmosphere in a process called denitrification. A relatively small amount of nitrogen gas is converted to usable forms by lightning in a process called atmospheric nitrogen fixation. Humans add nitrogen to the biosphere through the manufacture and use of fertilizers. Excess fertilizer is often carried into surface water or groundwater by precipitation.

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