The Nitrogen Cycle for Middle School Science

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The Nitrogen Cycle describes how nitrogen moves through the environment. Like carbon, nitrogen exists both in the atmosphere and on the surface of the Earth. In fact, nitrogen is the most common gas in our atmosphere. Nitrogen is also an important element for plant growth, but plants can't access it without help from bacteria.

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The Nitrogen Cycle

The Nitrogen Cycle describes how nitrogen moves through the environment. Nitrogen is an element found in the amino acids and DNA of all living things. It is also an essential part of chlorophyll, which plants use during photosynthesis. While nitrogen makes up nearly 80% of the atmosphere, organisms can't use nitrogen gas from the atmosphere.

Luckily, bacteria found in the soil and water can convert nitrogen gas into ammonia through a process called nitrogen fixation. Some of the bacteria that can fix nitrogen are free-floating, but others have a symbiotic relationship with plants. Plant cells and nitrogen-fixing bacteria live together at the root nodules of the plant.

Once the bacteria have converted the nitrogen gas to ammonia, the plant can use the ammonia as a source of nitrogen. The next step in the nitrogen cycle provides another source of nitrogen for the plants. A different type of bacteria converts the ammonia in the soil into nitrates. This process is called nitrification. Plants can use the nitrogen in nitrates to make amino acids, chlorophyll, or DNA.

When ammonia and nitrates from the soil become a part of the plant, we call it nitrogen assimilation. The plants take in the nitrogen and use it to make amino acids, chlorophyll, and DNA. Then, animals eat the plants and get the nitrogen to make their amino acids and DNA. Animals do not make chlorophyll because they do not photosynthesize.

After an organism dies, bacteria and other microorganisms in the soil decompose the body and release ammonia back into the ground for the plants to absorb. This process is called ammonification.

While some bacteria convert nitrogen gas into ammonia and nitrates, other bacteria convert ammonia and nitrates into nitrogen gas. Because this is the opposite of nitrification, it is called denitrification.

The nitrogen cycle also happens in lakes, rivers, and oceans, where bacteria can fix dissolved nitrogen gas into ammonia. All of the other steps in the nitrogen cycle remain the same in the water. When compounds containing nitrogen fall to the ocean floor, they become a part of sediments. Over millions of years, new sediments cover the older sediments, and the pressure turns the old sediments into sedimentary rock. Millions of years later, when the sedimentary rock eventually wears down through weathering and erosion, the nitrogen from the sediments is released into the soil for plants to use.

Like other environmental cycles, the steps in the nitrogen cycle do not always happen in the same order. Ammonia released through ammonification may re-enter the atmosphere through denitrification, or a plant might absorb it. The nitrogen cycle steps show what could happen to nitrogen in the environment.

Nitrogen is a limiting factor for plants because the amount of nitrogen available to plants limits how much they can grow. To make their plants grow larger, farmers add nitrogen in fertilizer to their fields. Extra nitrogen helps the plants grow, but it also has negative effects on the environment. With more nitrogen in the soil, denitrification bacteria release more nitrogen gas into the atmosphere. When nitrogen combines with water in the atmosphere, it can form acid rain. Nitrogen gas is also a greenhouse gas that traps heat in Earth's atmosphere and makes our planet warmer.

When water carries soil full of nitrogen into streams, rivers, and lakes, the extra nitrogen causes algae blooms because the algae need the nitrogen to grow too. When too many nutrients are in the water, it is called eutrophication. Eutrophication causes plants to grow too much, leading to animal deaths. Plants and algae use photosynthesis to make energy during the day, but they need oxygen to use the energy at night. When too many algae live in one area, they deplete the oxygen for the rest of the animals living in the water. As a result, many larger animals die.

Nitrogen gas can also be converted into nitrates through the energy from a lightning strike or ultraviolet radiation. However, this process only accounts for about 10% of the nitrogen fixation that takes place today.