Environmental Science: The Nitrogen and Phosphorus Cycles
How essential nutrients move through ecosystems
Environmental Science: The Nitrogen and Phosphorus Cycles
How essential nutrients move through ecosystems
Environmental Science - Grade 9-12
- 1
Explain why nitrogen is essential for living organisms, even though most organisms cannot use nitrogen gas directly from the atmosphere.
Think about what cells are made of and what form of nitrogen plants can absorb.
Nitrogen is essential because organisms use it to build proteins, DNA, RNA, and other important molecules. Most organisms cannot use atmospheric nitrogen gas because its atoms are held together by a very strong bond, so it must first be converted into usable forms such as ammonium or nitrate. - 2
Describe the role of nitrogen-fixing bacteria in the nitrogen cycle.
Nitrogen-fixing bacteria convert nitrogen gas from the atmosphere into ammonia or ammonium. This process makes nitrogen available to plants and begins the movement of usable nitrogen through food webs. - 3
A farmer plants soybeans in a field after several years of growing corn. Explain how soybeans can help add usable nitrogen to the soil.
Legumes often form a relationship with bacteria in their roots.
Soybeans are legumes that often have nitrogen-fixing bacteria living in nodules on their roots. These bacteria convert atmospheric nitrogen into forms that can enter the soil and be used by plants, which can improve soil nitrogen for future crops. - 4
Compare nitrification and denitrification. Include what happens to nitrogen in each process.
Nitrification converts ammonium into nitrite and then nitrate, which plants can absorb. Denitrification converts nitrate back into nitrogen gas, returning nitrogen to the atmosphere. - 5
Explain why phosphorus does not have a major atmospheric stage like nitrogen does.
Consider whether phosphorus commonly exists as a gas under normal Earth surface conditions.
Phosphorus does not have a major atmospheric stage because it is usually found in rocks, sediments, soil, water, and living organisms rather than as a common gas. It moves mainly through weathering, erosion, absorption by organisms, decomposition, and sediment formation. - 6
Describe how weathering of rocks contributes to the phosphorus cycle.
Weathering breaks down phosphate-containing rocks and releases phosphate into soil and water. Plants can absorb this phosphate, and it then moves through food webs when organisms eat plants or other organisms. - 7
A lake receives runoff from nearby lawns and farms after a heavy rainstorm. The water contains high levels of nitrate and phosphate. Predict what may happen in the lake and explain why.
Think about how extra nutrients affect algae and how decomposition affects oxygen.
The lake may experience an algal bloom because nitrate and phosphate are nutrients that can increase algae growth. When the algae die and decompose, bacteria use oxygen, which can lower dissolved oxygen levels and harm fish and other aquatic organisms. - 8
Define eutrophication and identify one human activity that can cause it.
Eutrophication is the enrichment of a body of water with excess nutrients, especially nitrogen and phosphorus, which can lead to algal blooms and low oxygen levels. One human activity that can cause it is applying too much fertilizer that washes into waterways. - 9
In an agricultural field, fertilizer is applied before a long period of heavy rain. Explain one way nitrogen from the fertilizer can be lost from the field and one environmental problem this can cause.
Nitrate is highly soluble in water.
Nitrogen can be lost when nitrate dissolves in water and leaches through the soil or runs off into streams. This can contribute to nutrient pollution, algal blooms, and low oxygen conditions in aquatic ecosystems. - 10
Compare the movement of nitrogen and phosphorus through ecosystems. Give one similarity and one difference.
One similarity is that both nitrogen and phosphorus are essential nutrients that cycle through soil, water, organisms, and decomposers. One difference is that nitrogen has a major atmospheric reservoir, while phosphorus cycles mainly through rocks, sediments, soil, water, and organisms. - 11
A dead fish sinks to the bottom of a pond. Explain how decomposers help return nitrogen and phosphorus from the fish to the ecosystem.
Decomposition changes nutrients in dead matter into forms that other organisms can use.
Decomposers break down the dead fish and release nutrients from its tissues. Nitrogen can be converted into ammonium and then into other nitrogen forms, while phosphorus can be released as phosphate that may be used by plants, algae, or microbes. - 12
Suggest two practices that can reduce nitrogen and phosphorus pollution from farms, and explain how each practice helps.
Farmers can use precise fertilizer application so crops receive only the nutrients they need, which reduces excess runoff and leaching. They can also plant buffer strips near waterways, which use vegetation to slow runoff and absorb nutrients before they enter streams or lakes.