Environmental Science: The Nitrogen and Phosphorus Cycles Worksheet

Question 1

Explain why nitrogen is essential for living organisms, even though most organisms cannot use nitrogen gas directly from the atmosphere.

Accepted Answer

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.

Question 2

Describe the role of nitrogen-fixing bacteria in the nitrogen cycle.

Accepted Answer

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.

Question 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.

Accepted Answer

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.

Question 4

Compare nitrification and denitrification. Include what happens to nitrogen in each process.

Accepted Answer

Nitrification converts ammonium into nitrite and then nitrate, which plants can absorb. Denitrification converts nitrate back into nitrogen gas, returning nitrogen to the atmosphere.

Question 5

Explain why phosphorus does not have a major atmospheric stage like nitrogen does.

Accepted Answer

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.

Question 6

Describe how weathering of rocks contributes to the phosphorus cycle.

Accepted Answer

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.

Question 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.

Accepted Answer

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.

Question 8

Define eutrophication and identify one human activity that can cause it.

Accepted Answer

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.

Question 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.

Accepted Answer

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.

Question 10

Compare the movement of nitrogen and phosphorus through ecosystems. Give one similarity and one difference.

Accepted Answer

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.

Question 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.

Accepted Answer

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.

Question 12

Suggest two practices that can reduce nitrogen and phosphorus pollution from farms, and explain how each practice helps.

Accepted Answer

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.

Environmental Science: The Nitrogen and Phosphorus Cycles

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