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Nitrogen Cycle Worksheets

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Nitrogen Cycle Worksheets

Work through the nitrogen-cycle investigation. Use the information and data provided; explain biological processes with precise scientific terms.

Investigation: nutrients near the station

After heavy rain, Aisha and Callum investigate a patch of land beside a busy train station. Run-off from nearby fields enters a stream. They test nitrogen compounds upstream and downstream.

Sample site

Nitrate concentration (mg/L)

Upstream of fields

2.1

Downstream of fields

12.6

Trace the nitrogen

Use this simplified cycle to help with Questions 1–3. Arrows show the direction nitrogen moves.

Atmospheric N₂ → nitrogen fixation → ammonia / ammonium in soil

Ammonia / ammonium → nitrification → nitrite → nitrate

Nitrate → assimilation into plants → animals; dead matter and waste → ammonification → ammonia / ammonium

Nitrate → denitrification → atmospheric N₂

Lightning can also fix some atmospheric nitrogen, forming compounds that reach soil in rain.

Nitrogen-fixing bacteria may live freely in soil or in root nodules of leguminous plants.

1.Which statement best explains why most plants cannot use atmospheric nitrogen gas directly?

N₂ is too reactive to remain in the atmosphere.
The triple bond in N₂ is very strong, so plants cannot convert it into usable nitrogen compounds.
Plants only absorb nitrogen through their leaves.
Nitrogen gas contains no nitrogen atoms.

2.Match each process to its main change: nitrogen fixation; nitrification; assimilation; ammonification; denitrification. Use each description once.

Nitrogen fixation
Nitrification
Assimilation
Ammonification
Denitrification

Plants take up inorganic nitrogen and incorporate it into organic molecules.
Decomposers convert organic nitrogen in waste or dead matter to ammonia / ammonium.
Nitrate is reduced to nitrogen gas, often in low-oxygen conditions.
Atmospheric N₂ is converted to ammonia or ammonium.
Ammonia / ammonium is oxidised to nitrite, then nitrate.

3.Aisha says, “Nitrifying bacteria fix nitrogen from the air.” Identify the error and correct her statement, naming the process nitrifying bacteria carry out.

4.Calculate the increase in nitrate concentration between the upstream and downstream samples. Include the correct unit.

5.Suggest how nitrate-rich run-off from fields could affect the stream. Explain a sequence of events that may lead to reduced oxygen availability for aquatic animals.

6.Which two conditions most favour denitrification in soil?

High nitrate availability and low oxygen
Low nitrate availability and bright light
High oxygen and very dry soil
No decomposers and low temperature

7.Give two distinct human activities that increase reactive nitrogen entering ecosystems, and state one possible consequence of either activity.

8.A farmer wants to reduce nitrate leaching from a field after harvest. Recommend one practical measure and explain how it reduces nitrogen loss.

9.Predict one likely long-term effect on plant growth and soil fertility if nitrogen-fixing bacteria disappeared worldwide. Explain your reasoning and include one qualification about the prediction.

10.A labelled flask contains ammonium ions and is supplied with oxygen. A culture of nitrifying bacteria grows in it. Predict the nitrogen compound expected to increase after nitrification, and name the intermediate compound.