Lesson Worksheet: The Nitrogen Cycle Biology

In this worksheet, we will practice describing the nitrogen cycle, and explaining the importance of nitrates for plants.


In what form do plants absorb the majority of nitrogen from the soil?

  • AGaseous nitrogen
  • BNitrous oxide
  • CNitrogen dioxide
  • DNitrites
  • ENitrates


Which of the following best explains why plants need to absorb nitrogen from the soil?

  • APlants require nitrogen to aid the movement of sucrose through the plant.
  • BPlants require nitrogen to synthesize important nitrogen-containing compounds, like proteins.
  • CPlants require nitrogen to be broken down during cellular respiration to release energy.
  • DPlants require nitrogen as it is a reactant in the process of photosynthesis.


The picture shows root nodules on a pea plant. Those root nodules are colonized by nitrogen-fixing bacteria.

Bacteria nodules of roots fix nitrogen for pea plants

The relationship between the bacteria and the pea plant is described as mutualistic; what does this mean?

  • AThe pea plant benefits from the relationship, but the bacteria do not.
  • BThe bacteria benefit from the relationship, but the pea plant does not.
  • CBoth the bacteria and the pea plant benefit from this relationship.
  • DNeither the bacteria nor the pea plant benefits from this relationship.


Legumes (e.g., pea plants) are able to increase the level of soil nitrogen where they are planted. Which of the following is a benefit this provides?

  • AAn increase in soil nitrogen means less nitrogen-fixing bacteria will be present in the soil.
  • BThere is more nitrogen available in the soil for other crops that are planted there.
  • CThere is less nitrogen in the soil after they are removed, so there is less to be converted into nitrogen gas.
  • DAn increase in soil nitrogen means this soil can be used as fertilizer elsewhere.


Which organisms convert nitrates in the soil back into atmospheric nitrogen?

  • ADecomposers
  • BNitrifying bacteria
  • CDenitrifying bacteria
  • DNitrogen-fixing bacteria


Which organisms in the nitrogen cycle convert ammonium ions into nitrites and then into nitrates?

  • ADenitrifying bacteria
  • BDecomposers
  • CNitrogen-fixing bacteria
  • DNitrifying bacteria


Which organisms in the nitrogen cycle convert proteins, contained in urea and the bodies of dead organisms, into ammonia?

  • ANitrogen-fixing bacteria
  • BDecomposers
  • CNitrifying bacteria
  • DDenitrifying bacteria


Why can plants not absorb nitrogen directly from the atmosphere?

  • AGaseous nitrogen is too unreactive to be absorbed and used by plants.
  • BGaseous nitrogen is too reactive to be taken in directly by plants and must go through the roots.
  • CNitrogen only makes up less than 1% of the gases in the atmosphere, so there is not enough to be absorbed.
  • DNitrogen in the atmosphere evaporates before it can be absorbed.


Which organisms involved in the nitrogen cycle convert nitrogen gas into nitrogen compounds in the soil?

  • ANitrifying bacteria
  • BNitrogen-fixing bacteria
  • CDecomposers
  • DDenitrifying bacteria


Farmers can use a technique where each year they plant a different crop in different fields in a fixed order. This helps maintain good levels of soil nitrogen. What is this technique called?

  • ACrop swap
  • BCrop alternating
  • CCrop migration
  • DCrop rotation

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