The Nitrogen Cycle

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

• Nitrites

• Nitrous oxide

• Nitrogen dioxide

• Gaseous nitrogen

• Nitrates

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

• Plants require nitrogen to synthesize important nitrogen-containing compounds, like proteins.

• Plants require nitrogen to aid the movement of sucrose through the plant.

• Plants require nitrogen to be broken down during cellular respiration to release energy.

• Plants 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.

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

• Both the bacteria and the pea plant benefit from this relationship.

• The pea plant benefits from the relationship, but the bacteria do not.

• The bacteria benefit from the relationship, but the pea plant does not.

• Neither 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?

• An increase in soil nitrogen means this soil can be used as fertilizer elsewhere.

• An increase in soil nitrogen means less nitrogen-fixing bacteria will be present in the soil.

• There is more nitrogen available in the soil for other crops that are planted there.

• There is less nitrogen in the soil after they are removed, so there is less to be converted into nitrogen gas.

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

• Nitrifying bacteria

• Nitrogen-fixing bacteria

• Denitrifying bacteria

• Decomposers

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

• Nitrogen-fixing bacteria

• Decomposers

• Nitrifying bacteria

• Denitrifying bacteria

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

• Decomposers

• Nitrogen-fixing bacteria

• Nitrifying bacteria

• Denitrifying bacteria

Why can plants not absorb nitrogen directly from the atmosphere?

• Nitrogen in the atmosphere evaporates before it can be absorbed.

• Gaseous nitrogen is too unreactive to be absorbed and used by plants.

• Nitrogen only makes up less than 1% of the gases in the atmosphere, so there is not enough to be absorbed.

• Gaseous nitrogen is too reactive to be taken in directly by plants and must go through the roots.

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

• Decomposers

• Nitrifying bacteria

• Denitrifying bacteria

• Nitrogen-fixing 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?

• Crop rotation

• Crop migration

• Crop swap

• Crop alternating