Round and Round It Goes: The Nitrogen Cycle

Match the following

• To supply water to land or crops

• Irrigate

• Convert atmospheric nitrogen into a usable form for plants

• Nitrogen-fixing bacteria

• Convert nitrates back into nitrogen gas

• Equilibrium

• Substance added to soil to enhance plant growth

• Denitrifying bacteria

• State of balance in a system

• Fertilizer

What gas makes up most of Earth’s atmosphere?

• Oxygen

• Carbon dioxide

• Nitrogen

• Hydrogen

Why can’t plants and animals use nitrogen directly from the air?

• It’s too cold

• It’s in a form they can’t use

• It is poisonous

• It mixes with oxygen

Which organisms can use free nitrogen directly?

• Plants

• Animals

• Bacteria

• Humans

What do nitrogen-fixing bacteria do?

• Destroy nitrogen in the soil

• Turn nitrogen into usable compounds for plants

• Absorb oxygen

• Stop plants from using nitrogen

Which compounds can plants absorb after bacteria fix nitrogen?

• Oxygen and hydrogen

• Ammonia, nitrates, and nitrites

• Carbon and water

• Methane and oxygen

How do animals get their nitrogen?

• By breathing air

• By eating plants or other animals

• By drinking water

• By absorbing it through their skin

What happens to nitrogen after animals die?

• It disappears

• It turns into water

• It returns to the soil through decay

• It becomes oxygen

What is the nitrogen cycle?

• A process that makes oxygen

• The movement of nitrogen through air, soil, and living things

• A weather pattern

• The food chain

What role do denitrifying bacteria play?

• Turn nitrogen into oxygen

• Return nitrogen to the air

• Absorb nitrogen from water

• Stop nitrogen from spreading