Here are the answers to your questions:
Question 1:
A) Nitrogen returns to the atmosphere through a process called
Answer: denitrification.
Question 2:
A) Nitrogen from animal waste can be stored in bodies of water as
Answer: ammonia.
Question 3:
A) Living organisms can incorporate ________ into their metabolism.
Answer: nitrates.
Question 4:
A) Usable forms of nitrogen can intentionally be increased in the soil by
Answer: allowing organic matter to decompose.