Nitrogen in the atmosphere must be "fixed" before it can be used by plants. atmospheric nitrogen is "fixed" by

Atmospheric nitrogen is "fixed" by bacteria.

Explanation:

Nitrogen gas (N2) is the most abundant in the earth's atmosphere, representing 79% of air volume. In organisms, nitrogen atoms are part of several organic substances, such as proteins and nucleic acids. However, most living things cannot use nitrogen in the N2 molecular form. Only a few species of bacteria are able to use it by incorporating nitrogen atoms into their organic molecules and making it available in other molecular forms for use by various species through a process called Biological Nitrogen Fixation. Similarly, the bacteria that perform the fixation are called nitrogen fixers.