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Which best defines nitrogen fixation?

the process of creating free nitrogen for plants to absorb

the breakdown of nitrogen in the soil

the conversion of nitrogen gas into a usable form

the destruction of the bonds between nitrogen and other elements

Nitrogen is converted from atmospheric nitrogen (N2) into usable forms, such as NO2-, in a process known as fixation.

In this case, the answer which best describes nitrogen fixation, is the third option, the conversion of nitrogen gas into a usable form.