Question

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

Answer 1

The enormous energy of lighting breaks nitrogen molecules and enables there atoms to combine with oxygen in the air forming nitrogen oxides. These dissolve in the rain, forming nitrates that are carried to Earth. 

Answer 2

Answer: nitrogen fixing bacteria

Explanation:

Nitrogen in the forms of nitrates, nitrites and ammonia are essential for the growth of the plants. Nitrogen present in the atmosphere is fixed in the soil by the nitrogen fixing bacteria. As plants cannot absorb nitrogen from the air. The bacteria convert atmospheric nitrogen into inorganic forms such as nitrates, nitrites and ammonia which are directly absorbed by the plants by their roots.