Ontario Grade 9 Science Class Practice Exam

Nitrogen fixation is the process of converting nitrogen gas into usable nitrogen compounds for plants. It is not the fixing of nitrogen levels in the atmosphere (option B) as that would imply controlling the levels rather than converting them. It is also not the decomposition of nitrogen from dead organisms (option C) as that is the breakdown of already existing nitrogen compounds. Lastly, it is not the release of nitrogen into the atmosphere by plants (option D) as that would be the opposite of fixing it. Instead, nitrogen fixation is the important process of converting nitrogen gas into a form that can be used by plants for growth and development.