Ontario Grade 9 Science Class Practice Exam

What is the first step in the nitrogen cycle?

1. Nitrification

2. Nitrogen fixation

3. Assimilation

4. Denitrification

The correct answer is: Nitrogen fixation

The first step in the nitrogen cycle is nitrogen fixation. This process involves the conversion of atmospheric nitrogen gas (N2), which cannot be used directly by most living organisms, into ammonia (NH3) or related compounds that can be assimilated by plants. Nitrogen fixation can occur abiotically through natural processes such as lightning or through biological means, primarily facilitated by certain bacteria that live in the soil or in symbiotic relationships with plants, particularly legumes. This step is crucial because it makes nitrogen, an essential nutrient for all living organisms, accessible in a form that can be utilized in biological processes, such as the synthesis of amino acids and proteins. Without nitrogen fixation, the nitrogen present in the atmosphere would not be available for the growth and development of plants, which form the base of most food chains.