Ontario Grade 9 Science Class Practice Exam 2024 - Free Grade 9 Science Practice Questions and Study Guide.

The first step in the nitrogen cycle is nitrogen fixation, which is the process where microbes convert nitrogen gas from the atmosphere into ammonia that can be used by plants. This option is correct because without nitrogen fixation, the other steps in the nitrogen cycle would not be possible. Option A, nitrification, is the second step in the nitrogen cycle. This involves the conversion of ammonia into nitrates and nitrites by bacteria. While this is an important step, it cannot occur without the first step of nitrogen fixation. Option C, assimilation, is the third step in the nitrogen cycle, where plants absorb nitrates and use them to build proteins. This step cannot occur without the first two steps of nitrogen fixation and nitrification. Option D, denitrification, is the final step in the nitrogen cycle. This is where bacteria convert nitrates back into nitrogen gas, which