Which of the following is NOT a method by which nitrogen is fixed in the environment?

Nitrification is a process that converts ammonia into nitrates and nitrites in the soil, but it does not actually fix nitrogen from the atmosphere into a usable form for organisms. Instead, nitrification involves the oxidation of nitrogen compounds that are already in the soil.

In contrast, other methods listed are indeed ways by which nitrogen is fixed. Lightning discharges can create nitrogen oxides, which eventually mix with rainwater and deposit into the soil. Combustion processes, such as those from engines or wildfires, can also produce nitrogen oxides that contribute to nitrogen fixation. Volcanic action can release gaseous nitrogen compounds as well. Thus, while nitrification is a crucial part of the nitrogen cycle, it is not a method of fixing atmospheric nitrogen, which is why it is the correct answer to the question.