Nitrogen is a common element found as a gas in the air we breathe as well as in other forms in water and soil. Nitrogen is essential for all life on Earth. This informative book explains how the Earth's supply of nitrogen moves in different forms in a cycle from the air to the soil to living things. Highly readable text and supportive images help explain such processes as fixation, nitrification, and dentrification, as well as the important role of bacteria in the nitrogen cycle. Feature boxes highlight examples of the ways in which human activity, such as adding more nitrogen to the soil to make plants grow faster, releases harmful greenhouse gases into the air interfering with the nitrogen cycle. Readers are encouraged to find ways to take action and find solutions.