The Evolution and Future of Earth’s Nitrogen Cycle

Nitrogen's Past and Future Microorganisms have been controlling Earth's nitrogen cycle since life originated. With life evolving around it, nitrogen became both an essential nutrient and a major regulator of climate. Canfield et al. (p. 192 ) review the major changes in the nitrogen cycle throughout Earth's history. Most of the time, perturbations typically coincided with the evolution of new metabolic pathways in various Bacteria or Archaea. The last century, however, has seen humans push the biological nitrogen cycle into a new stage altogether. The addition of large quantities of fixed nitrogen to crops in the form of fertilizer chokes out aquatic life that relies on runoff and adds significant amounts of N 2 O—a potent greenhouse gas—to the atmosphere. Although microorganisms may one day restore balance to the nitrogen cycle that they helped shape for billions of years, humans must modify their behavior or risk causing irreversible changes to life on Earth.