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 N2O—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. Atmospheric reactions and slow geological processes controlled Earth’s earliest nitrogen cycle, and by ~2.7 billion years ago, a linked suite of microbial processes evolved to form the modern nitrogen cycle with robust natural feedbacks and controls. Over the past century, however, the development of new agricultural practices to satisfy a growing global demand for food has drastically disrupted the nitrogen cycle. This has led to extensive eutrophication of fresh waters and coastal zones as well as increased inventories of the potent greenhouse gas nitrous oxide (N2O). Microbial processes will ultimately restore balance to the nitrogen cycle, but the damage done by humans to the nitrogen economy of the planet will persist for decades, possibly centuries, if active intervention and careful management strategies are not initiated.