Tufts OpenCourseware
Authors: Kathleen Merrigan, Ph.D., William Lockeretz, Ph.D.


  • Keeney, D.R., and J.L. Hatfield. The nitrogen cycle, historical perspective, and current and potential future concerns. In: Follett, R.F., and Hatfield, J.L. Nitrogen in the Environment: Sources, Problems, and Management. 2001.
  • Foth, H.D., and Ellis, B.G. Soil Fertility, 2nd.ed. Chapter 6. Nitrogen (part). 1997.
  • Council for Agricultural Science and Technology, “Gulf of Mexico Hypoxia: Land and Sea Interactions,” Task Force Report, June 1999.

Key Questions

The Nitrogen Cycle, Historical Perspective, and Current and Potential Future Concerns

  • How does the amount of nitrogen brought into global circulation from anthropomorphic sources compare to that which is fixed naturally?
  • Which human activity brings the maximum amount of fixed nitrogen into circulation?
  • What are the local and global consequences of the increased N levels in our surroundings?
  • What is the MCL allowed for nitrate nitrogen in drinking water? What are the arguments against this levels?
  • What factors complicate regulation of N levels in ground water?
  • Which are the nutrients of concern when we consider the problem of eutrophication and hypoxia in freshwater, and, ocean and estuaries?
  • What factors affect movement of nitrogen into ground and surface waters?
  • What is the general use of nitrogen use and manure production in the US over the 15 years?
  • What different routes does the author suggest for improving nitrogen fertilizer use efficiency?

Soil Fertility

  • What are the major storehouses of nitrogen?
  • What are the dominant forms of nitrogen in soils?
  • What is the importance of biologically fixed nitrogen with reference to the overall amount of nitrogen that is added to soils?
  • What is the difference between symbiotic and non symbiotic nitrogen fixation? What are some of the examples for the two different mechanisms?

Gulf of Mexico Hypoxia

  • What conditions define hypoxia?
  • What are the factors causing hypoxia?