Anthropogenic Nitrogen: Too Much of a Good Thing?
While human changes to the global carbon cycle have received a great deal of attention, humanity has altered several other cycles to a greater extent. More nitrogen is now fixed anthropogenically (mostly for fertilizer) than is fixed biologically in all terrestrial ecosystems. The resultant enrichment makes a relatively small contribution to climate change and a larger one to air quality—and it transforms the structure, dynamics, and diversity of many terrestrial and coastal marine ecosystems. Mitigating these changes will require far more efficient use of nitrogen in agricultural systems—a challenge that is complicated by the heterogeneity of intensive agricultural systems around the world.