Agricultural land use and climate change: Tipping points along the nitrogen cascade?

An increase in the flux of agriculturally-derived nutrients like nitrogen in rivers is blamed for eutrophication and bottom-water hypoxia in coastal waters worldwide. Nitrogen flux to the ocean, however, represents only a small fraction on nitrogen inputs. A large body of research has focused on the wide spatial variability in retention of nitrogen along a ‘cascade’ from the field to the riparian zone to the river system and, finally, to the coast. Since many of the retentive processes like denitrification are dependent on climatic and hydrologic conditions, the retention of nitrogen can also vary widely in time. Therefore variability in climate, particularly precipitation, can dramatically affect the delivery of agriculturally-derived nitrogen and the occurrence of eutrophication or hypoxia. This is crucial in intensively cultivated river basins like the Mississippi with consistently high inputs of nitrogen to the land surface.

In this presentation, I will use examples from research in the Mississippi River Basin to discuss the synergistic role of fertilizer application and climate variability along the nitrogen cascade. Evidence suggests there may be “tipping points” on land and in water beyond which the retention of nitrogen may dramatically decrease – or increase. When integrated together, the results of a wide variety of research suggests that the flux of nitrogen and health of downstream ecosystem could be highly sensitive to climate change in nitrogen saturated river basins like Mississippi.