Home
Dispatches
Climate
Water
Coral Reefs
Publications
Biography
What You Can Do
Blog
Links

And the waters turned to blood. And the fish in the river shall die and the Egyptians shall loathe to drink the water

          - Exodus 7:18

The production of enough food to satisfy the rapid population growth over the past 150 years would not have been possible without the mobilization of essential plant nutrients like nitrogen (N) and phosphorus (P). Nitrogen, for example, is a key building block for proteins, and thus a vital food for crops. The soil N used by plants is largely generated by microbes, which extract or “fix” inert di-nitrogen gas (N₂) from the air and convert it to more reactive forms.

Early in the last century, an industrial process was developed to generate reactive N and supplement the natural fixation process. Today, we actually create more reactive N each year, through the production of fertilizers, burning of fossil fuels and cultivation of N-fixing crops like soybeans, than is created naturally by the planet.

The increased loading of nutrients like N and P does come at a substantial cost to both terrestrial and aquatic ecosystems. As limiting nutrients, nitrogen and phosphorous can promote excess algae growth in freshwaters and coastal waters, leading to eutrophication and depletion of crucial oxygen from bottom waters. This is commonly observed in freshwater lakes and coastal regions at the outlet of large rivers like the Mississippi worldwide.

Research

I look at how climate and land use influences nutrient cycling in large watersheds or river basins, following the movement of nutrients through soils and vegetation into the river system and downstream to the ocean. Most of my work to date has focused on the problem of nitrogen use in the Mississippi River Basin and the development of the seasonal “dead zone” in the Gulf of Mexico (see image, right).

Along with colleagues Jon Foley and Chris Kucharik at the University of Wisconsin, and Michael Coe at the Woods Hole Research Centre, I have detailed how changes in climate and land use over the past fifty years contributed to the observed increase in nitrogen by the Mississippi and identified the “hot spots” of nitrogen loading to the river system. I have also recently looked at how the growth of the “dead zone” can be largely attributed to the extensive use of croplands to cultivate feed for meat production rather than food cultivation.

Ongoing work with Michael Oppenheimer at Princeton is examining how the weather and climate controls precipitation controls the year-to-year variability in the amount of nitrogen leached from land and sent the ocean, and what this may imply for the future.

I am also collaborating with Navin Ramankutty, Elena Bennett and Garry Peterson at McGill and Wisconsin on tradeoffs between aquatic ecosystem health and food production and global patterns in fertilizer use and nutrient loading by rivers.

In the future, I hope to expand this work to examine other large river basins like the Mekong, where the transport of nutrients and sediment play a crucial role in health of the ecosystems downstream.

 Water

Family farm,NY state

  Summer corn crop

 Upper Mississippi River

(Perrot State Park, Wisconsin)

  Mississippi River delta via

satellite (courtesy of NASA)

Cambodian village along the

Tonle Sap, a major  tributary

of the Mekong River