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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

My research team looks at how climate and land use influence hydrology and nutrient cycling in large watersheds or river basins, using physically-based computer models that track the movement of water and nutrients through soils and vegetation into the river system and downstream to the ocean.

Much of this work to date has focused on the Mississippi River Basin and the Gulf of Mexico (see image, right). My colleagues and I have detailed how changes in climate and fertilizer use over the past fifty years in the Central U.S. contributed to the development of the “Dead Zone” in the northern Gulf of Mexico. We have also evaluated the impact of US energy policy and trade-offs between feed, food and fuel production on nutrient pollution.

Ongoing projects include a study of the impacts of climate change on wetland area and river flow in the Mackenzie River Basin, and examinations of the sensitivity of stoichiometric ratios in streams to climate variability.

Mississippi River delta via satellite (courtesy of NASA)