The case of too much or too little nitrogen fertilizer

National Geographic

Many of the increases in food production during the Green Revolution can be attributed to a single element in the periodic table — nitrogen. Begun in the early 1900s as an effort to convert nitrogen gas from the air we breathe into a solid form that could propel ammunition farther, the Haber-Bosch process later became the key mechanism for boosting crop yields through mass production of nitrogen fertilizer. Unfortunately, excess nitrogen degrades our drinking water quality, causes many coastal areas to be oxygen-depleted “dead zones,” and adds a very powerful greenhouse gas to our atmosphere. How can we manage our farmlands more effectively?

Several colleagues and I develop data and analysis techniques to understand how nitrogen (and other fertilizers) can be used more efficiently to increase food production and reduce its environmental impacts. We recently collaborated with National Geographic to create an infographic on excess nitrogen in croplands based on our research.

Two points should be obvious from the map. First, China, India and the United States have the bulk of the world’s nitrogen that remains in the soil instead of being taken up by crops. This excess is generally a short-term resident in the soil and quickly leaches to steams, lakes, and eventually coastal areas. Second, most of Africa and much of Asia have a deficit of nitrogen. In these areas, more nitrogen is used to grow the crops than is replaced by manure and synthetic fertilizer. This process essentially mines the soil, depleting nutrients and lowering future crop yields.

However, a third point may not be immediately obvious. Farms with the highest excess nitrogen per area of cropland are not necessarily the biggest source of nitrogen reaching the coast. Think of it like vehicle fuel efficiency. Yes, gas-guzzling monster trucks use a lot of fuel for each mile they’re driven, but the total fuel used in a few trucks is small compared to millions of higher efficiency cars driven during long daily commutes. The same is true for agriculture. For example, crop production is more nitrogen efficient in the central than southeastern USA, but is a much larger source of nitrogen pollution because so much of the landscape is farmed.

The map below post presents the same data as excess nitrogen across the entire landscape (not just croplands). This alternate version of the map highlights the main source of excess nitrogen more effectively than does the version in National Geographic, but is less effective at depicting nitrogen use efficiency on croplands.
Excess Nitrogen on the Landscape

An alternate version of the excess nitrogen data, which highlights the major sources of excess nitrogen rather than the efficiency for each cropland area. This map was presented in a similar form in Foley et al. 2011, Nature.

Our team is currently working to identify places to target action for increasing food production while decreasing the negative effects of fertilizer and other management practices. Please check out the interactive map online at National Geographic and let us know what you think!

Special thanks to my colleagues James Gerber, Peder Engstrom, Nathan Mueller and Jon Foley, who helped me on this project.

Paul West is chief collaboration officer for the Institute on the Environment’s Global Landscapes Initiative at the University of Minnesota.