Nitrogen Fertilizer: A Growing Problem

Nitrogen is the crucial nutrient for agriculture, more prized than phosphorus, potassium, or any other fertilizer ingredient. I know that recommendations to reduce nitrogen applications are not taken lightly, from working with farmers on nutrient management issues. A farmer may be willing to change tillage, rotations, herbicide use, and many other practices, but don't mess with the nitrogen!

The industrial production of nitrogen fertilizer is a relatively recent event. Less than a century ago humans formulated a process for converting readily prevalent atmospheric nitrogen into a form that can be used as fertilizer. The development of a cheap (though energy intensive) nitrogen source provided the basis for the green revolution, where new crop varieties were developed that could take advantage of the available nitrogen. This dramatically increased the yields of grains and the way people farm around the world. Over the past 35 years, a six-fold increase in nitrogen fertilization has contributed to a doubling of food production.

Since then we have made staggering changes to the global nitrogen cycle. Through fertilizer production, burning fossil fuels, and cultivating nitrogen-fixing crops (such as soybeans), we have more than doubled the nitrogen available to ecosystems. And as industrial processes strip the soil of its natural fertility and less-fertile land is brought into production, the amount of nitrogen fertilizer applied will only increase.

Yet there is a flip side to this story that we are learning more about all of the time. Nitrogen does not sit in one place, and so the impacts go well beyond the farm field. Nitrogen fertilizer quickly converts to nitrate, which is water-soluble and often flows off of fields with the next rainstorm. Sixty one percent of U.S. streams have nitrogen concentrations above background levels.

This can be detrimental to drinking water supplies. Fifteen percent of all shallow groundwater sampled beneath agricultural areas exceeded the drinking water standard for nitrate. This is particularly dangerous for infants as it causes "blue baby" syndrome, where nitrates interfere with an infant's ability to absorb oxygen. Chronic exposure to high nitrate levels has also been linked to diuresis, increased starchy deposits, hemorrhaging of the spleen, and possibly some cancers.

Yet the impacts don't stop there. Nitrogen does not usually contribute to algae growth in freshwater ecosystems, but can cause algae to proliferate in coastal systems. The algae block sunlight into the waters and use up oxygen as they decompose. Most aquatic life cannot survive if dissolved oxygen levels drop below 2 milligrams per liter. Nitrogen is the likely culprit in an 8,000 square mile "dead zone" of low oxygen that develops each summer near the mouth of the Mississippi River in the Gulf of Mexico. The size of the zone varies each year depending on the flow of the Mississippi River and other variables, but has trended larger in recent years. This could seriously disrupt the Gulf fishing industry, the largest fishery in the U.S.

Then there are impacts that scientists are just beginning to understand. Ecosystems have evolved to efficiently use available resources. If nitrogen is not plentiful, plants that effectively obtain and use limited nitrogen tend to be successful. When a load of nitrogen is deposited on an ecosystem (as nitrate in water or as nitrate and ammonium in air), those plants lose their competitive advantage, and non-native plants that thrive on abundant nitrogen may take over. Excess nitrogen has been shown to reduce diversity in ecosystems, making them more susceptible to stresses like droughts or floods.

Speaking of droughts and floods, nitrogen is a significant contributor to climate change. Nitrous oxide is considered a major greenhouse gas and is emitted during fertilizer application and other agricultural practices. The production of commercial nitrogen fertilizer also contributes to climate change by consuming fossil fuels.

A 1954 Life Magazine touts anhydrous ammonia as "farmers' new miracle maker." For better or worse, agriculture could not have gone through its rapid industrialization without commercial nitrogen fertilizer. But as with many other environmental issues, it has taken years for us to understand the consequences of commercial nitrogen. Issues such as the dead zone, climate change, and impaired drinking water may very well force changes in agricultural nitrogen management. Organic farmers may soon be teaching the rest of agriculture their old tricks.