For decades, scientists have wondered what happens to the nitrogen that farmers apply to fields. On the farm, levels of the nutrient are high. But downstream, they’re lower — sometimes only half as much. In an attempt to figure out where it went, scientists have undertaken “mass balance studies” to solve the mystery.This isn't rocket science; it's more complicated. When in doubt, the usual answer is both:
Does the nitrogen dissipate into the air as a harmless gas? Does it convert to nitrous oxide, a potent greenhouse gas, and pollute the air? Does it stay in the soil — as the other plant nutrient, phosphorus, does— and eventually ooze into streams over time?
Two recent studies — one focused on the Mississippi River and one on the Chesapeake Bay — suggest at least partial answers.
Nandita Basu, an assistant professor at the University of Waterloo in Canada, and doctoral student Kim Van Meter looked at more than 2,000 soil samples throughout the Mississippi basin and found an accumulation of nitrogen in the soils. In the March issue of Environmental Research Letters, they reported finding that the nitrogen often was not in the first layer of soil, but 10 to 39 inches below the surface. There, it converts to nitrate, an inorganic compound that is the most common drinking water pollutant in the United States.However. . .
Closer to home, Tom Fisher, an ecologist with the University of Maryland Center for Environmental Science, began his own investigations into the missing nitrogen with graduate students John R. Gardner, Thomas Jordan and Karen L. Knee. The group found that 74 percent of the nutrient applied to the crops was leaving the fields in the Choptank and Nanticoke river watersheds where the team conducted its research. But most of it never got into streams because it was transformed through natural biological processes into harmless nitrogen gas and released into the air.Next question: why do the farms in the Bay region process nitrogen more completely than farms in the Mississippi?