Nitrogen is the single most important input a farmer can control to increase crop yields on nonirrigated fields. Given this, and the fact that nitrogen has been a relatively inexpensive input, farmers have an economic incentive to “apply a little extra” to ensure that crops have the necessary nutrients when they need them most. As a consequence, excess nitrogen remains in the soil and freely moves into water resources or into the atmosphere. Agriculture is the single largest source of nitrogen compounds entering the environment in the U.S., contributing 73 percent of nitrous oxide emissions, 84 percent of ammonia emissions, and 54 percent of nitrate emissions in recent years. The production and release of nitrogen, however, has greatly changed the Earth’s natural balance of nitrogen. The influx of nitrogen compounds that can change form and move easily between air, land, and water, such as nitrate, nitrous oxide, and ammonia, contributes to both beneficial and harmful changes to ecosystems. Increased productivity in agricultural systems is a benefit. On the other hand, ozone-induced injury to crops and forests, acidification and over-enrichment (eutrophication) of aquatic ecosystems, biodiversity losses, visibility-impairing haze, and global climate change are all considered harmful impacts (see box, “Pathways for Nitrogen Losses”). Hypoxia in the Gulf of Mexico and the declining health of the Chesapeake Bay are examples of the consequences of excess nitrogen in the environment, especially when compounded with other factors like the loss of wetlands and the increase in impervious surfaces, such as asphalt roads and parking lots.