Unlike phosporus- or potassium-based fertilizers, whose abundance is limited by the extraction of source materials by mining, nitrogen-based fertilizers are largely derived from the direct chemical conversion of inert elemental nitrogen, N2, in the atmosphere to biologically useable forms of nitrogen (typically compounds of nitrogen plus oxygen or hydrogen). Elemental nitrogen is the most abundant gas in the earth’s atmosphere, thus there is an essentially inexhaustible supply of inorganic, nitrogen-based fertilizer. The process used, originally developed to address Germany’s needs for nitrate to produce munitions during World War I, remains the most economical method for the commercial fixation of nitrogen, and with modifications is one of the basic processes of the chemical industry. Over the last decade, ammonia derived from natural gas has emerged as another important source of inorganic fertilizer.
When nitrogen-based fertilizer is applied to a field, it can move through a variety of flow paths to downstream aquatic ecosystems. Some fertilizer leaches directly to groundwater and surface waters, varying from 3 to 80 percent of the fertilizer applied, depending on soil characteristics, climate, and crop type (Howarth et al. 1996). On average for North America, about 20 percent leaches directly to surface waters (NRC 1993a). Some fertilizer is volatilized directly to the atmosphere; in the United States, this averages 2 percent of the fertilizer applied, but the value is higher in tropical countries and also in countries that use more ammonium-based fertilizers, such as China (Bouwman et al. 1997). Much of the nitrogen from fertilizer is incorporated into crops and is removed from the field in the crops when they are harvested. An NRC report (NRC 1993a) suggests that on average, 65 percent of the nitrogen applied to croplands in the United States is harvested, although other estimates are somewhat lower (Howarth et al. 1996). Given these paths and rates, about 13 percent of the nitrogen applied builds up in soils or is denitrified back to elemental nitrogen (a gas) and released to the atmosphere.
To fully understand nitrogen transport, it is important to trace the eventual fate of the nitrogen harvested in crops. Some nitrogen is consumed directly by humans eating vegetable crops—in North America this constitutes perhaps 10 percent of the amount of nitrogen originally applied to the fields (Bouwman and Booij 1998). Perhaps 10 percent of the nitrogen originally applied to fields is lost during food processing and ends up in landfills or released to surface waters from food-processing plants. The largest part of the nitrogen is fed to animals in feed crops, estimated to be about 45 percent (Bouwman and Booij 1998).
Of the nitrogen consumed by animals, much is volatilized from animal wastes to the atmosphere as ammonia. In North America, this volatilization is roughly one-third of the nitrogen fed to animals (Bouwman et al. 1997), or 15 percent of the amount of nitrogen originally placed on the fields. This ammonia is deposited back onto the landscape, often near the source of volatilization, although some of it travels long distances through the atmosphere (Holland et al. 1999). Some of the nitrogen in animals is consumed by humans, an amount roughly equivalent to 10