applied to agricultural fields leaches into ground and surface waters (Howarth et al. 1996), although much of that is lost to denitrification in downstream wetlands, streams, and rivers before reaching estuaries or coastal waters.

A variety of factors affect the volatilization of nitrogen from fertilizer to the atmosphere, including soil type, climate, farming practices, and type of fertilizer (Bouwman et al. 1997). For example, when ammonium sulfate is applied to a soil with a pH below 5.5, less than 2 percent of the ammonium is volatilized (in the form of ammonia) to the atmosphere. Conversely, when ammonium sulfate is applied to calcareous soil (which has a higher pH), up to 50 percent of the nitrogen can be volatilized as ammonia gas to the atmosphere (Whitehead and Raistrick 1990; Bouwman et al. 1997). For typical farming practices, climate, and soils in the United States and Europe, Bouwman et al. (1997) estimated that on average 8 percent of the nitrogen in ammonium sulfate and 15 percent of the nitrogen in urea is volatilized to the atmosphere. The percentages are greater in tropical countries, and the volatilization from nitrate-based fertilizers is much less. While emissions of nitric oxide to the atmosphere are an important nitrogen loss from fertilized fields in tropical areas, this is generally a very small flux in temperate regions, including the United States (Holland et al. 1999). Virtually all the nitrogen volatilized from agricultural fields is eventually redeposited back onto the landscape and can reach estuaries and coastal waters (Howarth et al. 1996). Generally, this nitrogen is redeposited quite close to the point of emission (Holland et al. 1999).

Since 45 to 75 percent of the nitrogen applied as fertilizer is harvested in crops, tracing the fate of nitrogen in food and feedstock is important for understanding nitrogen inputs to natural waters (Howarth et al. 1996). The nitrogen in foods that are consumed by humans becomes sewage and is released in sewage effluent, where it is volatilized to the atmosphere as ammonia from sewage treatment plants or is denitrified (converted to plant-unavailable nitrogen) in the sewage treatment plants. However, in the United States most crops are fed to animals (Bouwman and Booij 1998). Thus, most of the nitrogen in harvested crops is excreted by animals. For animals such as poultry, hogs, and cows kept in barns or sheds, 36 percent of the excreted nitrogen on average is volatilized to the atmosphere as ammonia; keeping cows in meadows instead of barns reduces the atmospheric volatilization by more than 50 percent (Bouwman et al. 1997).

Assuming that (1) 65 percent of the nitrogen applied as fertilizer is removed in crops (NRC 1993b); (2) two-thirds of the crop production in the United States is fed to animals (Bouwman and Booij 1998); (3) the nitrogen growth efficiency for animals is 10 percent (Bouwman and Booij



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