percent of the amount of nitrogen fed to the animals, or 4 percent of the nitrogen originally applied to fields. The rest of the nitrogen—over 25 percent of the amount of nitrogen originally applied to the fields—is in animal waste that is accumulating somewhere in the environment. Much of this may be leached to surface waters.
Of the nitrogen consumed by humans, either through vegetable crops or meat, some is released through wastewater treatment plants and from septic tanks. In North America, this is an amount equivalent to approximately 5 percent of the amount of nitrogen originally applied to fields (Howarth et al. 1996). The rest is placed as food wastes in landfills or is denitrified to nitrogen in wastewater treatment plants and septic tanks.
In conclusion, fertilizer leaching from fields is only a portion of the nitrogen that potentially reaches estuaries and coastal waters. Probably of equal or greater importance in many regions of North America is the nitrogen tied up in ammonia, which is volatilized to the atmosphere or released to surface waters from animals’ wastes and landfills. Since food is often shipped over long distances in the United States, the concentration and subsequent environmental effect of nitrogen over-enrichment can occur well away from the original fertilized cropland.
Although production of fertilizer is the most significant way human activity mobilizes nitrogen globally, other human-controlled processes, such as combustion of fossil fuels and production of nitrogen-fixing crops in agriculture, convert atmospheric nitrogen into biologically available forms of nitrogen. Overall, human fixation of nitrogen (including production of fertilizer, combustion of fossil fuel, and production of nitrogen-fixing agricultural crops) increased globally some 2- to 3-fold from 1960 to 1990, and continues to grow (Galloway et al. 1995). By the mid 1990s, human activities made new nitrogen available at a rate of some 140 Tg yr-1 (Vitousek et al. 1997), or a rate roughly equivalent to the natural rate of biological nitrogen fixation on all of the land surfaces of the world (Vitousek et al. 1997; Cleveland et al. 1999). Thus, the rate at which humans have altered nitrogen availability globally far exceeds the rate at which humans have altered the global carbon cycle.