2001). Urea in the urine of mammals can be hydrolyzed rapidly to ammonia and carbon dioxide by urease enzymes present in feces. On a global scale, animal farming systems emit to the atmosphere ~20 Tg N/yr as NH3 (Galloway and Cowling, 2002), an amount that comprises about 50 percent of total NH3 emissions from terrestrial systems (van Aardenne et al., 2001). Based on gridded emissions documented by van Aardenne et al. (2001), about 3 Tg N/yr was emitted from natural and anthropogenic sources in the United States in the mid-1990s. Emissions from animal waste (1.4 Tg N/yr) accounted for about 50 percent of the total. This figure is similar to the 1.9 Tg N/yr in Figure 1-1 (Howarth et al., 2002) estimated to come from animal emissions.
Once emitted, the NH3 can be converted rapidly to ammonium (NH4+) aerosol by reactions with acidic species (e.g., HNO3 [nitric acid] and H2SO4 [sulfuric acid]) found in ambient aerosols. Gaseous NH3 is removed primarily by dry deposition, while aerosol NH4+ is removed primarily by wet deposition. As an aerosol, NH4+ contributes directly to PM2.5 (particulate matter having an aerodynamic equivalent diameter of 2.5 μm or less) and, once removed, contributes to ecosystem fertilization, acidification, and eutrophication. After NH3 is emitted to the atmosphere, each nitrogen atom can participate in a sequence of effects, known as the nitrogen cascade (see discussion of environmental impacts and Figure 3-2 later in this chapter) in which a molecule of NH3 can, in sequence, impact atmospheric visibility, soil acidity, forest productivity, terrestrial ecosystem biodiversity, stream acidity, and coastal productivity (Galloway and Cowling, 2002). Excess deposition of reactive nitrogen can also decrease the biodiversity of terrestrial ecosystems (NRC, 1997). Since the residence times of NH3 and NH4+ in the atmosphere are on the order of days, a regional-scale perspective is necessary to assess the environmental effects of, and control strategies for, NH3 emissions.
Nitrous oxide (N2O) forms and is emitted to the atmosphere via the microbial processes of nitrification and denitrification. Global emissions in 1990 were ~15 Tg N/yr (Olivier et al., 1998), of which anthropogenic sources accounted for ~3 Tg N/yr. Of these, N2O emissions from animal excreta accounted for ~1 Tg N/yr (Olivier et al., 1998; van Aardenne et al., 2001). In the United States, total anthropogenic sources in 1990 were ~0.4 Tg N/yr, with animal excreta contributing about 25 percent (Table 3-1) (van Aardenne et al., 2001).
N2O diffuses from the troposphere to the stratosphere, where it is lost to photolysis and other processes. Once emitted, N2O is globally distributed because of its long residence time (~100 years); it contributes to both tropospheric warming and stratospheric ozone depletion. N2O has a global warming potential 296 times that of carbon dioxide (CO2) (IPCC, 2001).