Although virtually no N is volatilized directly from animals, the N in animal manure can be converted to ammonium (NH4+) by hydrolysis of urea or uric acid or deamination of amino acids after hydrolysis of proteins. This ammonium equilibrates with ammonia (NH3) which can be readily lost to air in a gaseous form. The urea (mammals) and uric acid (birds) in urine are rapidly hydrolyzed by enzymes present in the animal’s feces (Oenema et al., 2001).
Thus, a substantial amount of ammonium can be formed within hours of urination, and this can be readily emitted to air from the animal housing. Nitrous oxide (N2O) is formed from microbial processes of nitrification and denitrification that may occur when manure is stored or applied to land for crop production. Nitric oxide (NO) is released during nitrification in aerobic soils when manure or other fertilizer is applied.
Once emitted, the NH3 can be converted back to NH4+ in the atmosphere, and this NH4+ reacts with acids (e.g. nitric acid, sulfuric acid) to form aerosols with a diameter of fewer than 2.5 micrometers (PM 2.5). These small particles are considered a health concern for humans and a contributor to smog formation. Removal of ammonium by deposition contributes to soil and water acidity and ecosystem overfertilization or eutrophication. Nitric oxide and N2O are rapidly interconverted in the atmosphere and are referred to jointly as NOx. Nitrous oxide diffuses from the troposphere into the stratosphere, where it can remain for hundreds of years contributing to global warming and stratospheric ozone depletion. A molecule of nitrous oxide has a global warming potential that is 296 times that of a molecule of CO2 (Intergovernmental Panel on Climate Change, 2001).
A single molecule of ammonia or nitrous oxide once emitted to the environment can alter a wide array of biogeochemical processes as it is passed through various environmental reservoirs in a process known as the nitrogen cascade (Galloway et al., 2003). A single molecule of nitric oxide can continue regenerating in the stratosphere while sequentially destroying one ozone molecule after another. Likewise, as reactive nitrogen is passed through various environmental reservoirs a single atom can participate in a number of destructive processes before being converted back to N2. For example, a single molecule of reactive nitrogen can contribute sequentially to decrease atmospheric visibility (increase smog), increase global warming, decrease stratospheric ozone, contribute to soil and water acidity, and increase hypoxia in fresh and subsequently coastal waters.
Worldwide, more than half of the anthropogenic losses of reactive nitrogen to the air, and more than 70% of the ammonia losses are estimated to derive from agricultural production (van Aardenne et al., 2001).
About 50% of the anthropogenic ammonia losses to the environment derive directly from animal feedlots, manure storage, or grazing systems, with additional losses occurring indirectly from cropping systems used to feed domestic animals as well as feed humans directly. In addition, animals contribute 25% of the anthropogenic N2O production with an additional 25% coming from cropping systems. Only about 10% of the anthropogenic NO production derives from agriculture, most of it coming from crop-soil systems. The environmental problems caused by reactive nitrogen released into the environment are profound and ever-increasing, and agriculture is the biggest source of reactive nitrogen losses to air and water (van Aardenne et al., 2001).