Date of this Version
Nitrogenous fertilizers have contributed much to the remarkable increase in food production that has occurred during the past 50 years (Smil, 2001). Globally, however, N fertilizers also account for 33% of the total annual creation of Nr or 63% of all anthropogenic sources of reactive nitrogen (Nr) (Table 1). Reactive nitrogen is defined as all biologically, photochemically, and/or radiatively active forms of N -- a diverse pool of nitrogenous compounds that includes organic compounds (e.g. urea, amines, proteins, amides), mineral N forms, such as NO3- and NH4+ as well as gases that are chemically active in the troposphere (NOx, NH3, N2O) and contribute to air pollution and the greenhouse effect (Galloway et al., 1995). Asia alone accounts for more than 50% of the global N fertilizer consumption as well as 37% for the global Nr creation. Smil (1999) estimated that only about half of all anthropogenic N inputs to cropland are taken up by harvested crops and their residues, with the remainder contributing significantly to Nr enrichment of the atmosphere, ground and surface waters.