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Crop plants are inefficient in their uptake and use of fertilizer N, with only about 50% of the applied N taken up by non-legume crops such as corn and wheat, and only 30-40% by paddy rice. Approximately 7 x 106 metric tons of N is applied each year to 74 x 106 hectares of irrigated rice in Asia; less than 50% is usually recovered by the crop because of high rates of ammonia volatilization and nitrification (Kronzucker et al (1998) and references cited therein). The nitrogen not used by the crop may contribute to environmental problems such as pollution of ground water, and run-off to oceans stimulating the growth of marine phytoplankton resulting in an increased frequency of algal blooms (Mlot, 1997).
 Nitrogen oxide denitrification products can cause destructive effects on stratospheric ozone (O3) (Mengel, 1992):
NO + O3 ---> NO2 + O2 (2) 2O3 + NO + NO2 --hv--> 3O2 + NO2 + NO (3) Reactions 2 and 3 are ozone-consuming. Nitrogen dioxide (NO2) has a short life since it easily forms HNO3 with hydroxyl radicals:
Probably some 120 million tons of soil nitrogen and applied fertilizer nitrogen escape worldwide per year in the gaseous form as a result of the microbial process of denitrification (Jung, 1979). In W. Europe there has been a trend towards using nitrification inhibitors (e.g. nitrapyrin) to minimize the conversion of ammonia fertilizers to nitrates and the associated losses of fertilizer N by leaching and denitrification. However, substantial amounts of nitrogen can be released from the soil in the form of ammonia gas; this release rate depends greatly on soil pH, and is favored under alkaline conditions.
Kronzucker HJ, Kirk GJD, Siddiqi MY, Glass ADM 1998 Effects of hypoxia on 13NH4+ fluxes in rice roots. Plant Physiol. 116: 581-587. Jung J 1979 Possibilities of optimization of plant nutrition by new agrochemical substances - especially in cereals. In (TK Scott ed) "Plant Regulation and World Agriculture", Plenum Press, New York, pp. 279-307. Lea PJ, Ireland RJ 1999 Nitrogen metabolism in higher plants. In (BK Singh ed.) "Plant Amino Acids: Biochemistry and Biotechnology", Marcel Dekker, NY, pp. 1-47. Mengel K 1992 Nitrogen: agricultural productivity and environmental problems. In (K Mengel, DL Pilbeam eds) "Nitrogen Metabolism of Plants", Clarendon Press, Oxford, pp. 1-15. Mlot C 1997 The rise in toxic tides: what's behind the ocean blooms? Science News 152: 202-204.
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