Environmental hazards of nitrogen loading in wetland rice fields

Rice is the predominant crop in South and Southeast Asian countries, covering an area of 73.7 million ha. It is unique among the major food crops that, it grows well on flooded soil. When applied under flooded condition nitrogen, a major nutrient is prone to different types of losses, which are as high as 60%. With the increase in area under modern rice varieties in different countries, the usage of chemical fertilizer has been increased up to as high as 150 kg N/ha. About 10% of the total nitrogen fertilizer used globally are applied to rice. Chemical fertilizers supply nitrogen in ammonia, nitrate or amide forms. Among the N-fertilizers, about 80% of the demand are met by urea, which is highly water soluble and prone to losses. When any N compound is applied to a submerged paddy field, it is lost through leaching, denitrification, volatilization and runoff. Of the total N loss, leaching contributes about 30-50%, mostly as nitrate, denitrification, about 10-30% as N-2 and volatilization, about 2-30% as ammonia. The escaped nitrogen causes pollution to the atmosphere and water systems. In the lowlands, a dual rice-fish culture is also practised, where loading of high dose of nitrogen can be lethal to the fish. Studies have shown that nitrogen when applied in the form of ammonium sulfate was more lethal than urea. About 50% of the fish, Catla catla (common carp) were killed when ammonia concentration reached 29.4 mg NH3-N/L. The fish growth was higher under organic based than inorganic based nitrogen fertilizers. The number of phytoplankton species, which are fish food, was also less when chemical fertilizers were used as a nitrogen source. Besides, it is expected that the leached NO3-N may pollute the groundwater. Thus loading of nitrogenous compounds in rice ecosystems creates an unavoidable continuum of environmental hazards in rice growing countries.