Fertilizer-N use efficiency and nitrate pollution of groundwater in developing countries

Around 76% of the world's population lives in developing countries where more fertilizer-N is currently applied than in developed countries. Fertilizers are applied preferentially in regions where irrigation is available, and soil and climatic conditions are favorable for the growth of crop plants. Due to low N application rates during the last 3 or 4 decades, negative N balances in the soil are a characteristic feature of the crop production systems in developing countries. In the future, with increasing fertilizer-N application rates, the possibility of nitrate pollution of groundwater in developing countries will be strongly linked with fertilizer-N use efficiency. A limited number of investigations from developing countries suggest that, in irrigated soils of Asia or in humid tropics of Africa, the potential exists for nitrate pollution of groundwater, especially if fertilizer-N is inefficiently managed. In a large number of developing countries in West and Central Asia and North Africa, the small amount of fertilizer applied to soils (mostly Aridisols) that remain dry almost all the year, do not constitute a major threat for nitrate pollution of groundwater, except possibly when soils are irrigated. In Asia and the Pacific regions, where 70% of the fertilizers are used to grow wetland rice on soils with low percolation rates, leaching of nitrates is minimal. Climatic water balance and soil moisture conditions do not favor leaching of nitrates from the small amount of fertilizer-N applied to Oxisols and Ultisols in Latin America:rn developing countries located in the humid tropics, attempts have not been made to correlate fertilizer-N use with nitrate level in groundwater; however, fertilizers are being increasingly used. Besides high rainfall, irrigation is becoming increasingly available to farmers in the humid tropics and substantial leaching of N may also increase.