Increasing nitrogen fertilizer application has increased crop productivity and met the food demands of growing populations, but its use efficiency is very low. More than 50% of applied nitrogen is not utilized by crops,posing huge economical costs and environmental concerns. Therefore, fertilizer management should consider optimum time, rates, source and methods of application (the "4Rs" of nutrient stewardship) to increase use efficiency, crop yield, soil health and farm profits and to reduce negative environmental effects.