J.K. Ladha
International Rice Research Institute-India
Nitrogen is unique among the major nutrients in that it originates from the atmosphere, and its transformations and transport in an ecosystem are mediated by the water cycle and biological processes. The atmosphere contains a large, well-mixed biologically unavailable pool of nitrogen, of which a small part is converted to biologically available reactive nitrogen. Biological nitrogen fixation is the primary source of reactive nitrogen but, in recent years, chemical nitrogen fixation has become important in increasing crop productivity to alleviate the ever-increasing food insecurity. Since the Green Revolution, the application of nitrogen fertilizers on “modern crop cultivars” of cereals boosted food production by about 6.4% per year. Today, fertilizer nitrogen supplies 100 Tg year-1 for food production. Of this, 50% is applied to three major cereal (maize, rice and wheat). It is projected that annual total global nitrogen use will be around 171 Mt in 2050, assuming no change in nitrogen-use efficiency (NUE). Fertilizer nitrogen-recovery efficiency by cereals is 30% to 50%. The remaining surplus nitrogen is lost to the environment, causing disruptions in ecosystem functions. Much research has been conducted during the past decades to improve NUE by developing fertilizer management strategies based on a better synchronization between the supply and crop deman. This presentation will analyze the (1) different sources of N inputs and outputs in global cereal production, (2) effect of long-term addition of synthetic nitrogen on soil nitrogen storage, (3) NUE for the cereals grown across large agroclimatic regions, (4) strategies available to improve the NUE and reduce losses.