Changes in patterns of fertilizer nitrogen use in Asia and its consequences for N2O emissions from agricultural systems

In most soils, formation and emissions of N2O to the atmosphere are enhanced by an increase in available mineral nitrogen (N) through increased rates of nitrification and denitrification. Therefore, addition of N, whether in the form of organic or inorganic compounds eventually leads to enhanced N2O emissions. Global N2O emissions from agricultural systems have previously been related primarily to fertilizer N input from synthetic sources. Little attention has been paid to N input from other N sources or to the N2O produced from N that has moved through agricultural systems. In a new methodology used to estimate N2O emissions on the country or regional scale, that is briefly described in this paper, the anthropogenic N input data used include synthetic fertilizer, animal waste (feces and urine) used as fertilizer, N derived from enhanced biological N-fixation through N-2 fixing crops and crop residue returned to the field. Using FAO database information which includes data on synthetic fertilizer consumption, live animal production and crop production and estimates of N input from recycling of animal and crop N, estimates of total N into Asian agricultural systems and resulting N2O emissions are described over the time period 1961 through 1994. During this time the quantity and relative amounts of different types of materials applied to agricultural soils in Asia as nitrogen (N) fertilizer have changed dramatically. In 1961, using the earliest entry from the FAO database, of the approximately 15.7 Tg of fertilizer N applied to agricultural fields 2.1 Tg N (13.5% of total N applied) was from synthetic sources, approximately 6.9 Tg N from animal wastes, 1.7 Tg N from biological N-fixation, and another 5 Tg N from reutilization of crop residue. In 1994, 40.2 Tg from synthetic fertilizer N (57.8% of total), 14.2 Tg from animal wastes, 2.5 Tg from biological N-fixation and 12.6 Tg from crop residue totalling 69.5 Tg N were utilized within agricultural soils in all Asian countries. The increases in N utilization have increased the emission of nitrous oxide from agricultural systems. Estimated N2O from agricultural systems in Asia increased from about 0.8 Tg N2O-N in 1961 to about 2.1 in 1994. The period of time when increases in N input and resulting N2O emissions were greatest was during 1970-1990. This evaluation of N input into Asian agricultural systems and the resulting N2O emissions demonstrates the large change in global agriculture that has occurred in recent decades. Because of the increased need for food production increases in N input are likely. Although the rate of increase of N input and N2O emissions during the 1990s appears to have declined, we ask if this slowed rate of increase is a general long term trend or if global food production pressures will tend to accelerate N input demand and resulting N2O emissions as we move into the 21st century.