Potential impact on the global atmospheric N2O budget of the increased nitrogen input required to meet future global food demands

In most soils, biogenic formation of N₂O is enhanced by an increase in available mineral N through increased nitrification and denitrification. N-fertilization, therefore, directly results in additional N₂O formation. In addition, these inputs may lead to indirect formation of N₂O after N leaching or runoff, or following deposition of NO_y and NH_x from gaseous losses of NO_x and NH₃. Anthropogenic N input into agricultural systems includes N from synthetic fertilizer, animal wastes, increased biological N-fixation, mineralization of crop residue returned to the field and cultivation of organic soils through enhanced organic matter mineralization. Nitrous oxide may be emitted (1) directly to the atmosphere from agricultural fields, (2) from animal confinements or pastoral systems, or (3) from N applied to agricultural systems which is transported into ground and surface waters through atmospheric deposition, sewage and surface runoff and eventually into surface water (rivers and oceans) where additional N₂O is produced. Eventually, all N that moves through the soil system will be either terminally sequestered in soil or buried sediments or denitrified in aquatic systems.

Using Food and Agricultural Organization of the United Nations (FAO) databases for fertilizer input, crop and animal production, and human population and the IPCC (1997) methodology for estimating N₂O from soil, we first estimated the N input into food production and then calculated N₂O emissions derived from N input into food production systems from 1500 until the year 2020. Using these estimates for N₂O emissions (∼6 Tg N in 1990 and ∼9 Tg N in 2020) as input to a simple atmospheric box model we estimated global atmospheric N₂O concentrations over time. During the 20th century, a fast expansion of agricultural land coupled with intensification of land use probably caused about 80% of the net increase in atmospheric N₂O, from ∼275 ppbv in 1900 to ∼294 ppbv in 1970, to projected concentrations of ∼317 in 2000 and ∼345 in 2020. With the increasing amount of fertilizer N application needed to feed an additional 1.5 billion people in the next 20 years, an accelerated rate of N₂O accumulation in the atmosphere is calculated for the coming decades. This is in contrast with the observed trends during the past decade, which indicate a linear increase in atmospheric N₂O, but is in line with observed trends during the whole 20th century, which show a non-linear increase in atmospheric N₂O.