Policy implications of human-accelerated nitrogen cycling (Reprinted from Biogeochemistry, vol 52, pg 281-320, 2001)

The human induced input of reactive N into the global biosphere has increased to approximately 150 Tg N each year and is expected to continue to increase for the foreseeable future. The need to feed (similar to125 Tg N) and to provide energy (similar to25 Tg N) for the growing world population drives this trend. This increase in reactive N comes at, in some instances, significant costs to society through increased emissions of NOx, NH3, N2O and NO3- and deposition of NOy and NHx. \ In the atmosphere, increases in tropospheric ozone and acid deposition (NOy and NHx) have led to acidification of aquatic and soil systems and to reductions in forest and crop system production. Changes in aquatic systems as a result of nitrate leaching have led to decreased drinking water quality, eutrophication, hypoxia and decreases in aquatic plant diversity, for example. On the other hand, increased deposition of biologically available N may have increased forest biomass production and may have contributed to increased storage of atmospheric CO2 in plant and soils. Most importantly, synthetic production of fertilizer N has contributed greatly to the remarkable increase in food production that has taken place during the past 50 years. The development of policy to control unwanted reactive N release is difficult because much of the reactive N release is related to food and energy production and reactive N species can be transported great distances in the atmosphere and in aquatic systems. There are many possibilities for limiting reactive N emissions from fuel combustion, and in fact, great strides have been made during the past decades. Reducing the introduction of new reactive N and in curtailing the movement of this N in food production is even more difficult. The particular problem comes from the fact that most of the N that is introduced into the global food production system is not converted into usable product, but rather reenters the biosphere as a surplus. Global policy on N in agriculture is difficult because many countries need to increase food production to raise nutritional levels or to keep up with population growth, which may require increased use of N fertilizers. Although N cycling occurs at regional and global scales, policies are implemented and enforced at the national or provincial/state levels. Multinational efforts to control N loss to the environment are surely needed, but these efforts will require commitments from individual countries and the policy-makers within those countries.