Policy implications of human-accelerated nitrogen cycling

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 (?125 Tg N) and to provide energy (?25 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 NO_x, NH₃, N₂O and NO₃^- and deposition of NO_y and NH_x.In the atmosphere, increases in tropospheric ozone and acid deposition (NO_y and NH_x) 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 leachinghave led to decreased drinking water quality, eutrophication, hypoxia and decreases in aquatic plant diversity, for example. On the otherhand, increased deposition of biologically available N may have increased forest biomass production and may have contributed to increasedstorage 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 unwantedreactive 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 theN that is introduced into the global food production system is not converted into usable product, but rather reenters the biosphere as a surpl [ABSTRACT FROM AUTHOR]