Policy implications of human-accelerated nitrogen cycling

The human induced input of reactive N into the global biosphere has increas ed to approximately 150 Tg N each year and is expected to continue to incre ase for the foreseeable future. The need to feed (similar to 125 Tg N) and to provide energy (similar to 25 Tg N) for the growing world population dri ves this trend. This increase in reactive N comes at, in some instances, si gnificant 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 redu ctions 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 examp le. On the other hand, increased deposition of biologically available N may have increased forest biomass production and may have contributed to incre ased storage of atmospheric CO2 in plant and soils. Most importantly, synth etic production of fertilizer N has contributed greatly to the remarkable i ncrease in food production that has taken place during the past 50 years. The development of policy to control unwanted reactive N release is difficu lt because much of the reactive N release is related to food and energy pro duction and reactive N species can be transported great distances in the at mosphere 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 reactiv e N and in curtailing the movement of this N in food production is even mor e 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 inc rease food production to raise nutritional levels or to keep up with popula tion growth, which may require increased use of N fertilizers. Although N c ycling occurs at regional and global scales, policies are implemented and e nforced at the national or provincial/state levels. Multinational efforts t o control N loss to the environment are surely needed, but these efforts wi ll require commitments from individual countries and the policy-makers with in those countries.