Policy implications of human-accelerated nitrogen cycling

The human induced input of reactive N into the globalbiosphere has increased to approximately 150 Tg N eachyear and is expected to continue to increase for theforeseeable future. The need to feed (125 Tg N) andto provide energy (25 Tg N) for the growing worldpopulation drives this trend. This increase inreactive N comes at, in some instances, significantcosts 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 andacid deposition (NO_y and NH_x) have led toacidification of aquatic and soil systems and toreductions in forest and crop system production. Changes in aquatic systems as a result of nitrateleaching have led to decreased drinking water quality,eutrophication, hypoxia and decreases in aquatic plantdiversity, for example. On the other hand, increaseddeposition of biologically available N may haveincreased forest biomass production and may havecontributed to increased storage of atmospheric CO₂ inplant and soils. Most importantly, syntheticproduction of fertilizer N has contributed greatly tothe remarkable increase in food production that hastaken place during the past 50 years.The development of policy to control unwanted reactiveN release is difficult because much of the reactive Nrelease is related to food and energy production andreactive N species can be transported great distancesin the atmosphere and in aquatic systems. There aremany possibilities for limiting reactive N emissionsfrom fuel combustion, and in fact, great strides havebeen made during the past decades. Reducing theintroduction of new reactive N and in curtailing themovement of this N in food production is even moredifficult. The particular problem comes from the factthat most of the N that is introduced into the globalfood production system is not converted into usableproduct, but rather reenters the biosphere as asurplus. Global policy on N in agriculture isdifficult because many countries need to increase foodproduction to raise nutritional levels or to keep upwith population growth, which may require increaseduse of N fertilizers. Although N cycling occurs atregional and global scales, policies are implementedand enforced at the national or provincial/statelevels. Multinational efforts to control N loss tothe environment are surely needed, but these effortswill require commitments from individual countries andthe policy-makers within those countries.