Policy implications of human-accelerated nitrogen cycling

Abstract

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 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.