The global biogeochemical cycling of nitrogen (N) has been significantly impacted by anthropogenic activity. Perturbations to the global nitrogen cycle have implications that range from terrestrial ecology to atmospheric chemistry and global climate change. To understand these effects and predict future changes in the nitrogen cycle, it is important to examine the pre- and post-agroindustrial global nitrogen cycle as well as to estimate the potential effects of human activity in the future. To do this, Galloway has calculated the effect humans have and may have on the cycling of reactive nitrogen through the atmosphere and biosphere.

In a world unimpacted by human activity, the fluxes of N between reactive and unreactive forms would be roughly balanced. In this environment, denitrification fluxes would probably be about equal to fluxes via biological N fixation (converted from N₂ to a reactive form of N). The global atmospheric/terrestrial N budget would include approximately 110 Tg N per year in denitrification and N fixation, 3 Tg N per year of N₂ would be converted to NO due to lightning, 4 Tg N per year would be emitted to the troposphere as nitrogen oxides from soils and 6 Tg N of NH₃ would be emitted to the troposphere from vegetation and soils and about 3 Tg N of NH₃ per year from animals.

When human activities are added to these numbers, the global fluxes of nitrogen change dramatically. The combustion of fossil fuels results in the conversion of about 20 Tg N per year from N₂ to NO_x (NO + NO₂). But combustion is not the only human impact on the global N cycle. The use of commercial fertilizers has resulted in major global changes in N cycling. Use of fertilizers results in the conversion of roughly 80 Tg N of N₂ to reactive N annually. An additional ~40 Tg N of N₂ is fixed due to the cultivation of leguminous crops. Significant amounts of agricultural N are then lost to the atmosphere as NO_x or NH₃. When soil, vegetation and animal losses are combined, these fluxes may amount to ~60 Tg N per year. While there are large uncertainties in these flux estimates, it is clear that human activity has dramatically altered the global biogeochemical cycling of nitrogen.

Atmospheric emissions of NO_x and NH₃ are increasing. The increase in NO_x emissions began in the early 1900s and is closely related to increases in the extent of biomass burning and combustion. NH₃ emissions have not been increasing for as long but are increasing at a more rapid rate now as the result of increasing fertilizer use and generations of domestic animals. Generally, the highest rates of NH_x (NH₃ + NH₄ + aerosol) deposition also occur in the areas with the highest agricultural activity but there is also substantial hemispheric dispersion of NH₃ emissions.

On a global scale, about 140 Tg of N are fixed annually by human activity. Of the 140 Tg total, an estimated 40 Tg are injected into coastal regions by rivers, 18 Tg are deposited onto the oceans from the atmosphere, 3 Tg accumulate as atmospheric N₂O. This leaves about 80 Tg N unaccounted for. The possible sinks of this 80 Tg of nitrogen are increases in N in biomass, N in ground water, and denitrification to N₂. It is worth noting that biological and social consequences of these potential sinks are very different. If reactive nitrogen is converted back to non-reactive N₂ by denitrification, this nitrogen will be removed from the reactive nitrogen cycle and will become benign. If however, the reactive nitrogen ends up in ground water or biomass, there will be significant implications for water quality and aquatic and terrestrial ecosystems.

Emissions of NO_x and NH₃ are currently high in Asia and are projected to rise dramatically in the future. To understand the potential for changes in the reactive N cycle in Asia, it is useful to compare China with the United States and examine both the present alterations of the N cycle in the two countries and the potential for future change (see figure 5.1). Current emissions of NO_x  due to energy use in the United States are relatively certain as both total energy use and NO_x  emission factors are known fairly well. Estimates for China are more problematic as both energy use patterns and emission factors are not as well documented. However, it is possible to make minimum and maximum estimates of NO_x production using estimates for energy consumption and maximum and minimum NO_x emission factors. Using this approach, current U.S. NO_x emissions are about 6 Tg N per year while Chinese emissions are about 2 Tg N per year. Looking forward to 2020, U.S. emissions will likely increase only slightly while Chinese emissions (from NO_x) are projected to increase to about 5 Tg N per year.

Currently, Chinese fertilizer use is increasing at a rate of 2.7% annually. In contrast, U.S. fertilizer use is relatively constant and is projected to remain flat. To calculate losses of ammonia from fertilizer use again requires the use of an emission factor. Unfortunately, emission factors vary with the type of fertilizer used. Unlike the U.S., much of the fertilizer used in China is in the form of ammonium bicarbonate for which published emission factors are uncertain. Using urea emission factors for ammonium bicarbonate (probably an underestimate of losses) it is possible to estimate NH₃ losses from fertilizer use. Emissions of NH₃ from fertilizer use in China currently are about 3 Tg N per year. These emissions are already higher than U.S. emissions of about 0.5 Tg N per year. In the future, China's emissions (from NH₃) are projected to increase to about 6 Tg N per year by 2020 while U.S. emissions remain relatively flat. The tremendous potential for NH₃ loss due to fertilizer application in China is likely to play a dominant role in the Chinese N cycle in the future. Atmospheric NH₃ in China will also be significantly affected by emissions of NH₃ due to animals. These emissions could reach as high as 6 Tg N per year with significant uncertainty (see figure 5.2).

The comparison of the U.S. versus China demonstrates that human effects on nitrogen cycling in the two countries occur with different combinations of agricultural and industrial pathways. While fertilizer use dominates the current and future N cycle in China, combustion and fertilizer use dominate in the U.S. These two types of human intrusions on the N cycle have very different pathways for mitigation. Emissions of NO_x due to combustion can be reduced by reducing the amount of material combusted or by controlling the combustion characteristics. NH₃ emissions from agricultural systems may be reduced through a variety of management techniques.