Effects of global change during the 21st century on the nitrogen cycle

Abstract. The global nitrogen (N) cycle at the beginning of the 21st century has been shown to be strongly influenced by the inputs of reactive nitrogen (N_r) from human activities, including combustion-related NO_x, industrial and agricultural N fixation, estimated to be 220 Tg N yr⁻¹ in 2010, which is approximately equal to the sum of biological N fixation in unmanaged terrestrial and marine ecosystems. According to current projections, changes in climate and land use during the 21st century will increase both biological and anthropogenic fixation, bringing the total to approximately 600 Tg N yr⁻¹ by around 2100. The fraction contributed directly by human activities is unlikely to increase substantially if increases in nitrogen use efficiency in agriculture are achieved and control measures on combustion-related emissions implemented.

Some N-cycling processes emerge as particularly sensitive to climate change. One of the largest responses to climate in the processing of N_r is the emission to the atmosphere of NH₃, which is estimated to increase from 65 Tg N yr⁻¹ in 2008 to 93 Tg N yr⁻¹ in 2100 assuming a change in global surface temperature of 5 °C in the absence of increased anthropogenic activity. With changes in emissions in response to increased demand for animal products the combined effect would be to increase NH₃ emissions to 135 Tg N yr⁻¹. Another major change is the effect of climate changes on aerosol composition and specifically the increased sublimation of NH₄NO₃ close to the ground to form HNO₃ and NH₃ in a warmer climate, which deposit more rapidly to terrestrial surfaces than aerosols. Inorganic aerosols over the polluted regions especially in Europe and North America were dominated by (NH₄)₂SO₄ in the 1970s to 1980s, and large reductions in emissions of SO₂ have removed most of the SO₄²⁻ from the atmosphere in these regions. Inorganic aerosols from anthropogenic emissions are now dominated by NH₄NO₃, a volatile aerosol which contributes substantially to PM₁₀ and human health effects globally as well as eutrophication and climate effects. The volatility of NH₄NO₃ and rapid dry deposition of the vapour phase dissociation products, HNO₃ and NH₃, is estimated to be reducing the transport distances, deposition footprints and inter-country exchange of N_r in these regions.

There have been important policy initiatives on components of the global N cycle. These have been regional or country-based and have delivered substantial reductions of inputs of N_r to sensitive soils, waters and the atmosphere. To date there have been no attempts to develop a global strategy to regulate human inputs to the nitrogen cycle. However, considering the magnitude of global N_r use, potential future increases, and the very large leakage of N_r in many forms to soils, waters and the atmosphere, international action is required. Current legislation will not deliver the scale of reductions globally for recovery from the effects of N_r deposition on sensitive ecosystems, or a decline in N₂O emissions to the global atmosphere. Such changes would require substantial improvements in nitrogen use efficiency across the global economy combined with optimization of transport and food consumption patterns. This would allow reductions in N_r use, inputs to the atmosphere and deposition to sensitive ecosystems. Such changes would offer substantial economic and environmental co-benefits which could help motivate the necessary actions.