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

Abstract

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 NH 3 , 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 NH 3 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 NH 4 NO 3 close to the ground to form HNO 3 and NH 3 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 (NH 4 ) 2 SO 4 in the 1970s to 1980s, and large reductions in emissions of SO 2 have removed most of the SO 2− 4 from the atmosphere in these regions. Inorganic aerosols from anthropogenic emissions are now dominated by NH 4 NO 3 , a volatile aerosol which contributes substantially to PM 10 and human health effects globally as well as eutrophication and climate effects. The volatility of NH 4 NO 3 and rapid dry deposition of the vapour phase dissociation Published by Copernicus Publications on behalf of the European Geosciences Union. D. Fowler et al.: Effects of global change during the 21st century on the nitrogen cycleproducts, HNO 3 and NH 3 , 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 re...

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