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The nitrogen cycle and its influence on the European greenhouse gas balance



- Humans have more than tripled the circulation of reactive nitrogen (Nr) in Europe, mainly through the production and release of fertilizers, in addition to fossil fuel combustion and biological nitrogen fixation.\ud - NitroEurope has combined experiments and modelling to quantify nitrogen fluxes and their influence on the European greenhouse gas budget. Additional synthesis activities have shown how these effects compare with the climate effects of Nr via aerosol and ozone.\ud - The new methods developed and comprehensive datasets obtained show how Nr interacts with other drivers of change at site, landscape, regional and European scales, pointing to opportunities for better Nr management and the development of mitigation options.\ud - A comprehensive nitrogen budget has been established, showing that Europe produces 15.6 Tg Nr annually, with 11.2 Tg from fertilizers, 3.4 Tg from combustion sources and 1 Tg from biological nitrogen fixation. In addition to the combustion emissions, agricultural Nr use is very leaky, wasting about 13 Tg to air and water annually. \ud - Atmospheric deposition of Nr increases the carbon storage of European forests, but this is constrained by an increase in Nr losses, while the ammonia (NH3) fraction of Nr deposition also represents a loss of productivity from agricultural systems.\ud - Comparing the warming effects of Nr emissions (N2O formation, ozone warming and phytotoxic effects) with the cooling effects (faster forest growth, altered methane lifetime and aerosol formation) leads to a rough balance over Europe (-16 m (-47 to +15) mW m-2). \ud - Cost-benefit analysis indicates that the threats of Nr particles to human health and of Nr deposition to biodiversity loss greatly outweigh their potential climate benefits. \ud - There are many opportunities to reduce the climate warming effects of Nr losses. \ud - Efforts to reduce the Nr-related warming effects of tropospheric ozone must decrease both NOx and volatile organic compounds (VOCs), requiring ongoing improvement in combustion technologies and further efforts to reduce transport mileage and energy use.\ud - Efforts to reduce N2O emissions must focus on improving overall nitrogen use efficiency in agriculture, for which the implementation of technical measures to reduce ammonia emissions, denitrification to N2 and nitrate leaching is essential.\ud - The overall European nitrogen cycle is driven by the human quest for luxury consumption of animal products. Of the Nr in crops produced or imported to Europe (12 Tg) only 15% is used to feed people directly, with 85% going to feed animals.\ud - Avoiding dietary excess of meat and diary products would provide a major contribution to decreasing the climate warming effects of N2O, while reducing the threat of Nr emissions to human health and biodiversity at the same time. \u

Topics: Agriculture and Soil Science, Ecology and Environment, Atmospheric Sciences
Publisher: NERC/Centre for Ecology & Hydrology
Year: 2011
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