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Human Acceleration of the Nitrogen Cycle Managing Risks and Uncertainty

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Human Acceleration of the Nitrogen Cycle Managing Risks and Uncertainty Human Acceleration of the Nitrogen Cycle Managing Risks and Uncertainty POLICY HIGHLIGHTS NITROGEN POLICY HIGHLIGHTS Three ways to tackle nitrogen pollution Despite immense benefits in terms of food and energy security, the use of fertilisers and the burning of fossil fuels Did you know? release excess reactive nitrogen that poses serious threats Until one century ago, to health and the environment. Chilean saltpetre (and The use of chemically-reactive nitrogen in fertilisers has before it Peruvian brought immense benefits in terms of agriculture productivity guano) was the main and food security. Nitrogen also has many industrial uses and source of nitrogen for the combustion of fossil fuels releases reactive nitrogen to the world agriculture and atmosphere as a by-product. Since the start of the 20th century, industry. humans have doubled the inputs of reactive nitrogen to the environment (Fowler et al., 2013) and this now poses serious threats to health and the environment. Excess nitrogen pollutes air, soil and water; increases greenhouse gas emissions; and impacts biodiversity and ecosystem functioning. Reactive nitrogen stimulates the formation of ground- level ozone and particles in the atmosphere; increases soil acidification; and, the transfer of nitrogen from terrestrial to coastal systems is leading to algal blooms and a decline in the quality of aquatic ecosystems. Not only is nitrogen a greenhouse gas (GHG) itself (in the form of nitrous oxide), but it also contributes to depletion of the stratospheric ozone. Countering nitrogen pollution in a cost-effective way requires a threefold approach: l Implement impact-pathway analysis (IPA) to better managing the risks of air, soil, water and ecosystem pollution. l Manage nitrous oxide as part of GHG mitigation policies. l Monitor and manage any residual nitrogen excess by measuring the effect of the above measures on the national nitrogen budget. Measures to counter nitrogen pollution should be based on their impact and applied as close as possible to the point of emission to maximise effectiveness and cost-efficiency. Often, the best environmental outcome and greater political acceptability will require a combination of measures: “polluter pays” and “beneficiary pays” policies, alongside direct environmental regulation. THREE WAYS TO TACKLE NITROGEN POLUTION . 1 How excess nitrogen threatens 1health and the environment AIR Human activity – especially the burning of fossil fuels Air motions play a key role in the distribution of – is a major source of nitrogen oxides (NOx) in the reactive nitrogen in the atmosphere and in its troposphere, the lowest region of Earth’s atmosphere. transport distance before “deposition” on the ground. In such combustion processes, under high temperature In the United States, the National Atmospheric and pressure conditions, atmospheric nitrogen (N2) Deposition Program (NADP) has mapped the risk combines with oxygen atoms to create NOx. Human of nitrogen aerosol deposition. As evidenced by the activity – especially agricultural practices – is also a risk assessment, it can be expected that ammonium + major source of ammonia (NH3). (NH4 ) wet deposition occurs near or downwind of – major agricultural centres, and that nitrate (NO3 ) In the atmosphere, nitrogen dioxide (NO2) and, to a levels in wet deposition are consistent with NOx lesser extent, NH3, are directly harmful to human emissions (USEPA-SAB, 2011). health, increasing likelihood of respiratory problems. NH3 and NOx can react with each other or with The health impact of air pollution by nitrogen incurs atmospheric components, contributing to the most social costs that are already in the hundreds of serious air pollution problems for human health billions of USD. This is because nitrogen compounds – airborne particulate matter (PM). The effects of represent a significant part of urban pollution with PM can range from eye and respiratory irritation to fine particles (PM2.5) whose health cost in terms of cardiovascular disease, lung cancer and consequent premature deaths is estimated at USD 1.8 trillion in premature death. Exposure to high levels of ground-level OECD countries and USD 3.0 trillion in the BRIICS ozone, caused by NOx and volatile organic compounds countries (Brazil, Russia, India, Indonesia, China that react under the influence of heat and the sun, and South Africa) (Roy and Braathen, 2017). For increases the risk of premature death from lung disease example, nitrogen-containing aerosols accounted and also affects vegetation by damaging leaves and for an estimated 30% of PM2.5 emissions measured reducing growth. NH3 can also be harmful to vegetation, in Beijing from June 2014 to April 2015 (Huang et al., especially lower plants, through direct damage to leaves. 2017). Did you know? The atmosphere we breathe is 78% dinitrogen (N2). However, if we had to rely on dinitrogen only, it would be like floating on the sea, dying of thirst. 2 . OECD POLICY HIGHLIGHTS | Human Acceleration of the Nitrogen Cycle: Managing Risks and Uncertainty POLICY HIGHLIGHTS When adding ground-level ozone pollution, of which N2O contributes not only to climate change but also nitrogen is also a precursor, health costs amount to to the depletion of the stratospheric ozone layer, USD 1.9 trillion in OECD countries and USD 3.2 trillion which protects life on Earth by absorbing some of in BRIICS countries (Roy and Braathen, 2017). the ultraviolet rays from the sun. With the phasing- out of chlorofluorocarbons (CFCs), 2N O has become a Despite a steady decline in NOx emissions since major depleting threat for ozone in the stratosphere 2000, most EU countries have at least one city where (Ravishankara et al., 2009) and is currently unregulated the annual average concentration of NO2 exceeds by the Montreal Protocol on Substances that Deplete (sometimes considerably) the EU’s legal limit values, the Ozone Layer. which are set equal to the World Health Organisation (WHO) Air Quality Guideline (OECD, 2017). SOIL GREENHOUSE BALANCE AND THE STRATOSPHERIC Many of the natural transformations involved in OZONE LAYER the biogeochemical nitrogen cycle are carried out by microorganisms in soil. Conversion of N2 to Microbial “denitrification” is a key pathway in the biologically available (“reactive”) nitrogen, a process nitrogen cycle of soils and oceans, but a pathway called “fixation”, involves several processes, as part of that is still poorly understood. Denitrification returns the nitrogen cycle. nitrogen from the biosphere to the atmosphere as N2, thereby closing the nitrogen cycle; this may Nitrogen-fixing bacteria convert 2N to NH3, which + involve the production of nitrous oxide (N2O) as an tends to converts into NH4 in non-alkaline soils. intermediate product. Human activity – especially Under aerobic conditions (in the presence of oxygen), + - agriculture – is a major source of N2O production. nitrifying bacteria can oxidise NH4 to NO3 as an N2O emissions of crops depend on the type of energy-yielding process. Because it releases hydrogen, fertiliser (with emissions typically being higher for nitrification contributes to the acidification of soils. urea than for ammonium nitrate) and soil type, with emissions generally high for clay soils with poor Under anaerobic conditions (in the absence of - drainage. oxygen), denitrifying bacteria can use NO3 in place of oxygen, reducing it to N2. In the process, N2O is the third most common long-lived GHG denitrification may generate 2N O. after carbon dioxide (CO2) and methane. The N2O greenhouse effect is partially offset by increased CO2 Microbial activity in soils is also driven by by ecosystems uptake due to nitrogen deposition. “rhizodeposition” – the release of organic carbon and Did you know? Microbial denitrification is a key pathway in the nitrogen cycle of soils and oceans, but a pathway that is still poorly understood. HOW EXCESS NITROGEN THREATENS HEALTH AND THE ENVIRONMENT . 3 THE MODELLING PHILOSOPHY . 4 organic nitrogen (e.g. amino acids) by plant roots Nitrogen finds its way into fresh water via pathways into their surrounding environment – and microbial that are mainly fed by agriculture, such as mineralisation of soil organic matter. While the groundwater, drainage water and runoff, as well available organic carbon and oxygen have a major as direct discharges of wastewater from firms and impact on total denitrification, the soil pH mainly sewage treatment plants, and atmospheric deposition. influences the 2N O/N2 ratio (the ratio tends to increase Nitrogen pathways in marine water include nitrogen with decreasing pH). imports by river discharge and precipitation; microbial fixation of 2N ; and bacterial remineralisation of dead The major nitrogen threats to soil quality, for both particulate biomass in sediments. agricultural soils and natural soils, are related to acidification and loss of soil biodiversity. Soil Oceanic currents may shift the impact of excess acidification may lead to a decrease in crop and nitrogen over long distances. For example, nitrogen forest growth and leaching of components negatively from the Amazonian Basin, where soils are washed affecting water quality, including heavy metals. away by the rains as a result of deforestation and - Nitrogen may also be lost from soils as NO3 , via intensive agriculture, has largely contributed to the leaching, or in gaseous forms such as NH3, by proliferation of algae in coastal areas of the Caribbean. volatilisation, or else N2O and N2, via denitrification. Climate change could
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