Humanity’s heavy load on the nitrogen cycle

Scientists are working to build a holistic picture of the environmental impacts of nitrogen-based pollutants in air, water and land. The approach could lead policymakers to reassess the current crowded policy framework, says Simon Evans

Nitrogen may be essential for life, but you can have too much of a good thing: man-made reactive nitrogen is linked to a range of environmental problems including climate change, acid rain, ozone depletion, biodiversity loss and reduced water quality, as well as impacts on health.

As with carbon emissions, humanity’s increasingly heavy load on the nitrogen cycle is closely linked to economic and population growth. Sooner or later, these will have to be decoupled.

A large number of European Directives and international treaties impose controls on "reactive nitrogen" - not the gas itself, which is inert, but nitrogen-containing compounds that are active in the environment. Such pieces of law include the Gothenburg Protocol on acidification, eutrophication and ground-level ozone, established by the UN Economic Commission for Europe (UNECE); the UN Kyoto Protocol on climate change; and EU Directives such as those on water, nitrates, integrated pollution prevention and control and emissions from power stations, among others.

This crowded policy landscape means some observers are arguing for a more integrated approach to nitrogen policy. According to Anke Lükewille, project manager for air pollution at the European Environment Agency (EEA), nitrogen "is covered by too many legislative instruments at the moment".

Without a comprehensive overview, the argument goes, the current array of discrete policies addressing each adverse effect of nitrogen separately could be at risk of pulling in opposite directions. "But how to bring them together into one package - that is certainly a challenge", says Dr Lükewille.

Nitrogen cycle
Elemental nitrogen (N2) is all around us, making up 78% of the air. This unreactive nitrogen is inaccessible to most living things. Indeed the French call it azote, meaning ‘lifeless’.

The term reactive nitrogen covers compounds that are biologically, chemically or physically active, such as nitrate (NO3-) compounds, ammonia (NH3), nitric acid, nitrogen oxides (NOx - mainly NO and NO2) and nitrous oxide (N2O).

At first glance, the Earth’s nitrogen cycle appears relatively simple. Nitrogen-fixing bacteria and algae turn unreactive N2 in the air into reactive nitrogen in soil and water. Lightning makes another small contribution to natural nitrogen fluxes by converting N2to reactive nitrogen, which is then deposited via rainfall.

Once deposited, reactive nitrogen is taken up by plants and the animals that eat them. Eventually, denitrifying bacteria convert reactive nitrogen in soil, water or decomposing organic matter into elemental nitrogen, which returns to the atmosphere and starts the cycle again.

Paul Crutzen, Nobel Prize winning atmospheric chemist coined the term ‘anthropocene’ to describe the current era in which humans are having massive impacts on entire Earth systems.

While human interference with the natural carbon cycle is widely known, the scale of the interference with the nitrogen cycle is less appreciated. Human activity contributes just 5-10% of total carbon dioxide emissions, but today man-made or ‘anthropogenic’ creation of reactive nitrogen is almost double the amount produced by natural sources.

This has altered the flow and balance of the nitrogen cycle to an enormous extent, and the pace of change is increasing. Since the mid-19th century, when made-made emissions were about 15 million tonnes per year, human-generated reactive nitrogen has risen by about one million tonnes per year. But in recent years the amount has gone up nearly three times as fast. In 2005 the amount of man-made reactive nitrogen reached 187Mt/year, compared with about 100Mt/year from natural sources (see figure).1

Thinktank Green Alliance highlighted the failure to effectively manage the nitrogen (and phosphorous) cycle in a report last year (ENDS Report 389, p 22).

Reactive nitrogen is emitted from two main sources: agriculture - fertiliser use and intensive livestock farming - and NOxfrom burning fossil fuels. The first route is deliberate: Malthusian predictions of apocalyptic food shortages were averted with the discovery that reactive nitrogen applied to agricultural land could boost yields. In the late 19th century demand was met by natural nitrate sources from South America, but by the 1930s these were largely replaced by man-made fertilisers.

However, only a small amount of the nitrogen in fertilisers actually finds its way into food. Most is lost to the environment as nitrates in water or as emissions of ammonia and nitrous oxide.

As for the fossil fuel sources of nitrogen, cutting NOx emissions from power stations and vehicles has been relatively successful in Europe but the continuing growth in traffic is undermining progress. The EEA recently identified road transport as the main source of NOx in the EU.2

Cascading adverse effects
Reactive nitrogen operates in a range of settings (see figure). The unique aspect of the nitrogen cycle is the ability of each nitrogen atom to ‘cascade’ through the cycle, causing adverse effects at each stage. Individual nitrogen atoms could, for example, travel from car exhaust fumes to the atmosphere, then fall in acid rain into a river, reach the sea and be emitted back to the atmosphere when marine bacteria digest nitrogen-containing compounds.

The cascade begins in the atmosphere: NOx from fossil fuels contributes to the formation of ground-level ozone and particulates. This air pollution is causing big problems for the Beijing Olympics in terms of visibility and more importantly has well-known consequences for respiratory and cardiovascular health (ENDS Report 397, pp 15-16). Low-level ozone also stunts plant growth, which damages ecosystems and agricultural productivity.

The interactions of air pollutants are complex and remain uncertain, for example in relation to global warming. But governments are becoming more aware that policies addressing air quality and climate change have developed in isolation and need to be brought together (ENDS Report 387, p 12).

The same nitrogen atoms can then be converted in clouds to produce acid rain. When it reaches land and water, the nitric acid can harm plants, insects and fish. Acidic soils also often release soluble aluminium and other toxic metals.

Atmospheric deposition and nitrate run-off from farms, caused by fertilisers, manure and slurry, leads to damagingly high nitrate levels in groundwater, lakes and rivers. Resulting eutrophication - excessive nutrient levels - is a serious problem in many inland and marine waters. In some areas, such as the Baltic Sea and Black Sea in Europe and Chesapeake Bay and the Gulf of Mexico in North America, excess reactive nitrogen can trigger potentially toxic algal blooms and so-called dead zones with insufficient oxygen to support marine life. Dead zones have doubled each decade since the 1960s, now covering more than 245,000 square kilometres, an area equal to the entire UK.3

Great uncertainty remains over the net climate effect of additional reactive nitrogen deposition. For instance increased nitrogen input into the ocean mainly occurs through atmospheric deposition. This increases the production of marine organisms, which in turn absorbs considerable amounts of CO2. But the higher levels of nitrogen also mean oceans emit more nitrous oxide, itself a potent greenhouse gas with a warming potential equivalent to some 300 molecules of CO2.4Nitrous oxide is also a major source of nitric oxide in the stratosphere, a key chemical in the ozone-depletion process.

A recent study in Nature Geoscience predicts that even if increased nitrogen deposition has a net positive effect in staving off climate change, "increases in the strength of land and ocean sinks are unlikely to keep pace with future increases in CO2".5

Such uncertainty assumes even greater significance when it comes to assessing the net impact of biofuels. Nitrogen-intensive agricultural methods used to produce current biofuel crops mean CO2 savings are likely to be wiped out or even exceeded by nitrous oxide emissions (ENDS Report 393, pp 13-14).6

Faced with the growing problems nitrogen is causing, scientists and researchers in several countries are striving to bring the issue to governments’ attention. In 2003 a group of academics established the International Nitrogen Initiative (INI) to "optimise the use of nitrogen in food production, while minimising the negative effects of nitrogen on human health and the environment as a result of food and energy production".7

Most governments are only just starting to look at nitrogen holistically. The European Commission’s thematic strategy on air pollution says: "Given that nitrogen plays a role in several environmental problems, the Commission will pursue a coherent and integrated approach to nitrogen management". The Commission’s Environment Directorate emphasises it is "still early days", although an official is about to be given the task of examining nitrogen’s impacts across the board.

Mark Sutton, the INI’s European coordinator, says it is still far from clear whether the best policy option would be to develop an overarching nitrogen strategy from scratch or to update the existing pieces of legislation relevant to nitrogen’s impacts.

Dr Sutton is co-chairing a task force on reactive nitrogen established earlier this year under the Gothenburg Protocol. The task force will assess the latest scientific evidence to develop strategy options for "the coordination of air pollution policies on nitrogen in the context of the nitrogen cycle" by UNECE and other bodies.

Although the Protocol focuses on NOxand ammonia, it states "measures to reduce the emissions… should involve consideration of the full biogeochemical nitrogen cycle and, so far as possible, not increase emissions of reactive nitrogen including nitrous oxide which could aggravate other nitrogen-related problems".

The task force is assessing whether explicit consideration should be given to the potential for synergies and trade-offs between different forms of reactive nitrogen. So-called pollution swapping - where, for instance, reductions in ammonia emissions are accompanied by increases in N2O - is a potential outcome of separate policy initiatives that fail to take a broad perspective.

In five to ten years Dr Sutton wants the Protocol to be engaged with other UN conventions such as one on Biological Diversity. But "the biggest challenge", he says, will be working with the UN Framework Convention on Climate Change and grappling with issues such as the environmental impacts of biofuels.

Nitrogen in the shadows?
The idea that reactive nitrogen ought to take a prominent place on the global warming agenda might seem strange. But for now nitrogen remains "in the shadow of climate change" says Dr Lükewille.

For example, the much-delayed review of the national emissions ceilings Directive - an important part of EU policy on nitrogen - is being held up by discussion of costs and consideration of the Commission’s recent climate and energy package.

Despite slow progress, there is some cause for optimism. Professor James Galloway, founding chair of the INI and a prominent nitrogen researcher at the University of Virginia, thinks "the Europeans are to be commended for their aggressive leadership in this area". Earlier this year the European Science Foundation launched a three-year European Nitrogen Assessment to review the role of excess nitrogen in environmental problems and the potential for integrated solutions.

Dr Sutton, who is part of the assessment’s coordination team, describes it as equivalent to the reports of the Intergovernmental Panel on Climate Change - only shorter. He hopes it will help distil the key messages for policymakers.

Following in carbon’s footsteps
Nitrogen’s impacts could also become a consumer issue, with the public treated to a graphic reminder of their personal impact on the nitrogen cycle. "If we are doing carbon footprinting, why not nitrogen?" says Professor Galloway, who has recently given up his role as INI chair to develop an online nitrogen calculator.

The calculator should be ready in basic form later this year. It will highlight how simple behavioural changes affect the amount of reactive nitrogen needed to support our lifestyle. Many of us are in for a surprise. The relatively high nitrogen intensity of livestock farming means the simplest and most effective change we can make is to eat less meat - a move that would also reduce other environmental impacts. But worldwide meat demand is expected to grow 85% by 2030 (ENDS Report 402, pp 47-48).

With more research, Professor Galloway sees his calculator becoming more sophisticated, for instance by taking into consideration relative contributions to different parts of the nitrogen cascade. Eventually it could be scaled up from individuals and institutions to a regional model that might serve as the basis for nitrogen cap-and-trade schemes. Such schemes would probably be national or regional, rather than global, reflecting the range of many of nitrogen’s adverse effects. The prospect of nitrogen markets and nitrogen offsets remains a distant one for now. But the day may yet come.

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