The nitrogen cascade

European ecosystems are being threatened by numerous factors such as changes in the way land is used, the composition of the atmosphere and the climate. These man-made disruptions lead to major changes in the water and food cycle. From a global perspective, the atmospheric nitrogen cycle is being disrupted the most: by more than 80% whereas the carbon cycle, in comparison, is affected by no more than 10%, according to estimates. More intensive farming and the combustion of fossil fuels increase the amount of reactive nitrogen, which has a snowball effect on human health and ecosystems when released. Agricultural activities cause the emission of ammonia (NH3), nitrous oxide (N2O) and nitrogen monoxide (NO) into the atmosphere, and nitrate (NO3-) and ammonia (NH4+) into groundwater and surface water. The combustion of fossil fuels by fixed sources (such as households and industries) and mobile sources (such as road traffic, aviation and shipping) is generally known as a cause of NO and NO2 emissions, which are collectively referred to as nitrogen oxides (NOx). Devices for reducing NOx emissions, such as catalysers, produce small yet non-negligible amounts of NH3 and N2O.  

The primary driving forces behind changing the European nitrogen policy are:

• The fixation of nitrogen for and by agriculture, which results in the release of NH3, N2O, NO and NO3.
• The import of foods from other parts of the world via nutritional concentrates and food.
• Combustion processes at high temperatures, during which a small amount of atmospheric nitrogen is oxidised into NOx.  

The presence of excess nitrogen in these reactive forms results in a range of highly diverse problems:

• NO and NO2 react with volatile organic compounds (VOCs), thereby increasing ozone concentrations (O3) at ground level. This has an impact on harvests, natural vegetation and human health. The elevated tropospheric ozone concentration also contributes significantly to the greenhouse effect.
• NH3 reacts with gaseous acids – including HNO3 originating from NOx emissions – to form a finely dispersed aerosol (“particulate matter”) such as ammonium nitrate (NH4NO3) and ammonium sulphate (NH4HSO4 and (NH4)2SO4). These are transported across great distances in the atmosphere, resulting in the precipitation of reactive nitrogen many thousands of kilometres away from the source.
• Aerosols increase the diffusion of light. This reduces visibility and has a direct negative effect (opposite to the greenhouse effect) on the global radiation balance. In addition, aerosols acts as condensation nuclei that cause clouds to form, which also has an indirect effect on the radiation balance.
• Aerosols containing nitrogen can be inhaled, and may be linked to cardiovascular and respiratory diseases.
• The precipitation of oxidised (NOy) and reduced (NHx) nitrogen leads to the fertilisation of nutrient-poor ecosystems in water and on land, which affects biodiversity. Locally, the influx of NHx in particular can be exceptionally substantial and destroy fragile habitats entirely.
• NOy and NHx precipitation in ecosystems on land can acidify the soil, causing changes within communities and reducing water quality. On an EU level, nitrogen now plays a greater role in acidification than sulphur.
• The drainage and leaching of nitrogen from farming increases NO3- concentrations in groundwater and surface water. This poses potential health risks for drinking water and alters freshwater ecosystems.
• The excess outflow of nitrogen from rivers and the precipitation of nitrogen from the atmosphere in coastal waters fertilise areas in the sea, increasing the likelihood of algal blooms and oxygen deficiency. In addition to that, atmospheric deposition is also a key source of nitrogen in sea ecosystems.
• N2O is a powerful greenhouse gas, accounting for 12% of mankind’s potential contribution to global warming. It also plays a role in the chemistry of the stratosphere, destroying O3 at high altitudes.
• The addition of nitrogen from the atmosphere also influences the exchange of CO2 and CH4 between ecosystems and the atmosphere.  

When released into the environment, nitrogen compounds can cause an avalanche effect before they are eventually converted into neutral nitrogen (N2) or absorbed into soils and sediments. The nitrogen snowball (see diagram) can be explained using the path followed by reactive nitrogen after it has been used as fertiliser on farmland or after it has been released when fossil fuels are burned. Firstly, a considerable amount is released into the atmosphere in the form of NH3, NO, N2O or NO2. Secondly, some is washed away into groundwater and surface water in the form of NO3-. Thirdly, some is converted into vegetable biomass used to feed humans or animals. The amount that is eaten by people (possibly after transport across major distances and transformation by the food industry) is released into waste water and discharged into surface water after some form of treatment. The amount that is consumed by animals ends up in farmland: a significant amount is excreted by livestock and released as NH3, with additional losses in the form of N2O, NO and NO3-. Subsequent NH3 precipitation from the atmosphere causes more emissions of N2O and NO, and NO3- leaching. The atmosphere therefore helps spread reactive nitrogen, thereby disrupting the nitrogen cycle in areas far away from direct human interference.  

It is clear from all of the topics mentioned here that nitrogen is an important theme that involves numerous areas. It covers the majority of key social domains and the indirect effect that these have on Europe’s environmental problems: climate change, biodiversity, the health of ecosystems, public health, contamination of and groundwater contamination, etc. 

Questions to be answered  

• Insight into the nitrogen cycle:
  • Which chemical and physical processes does this cycle comprise?
  • How are these processes related?
  • How large is every mass flow in each geographical area that enters and leaves these processes?
• In which agricultural and other production processes do reactive nitrogen losses occur, how great are these and which paths do they follow afterwards?
• What is the interaction between the production of food and the utilisation of biomass, and how do these influence the nitrogen cycle?
• What does the nitrogen balance for the Netherlands look like?
• What is the relationship between elements of the nitrogen cycle and emission or absorption of greenhouse gases? (not only N2O but also CO2 and CH4)

Examples of recent results

Deposition monitoring in forests

The Pan-European Programme for Intensive and Continuous Monitoring of Forest Ecosystems is the level II programme of the International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (“ICP Forests of UN/CE”). It provides a framework for analysing the impact of harmful substances from the atmosphere on forests and the variations therein. The programme in the Netherlands involves five locations: Dwingeloo (1), Hardenberg (2), Speuld (3), Zeist (4) and Leende (5). The composition of throughfall is measured at all locations: water drops that fall through the tops of trees (part of which comprises the original precipitation, and dripping water that flows from tree trunks). 

A comparison was made between measured depositions and emissions reported in the Netherlands. The aim was to determine the effectiveness of the Dutch policy for emission reductions. It must be noted that this can be complicated by the fact that depositions in the Netherlands do not only originate from Dutch sources, but also from other EU countries (60%). There is a five-year gap between emission figures because these are made available once every five years.

The comparison is based on the situation in 1995. There is a distinct difference in the trends. Emissions decrease between 1955 and 2005, but depositions remain more or less the same initially (NHx, NOy) or increase first (SOx) and then only decrease in the final five years. This could be caused by:

• Foreign sources
• Differences between reported and actual emissions
• Shifts in chemical reactions en route between the source and measurement locations, which can depend on concentrations  

Further research is required.

Visualisation of the nitrogen problem

We have created software that can offer a quick insight into the sources and effects of reactive nitrogen and the links between them. This also reveals how this would develop over the course of time.

One of the most important aspects of reactive nitrogen is its role in food supply. According to a rough calculation, 40% of the world population could not be adequately fed if fertiliser did not exist. Even with the current use of fertiliser, millions of people still suffer from malnutrition. In the animation, however, we have not focused on problems caused by reactive nitrogen deficiencies, but rather on the problem areas that arise due to the consequences of excess nitrogen.

The visualisation comprises three parts:

1. The development of sources of reactive nitrogen.
2. The effects thereof.
3. A cockpit in which you can enter policy choices and see the consequences thereof.