Forms of reactive nitrogen
Reactive nitrogen, unlike nonreactive molecular nitrogen (N2), forms a wide variety of bonds with organic and inorganic substances and thus is of vital importance to all life processes. The availability of reactive nitrogen often limits plant growth, which is why sufficient supply of reactive nitrogen is so important in food production. If excessive amounts of reactive nitrogen are deposited to the environment, however, it poses a danger to human health and the viability of ecosystems
Forms of reactive nitrogen which are especially relevant to the environment are
- the gases ammonia (NH3), nitrogen monoxide (NO), nitrogen dioxide (NO2 ), nitrous oxide (N2O), and
- ammonium (NH4+) and nitrate (NO3), which occur in dissolved form and in atmospheric particulate matter.
The different forms of reactive nitrogen are very mobile and can convert into one another. They circulate in a biogeochemical cycle between air, soil, water and organisms.
Sources of reactive nitrogen
A large share of the planet’s nitrogen supply is in gaseous atmospheric nitrogen compounds. The majority of it is the inert molecular nitrogen (N2), a chemical form that is not usable by most organisms including plants. In the prehuman world molecular nitrogen was converted to reactive nitrogen primarily by a very few bacterial strains. Since the age of the Industrial Revolution, however, human activities have caused far more elemental nitrogen to change into reactive forms, probably at a rate ten times higher than in the pre-industrial era. The major conversion processes are
- combustion of fossil fuels and corresponding emissions of nitrogen oxides (NOx ),
- synthesis of ammonia (NH 3 ) through the Haber-Bosch process (primarily for fertiliser production), and
- cultivation of legumes.
Some of the anthropogenic reactive nitrogen is released into the environment. The German Environment Agency (UBA) regularly analyses the emissions and contributions of various polluter sectors. According to the most recent assessment, the total annual emission of gaseous nitrogen compounds or nitrate into the environment in Germany is 1,369 kt N a-1. 65 % of this is released by agriculture, 13 % from the transport sector, 13 % from industrial and energy sector processes and 9 % from households, municipal wastewater management and surface runoff. The majority of reactive nitrogen is released into the hydrosphere as nitrate input followed by ammonia, nitrogen oxide emissions and the release of nitrous oxide into the atmosphere (table).
However, in order to present the long-term indicator showing total nitrogen input and the sectoral shares of these emissions in a methodologically consistent manner between 1995 - 2020, the figure does not include inputs from agriculture into groundwater, as these data was not yet inventoried prior to 2010. When these inputs are included, the total nitrogen amount actually rises to 1,963 kt N, with agriculture accounting for as much as 74%.
Effects of reactive nitrogen
The excessive release of reactive nitrogen compounds disrupts natural biogeochemical cycles and the delicate balance in ecosystems, which results in considerable negative consequences for man and the environment.
- Atmospheric deposition of reactive nitrogen species contribute to ecosystem eutrophication in German and acidification in German and, as such, to habitat degradation and loss of biodiversity.
- Nitrate leaching in German
- jeopardises drinking water quality.
- Higher emissions of nitrous oxide promote global warming.
- Gaseous nitrogen compounds are precursors of ground-level ozone and secondary particulates and, as such, pose a risk to human health.
- High atmospheric ammonia and ozone concentrations cause widespread damage to sensitive plants in Europe.
- Nitrogen dioxide is harmful to the respiratory tract and human health.
Reduction strategies
Because of the mutability of nitrogen, reduction measures in one environmental medium might shift an existing problem to another area to become more acute there. Effective reduction of reactive nitrogen compound emissions and corresponding adverse environmental impact requires adoption of integrated concepts. The German Environment Agency UBA) has been working on this for many years and is supporting the Federal Ministry for the Environment. In 2015, the German Advisory Council on the Environment also urged the German government to develop a strategy to solve the problem. Recently, supra-regional and cross-media solutions have also been increasingly developed at international level.
Integrated Nitrogen emission reduction
Despite many years of efforts to reduce nitrogen emissions into the environment, most nitrogen-related environmental quality and action targets have not yet been achieved.
- Due to the persistent over-fertilisation of ecosystems as a result of nitrogen inputs, the loss of biodiversity continues unabated.
- Existing limit values for the protection of water bodies from nitrate inputs and guideline values for the protection of human health from the risks posed by NO2 in the ambient air are being exceeded.
- Germany faces further reduction obligations for the year 2030 of the NEC Directive to reduce ammonia and nitrogen oxide emissions.
- Stabilising greenhouse gas concentrations in the atmosphere in order to prevent dangerous anthropogenic interference with the climate system also requires a reduction in the release of nitrous oxide (N2O).
Previous measures to reduce nitrogen emissions were often not effective enough, as legal requirements and regulations were limited to individual emission areas or only affected one environmental impact or one protected good.
Due to the complexity of the nitrogen problem, it makes sense to consider all polluter areas and affected environmental media together. Independent assessments of individual nitrogen-related environmental problems, such as the impairment of water or air quality, contain the risk of overlooking important interactions. For policymakers, an integrated approach to solving the nitrogen problem is worthwhile because a cross-sectoral, standardised understanding of policy is a prerequisite for joint action. Recognising, understanding and communicating the problem as a whole can increase the willingness to change lifestyles and thus support social transformation processes in the areas of nutrition, mobility and energy use. This is because changes in these areas can contribute to reducing nitrogen surpluses in the environment. An integrated approach also supports effective solutions within an individual sector, as the synergetic effects of measures can be accounted for with an overarching view. In the case of nitrogen-reducing measures in agriculture, for example, it makes sense to assess the potential for air pollution control, climate protection and water protection together, in order to recognise efficient combinations of measures.
The Federal Ministry for the Environment is also working on the development of a national nitrogen emission target. The German Environment Agency (UBA) is supporting this process through several research projects. Most recently, an impact-based nitrogen boundary for Germany was calculated; compliance with this boundary is expected to eliminate any harmful effects of nitrogen on humans and the environment. To demonstrate how this boundary can best be achieved, researchers are collaborating with UBA on a current project to develop a cross-sectoral measures database.
A national nitrogen boundary
The impact-oriented, integrated boundary, which was developed by the German Environment Agency within a research project marks the maximum amount of nitrogen that may be released in Germany per year in order to achieve the environmental and health targets currently set. It therefore sets a quantitative framework for the sustainable use of nitrogen compounds in the various economic sectors, without emphasising any individual sector.
To take account of the heterogeneity of the nitrogen cycle, these upper limits have also been broken down regionally for 302 district regions in Germany. This enables local stakeholders and decision-makers to see the scale of the problem and what is causing it.
The resulting ecologically acceptable national boundary amounts to 932 kt of reactive nitrogen. Compared to the current total release of 1.963 kt N mentioned above, this corresponds to a reduction requirement of more than 50%. A look at the regional differences shows that reduction contributions must be made in all district regions. In most regions, covering approximately 80% of Germany’s area, the total area-based reduction requirement is relatively low at less than 40 kg per hectare per year. In contrast, 16 district regions show a high area-based reduction requirement of more than 80 kg per hectare per year. Nine of the district regions with high reduction requirements are predominantly agricultural, whilst seven are more urban in character. For districts wishing to find out more about the composition of nitrogen emissions and the calculated reductions in their area, a tabular overview is available in the research report and as an Excel download.
With these results, Germany now has an easily communicable and clearly understandable long-term scientific orientation that can serve comparable to the 1.5-degree target for climate protection. This makes Germany the first country in the EU to have calculated a national nitrogen boundary to complement the planetary boundary for nitrogen.
International initiatives to reduce nitrogen emissions
The overloading of ecosystems with reactive nitrogen compounds does not only affect Germany and Europe. Humans have a massive impact on the global nitrogen cycle. Worldwide, around four times more nitrogen is currently converted into the reactive form than is sustainable for long-term survival. Increasing energy requirements and population growth are expected to further exacerbate the problem.
There are regional differences, according to which the problems and solutions in Africa, Europe, South America and South-East Asia differ fundamentally from one another. However, the continents are also interlinked via global nitrogen flows. While industrially developed regions have an abundance of nitrogen as a resource for agriculture and at the same time have been working on reducing nitrogen emissions (nitrogen dioxide, nitrate, ammonia) for decades, other regions (e.g. Africa) lack sufficient fertilisers and the implementation of emission reduction measures. In order to solve central humanitarian problems such as hunger, access to clean drinking water and life in clean air, inadequate wastewater management, loss of biodiversity, pollution of coastal waters and climate change, coordinated nitrogen-related policies are also required at international level. Comprehensive global management with the help of integrated solutions and cooperation between all stakeholders is urgently needed and particularly promising in order to minimise the adverse effects of nitrogen and at the same time make greater use of the benefits of nitrogen in regions of the world suffering from hunger.
Various European and global networks of scientists and policy-makers have therefore been increasingly drawing attention to the global problems arising from the overloading of the nitrogen cycle for several years.
The International Nitrogen Initiative (INI), a global network of scientists, aims to strengthen the use of reactive nitrogen in the context of sustainable food production while minimising negative effects on people and the environment. Together with the Federal Environment Ministry, the German Environment Agency organised the 8th INI Global Conference as a virtual conference. Several hundred scientists and political representatives from all over the world came together in 2021 to exchange the latest knowledge on assessment and solution approaches under the heading "Nitrogen and the UN Sustainable Development Goals". The conference participants' recommendations to policymakers are set out in the Berlin Declaration. In November 2026 the 10th International Nitrogen Conference is going to take place in Kyoto, Japan.
At European level, the Task Force on Reactive Nitrogen is working on integrated approaches to nitrogen reduction and advises the UNECE Geneva Convention on Long-range Transboundary Air Pollution on this matter. The UBA acts as the national representative in this working group. Furthermore, Germany holds the co-chairmanship of the associated ‘Expert Panel on Nitrogen Budgets (EPNB)’. Supported by a UBA research project, the Expert Panel in 2025 updated the Guidance Document on National Nitrogen Budgets and the related detailed technical annexes. A call for data is currently open among the parties to the Geneva Convention on Long-range Transboundary Air Pollution and will run until 2027.
