Reactive nitrogen

Forms of reactive nitrogen

Reactive nitrogen, unlike nonreactive molecular nitrogen (N₂), is in a great many organic and inorganic substance compounds and 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 (NO_x ),
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 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 1,547 kt N a^-1. 67 % of this is released by agriculture, 11 % from the transport sector, 16 % from industrial and energy sector processes and 6 % from households, wastewater management and surface runoff. The majority is released into the air as ammonia, followed by nitrate input into surface waters, nitrogen oxide emissions and the release of nitrous oxide into the atmosphere.

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 has been working on this for many years and is supporting the Federal Ministry for the Environment in developing a nitrogen strategy and an action programme for nitrogen reduction. 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 guideline and limit values for the protection of water bodies from nitrate inputs and for the protection of human health from the risks posed by NO₂, particulate matter and ozone 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 currently also working on an integrated solution for national nitrogen reduction. The German Environment Agency is supporting this process continuously with research projects.

A national nitrogen target

In order to visualise and verify the extent of the necessary reduction requirements for the various nitrogen compounds at a glance, the German Environment Agency has derived an impact-oriented, integrated nitrogen target for Germany as part of a research project. The integrated target 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.

In order to better protect people and the environment, the release of nitrogen should be limited to 1,000 kt N a^-1 (or 1 million tonnes) per year for Germany as a first step from 2030. Compared to the current release, the total annual release of reactive nitrogen would have to be reduced by around a third. With a population of 83 million, this means limiting the release of nitrogen to around 12 kg N per person per year. Current forecasts suggest that the upper limit could be reached if the current rules on air pollution control, climate protection and fertiliser legislation are complied with in practice in all areas by 2030.

However, this would only be the first stage, as further reductions will be required in the long term in order to achieve a good environmental status for all nitrogen-polluted areas in Germany. Nevertheless, the instrument sends out an important signal for co-operation and action across different areas of politics and society.

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. This is a global cross-sectoral problem of environmental protection.

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.

A particularly promising example for global nitrogen management is a resolution of the United Nations Environment Assembly on "Sustainable Nitrogen Management". Building on the resolution, a co-operation between the various UN conventions relating to nitrogen called Interconvention Nitrogen Coordination Mechanism (INCOM) is currently being developed. Participants in this process include the Convention on Biological Diversity (CBD), the Convention on Transboundary Air Pollution (CLRTAP), the United Nations Framework Convention on Climate Change (UNFCCC), the Food and Agriculture Organisation (FAO), the World Health Organisation (WHO) and the United Nations Environment Programme (UNEP).

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 8^th 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.