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Frequently Asked Questions

General questions on Integral Nitrogen

Questions on acceleration of the nitrogen cycle

What is nitrogen?

Nitrogen is a chemical element with the symbol N and atomic number 7. Elemental nitrogen (N2) is a colourless, odourless, tasteless and mostly inert diatomic gas under standard conditions. N2 constitutes 78.1% by volume of the earth's atmosphere. The triple bond in molecular nitrogen (N2) is the strongest in nature. Breaking the triple bond requires a significant amount of energy that can be mustered only in high-temperature processes or by a small number of specialised N-fixing microbes. The resulting difficulty of converting (N2) into other compounds, and the ease (and associated high energy release) of converting nitrogen compounds into elemental N2  (denitrification and the transformation of nitrate to N2  by denitrifying bacteria) have dominated the role of nitrogen in both nature and human economic activities.

Molecular nitrogen in the atmosphere cannot be used directly by plants and animals, but needs to be converted to other compounds, or 'fixed', in order to be used by living organisms. Nitrogen fixation is the transformation of the highly abundant, but biologically unavailable, atmospheric dinitrogen (N2) by breaking the triple bond in N2 to 'reactive' reduced and oxidized N forms such as: 

  • ammonia (NH3); 
  • nitrate (NO3-); 
  • nitrous oxide (N2O);
  • nitric oxide (NO). 

Biological N fixation occurs through specialised bacteria and algae, either free-living or in a symbiotic relationship with higher and especially leguminous, plants. Further natural N fixation occurs during lightning. N fixation also occurs in N fertiliser and energy production.

Once the triple bond of N2 is broken by nitrogen fixation, nitrogen becomes a constituent element of all living tissues and amino acids. Nitrogen makes up a large proportion of animal waste (for example, guano), usually in the form of urea, uric acid and compounds of these nitrogenous products. Many industrially important compounds, such as ammonia, urea, nitric acid and cyanides contain nitrogen.

Why is nitrogen so important?

Nitrogen forms an essential part of amino acids and nucleic acids, both of which are essential to support of all life. Molecular nitrogen in the atmosphere cannot be used directly, whether by plants or animals,  but needs to be converted to other compounds, or ‘fixed’, in order to be used by life. So nitrogen is an essential nutrient for plant growth and therefore very important for food production. In addition to its role in supporting life, many industrially important compounds, such as ammonia, nitric acid, urea and cyanides contain nitrogen.

In the early part of the 19th century, the use of nitrogen from droppings of sea birds (guano) on small islands off the Peruvian [is het niet ‘Ecuadorian’] coast increased, but the guano deposits were rapidly depleted. For this reason, it is understandable that the English chemist, Sir William Crookes, claimed in a widely published lecture in1898 that without a new source of nitrogen fertiliser famine would be inevitable within two to three decades.

Crookes then called on all chemists to find way to fix nitrogen chemically from the unlimited reserves in the air. Several attempts were made, and in 1908 Fritz Haber patented his synthesis of ammonia from the elements, nitrogen and hydrogen. The equipment used by Haber was later scaled up to a pilot plant and further to a commercial plant. By 1930, the amount of Haber-Bosch  nitrogen was already equal to that from all other sources put together. However, the real increase in nitrogen fertiliser production took place between 1960 and 1970. The use of nitrogen fertilisers has played an essential role in the more than doubling of the global food production (and the world population) in the past 50 years. Of course,  industrial nitrogen fixation has also led to large surpluses of nitrogen in agriculture.

What is reactive nitrogen (Nr)?

Reactive N (Nr) includes all biologically, photochemically and radiatively active N compounds in the earth’s atmosphere and biosphere. In other words, Nr includes:

  • inorganic reduced forms of N (e.g. ammonia [NH3] and ammonium [NH4 +]);
  • inorganic oxidized forms (e.g. nitrogen oxide [NOx], nitric acid [HNO3], nitrous oxide [N2O] and nitrate [NO3  –]);
  • organic compounds (e.g. urea, amines, proteins and nucleic acids). 

In the prehuman world Nr was created from N2 primarily through two processes: lightning and biological nitrogen fixation. Reactive N did not accumulate in environmental reservoirs because microbial N fixation and denitrification (the transformation of nitrate to N2 by denitrifying bacteria) processes were approximately equal. 

This is no longer the case. Reactive N now accumulates in the environment on all local and regional scales. During the last few decades, production of Nr by humans has been greater than production from all natural terrestrial systems. The global increase in Nr production has three main causes: 

  1. widespread cultivation of legumes, rice and other crops that promote conversion of N2 to organic N through BNF;
  2. combustion of fossil fuels, which converts both atmospheric N2 and fossil N to reactive NOx;
  3. the Haber-Bosch process, which converts nonreactive N2 to reactive NH3 to sustain food production and some industrial activities.   

How does crop production (and energy crops or biofuels) influence reactive nitrogen (Nr)?

With current fertiliser use efficiency, about half the fertiliser nitrogen used in crop production will end up in the environment as reactive N (nitrate, ammonia, nitrous oxide and nitric oxide). Just like food and feed crops, energy crops (often called biofuels or biofuel crops) require nitrogen and other nutrients for good growth. In modern crop production systems, fertilisers are needed to replenish the nutrient stocks in the soil.

MNP has done several studies on the crop uptake of nitrogen and nitrogen use efficiency. The main purpose of these studies was to inventory differences in nitrogen use efficiency across different agricultural systems, crops and climates. In these studies,  the overall system N recovery and fertiliser use efficiency showed a slow increase in the industrialised countries between 1970 and 1995, while the figures for developing countries decreased in the same period. For the coming three decades results indicate a rapid increase in both the industrialised and developing countries. In many countries there may even be surface N balance deficits, leading to soil degradation through depletion of soil N and loss of soil fertility. The projected intensification for the coming decades in most developing countries will cause a gradual shift from deficits to surpluses. The projected fast growth of crop and livestock production, and intensification and associated increase in fertiliser inputs, will cause a major increase in the surface N balance surplus in the coming three decades. This increase will mean increasing losses of N compounds to air (ammonia, nitrous oxide and nitric oxide), and to groundwater and surface water (nitrate).

More information

Is nitrogen in surplus everywhere?

No, in fact there may be a net depletion of soil nitrogen in many developing and transition countries, as indicated by the global inventory of nitrogen use efficiency presented in the MNP publications below:

Early detailed analyses of soil N depletion were presented in the 1990s:

These papers were succeeded by other more in-depth studies at the regional and country-scale, as summarized by FAO in "Assessment of soil nutrient balance, Approaches and Methodologies" (Fertilizer And Plant Nutrition Bulletin 14, 2003) and "Scaling of soil nutrient balances" (Fertilizer and Plant Nutrition Bulletin 15, 2004).

What are the problems associated with reactive nitrogen (Nr)?

There are significant worrisome consequences from an increase of the release of Nr into the environment. These are listed below. 

  1. Nr is widely dispersed by hydrologic and atmospheric transport processes.
  2. Nr accumulates in the environment because Nr creation rates are higher than Nr removal rates through denitrification to nonreactive N2.
  3. Nr creation and accumulation have been projected to continue to increase in the future as human populations and per capita resource use increase.
  4. Nr accumulation contributes to many contemporary environmental problems. For example:
  • Increases in Nr lead to the production of tropospheric ozone and aerosols that induce serious respiratory illness, cancer and cardiac disease in humans. 
  • Forest and grassland productivity increase and then decrease wherever atmospheric Nr deposition increases significantly and critical thresholds are exceeded
  • Nr additions probably also cause biodiversity to decrease in many natural habitats.
  • Reactive N is responsible (together with S) for acidification and loss of biodiversity in lakes and streams in many regions of the world.
  • Reactive N is one cause of groundwater pollution.
  • Reactive N is responsible for eutrophication, hypoxia, loss of biodiversity, and habitat degradation in coastal ecosystems. It is now considered the biggest pollution problem in coastal waters.
  • Reactive N contributes to global climate change and stratospheric ozone depletion, both of which have impacts on human and ecosystem health.    

MNP has carried out several studies on these environmental problems. These are grouped as follows:

Eutrophication and acidification of terrestrial ecosystems

Effects of nitrogen on terrestrial biodiversity

Eutrophication of coastal ecosystems 

Emissions of nitrogen greenhouse gases