Nitrogen Budgets

Abstract

The compilation of inputs and outputs of nitrogen across the boundaries of a system defined in time and space comprises a nitrogen budget. This budget contains information about internal nitrogen fluxes within the system and can be used as an efficiency indicator or policy instrument. Nitrogen cycling at global and ecosystem scale is described as well as transformations of nitrogen between different chemical forms occurring in nature. Chemical and biological processes governing the distribution and transformation of nitrogen within ecosystems are presented including ammonification, nitrification, denitrification and plant uptake. Examples for agricultural systems are shown for illustrating how different nitrogen fluxes can be selected for calculating nitrogen budgets depending on definitions of system boundaries. Environmental consequences due to human interactions with the nitrogen cycle are also presented and discussed.

Key Concepts:

  • The compilation of inputs and outputs of nitrogen across the boundaries of a system defined in time and space comprises a nitrogen budget.

  • Nitrogen is an essential element of all living matter and, in addition to carbon, the main building block of many macromolecules, such as proteins and nucleic acids.

  • Although nitrogen makes up about 78% of the air we breathe it must be transformed into reactive chemical forms for being available for vascular plants.

  • The amount of nitrogen cycled within ecosystems is about one order of magnitude larger than the amounts crossing ecosystem boundaries.

  • Generally, more than 90% of the nitrogen needed for primary production is recycled through within ecosystems.

  • Production of mineral fertilisers is essential for feeding a growing human population.

  • Human impacts on nitrogen cycling are significant.

  • Environmental concerns due to human interactions with the nitrogen cycle are mainly related to the presence of nitrate in water and emissions of nitrogen oxides contributing to stratospheric ozone depletion and to the greenhouse effect.

  • Farmgate nitrogen balances, defined as the difference between nutrient input and output at farm level, are increasingly used as environmental performance indicators or as policy support tools for emission control.

Keywords: cycling; fluxes; nitrogen; processes; transformations

Figure 1.

Atmospheric N2 is made available to ecosystems through biological nitrogen fixation and lightning. The organically bound nitrogen (N‐org) in organisms is released as NH4+ during decomposition of organic material (ammonification or mineralisation). Ammonium can be fixed in clay minerals or exchangeably adsorbed to mineral particles and organic colloids. In most soils or sediments, ammonium is oxidised to nitrate under aerobic conditions (nitrification). Both ammonium and nitrate can be assimilated by plants and microorganisms (immobilisation) to support the build‐up of new biomass. If nitrate is transported to anaerobic zones, it can be reduced to N2 or N2O by denitrifying microorganisms (denitrification). The release of N2 to the atmosphere closes the natural nitrogen cycle. However, the natural nitrogen fluxes are increased by human impact through the production of nitrogenous fertilisers (industrial nitrogen fixation), by ammonia emissions from the excreta of domestic animals (ammonia volatilisation) and the use of combustion engines producing NOx that is deposited on ecosystems (acidifying deposition).

Figure 2.

Conceptual model of the cycling of nitrogen within an agricultural ecosystem.

Figure 3.

Principles for nitrogen release from crop residues with different C/N ratios.

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References

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Further Reading

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Kätterer, Thomas(Oct 2011) Nitrogen Budgets. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0003189.pub2]