Agriculture Reference
In-Depth Information
3.1.2.2 Nitrogen
Nitrogen is an element of major biological importance in all terrestrial and aquatic
ecosystems. It is the dominant element in the atmosphere (Table I.9) where it occurs
principally as dinitrogen gas and in low concentrations as the various gaseous
oxides and as ammonia. Large reservoirs of these gases also occur in dissolved form
(and in suspension)‚ often at saturation concentrations‚ in the oceans and other water
bodies and in living organisms. Substantial amounts of nitrogen also occur in
the lithosphere‚ in coal‚ shales and in liquid and gaseous hydrocarbons. Table I.17
contrasts the estimated sizes of the world's major pools of carbon and nitrogen.
The overall carbon: nitrogen ratios in the vegetation biomass‚ the top 100 cm of the soil
and the microbial biomass are thus 55‚ 8-13 and 5‚ respectively.
As part of the global nitrogen cycle‚ constant exchanges occur between
the atmosphere‚ the biosphere and the oceans and other water bodies. Ecosystem level
cycles occur in all terrestrial environments and internal cycles also occur in such
large water bodies as the oceans (Schlesinger‚ 1997).
Substantial modification to the global nitrogen cycle has occurred in recent years
through mans' activities. These include a greater than doubling of the rate of nitrogen
transfer from the atmosphere to biologically-available terrestrial pools (Vitousek et al .‚
1997) and a decoupling of the close linkages between the carbon and nitrogen cycles
(Asner et al .‚ 1997). Very high rates of nitrogen deposition in certain North American
and European environments (Holland and Lamarque‚ 1997) have contributed to soil
acidification‚ nutrient element impoverishment‚ ecosystem degradation and damage to
downstream environments (Vitousek et al .‚ 1997).
Nitrogen is critical to all forms of life. It is an essential component of such basic
biological building blocks as amino acids and thus their polymers: peptides and proteins.
In the micro-organisms‚ nitrogen-containing materials such as chitin (polymers of amino
sugars) are major cell wall constituents of the fungi and the bacteria. This polymer is also
a major component of the endo- and exo-skeleton of the arthropods. Nitrogen is a com-
ponent of the nucleic acids DNA and RNA and is thus of importance in reproduction and
evolution. As an important constituent of many biologically-active compounds‚ nitrogen
plays a part in the integration of life at levels from the cell to the population. There is no
major category of biological function in which nitrogen
does not play
a major part.
The productivities of micro-organisms‚ higher plants and animals are commonly
limited by low nitrogen concentrations (Vitousek and Howarth‚ 1991). Nitrogen is
found in soil mainly within the organic matter fraction where it occurs largely in humic
compounds but also in plant roots‚ the microbial biomass and in decomposing organic
materials. While low concentrations of inorganic nitrogen (nitrates‚ ammonium) are
found in most soils‚ their turnover rates are usually high and seasonally-related changes
follow from variation in environmental conditions and thus micro-organism activity.
A variety of other nitrogenous compounds are present in soils and include dinitrogen gas‚
the gaseous oxides of nitrogen‚ ammonia and other excretory products.
The nitrogen concentrations of higher plants range from less than 0.10 % of dry
weight in woody tissues to more than 4 % in actively-growing tissues (Table I.15) and
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