Ions and molecules of nutrients are continuously transferred from the physical environment to organisms through circular paths: the Biogeochemical Cycles. These include nitrogen
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Only 12% of nitrogen compounds come from fixation, all the rest is provided by decomposition processes.
The nitrogen cycle starts with the fixation of nitrogen and the release of NH4 + which can be incorporated into organic matter, by assimilation or nitrified to NO2− and NO3−.
Both NH4 +, NO2− and NO3− can be absorbed by the producers (assimilation), thus becoming part of the organic matter and through decomposition can be released with production of NH4 + (ammonification).
On the contrary, in anaerobic conditions, the return of nitrogen to the atmosphere takes place through the denitrification process, i.e. NO3 – is reduced to N2.
Denitrification is also called dissimilative reduction as it contrasts with the reductive assimilation of nitrates by plants.
Nitrogen cycle in terrestrial and aquatic environments: nitrates in the groundwater
In the terrestrial environment, mineral nitrogen (NH4+ and NO3-) is present in small quantities either because it is quickly absorbed or because it is attacked by nitrifying agents, denitrificants etc. or because it is washed away by water and leached into the aquifers. In the aquatic environment, more mineral nitrogen is present, and the cycle is slower.
Nitrogen-fixing agents obtain the energy required for fixation from photosynthesis (autotrophs), dead organic matter (heterotrophs) or organic compounds provided by the plant with which they achieve symbiosis.
In the euphotic zone, where there is photosynthetic activity due to the presence of light, decomposition and fixation prevail, therefore the phenomena of mineralization. In intermediate and deep waters where oxygen is scarce, mineralization is accompanied by denitrification.
In the bottom sediments, accumulation is added to the mineralization and denitrification, thanks to which part of the nitrogen is blocked in the lithosphere compartment.
The plant doesn’t absorb NH4+, NO2− and NO3− in the same way, preferentially absorbs NO2− e NO3−. In fact, NH4+ even at relatively low concentrations, can be toxic. It is much easier for the plant to find NO2− and NO3− as more soluble at physiological pH, while NH4+ is abundant only in asphyxiated and acidic soils and being a cation binds with negative charge particles of the soil.
In addition, NH4+ must compete more to be absorbed because most of the compounds that the plant absorbs are cations. The plant, however, from an energy standpoint, should absorb nitrogen as reduced NH4+ because once absorbed NO2− and NO3− must be reduced to be assimilated. Nitrogen absorbed by producers is transferred to consumers by feeding.
Nitrogen is often a limiting factor, so plants (particularly conifers) retain nitrogen compounds in their biomass by the process of translocation and immobilization.
Dead organic matter, mostly composed of -NH2 , is degraded by ammonification, performed by bacterial proteases.