The nitrogen cycle

Nitrogen does not occur in a substantial amount in a geographical deposit. The only natural accumulations of nitrate which take place on some islands off the Chilean coast, and these are derived from the guano deposited by seabirds. The dry climate of these habitats has prevented the possibility of leaching of nitrate. The organic salts of nitrogen are highly water soluble and are consequently distributed in dilute aqueous solution throughout the ecosphere. These organic salts form a small, very actively cycled reservoir and availability of combined nitrogen is a prime limiting factor for primary production in many ecosystems. Living and dead organic matters are also relatively small, actively cycled reservoirs of nitrogen. At least in the region of temperate climates, stabilized soil organic matter forms a substantial relatively stable reservoir of nitrogen. The nitrogen of humus becomes available for uptake by living organisms only through its slow way of mineralization, a process measured in decades and centuries. In tropical climates, the temperature and humidity conditions favor the rapid direct mineralization of organic matter and limit the accumulation of litter humus.

A very large, slowly cycled reservoir for nitrogen is N₂ gas of the atmosphere (79%). This nitrogen reservoir is accessible only to micro organism or symbiotic associations with specific nitrogen fixing ability. Nitrogen fixation is the process that requires a substantial energy input. Small amounts of combined nitrogen are introduced into the ecosphere by volcanic activity, by atmospheric lighting discharges and ionizing radiation, the combined nitrogen generated in this manner is introduced into the ecosphere by means of precipitation. Chemical fixation of molecule nitrogen by man, using the Haber Bosch synthesis developed shortly, before the First World War, is assuming an increasing important role in the global nitrogen cycle.

The biochemical cycling of nitrogen is highly depended on the activities of micro organisms. Nitrogen is one of the essential components of protein, nucleic acids and other cell biochemical’s. The vast supply of molecular nitrogen is inaccessible to most biological systems. Plants, animal and most of the micro – organisms required combined forms of nitrogen for incorporation into cellular biomass. The ability to fix atmospheric nitrogen is restricted to a limited number of bacteria. While many habitats depend on plant for the supply of organic carbon that can be used as source of energy, all habitats are depended on the bacteria fixation of atmospheric nitrogen or on the intervention of man through the synthetic production of nitrogen fertilizer, for providing a supply of fixed nitrogen. Plants could not continue their photosynthesis metabolism without the availability of fixed forms of nitrogen providing by micro – organism or by synthetic fertilizer. Micro – organisms are also responsible for the return of molecular nitrogen to the atmosphere through the means of dentrification and for the transformations that affect the mobility and accessibility of fixed nitrogen to the inhabitants of the litho and hydro ecospheres.