The Nitrogen cycle
[ammonification, nitrogen assimilation, denitrification, nitrification, nitrogen fixing](a) The nitrogen cycle, like the carbon cycle, involves a gaseous form, i.e., N2 or nitrogen
gas.
(b) Nitrogen gas may be removed from the atmosphere, particularly by bacteria, in a
process called nitrogen fixing [which is relatively expensive since nitrogen gas is
quite stable].
(c) Nitrogen gas may be returned to the atmosphere, again particularly by bacteria, in a process called denitrification (a form of anaerobic respiration)
(d) More typically, bioavailable nitrogen is found as ammonium ion (NH4+), nitrate ion
(NO32-), and various organic, nitrogen-containing compounds (e.g., amino acids and nucleic acids).
(e) Nitrate and ammonium ion are converted back and forth between each other (and nitrite, NO2 2-), also by various bacteria via processes termed nitrification and ammonification.
(f) The nitrogen cycle thus involves
(i) Nitrogen fixing, the fixing of nitrogen from the atmosphere [typically by free-living or plant-associated nitrogen-fixing bacteria]
(ii) Assimilation, the uptake of ammonium ion and nitrate ion from soil by plants
and the uptake of organic nitrogen by animals from plants (amino acids, nucleic
acids).
(iii) Ammonification, the conversion of organic nitrogen back to ammonium ion
by decom posers (nitrogenous waste) (4NH4
+); “The decomposition of organic nitrogen back to ammonium, a process called ammonification, is carried out mainly by bacterial and fungal decomposers.”
(iv) Nitrification, the various conversions of nitrogen within the soil from ammonium
ion (NH4+ 4NO2 2– 4NO3 2– ; note that this represents an oxidation of nitrogen);
“Although plants can use ammonium directly, most of the ammonium in soil is
used by certain aerobic bacteria as an energy source; their activity oxidizes
ammonium to nitrite (NO2 2–) and then to nitrate (NO3 2–).”
(v) Denitrification, also by soil bacteria (4N2
; note that this process involves the
reduction of nitrogen); “Some bacteria can obtain the oxygen they need for metabolism from nitrate (NO3 2–) rather than from O2 under anaerobic conditions.”
(g) [A portion of the nitrogen cycle as it occurs within fish tanks (here nitrification is a good thing and ammonification, the production and subsequent build up of ammonium ion, is a bad thing): The nitrogen cycle has some important practical considerations, as anyone who has ever set up a saltwater fish tank has found out. It takes several weeks to set up such a tank, because you must have sufficient numbers of nitrite and nitrate bacteria present to detoxify the ammonia produced by the fish and decomposers in the tank.
Otherwise, the ammonia levels in the tank will build up and kill the fish.
This is usually not a problem in freshwater tanks for two reasons. One, the pH in a
freshwater tank is at a different level than in a saltwater tank. At the pH of a freshwater tank, ammonia is not as toxic.
Second, there are more multicellular plant forms that can grow in freshwater, and these plants remove the ammonia from the water very efficiently. It is hard to get enough plants growing in a saltwater tank to detoxify the water in the same way.
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