10.16
Nitrogen changes between different chemical forms and oxidation states through fixation, ammonification, nitrification, denitrification, and anammox.
The nitrogen cycle begins with nitrogen gas, which most organisms cannot use directly.
Nitrogen fixation by free-living bacteria, like Azotobacter, converts nitrogen gas into ammonia, which enters the soil.
Symbiotic bacteria, like Rhizobium, fix nitrogen into ammonia in the root nodules of legumes.
During ammonification, decomposers break down amino acids and nucleotides from dead organisms and waste, releasing nitrogen as ammonium ions.
In aerobic zones, nitrification occurs in two steps. First, ammonia-oxidizers like Nitrosomonas oxidize ammonia to nitrite. Then, Nitrobacter converts nitrite to nitrate.
In low-oxygen zones, denitrification by Pseudomonas species converts nitrate into nitrogen gas.
Whereas in anoxic zones, the anammox process by the family Brocadiaceae combines ammonia and nitrite to form nitrogen gas.
This gas then returns to the atmosphere.
The nitrogen cycle is a complex biogeochemical process critical to maintaining the balance of nitrogenous compounds in ecosystems. This cycle involves multiple microbial-mediated transformations through which nitrogen changes oxidation states, supporting essential ecological functions and contributing to plant and microbial growth.
Nitrogen Fixation and Ammonification
Nitrogen fixation initiates the cycle by converting inert atmospheric nitrogen (N₂) into bioavailable ammonia (NH₃), a process mediated by nitrogenase enzymes. This transformation is carried out by both free-living and symbiotic diazotrophs. Free-living aerobic bacteria such as Azotobacter spp. and anaerobic bacteria like Clostridium spp. independently fix nitrogen under respective environmental conditions. On the other hand, symbiotic bacteria, notably Rhizobium spp., form mutualistic associations with leguminous plants, localizing nitrogen fixation within specialized root nodules.
Ammonification follows, releasing ammonia from organic nitrogen during the microbial decomposition of dead biomass and waste. This step recycles nitrogen back into a form suitable for plant uptake or further microbial processing.
Nitrification and Comammox
Under aerobic soil conditions, nitrification oxidizes ammonia to nitrate (NO₃⁻) via a two-step bacterial pathway. Nitrosomonas spp. first convert ammonia into nitrite (NO₂⁻), followed by Nitrobacter spp., which oxidize nitrite to nitrate. Recently, complete ammonia oxidizers (comammox), such as Nitrospira spp., have been identified as capable of performing both steps autonomously, thereby simplifying the nitrification pathway and enhancing nitrogen turnover in various environments.
Denitrification and Anammox
To prevent excessive nitrate accumulation, denitrification processes reduce nitrate back to nitrogen gas (N₂), primarily under oxygen-limited conditions. Facultative anaerobes such as Pseudomonas spp. facilitate this reduction, closing the nitrogen loop. Additionally, under strictly anoxic environments, bacteria within the Brocadiaceae family perform anaerobic ammonium oxidation (anammox), coupling nitrite and ammonia to directly yield nitrogen gas. This reaction is energetically efficient and vital in nitrogen removal from aquatic ecosystems.
Nitrogen changes between different chemical forms and oxidation states through fixation, ammonification, nitrification, denitrification, and anammox.
The nitrogen cycle begins with nitrogen gas, which most organisms cannot use directly.
Nitrogen fixation by free-living bacteria, like Azotobacter, converts nitrogen gas into ammonia, which enters the soil.
Symbiotic bacteria, like Rhizobium, fix nitrogen into ammonia in the root nodules of legumes.
During ammonification, decomposers break down amino acids and nucleotides from dead organisms and waste, releasing nitrogen as ammonium ions.
In aerobic zones, nitrification occurs in two steps. First, ammonia-oxidizers like Nitrosomonas oxidize ammonia to nitrite. Then, Nitrobacter converts nitrite to nitrate.
In low-oxygen zones, denitrification by Pseudomonas species converts nitrate into nitrogen gas.
Whereas in anoxic zones, the anammox process by the family Brocadiaceae combines ammonia and nitrite to form nitrogen gas.
This gas then returns to the atmosphere.
From Chapter 10:
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