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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.
