The nitrogen cycle is a biogeochemical process that describes how nitrogen moves through the Earth’s atmosphere, land, and living organisms. It involves a series of transformations essential for life, as nitrogen is a core component of biomolecules like amino acids (the building blocks of proteins) and nucleic acids (DNA and RNA). Although nitrogen makes up approximately 78% of the Earth’s atmosphere as dinitrogen gas (N₂), most organisms cannot directly use it in this form. This is because atmospheric nitrogen is highly stable due to a strong triple bond. The cycle converts this unusable atmospheric nitrogen into various reactive forms that can be utilized by different life forms.
Nitrogen Fixation
Nitrogen fixation is the initial step in the cycle, converting atmospheric nitrogen gas (N₂) into more reactive forms, primarily ammonia (NH₃) or ammonium (NH₄⁺). This transformation makes nitrogen available for biological systems. The process is predominantly carried out by specific microorganisms, known as diazotrophs, which possess the enzyme nitrogenase.
These nitrogen-fixing bacteria can be free-living in the soil, such as Azotobacter and certain cyanobacteria, or they can form symbiotic relationships with plants. A prominent example involves Rhizobium bacteria, which reside in root nodules on leguminous plants like peas, beans, and clover. Within these nodules, the bacteria convert N₂ into ammonia, which the host plant absorbs and utilizes for its growth. This symbiotic relationship is mutually beneficial, with the plant providing carbohydrates and the bacteria supplying usable nitrogen compounds.
Nitrification
Nitrification is a two-step microbial process where ammonia (NH₃) or ammonium (NH₄⁺) is oxidized into nitrites (NO₂⁻) and then further into nitrates (NO₃⁻). This process occurs in aerobic, oxygen-rich environments within the soil and water. The first step involves ammonia-oxidizing bacteria (AOB), such as Nitrosomonas, which convert ammonium to nitrite.
Following this, nitrite-oxidizing bacteria (NOB), such as Nitrobacter and Nitrospira, convert the nitrites into nitrates. Nitrates are the most common and easily absorbed form of nitrogen for most plants, allowing them to take up this nutrient from the soil through their roots.
Assimilation
Assimilation is the process by which plants absorb usable forms of nitrogen from their environment, primarily nitrates (NO₃⁻) and, to a lesser extent, ammonium (NH₄⁺). Once absorbed, this nitrogen is incorporated into organic molecules within the plant’s cells.
This incorporation involves the synthesis of biological compounds such as amino acids, which are the building blocks of proteins. Nitrogen is also used to form nucleic acids like DNA and RNA, as well as chlorophyll, essential for photosynthesis. When animals consume plants, or other animals consume plant-eating animals, this assimilated nitrogen moves up the food chain, becoming part of their own tissues and biomolecules.
Ammonification
Ammonification is the process where organic nitrogen from dead organisms and waste products is converted back into inorganic forms, specifically ammonia (NH₃) or ammonium ions (NH₄⁺). This decomposition process is carried out by decomposers, primarily bacteria and fungi in the soil and water. They break down complex nitrogenous compounds found in decaying plant and animal matter, as well as animal waste products like urea.
This conversion releases ammonia, which quickly reacts with water in the soil to form ammonium. Ammonification recycles nitrogen within ecosystems. It ensures that nitrogen, which would otherwise be locked up in organic forms, is returned to the soil, making it available for subsequent steps in the nitrogen cycle, such as nitrification, or for direct uptake by plants.
Denitrification
Denitrification is the final step in the nitrogen cycle, returning nitrogen gas (N₂) to the atmosphere, thus completing the cycle. This process involves specific types of denitrifying bacteria, such as Pseudomonas, Thiobacillus denitrificans, and Micrococcus denitrificans. These bacteria convert nitrates (NO₃⁻) and nitrites (NO₂⁻) back into gaseous nitrogen compounds, including nitrous oxide (N₂O) and molecular nitrogen (N₂).
Denitrification occurs in anaerobic environments, meaning areas with low or no oxygen. Such conditions can be found in waterlogged soils, wetlands, or deeper layers of aquatic sediments where oxygen is depleted. The bacteria use nitrates as an alternative electron acceptor in their respiration processes when oxygen is unavailable. This microbial activity helps to balance the amount of fixed nitrogen in ecosystems and replenish the atmospheric nitrogen reservoir.