Nitrogen fixation is a process by which nitrogen (N₂) in the atmosphere is converted into ammonium (NH₄₊).^[1] Atmospheric nitrogen or elemental nitrogen (N₂) is relatively inert: it does not easily react with other chemicals to form new compounds. Fixation processes free up the nitrogen atoms from their diatomic form (N₂) to be used in other ways.

Nitrogen fixation, natural and synthetic, is essential for all forms of life because nitrogen is required to biosynthesize basic building blocks of plants, animals and other life forms, e.g., nucleotides for DNA and RNA and amino acids for proteins. Therefore nitrogen fixation is essential for agriculture and the manufacture of fertilizer. It is also an important process in the manufacture of explosives (e.g. gunpowder, dynamite, TNT, etc.) Nitrogen fixation occurs naturally in the air by means of lightning.^{[2][3][dead link]}

Nitrogen fixation also refers to other biological conversions of nitrogen, such as its conversion to nitrogen dioxide. Microorganisms that fix nitrogen are bacteria called diazotrophs. Some higher plants, and some animals (termites), have formed associations (symbioses) with diazotrophs. Biological nitrogen fixation was discovered by the German agronomist Hermann Hellriegel and Dutch microbiologist Martinus Beijerinck.

Biological nitrogen fixation[link]

Schematic representation of the nitrogen cycle. Abiotic nitrogen fixation has been omitted.

Biological nitrogen fixation (BNF) occurs when atmospheric nitrogen is converted to ammonia by an enzyme called nitrogenase.^[1] The reaction for BNF is:

N₂ + 8 H⁺ + 8 e⁻ → 2 NH₃ + H₂

The process is coupled to the hydrolysis of 16 equivalents of ATP and is accompanied by the co-formation of one molecule of H₂. In free-living diazotrophs, the nitrogenase-generated ammonium is assimilated into glutamate through the glutamine synthetase/glutamate synthase pathway.

Enzymes responsible for nitrogenase action are very susceptible to destruction by oxygen. Many bacteria cease production of the enzyme in the presence of oxygen.^[1] Many nitrogen-fixing organisms exist only in anaerobic conditions, respiring to draw down oxygen levels, or binding the oxygen with a protein such as Leghemoglobin.^[1]

Microorganisms that fix nitrogen (diazotrophs)[link]

• Cyanobacteria
• Green sulfur bacteria
• Azotobacteraceae
• Rhizobia
• Frankia

Nitrogen fixation by rhizobia and frankia[link]

Rhizobia are Gram-negative with the ability to establish a N₂-fixing symbiosis on legume roots and on the stems of some aquatic legumes. During this interaction bacteroids, as rhizobia are called in the symbiotic state, are contained in intracellular compartments within a specialized organ, the nodule, where they fix N₂. Similarly, Frankia, Gram-positive soil bacteria induce the formation of nitrogen-fixing nodules in actinorhizal plants.^[4]

Nitrogen fixation by cyanobacteria[link]

Cyanobacteria inhabit nearly all illuminated environments on Earth and play key roles in the carbon and nitrogen cycle of the biosphere. In general, cyanobacteria are able to utilize a variety of inorganic and organic sources of combined nitrogen, like nitrate, nitrite, ammonium, urea, or some amino acids. Several cyanobacterial strains are also capable of diazotrophic growth, an ability which may have been present in their last common ancestor in the Archaean.^[5] Genome sequencing has provided a large amount of information on the genetic basis of nitrogen metabolism and its control in different cyanobacteria. Comparative genomics, together with functional studies, has led to a significant advance in this field over the past years. 2-Oxoglutarate has turned out to be the central signalling molecule reflecting the carbon/nitrogen balance of cyanobacteria. Central players of nitrogen control are the global transcriptional factor NtcA, which controls the expression of many genes involved in nitrogen metabolism, as well as the P_II signalling protein, which fine-tunes cellular activities in response to changing C/N conditions. These two proteins are sensors of the cellular 2-oxoglutarate level and have been conserved in all cyanobacteria. In contrast, the adaptation to nitrogen starvation involves heterogeneous responses in different strains.^[6] Nitrogen fixation by cyanobacteria in coral reefs can fix twice the amount of nitrogen than on land–around 1.8 kg of nitrogen is fixed per hectare per day.

Root nodule symbioses[link]

Legume family[link]

Plants that contribute to nitrogen fixation include the legume family – Fabaceae – with taxa such as clovers, soybeans, alfalfa, lupines, peanuts, and rooibos. They contain symbiotic bacteria called Rhizobia within nodules in their root systems, producing nitrogen compounds that help the plant to grow and compete with other plants. When the plant dies, the fixed nitrogen is released, making it available to other plants and this helps to fertilize the soil^[1][7] The great majority of legumes have this association, but a few genera (e.g., Styphnolobium) do not. In many traditional and organic farming practices, fields are rotated through various types of crops, which usually includes one consisting mainly or entirely of clover or buckwheat (non-legume family Polygonaceae), which are often referred to as "green manure."

Inga alley farming relies on the leguminous genus Inga, a small tropical, tough-leaved, nitrogen-fixing tree.^[8]

Non-leguminous[link]

A sectioned alder tree root nodule.

A whole alder tree root nodule.

Although by far the majority of plants able to form nitrogen-fixing root nodules are in the legume family Fabaceae, there are a few exceptions:

• Parasponia, a tropical Celtidaceae also able to interact with rhizobia and form nitrogen-fixing nodules^[9]
• Actinorhizal plants such as alder and bayberry, can also form nitrogen-fixing nodules, thanks to a symbiotic association with Frankia bacteria. These plants belong to 25 genera^[10] distributed among 8 plant families. The ability to fix nitrogen is far from universally present in these families. For instance, of 122 genera in the Rosaceae, only 4 genera are capable of fixing nitrogen. All these families belong to the orders Cucurbitales, Fagales, and Rosales, which together with the Fabales form a clade of eurosids. In this clade, Fabales were the first lineage to branch off; thus, the ability to fix nitrogen may be plesiomorphic and subsequently lost in most descendants of the original nitrogen-fixing plant; however, it may be that the basic genetic and physiological requirements were present in an incipient state in the last common ancestors of all these plants, but only evolved to full function in some of them:

There are also several nitrogen-fixing symbiotic associations that involve cyanobacteria (such as Nostoc):

• Some lichens such as Lobaria and Peltigera
• Mosquito fern (Azolla species)
• Cycads
• Gunnera

Industrial nitrogen fixation[link]

Haber process[link]

Artificial fertilizer production is now the largest source of human-produced fixed nitrogen in the Earth's ecosystem. Ammonia is a required precursor to fertilizers, explosives, and other products. The most common method is the Haber process. The Haber process requires high pressures (around 200 atm) and high temperatures (at least 400 °C), routine conditions for industrial catalysis. This highly efficient process uses natural gas as a hydrogen source and air as a nitrogen source.^[11]

Much research has been conducted on the discovery of catalysts for nitrogen fixation, often with the goal of reducing the energy required for this conversion. However, such research has thus far failed to even approach the efficiency and ease of the Haber process. Many compounds react with atmospheric nitrogen to give dinitrogen complexes. The first dinitrogen complex to be reported was based on ruthenium,[Ru(NH₃)₅(N₂)]²⁺.^[12]

Ambient nitrogen reduction[link]

Catalytic chemical nitrogen fixation at temperatures considerably lower than the Haber process is an ongoing scientific endeavor. Nitrogen was converted to ammonia and hydrazine by Alexander E. Shilov in 1970.^[13][14]

Few compounds will cleave the N₂ molecule. Under an atmosphere of nitrogen, lithium metal converts to lithium nitride. Treatment of the resulting nitride gives ammonia. Another example of homolytic cleavage of dinitrogen under mild conditions was published in 1995. Two equivalents of a molybdenum complex reacted with one equivalent of dinitrogen, creating a triple bonded MoN complex.^[15] Since then, this triple bonded complex has been used to make nitriles.^[16]

Catalytic systems for converting nitrogen to ammonia have been developed since the 1980s.^[17] In 2003 another was reported based on molybdenum compound, a proton source, and a strong reducing agent.^{[18][19][20][21]} However, this catalytic reduction fixates only a few nitrogen molecules.

In 2011 Arashiba et al. reported yet another system with a catalyst again based on molybdenum but with a diphosphorus pincer ligand.^[22]

See also[link]

• Birkeland–Eyde process
• Ostwald process
• Haber process
• Denitrification
• George Washington Carver
• Nif gene
• Nitrification
• Nitrogen cycle
• Nitrogen deficiency
• Nitrogenase
• Push–pull technology

References[link]

External links[link]

• A Brief History of the Discovery of Nitrogen-fixing Organisms, Ann M. Hirsch (2009)
• Marine Nitrogen Fixation laboratory at the University of Southern California