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The nitrogen-fixing clade consists of four orders of flowering plants: Cucurbitales, Fabales, Fagales and Rosales. [a] This subgroup of the rosids encompasses 28 families of trees, shrubs, vines and herbaceous perennials and annuals. The roots of many of the species host bacteria that fix nitrogen into compounds the plants can use. [4] [5]
Nitrifying bacteria are chemolithotrophic organisms that include species of genera such as Nitrosomonas, Nitrosococcus, Nitrobacter, Nitrospina, Nitrospira and Nitrococcus. These bacteria get their energy from the oxidation of inorganic nitrogen compounds. [1] Types include ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB).
Some nitrogen-fixing bacteria have symbiotic relationships with plants, especially legumes, mosses and aquatic ferns such as Azolla. [4] Looser non-symbiotic relationships between diazotrophs and plants are often referred to as associative, as seen in nitrogen fixation on rice roots. Nitrogen fixation occurs between some termites and fungi. [5]
Plants that contribute to nitrogen fixation include the legume family – Fabaceae – with taxa such as clover, soybeans, alfalfa, lupins, 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.
Sulphate-reducing bacteria are important in ocean sediments (e.g. Desulfovibrio), and some Archean methanogens, like Methanococcus, fix nitrogen in muds, animal intestines [3] and anoxic soils. [4] Facultative anaerobes—these species can grow either with or without oxygen, but they only fix nitrogen anaerobically.
Rhizobium: Symbiotic nitrogen fixation by Rhizobium with legumes contributes substantially to total nitrogen fixation. Rhizobium inoculation is a well-known agronomic practice to ensure adequate nitrogen. [5] [6] One of the most widespread species is R. leguminosarum. Bradyrhizobium spp. (in particular Bradyrhizobium japonicum). [7]
Rhizobium is a genus of Gram-negative soil bacteria that fix nitrogen. Rhizobium species form an endosymbiotic nitrogen-fixing association with roots of (primarily) legumes and other flowering plants. The bacteria colonize plant cells to form root nodules, where they convert atmospheric nitrogen into ammonia using the enzyme nitrogenase.
The rhizobia, which fix nitrogen and are symbiotic with plant roots, appear in several different families. The four families Nitrobacteraceae, Hyphomicrobiaceae, Phyllobacteriaceae, and Rhizobiaceae contain at least several genera of nitrogen-fixing, legume-nodulating, microsymbiotic bacteria. Examples are the genera Bradyrhizobium and Rhizobium.