Introduction
The fixation of atmospheric nitrogen is one of the great discoveries, awaiting the genius of chemists.Sir William Crookes Presidential Address to the British Association for the Advancement of Science, 1898
Of course, plants discovered how to fix nitrogen (convert N 2 gas to a form that can be incorporated in biological molecules such as nucleic acids and proteins) a long time ago. Or rather, they recruited nitrogen-fixing bacteria into a mutualistic relationship, where the bacteria fix nitrogen and provide nitrogen-containing compounds to the plant in exchange for food and shelter.
Plants may also enlist the help of other microbial partners in nutrient acquisition. Particular species of bacteria and fungi have evolved along with certain plants to create a mutualistic symbiotic relationship with roots. This improves the nutrition of both the plant and the microbe. The formation of bacteria-containing nodules on the roots for nitrogen fixation, or the association of mycorrhizal fungi with roots for enhanced nutrient and water uptake, can be considered among the nutritional adaptations of plants. However, these are not the only type of adaptations that we may find; many plants have other adaptations that allow them to thrive under specific conditions.
Nitrogen fixation: root and bacteria interactions
Nitrogen is an important macronutrient because it is part of nucleic acids and proteins. Atmospheric nitrogen, which is the diatomic molecule N 2, or dinitrogen, is the largest pool of nitrogen in terrestrial ecosystems. However, plants cannot take advantage of this nitrogen because they do not have the necessary enzymes to convert it into biologically useful forms. However, nitrogen can be “fixed,” which means that it can be converted to ammonia (NH 3 ) through biological, physical, or chemical processes. As you have learned, biological nitrogen fixation is the conversion of atmospheric nitrogen (N 2 ) into ammonia (NH 3 ), exclusively carried out by prokaryotes such as soil bacteria or cyanobacteria. Biological processes contribute 65 percent of the nitrogen used in agriculture.
Nitrogen-fixing soil bacteria, collectively called rhizobia , symbiotically interact with legume roots to form specialized structures called nodules , in which nitrogen fixation takes place. This process entails the reduction of atmospheric nitrogen to ammonia, by means of the enzyme nitrogenase. Therefore, using rhizobia is a natural and environmentally friendly way to fertilize plants, as opposed to chemical fertilization that uses a nonrenewable resource, such as natural gas. Through symbiotic nitrogen fixation, the plant benefits from using an endless source of nitrogen from the atmosphere. The process simultaneously contributes to soil fertility because the plant root system leaves behind some of the biologically available nitrogen. As in any symbiosis, both organisms benefit from the interaction: the plant obtains ammonia, and bacteria obtain carbon compounds generated through photosynthesis, as well as a protected niche in which to grow ( [link] ).