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Nutrient deposition is of particular issue to legumes as the increased availability of nitrogen allows nitrogen-limited plant species to quickly out compete legumes for light. This increases photosynthesis costs, further destabilizing the legume-rhizobia mutualism as the legume suffers fitness consequences and is unable to provide benefits to ...
Nitrogen is the most commonly limiting nutrient in plants. Legumes use nitrogen fixing bacteria, specifically symbiotic rhizobia bacteria, within their root nodules to counter the limitation. Rhizobia bacteria convert nitrogen gas (N 2) to ammonia (NH 3) in a process called nitrogen fixation.
Some plants, called legumes, can form simultaneous symbiotic relationships with both AM fungi and the nitrogen-fixing bacteria Rhizobia. In fact, both organisms trigger the same pathways in plants during early colonization, indicating that the two very different responses could share a common origin.
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]
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.
Plants that contribute to nitrogen fixation include those of the legume family—Fabaceae— with taxa such as kudzu, clover, soybean, alfalfa, lupin, peanut and rooibos. [45] They contain symbiotic rhizobia bacteria within nodules in their root systems, producing nitrogen compounds that help the plant to grow and compete with other plants. [64]
As with most legumes, snow peas host beneficial bacteria, rhizobia, in their root nodules, which fix nitrogen in the soil—this is called a mutualistic relationship—and are therefore a useful companion plant, especially useful to grow intercropped with green, leafy vegetables that benefit from high nitrogen content in their soil.
Soybeans, like most legumes, perform nitrogen fixation by establishing a symbiotic relationship with the bacterium Bradyrhizobium japonicum (syn. Rhizobium japonicum; Jordan 1982). This ability to fix nitrogen allows farmers to reduce nitrogen fertilizer use and increase yields when growing other crops in rotation with soy. [26]