Topics : Successful Forage Establishment : Inoculation of Legumes for Maximum Nitrogen Fixation
Inoculation of Legumes for Maximum Nitrogen Fixation
Additional inoculation information via the Forage Information System (FIS)
The Symbiotic Nitrogen Fixation Process
Air is almost 80 percent nitrogen (N). Although air-supplied nitrogen is the primary source of N for the fertilizer industry, as well as the source used by legumes, it does not come free. Nitrogen in air exists as two N atoms that are tripled-bonded together; in effect, the atoms are glued, stapled, and taped together, not to be undone without a great expenditure of energy.
In the industrial (Haber) process, petroleum energy is used to break the triple bond, and three hydrogen ions from natural gas or another petroleum product are added to each N atom. Therefore, N fertilizer prices increase as energy prices rise.
In symbiotic N fixation, as is the case with legumes, these same steps are required of the Rhizobia bacteria. Sugars or carbohydrates of a legume infected with Rhizobia are the energy and hydrogen source used by the bacteria to fix N from the air at high energy costs to the plant. If the plant can avoid these costs by taking up N from the soil, it will. Therefore, N fertilization inhibits nodule formation and N fixation.
Infection by the Rhizobia bacteria is somewhat similar to an infection in the body. The bacteria enter through a susceptible location travel inward, establish themselves, and multiply. In the case of legumes, the bacteria are welcome as long as they are of a certain species that the plant somehow recognizes. Infection enters through a root hair, then grows back to the base of the root hair. Multiplication of the bacteria and enlargement of the root cells form a nodule.
Inside the nodule, an enzyme called nitrogenase drives the N fixation reaction. Similar to the catalyst used in the industrial process, the enzyme also contains molybdenum (Mo). Therefore, availability of this element in the soil is important to the legume. Nitrogenase is peculiar in that contact with oxygen ruins the enzyme. The bacteria and the plant are faced with an engineering problem: how to get the nitrogen out of air that also contains oxygen. A sophisticated system to accomplish this involves a protein called leghemoglobin that is capable of binding to oxygen and removing it from the presence of nitrogenase. Leghemoglobin, like the protein hemoglobin in our blood, binds to and transports oxygen. The combination of the protein with oxygen makes both blood and the inside of an oxygen-free nodule red or pink, which is why these colors indicate an active N-fixing nodule.
Fixed N is used in the plant to make amino acids, the building blocks of proteins. The amount of N that is fixed depends on many factors that include the specific legume and the health of the plant. Because the Rhizobia rely on the plant to supply carbohydrate, maintaining a healthy stand fosters N fixation. The majority of the N fixed is removed with the crop; however, residual N is often available to the next crop. The amount of nitrogen available to the following crop depends on the legume species, density of the legume stand, and harvest management of the legume. That residual N reduces the requirements for applied N.