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Approximately 80% of the atmosphere is nitrogen gas (N2). Unfortunately N2 is unusable by most living organisms. Plants, animals, and micro-organisms can die of nitrogen deficiency, surrounded by N2 they cannot use. All organisms use the ammonia (NH3) form of nitrogen to manufacture amino acids, proteins, nucleic acids, and other nitrogen-containing components necessary for life.
Biological nitrogen fixation is the process that changes inert N2 to biologically useful NH3. This process is mediated in nature only by bacteria. Other plants benefit from nitrogen fixing bacteria when the bacteria die and release nitrogen to the environment, or when the bacteria live in close association with the plant. In legumes and a few other plants, the bacteria live in small growths on the roots called nodules. Within these nodules, nitrogen fixation is done by the bacteria, and the NH3 produced is absorbed by the plant. Nitrogen fixation by legumes is a partnership between a bacterium and a plant.
Biological nitrogen fixation can take many forms in nature including bluegreen algae (a bacterium), lichens, and free-living soil bacteria. These types of nitrogen fixation contribute significant quantities of NH3 to natural ecosystems, but not to most cropping systems, with the exception of paddy rice. Their contributions are less than 5 lbs of nitrogen per acre per year. However, nitrogen fixation by legumes can be in the range of 25-75 pounds of nitrogen per acre per year in a natural ecosystem, and several hundred pounds in a cropping system.
Legume nitrogen fixation starts with the formation of a nodule. A common soil bacterium, Rhizobium, invades the root and multiplies within the cortex cells. The plant supplies all the necessary nutrients and energy for the bacteria. Within a week after infection, small nodules are visible with the naked eye. In the field, small nodules can be seen 2-3 weeks after planting, depending on legume species and germination conditions. When nodules are young and not yet fixing nitrogen, they are usually white or grey inside. As nodules grow in size they gradually turn pink or reddish in color, indicating nitrogen fixation has started. The pink or red color is caused by leghemoglobin (similar to hemoglobin in blood) that controls oxygen flow to the bacteria.
Nodules on many perennial legumes such as alfalfa and clover are finger-like in shape. Mature nodules may actually resemble a hand with a center mass (palm) and protruding portions (fingers), although the entire nodule is generally less than 1/2 inch in diameter. Nodules on perennials are long-lived and will fix nitrogen through the entire growing season, as long as conditions are favorable. Most of the nodules (10-50 per large alfalfa plant) will be centered around the tap root.
Nodules on annual legumes such as beans, peanuts, and soybeans are round and can reach the size of a large pea. Nodules on annuals are short-lived and will be replaced constantly during the growing season. At the time of pod fill, nodules on annual legumes generally lose their ability to fix nitrogen because the plant feeds the developing seed rather than the nodule. Beans will generally have less than 100 nodules per plant, soybeans will have several hundred per plant, and peanuts may have 1,000 or more nodules on a well-developed plant.
Legume nodules that are no longer fixing nitrogen usually turn green, and may actually be discarded by the plant. Pink or red nodules should predominate on a legume in the middle of the growing season. If white, grey, or green nodules predominate, little nitrogen fixation is occurring as a result of an inefficient Rhizobium strain, poor plant nutrition, pod filling, or other plant stress.
The nitrogen fixed is not free. The plant must contribute a significant amount of energy in the form of photosynthate (photosynthesis derived sugars) and other nutritional factors for the bacteria. A soybean plant may divert 230% of its photosynthate to the nodule instead of to other plant functions when the nodule is actively fixing nitrogen. Any stress that reduces plant activity will reduce nitrogen fixation. Factors like temperature and water may not be under the control of the farmer. But nutrition stress (especially phosphorus, potassium, zinc, iron, molybdenum, and cobalt) can be corrected with fertilizers. When a nutritional stress is corrected, the legume responds directly to the nutrient, and indirectly to the increased nitrogen nutrition resulting from enhanced nitrogen fixation. Poor nitrogen fixation in the field can be easily corrected by inoculation, fertilization, irrigation, or other management practices.
Some legumes are better at fixing nitrogen than others. Common beans are poor fixers (less than 50 lbs per acre) and fix less than their nitrogen needs. Maximum economic yield for beans in New Mexico requires an additional 30-50 lbs of fertilizer nitrogen per acre. However, if beans are not nodulated, yields often remain low, regardless of the amount of nitrogen applied. Nodules apparently help the plant use fertilizer nitrogen efficiently.
Other grain legumes such as peanuts, cowpeas, soybeans, and faba beans are good nitrogen fixers, and will fix all of their nitrogen needs other than that absorbed from the soil. These legumes may fix up to 250 lbs of nitrogen per acre and are not usually fertilized. In fact, they usually don't respond to nitrogen fertilizer as long as they are capable of fixing nitrogen. Nitrogen fertilizer is applied at planting to these legumes when grown on sandy or low organic matter soils to supply nitrogen to the plant before nitrogen fixation starts. If nitrogen is applied, the rate is low, 10-15 lbs per acre. When large amounts of nitrogen are applied, the plant literally slows or shuts down the nitrogen fixation process. It is easier and less energy consuming for the plant to absorb nitrogen from the soil than to fix it from the air.
Perennial and forage legumes such as alfalfa, sweetclover, true clovers, and vetches may fix 250-500 lbs of nitrogen per acre. Like the grain legumes previously discussed, they are not normally fertilized with nitrogen. They occasionally respond to nitrogen fertilizer at planting or immediately after a cutting when the photosynthate supply is too low for adequate nitrogen fixation.
The amount of nitrogen returned to the soil during or after a legume crop can be misleading. Almost all of the nitrogen fixed goes directly into the plant. Little leaks into the soil for a neighboring non-legume plant. However, nitrogen eventually returns to the soil for a neighboring plant when vegetation (roots, leaves, fruits) of the legume die and decompose.
When the grain from a grain legume crop is harvested, little nitrogen is returned for the following crop. Most of the nitrogen fixed during the season is removed from the field. The stalks, leaves, and roots of grain legumes such as soybeans and beans contain about the same concentration of nitrogen as found in non-legume crop residue. In fact, the residue from a corn crop contains more nitrogen than the residue from a bean crop, simply because the corn crop has more residue.
A perennial or forage legume crop only adds significant nitrogen for the following crop if the entire biomass (stems, leaves, roots) is incorporated into the soil. If a forage is cut and removed from the field, most of the nitrogen fixed by the forage is removed. Roots and crowns add little soil nitrogen, compared to the above ground biomass.
Measurement of nitrogen fixation in the field is difficult. However, a grower can make some field observations that can help indicate if nitrogen fixation is adequate in some of the common legumes.
B. Try to inoculate the plants in the field through the irrigation system or by other means. Caution: this technique often does not work and expert advice is needed.
C. Consider nitrogen fertilization to meet all of the plant's nitrogen needs. This may not be an option for a perennial legume such as alfalfa if the field is kept in alfalfa for several years. Also, some legumes use soil or fertilizer nitrogen more efficiently if nodules are present.
If young nodules are present, sufficient soil nitrogen may not be available for the young plant before nitrogen fixation starts. The plant usually grows out of this condition, or a small amount of nitrogen can be applied. Also, inefficient native Rhizobium may result in poor nitrogen fixation. Consider other soil stresses that may be inhibiting plant growth, especially plant nutrition and water stress.
New Mexico State University Extension Guide A-130, "Inoculation of Legumes," describes when and how to inoculate legumes.
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