Soybeans fix nitrogen, a biological process known as nitrogen fixation. This process converts atmospheric nitrogen gas (N₂) into ammonia (NH₃), a form plants can use to build proteins and DNA. Although the air is nearly 78% nitrogen, this gas is chemically inert and inaccessible to the plant. As a leguminous crop, the soybean partners with specific soil bacteria to accomplish this transformation, which often supplies between 40% and 70% of its total nitrogen requirement.
The Symbiotic Relationship
The ability of soybeans to fix nitrogen depends on a specific partnership with soil microbes, primarily the bacterium Bradyrhizobium japonicum. This interaction begins when the bacteria colonize the soybean root hairs, leading to the formation of specialized structures called root nodules. These nodules house the nitrogen-fixing bacteria, sometimes called bacteroids. The soybean plant provides the bacteroids with carbohydrates produced through photosynthesis. In return, the bacteria supply the plant with the fixed nitrogen compounds needed for growth. This exchange ensures both partners benefit.
Converting Nitrogen Gas to Usable Forms
The conversion of atmospheric nitrogen gas takes place within the root nodules through the action of the enzyme complex called nitrogenase. Nitrogenase is highly sensitive to oxygen, which can quickly deactivate it. The nodule must maintain a very low-oxygen environment to protect the enzyme while still supplying enough oxygen for the bacteria’s respiration. This delicate balance is managed by leghemoglobin, a specialized protein produced by the plant. Leghemoglobin binds to oxygen and controls its concentration within the nodule. An active nodule is often pink or reddish inside due to the presence of this oxygen-carrying protein. Nitrogenase converts the nitrogen gas into ammonia, which the plant metabolizes into amino acids and other nitrogen compounds.
Benefits for Sustainable Agriculture
Biological nitrogen fixation offers significant practical advantages for farming systems. By obtaining nitrogen from the atmosphere, soybeans reduce the need for synthetic nitrogen fertilizers. This lowers production costs for farmers and decreases the environmental impact associated with manufacturing these fertilizers, a process that generates greenhouse gas emissions.
The reduced reliance on synthetic nitrogen also helps mitigate nutrient runoff into waterways, which can lead to eutrophication. Beyond meeting the soybean’s own needs, this fixation process leaves behind residual nitrogen in the soil when the plant residues decompose. This residual nitrogen, often referred to as “N carryover,” provides a valuable nutrient source for the subsequent crop in a rotation, such as corn or wheat. Farmers can often subtract a nitrogen credit from the fertilizer recommendation for the following crop, improving overall soil fertility and supporting a resource-efficient agricultural cycle.
Factors Affecting Fixation Efficiency
The efficiency of nitrogen fixation is highly dependent on environmental and soil conditions. The bacteria require a specific range of soil pH, with optimal activity occurring between pH 6 and 7. Soils that are too acidic or too alkaline can stress the Bradyrhizobium bacteria, hindering their ability to form nodules and fix nitrogen.
Moisture and temperature also play important roles; fixation is reduced under conditions of drought, excessive water, or high soil temperatures. Another major factor is the existing level of mineral nitrogen in the soil, such as nitrate. If the soil contains high levels of available nitrogen, the soybean plant will suppress the symbiotic relationship, as fixation is energetically costly when a usable form is already present.
For fields where soybeans have not been grown recently, farmers often inoculate the seeds with commercial preparations of Bradyrhizobium japonicum. This practice ensures a sufficient population of the correct bacteria is present. Micronutrients like molybdenum are also necessary for the nitrogenase enzyme to function, and a deficiency in these elements can limit the rate of nitrogen fixation.