Legumes, such as peas, beans, and clover, acquire nitrogen from the atmosphere and incorporate it into the soil. These plants form a cooperative relationship with certain soil microbes to perform this function. This process, known as symbiotic nitrogen fixation, is a natural and energy-efficient way to convert unusable atmospheric nitrogen gas into a form that plants can absorb. This natural fertilization ability makes legumes a foundational element of sustainable agriculture and crop rotation systems.
The Symbiotic Mechanism of Nitrogen Fixation
The ability of legumes to gather atmospheric nitrogen results from a specialized partnership with soil-dwelling bacteria known as Rhizobia. This symbiosis begins when legume roots release chemical signals that attract the appropriate species of Rhizobia. The bacteria then infect the root hairs, prompting the plant to develop specialized structures called root nodules where nitrogen fixation takes place.
Inside these root nodules, the Rhizobia differentiate into specialized forms called bacteroids, protected within the plant cells. The plant supplies the bacteroids with carbohydrates derived from photosynthesis to fuel the energy-intensive conversion process. In return, the bacteroids use the enzyme nitrogenase to convert atmospheric nitrogen gas (\(N_2\)) into ammonia (\(NH_3\)), a form readily usable by the host plant.
The nitrogenase enzyme is extremely sensitive to oxygen, necessitating a protective mechanism within the nodule. Legumes produce a molecule called leghemoglobin, which is structurally similar to hemoglobin in animal blood, to manage the oxygen supply. Leghemoglobin binds to free oxygen, maintaining a low-oxygen environment sufficient for the nitrogenase to function while still allowing the bacteroids to respire.
How Fixed Nitrogen Becomes Available to Soil
The nitrogen fixed inside the root nodules is primarily integrated into the legume’s own biomass, including the roots, stems, and leaves. During the plant’s active growth, a small amount of fixed nitrogen may be released into the soil through root exudates and the natural sloughing off of root cells and dead nodules. However, this minor release contributes only a fraction of the total nitrogen benefit to other plants.
The majority of the nitrogen fixed by the legume becomes available to the rest of the soil ecosystem through the decomposition of the plant residue. When the legume plant dies or is terminated and incorporated into the soil, the breakdown of its organic matter begins. This practice is often referred to as “green manuring” when the crop is purposefully grown and tilled under for soil improvement.
Since the fixed nitrogen is contained within high-protein plant tissue, soil microbes gradually mineralize this organic nitrogen into inorganic forms like nitrate, which subsequent crops can easily take up. This slow release process is referred to as “carryover nitrogen” because the benefit is transferred to the following crop in the rotation. Studies suggest that approximately half of the nitrogen contained in the residue is released within the first year of decomposition, with the remainder becoming available in subsequent seasons.
Maximizing Nitrogen Contribution: Key Influencing Factors
The actual amount of nitrogen a legume contributes to the soil is highly variable and depends on several environmental and management factors. One of the most significant factors is the existing level of mineral nitrogen in the soil. If the soil already contains a high concentration of available nitrogen, the legume plant will prioritize absorbing that nitrogen, which suppresses the activation of the symbiotic fixation process.
Soil conditions also heavily influence the Rhizobia bacteria and nodule function. The nitrogenase enzyme and the bacteria are sensitive to acidic environments, with nitrogen fixation becoming significantly reduced in soils with a pH below 5.0. Furthermore, extreme soil moisture conditions, such as severe drought or waterlogged conditions, inhibit bacterial activity and nodule formation, directly limiting nitrogen output.
The choice of legume species and the timing of its management are also determinants of nitrogen contribution. Different legume species fix varying amounts of nitrogen; for instance, alfalfa and clover typically contribute more nitrogen per acre than certain grain legumes like navy beans. For maximum nitrogen transfer, the plant should generally be allowed to grow until it reaches the early flowering stage before being terminated and incorporated into the soil, as this point typically represents the peak accumulation of fixed nitrogen in the plant’s biomass.