Nitrogen is a fundamental building block for all life, forming the structure of amino acids, proteins, and the nucleic acids that make up DNA and RNA. While the Earth’s atmosphere is composed of approximately 78% nitrogen gas (\(\text{N}_2\)), this form is chemically inert and unusable by most plants. Plants require nitrogen in a “fixed” form, meaning it must be combined with hydrogen or oxygen to create compounds like ammonia or nitrate. Certain specialized plants have evolved an ingenious biological process that facilitates the conversion of inert atmospheric nitrogen into a usable form, thereby enriching the soil around them.
The Essential Role of Nitrogen Fixing Organisms
The ability to perform this conversion, known as biological nitrogen fixation, is carried out by specialized microorganisms. These prokaryotic organisms, collectively called diazotrophs, possess the unique genetic machinery required to break the strong triple bond of the \(\text{N}_2\) molecule. The most effective and agriculturally relevant method of fixation involves a close, mutually beneficial relationship, or symbiosis, between certain plants and these microbes.
The best-known examples involve leguminous plants, such as beans, peas, and clover, which form partnerships with Rhizobium bacteria. In this exchange, the bacteria convert atmospheric nitrogen into ammonia for the plant’s use. In return, the host plant supplies the bacteria with carbohydrates, which are energy-rich compounds created during photosynthesis. The plant’s role is therefore not to perform the chemistry, but to create and maintain a protective “factory” for the microorganisms.
Creating Specialized Root Structures for Fixation
The symbiotic relationship begins with a chemical conversation between the plant and the bacteria in the soil. The host plant, often a legume, releases specific organic compounds called flavonoids from its roots, which act as a signal to attract the appropriate strain of Rhizobium. In response, the bacteria release Nod factors, which are signaling molecules that induce the root hairs to curl and deform. This intricate molecular exchange ensures that the plant only engages with compatible microbial partners.
The curling root hair traps the bacteria, and the plant then forms a specialized inward tube called the infection thread, allowing the microbes to penetrate the root tissue. The bacteria travel through this thread into the inner root cells, where they are released and become encased within a plant-derived membrane, forming structures called symbiosomes. This coordinated process culminates in the formation of a root nodule, a specialized organ on the plant root that houses millions of nitrogen-fixing bacteria, now referred to as bacteroids.
The Biochemical Conversion of Atmospheric Nitrogen
The actual conversion of atmospheric nitrogen (\(\text{N}_2\)) into ammonia (\(\text{NH}_3\)) is performed by a complex metalloenzyme called nitrogenase. Nitrogenase is an extremely sensitive enzyme that is irreversibly inactivated by the presence of oxygen. This characteristic explains the need for the specialized, oxygen-controlled environment within the root nodule.
The nitrogenase enzyme complex reduces the \(\text{N}_2\) molecule by sequentially adding electrons and protons. This process is highly demanding, requiring a considerable energy investment from the plant. For every single molecule of \(\text{N}_2\) reduced to two molecules of \(\text{NH}_3\), the reaction consumes a minimum of 16 molecules of the energy compound adenosine triphosphate (ATP). The enzyme complex uses this energy to break the robust triple bond that holds the two nitrogen atoms together.
To maintain the necessary low-oxygen state while still supplying the bacteroids with enough oxygen for their own respiration (which produces the required ATP), the plant synthesizes a red, oxygen-binding protein called leghemoglobin. Leghemoglobin scavenges free oxygen within the nodule, buffering the concentration to a very low level that protects the nitrogenase. Once the ammonia is produced, it is quickly converted into ammonium (\(\text{NH}_4^+\)) and then assimilated into amino acids and other nitrogen-containing compounds for the host plant’s growth.
How Fixed Nitrogen Becomes Available to Other Plants
The nitrogen fixed inside the root nodules is initially used almost entirely by the host plant for its own physiological needs. The primary pathways through which this enriched nitrogen enters the soil to benefit surrounding and subsequent plants are through decomposition and root turnover. When the nitrogen-fixing plant, such as a clover or alfalfa, completes its life cycle and dies, the entire plant structure, including the nitrogen-rich roots and nodules, begins to decompose.
Soil microbes break down this plant matter, releasing the accumulated nitrogen compounds, now in the form of ammonium and nitrate, directly into the soil. This organic material functions as a natural fertilizer, making the nitrogen available for non-fixing plants that follow in the next growing season. A small amount of fixed nitrogen may also be released into the soil through the plant’s roots during its active growth.
Agricultural practices like crop rotation and the use of cover crops are based on leveraging this natural mechanism. By alternating nitrogen-fixing crops with non-fixing crops, farmers can naturally replenish the soil’s nitrogen content, reducing the reliance on synthetic fertilizers.