The air is approximately 78% nitrogen gas (N2), a molecule fundamental to all life. Despite this abundance, plants cannot directly absorb or metabolize gaseous nitrogen because the triple bond linking the atoms makes it chemically inert. For plants to construct proteins and nucleic acids, this atmospheric nitrogen must first be converted into reactive compounds like ammonia or nitrate. This conversion, known as nitrogen fixation, is primarily carried out by specific microorganisms, and certain plants have evolved a partnership to facilitate it.
The Process of Biological Nitrogen Fixation
Biological nitrogen fixation is a complex biochemical process accomplished by a specialized enzyme complex called nitrogenase, which is only found in certain prokaryotes, such as bacteria. This enzyme catalyzes the reduction of atmospheric nitrogen (N2) into ammonia (NH3), a compound readily usable by the plant. The reaction requires a significant amount of energy, which the host plant provides in the form of carbohydrates produced during photosynthesis.
The process begins when plant roots release chemical signals, known as flavonoids, which attract symbiotic bacteria from the soil. Once attracted, the bacteria penetrate the root hairs and stimulate the formation of root nodules. These nodules act as protected factories where the fixation process takes place.
The nitrogenase enzyme within the nodule is extremely sensitive to oxygen, which can quickly deactivate it. To manage this, the host plant produces leghemoglobin, similar to animal hemoglobin. Leghemoglobin controls the oxygen concentration inside the nodule, maintaining a low-oxygen environment to protect the enzyme while still allowing the bacteria to respire and generate energy. The resulting ammonia is quickly converted into ammonium (NH4+) and then into amino acids, which are transported throughout the plant for growth.
Key Nitrogen-Fixing Plants: The Legume Family
The most widely recognized and agriculturally significant nitrogen-fixing plants belong to the Fabaceae family, commonly known as legumes. This family includes thousands of species that form a highly effective symbiotic relationship with Rhizobia bacteria. This partnership is so successful that the legume family is often considered the primary terrestrial source of biologically fixed nitrogen, making them a staple in sustainable agriculture.
Many familiar food crops are legumes, including garden peas, snap beans, broad beans, and lentils. Larger-scale agricultural examples include soybeans, the most widely grown nitrogen-fixing crop globally, and alfalfa, a forage crop valued for its deep roots and nitrogen contribution. Other important cover crops and forages include clovers (crimson, red, and white) and vetches (hairy vetch).
The success of the legume-Rhizobia symbiosis is due to the highly evolved signaling and nodule formation pathways. The bacteria are housed in the root nodules, where they receive a steady supply of energy from the plant in exchange for fixed nitrogen. This allows legumes to thrive even in nitrogen-poor soils, giving them a competitive advantage over other plant species.
Nitrogen-Fixing Plants Outside the Legume Family
While legumes are the most numerous and well-known nitrogen fixers, the ability to form this symbiosis is not exclusive to the Fabaceae family. A diverse group of non-leguminous plants, collectively called actinorhizal plants, also fix nitrogen through a partnership with a different type of microorganism, the filamentous bacterium Frankia. These plants represent about 220 species across eight plant families.
Examples of these non-legume fixers often include woody shrubs and trees, which are important for soil enrichment in natural ecosystems. Common examples include:
- Trees in the genus Alnus (alders), often pioneer species in poor soils.
- Shrubs like sea buckthorn (Hippophae).
- Bayberry (Myrica).
- California lilac (Ceanothus).
- Australian pine (Casuarina).
The nodules formed by Frankia in these non-legumes serve the same function but differ structurally from those found in legumes. In many Frankia associations, the bacteria form protective vesicles within the nodule to shelter the nitrogenase enzyme from oxygen. This demonstrates that the ability to utilize atmospheric nitrogen through symbiosis has evolved multiple times.
Integrating Nitrogen Fixers for Soil Health
Knowing which plants fix nitrogen is foundational to improving soil health through regenerative agricultural and gardening practices. The primary application is using nitrogen-fixing plants in crop rotation and as cover crops. Cover crops are grown to protect and enrich the soil when cash crops are not growing, and legumes like clover or vetch are frequently used for this purpose.
Nitrogen fixers are also used as green manure, where the crop is grown and then incorporated directly into the soil while still green. Tilling the green manure accelerates nutrient release, making the fixed nitrogen in the plant biomass and nodules quickly available to subsequent crops as the material decomposes. Nitrogen is also transferred through the slow decomposition of root nodules that naturally slough off during the plant’s life cycle. The released fixed nitrogen becomes a renewable nutrient source, effectively cycling nitrogen within the ecosystem. Utilizing these plants in rotation with heavy nitrogen feeders, such as corn or brassicas, is a strategy for maintaining long-term soil fertility.