Plants primarily obtain nitrogen to synthesize proteins through two main pathways: absorption from the soil and symbiotic relationships with nitrogen-fixing bacteria.
Nitrogen Acquisition Methods for Plants
Plants require nitrogen in a usable form to create essential proteins, nucleic acids (DNA and RNA), and other vital biomolecules. Because atmospheric nitrogen (N₂) is largely inaccessible to plants in its gaseous form, it must be converted into usable forms like ammonia (NH₃), ammonium (NH₄⁺), nitrite (NO₂⁻), or nitrate (NO₃⁻).
1. Absorption from the Soil
- Nitrates (NO₃⁻): This is the most common form of nitrogen absorbed by plants from the soil. Soil nitrates are produced through nitrification, a process carried out by soil bacteria.
- Ammonium (NH₄⁺): Plants can also absorb ammonium directly from the soil. Ammonium is produced during the decomposition of organic matter.
- Nitrites (NO₂⁻): While less common, plants can also absorb nitrites. However, high concentrations of nitrites can be toxic.
2. Symbiotic Nitrogen Fixation
- Rhizobium Bacteria: This is the most well-known symbiotic relationship. Rhizobium bacteria live in nodules on the roots of legumes (e.g., beans, peas, alfalfa). These bacteria convert atmospheric nitrogen (N₂) into ammonia (NH₃), which is then converted to ammonium (NH₄⁺) usable by the plant. In return, the plant provides the bacteria with carbohydrates for energy.
- Other Nitrogen-Fixing Bacteria: Besides Rhizobium, other bacteria like Azotobacter and Azospirillum can also fix nitrogen, though not always in a symbiotic relationship with plants. Some cyanobacteria are also capable of nitrogen fixation and can form symbiotic relationships with plants like Azolla (water fern) often found in rice paddies.
Table summarizing nitrogen sources for plants
| Source | Form of Nitrogen | Mechanism | Example |
|---|---|---|---|
| Soil | Nitrate (NO₃⁻) | Absorption from soil water | Most plants |
| Soil | Ammonium (NH₄⁺) | Absorption from soil water | Many plants, especially in acidic soils |
| Rhizobium | Ammonium (NH₄⁺) | Symbiotic nitrogen fixation in root nodules | Legumes (beans, peas, alfalfa) |
| Other Bacteria | Ammonium (NH₄⁺) | Nitrogen fixation (symbiotic or free-living) | Azotobacter, Azospirillum |
In summary, plants access nitrogen primarily through absorption from the soil (as nitrates, nitrites, or ammonium) and through symbiotic relationships with nitrogen-fixing bacteria like Rhizobium, which convert atmospheric nitrogen into a usable form.
