Nitrogen is a fundamental element for all living organisms, playing a central role in biological processes. Although the atmosphere is 78% nitrogen gas (N2), plants cannot directly use this form. They require nitrogen in specific converted forms for growth and development. This article explores how plants acquire this essential nutrient, from soil absorption to microbial activity.
Why Plants Need Nitrogen
Nitrogen is a major component of crucial molecules within plant cells. It is a key building block for amino acids, which construct proteins and enzymes. Proteins perform functions like structural support and facilitating metabolic reactions. Chlorophyll, the green pigment for photosynthesis, also contains nitrogen. Without sufficient nitrogen, plants cannot produce enough chlorophyll, impacting their ability to convert light energy into sugars. Nitrogen is also a component of nucleic acids, such as DNA and RNA, which carry genetic information essential for plant growth and development.
Nitrogen from the Soil
Plants primarily absorb nitrogen from the soil through their root systems. The main forms available for uptake are nitrate (NO3-) and ammonium (NH4+). Nitrate is often predominant in well-aerated soils, while ammonium is common in acidic or anaerobic environments. Plants possess specialized root transporters to facilitate the uptake of these forms.
Usable nitrogen forms become available in the soil largely through organic matter decomposition. Soil microorganisms break down dead plant and animal material, converting organic nitrogen into ammonium via ammonification or mineralization. Other soil bacteria then convert ammonium into nitrites and nitrates through nitrification. This microbial activity ensures a continuous supply of absorbable nitrogen for plants.
Nitrogen from the Air: The Role of Microbes
Atmospheric nitrogen gas (N2) is abundant but chemically inert; plants cannot directly utilize it. The conversion of this nitrogen into a usable form, like ammonia (NH3), is nitrogen fixation. Microorganisms carry out this process, accounting for about 90% of natural nitrogen fixation on Earth.
Symbiotic nitrogen-fixing bacteria, particularly Rhizobium species, form a mutualistic relationship with leguminous plants like peas and beans. These bacteria reside within root nodules on legume roots. Inside, Rhizobium convert atmospheric N2 into ammonia using nitrogenase, supplying fixed nitrogen directly to the plant in exchange for carbohydrates.
Free-living nitrogen-fixing bacteria also contribute to soil nitrogen. Genera like Azotobacter and Azospirillum live independently in the soil, converting N2 gas into ammonia. Unlike symbiotic bacteria, they obtain their own energy source from the soil. The ammonia produced is then converted into ammonium or nitrate, making it available for plant uptake.
Signs of Nitrogen Deficiency
When plants lack adequate nitrogen, they exhibit visual symptoms. A common indicator is the yellowing of older leaves, known as chlorosis, which typically begins at the leaf tips and progresses inward. Nitrogen is a mobile nutrient; when supplies are low, the plant reallocates it from older leaves to support new, metabolically active tissues.
Affected plants often display stunted growth and a pale green appearance. This occurs because insufficient nitrogen limits chlorophyll production, reducing photosynthesis and energy generation. Severe deficiency can also result in smaller leaves, reduced fruit or flower production, and premature leaf drop.