Importance of nitrogen in plants
Nitrogen is one of the most important essential elements for plant growth and development. It is a fundamental component of key molecules such as chlorophyll, nucleic acids, and proteins. Its adequate presence determines the vigor, color, size, and productivity of crops, as well as directly influencing the process of photosynthesis and the synthesis of new tissues.
In the atmosphere, nitrogen is abundant in the form of Nâ‚‚ , although the high stability and inert nature of this molecule make it unusable directly by most plants. Therefore, the conversion of this nitrogen into assimilable forms is an essential process for plant life and for the maintenance of agricultural and natural ecosystems.
Nitrogen deficiencies manifest as poor growth, pale coloration in older leaves, and reduced plant size . Conversely, an excess can lead to excessive vegetative growth, weakness, and susceptibility to pests.
Balancing nitrogen supply, absorption, and assimilation is essential for healthy crops, optimizing fertilization, and minimizing the environmental impact of nitrogen fertilizers.
Natural sources and chemical forms of nitrogen
Nitrogen can be found in nature and in the soil in different chemical forms , although only certain forms are directly available to plants. The main sources and forms are:
- Atmospheric nitrogen (N2): It constitutes about 78% of the atmosphere. It is not directly assimilated by most plants.
- Nitrate (NO3 ) and ammonium (NH4+): They are the main ways in which plants absorb nitrogen from the soil. They can also take small amounts of nitrite (NO2 ), although it is less common.
- Organic nitrogen compounds: Derivatives of the decomposition of organic matter, such as amino acids and urea, which require mineralization to be converted into useful inorganic forms.
The transformation between these forms occurs through biological and chemical processes, involving specific microorganisms and enzymes, and is fundamental for maintaining soil fertility and plant nutrition. To better understand these processes, we recommend consulting our article on plant nutrition.
Nitrogen transformation and cycle processes in the soil
Nitrogen follows a biogeochemical cycle involving different stages and soil microorganisms. The most relevant for plant assimilation are:
- Biological nitrogen fixation: Some microorganisms (free bacteria or in symbiosis with legumes) transform N2 atmospheric ammonia (NH3) through the action of the enzyme nitrogenase. This ammonia is then converted into ammonium (NH4+), an assimilable form.
- Mineralization: Microorganisms decompose organic matter, releasing nitrogen in the form of ammonia and ammonium after decomposition and mineralization processes.
- Nitrification: Receives the generated ammonium and converts it into nitrite (NO2 ) by bacteria such as Nitrosomonas, and later into nitrate (NO3 ) by action of Nitrobacter.
- Denitrification: Under anaerobic conditions, other bacteria return some of the nitrogen to the gaseous state, closing the cycle.
Plants primarily utilize nitrogen during the mineralization and nitrification stages , when it is in the form of ammonium and nitrate. To learn more about how plants manage these processes, you can visit our section on xylem and phloem.
Mechanisms of nitrogen absorption by plants
Plant roots absorb nitrogen in the form of ammonium and nitrate through various mechanisms:
- Nitrate absorption (NO3 ): An active, energy-consuming process mediated by specific enzymes and transporters present primarily in root hairs. Nitrate is stored in the tissues or transported to the stem through the xylem.
- Ammonium absorption (NH4+): It occurs by both active and passive processes, depending on the concentration gradient and energy supply. Ammonium uptake can acidify the rhizosphere due to the release of H+ to maintain electrical neutrality.
Ammonium is rapidly incorporated into organic compounds in the roots , while nitrate can be transported to the aerial parts of the plant before being assimilated. To learn more about how plants transport these nutrients, you can consult our article on lamb's quarters plants.
Metabolic pathways and nitrogen assimilation
Nitrogen assimilation in plants involves the conversion of absorbed inorganic forms into essential organic molecules. The process differs for each ion:
- Nitrate: After absorption, it is reduced to nitrite by the enzyme nitrate reductase. Nitrite is subsequently reduced to ammonia by nitrite reductase, usually in the chloroplasts of the leaves. To better understand these processes, we suggest you visit our section on Physalis.
- Ammonium: It is incorporated very rapidly through the glutamine synthase and glutamate synthase pathways, forming amino acids such as glutamine and glutamate.
The synthesis of amino acids through these pathways is fundamental for the subsequent production of proteins, nucleic acids, and other biomolecules vital to the structure and functions of plant cells.
Factors influencing nitrogen assimilation
Several variables affect the efficiency and manner in which plants assimilate nitrogen , including:
- Temperature and humidity: They affect the speed of microbial processes (mineralization and nitrification) and the activity of enzymes related to assimilation.
- soil pH: A slightly acidic pH favors the availability of ammonium, while a neutral to alkaline pH increases the prevalence of nitrate. Furthermore, rhizosphere pH can vary depending on the nitrogen source absorbed.
- Energy and carbohydrate availability: The conversion of nitrate into ammonium and its assimilation consume energy resources and require the plant to have sufficient photosynthates.
- Presence of micronutrients: Elements such as molybdenum are essential for the proper function of enzymes such as nitrate reductase.
- Soil microbial conditions: Microbial biodiversity determines the efficiency of the transformation of organic and inorganic nitrogen compounds.
Interaction between nitrogen forms and other nutrients
The way plants absorb nitrate or ammonium has effects on the absorption of other nutrients:
- Nitrate (NO3 ) competes with anions such as chloride (Cl ) and can affect the absorption of certain cations. In soils with excess chloride, nitrate absorption may be reduced.
- Ammonium (NH4+) competes with potassium (K+) and other positive ions. If ammonium accumulates in excess, it can cause symptoms of potassium toxicity and deficiency.
- Plants usually benefit from a combination of nitrate and ammonium in the substrate, since a balance between both forms optimizes nutrition and growth.
Regulation of assimilation and control mechanisms
It has recently been discovered that nitrogen assimilation is regulated by signaling molecules such as nitric oxide (NO) . This molecule acts as a sensor, indicating when the plant should limit or enhance nitrate uptake and assimilation based on its internal nitrogen availability.
The enzyme S-nitrosoglutathione reductase 1 (GSNOR1) regulates the availability of nitric oxide within the plant. An increase in nitric oxide reduces GSNOR1 activity and limits nitrate assimilation, while a higher GSNOR1 level allows for increased absorption and plant growth. To learn more about regulatory mechanisms, you can consult our section on endotherapy.
These mechanisms open up new avenues for the development of more efficient plant varieties, capable of performing better with lower doses of nitrogen fertilizers, thereby reducing costs and environmental impact.
Agricultural technology and nitrogen fertilization
In agriculture, nitrogen fertilization is crucial to ensure optimal yields and quality products . However, only a portion of the applied nitrogen is actually utilized by crops; the rest can be lost through leaching, volatilization, or denitrification, contaminating groundwater and the environment.
Sources of nitrogen fertilizers:
- Organic: Manure, compost, blood meal, guano, and other organic waste. They provide slow-release nitrogen and improve soil structure.
- Minerals: Urea, ammonium nitrate, ammonium sulfate, potassium nitrate, calcium nitrate, monoammonium phosphate, among others. They provide nitrogen in forms that are quickly and easily available to the plant.
Choosing the right fertilizer depends on soil conditions, crop type, growth stage, and production goals. It is also essential to consider incompatibilities in mixtures, volatilization risks, and integrated soil and water management.
Nitrogen deficiency and toxicity symptoms in plants
Observing symptoms in plants can help diagnose nitrogen status and the need for intervention:
- Deficiency: Slow growth, older pale green or yellowish leaves, premature drop. In severe cases, stems may turn reddish or purple, and there may be a general loss of vigor.
- Toxicity: Excessive development of leaves with soft tissues, decreased flowering and fruiting, susceptibility to pests, nitrate accumulation, and growth arrest due to ionic imbalances and deficiencies of other nutrients.
Sustainable strategies for nitrogen assimilation
Responsible nitrogen nutrition management involves the use of sustainable agricultural practices :
- Crop rotation and use of legumes: They promote biological fixation and reduce dependence on chemical fertilizers.
- Fractional application based on soil analysis: Maximizes efficiency of use and minimizes losses and contamination.
- Use of controlled release technologies: They allow a progressive offer adapted to the demands of the crop.
- Promoting microbial biodiversity: Improves mineralization and maintains soil health.
A thorough understanding of the nitrogen cycle, its assimilable forms, and the factors that modulate its absorption and metabolism is essential for optimizing agricultural production, ensuring environmental sustainability, and enhancing plant development. Recent advances in understanding regulatory mechanisms and in the design of fertilizer strategies are making it possible to advance toward more efficient and environmentally friendly agriculture.
