Comprehensive Notes on Nitrogen and Biological Nitrogen Fixation

Macronutrients and the Role of Nitrogen in Plant Physiology

• Primary Macronutrients: The essential elements for plant growth include Nitrogen ($N$), Phosphorus ($P$), Potassium ($K$), Calcium ($Ca$), Magnesium ($Mg$), and Sulfur ($S$).
• Abundance and Uptake: After Carbon ($C$), Nitrogen is the element that plants uptake in the largest quantity. Carbon content is the most abundant, making Nitrogen the second largest element that the plant assimilates or uptakes.
• Biomass Composition: Analysis of plant tissue normally reveals that between $1\%$ and $5\%$ of total plant dry biomass consists of Nitrogen ($N$).
• Biological Roles: Nitrogen is an integral component of protein structures, hormones, metabolic products, nucleic acids ($DNA$ and $RNA$), chlorophyll, and various coenzymes.
• Growth Dependency: Plant growth and the efficiency of Nitrogen assimilation depend heavily on the availability of Nitrogen within the soil itself. The specific amount of available Nitrogen in the soil determines the rate of plant uptake.
• Atmospheric Nitrogen Utilization: Very few plants are capable of utilizing atmospheric Nitrogen ($N_2$). Legumes are a notable example, as they form a symbiosis with bacteria, such as Rhizobium, which can fix atmospheric Nitrogen into a plant-available form.

Forms and Mobility of Nitrogen in the Soil

• Soil as a Nitrogen Source: The soil serves as the primary reservoir for Nitrogen used by plants.
• Forms of Soil Nitrogen:   - Organic Form: Approximately $75\%$ to $95\%$ of soil Nitrogen exists in an organic form. This organic matter releases Nitrogen slowly into the soil over a long duration.   - Inorganic Form: Only a small fraction, roughly $0.1\%$ to $0.3\%$, exists in inorganic forms available for immediate plant uptake.
• Plant-Available Inorganic Nitrogen:   - Nitrate ($NO_3^-$): This is the form most preferred by plants, with some exceptions. In typical soil, Nitrate is present in higher concentrations ($1-5\,mM$).   - Ammonium ($NH_4^+$): This form is generally found in lower concentrations ($20-200\,\mu M$).
• Mobility and the Clay Lattice:   - Nitrogen mobility is determined by electrical charges. The clay lattice in soil possesses a negative charge.   - Ammonium ($NH_4^+$): Because it is positively charged, Ammonium has a tendency to stick to the clay surface through electrostatic attraction.   - Nitrate ($NO_3^-$): Because it is negatively charged, Nitrate is repelled by the negative charge of the clay lattice (negative and negative repulse). Consequently, Nitrate is significantly more mobile in the soil than Ammonium.

The Nitrogen Cycle: Industrial and Natural Processes

• Industrial Fixation (The Haber-Bosch Process): This process involves industrial synthesis under high temperature ($1200^{\circ}C$) and high pressure ($500\,atm$). The chemical reaction combines Hydrogen and Nitrogen to produce ammonia:   - $3H_2 + N_2 \rightarrow 2NH_3$
• Natural Nitrogen Inputs:   - Lightning: Approximately $8\%$ of natural Nitrogen fixation occurs via lightning. The massive energy released during a thunderclap breaks the triple bond of the inert Nitrogen molecule ($N_2$) in the environment. This Nitrogen reacts with ozone/oxygen to form Nitrogen dioxide ($NO_2$) or other oxides, which then react with water to form nitric acid ($HNO_3$). This acid is soluble and is deposited onto the soil through rainfall.   - Biological Nitrogen Fixation: Microbes (symbiotic, free-living, or associative) fix atmospheric Nitrogen.   - Decomposition and Mineralization: Adding organic matter to soil acts as a slow-release fertilizer. Mineralization occurs over time as the organic matter decomposes:     - Year 1: Maximum release occurs, with approximately $35\%$ mineralization.     - Year 2: Mineralization drops to approximately $20\%$.     - Year 3: Mineralization drops to approximately $15\%$.

Biological Nitrogen Fixation (BNF) and Microorganisms

• The Capability of Microbes: Microbes are the only organisms capable of utilizing free atmospheric Nitrogen ($N_2$), which comprises over $70\%$ of the air. Fertilizers are necessary because most crops cannot utilize atmospheric Nitrogen directly.
• General Reaction for N-Fixation: $N_2 \rightarrow NH_3$.
• Categories of Nitrogen-Fixing Microbes:   - Symbiotic: These involve close association with a host plant, often forming nodules. Examples include Rhizobium and Frankia. The plant provides Carbon ($C$) to the bacteria, and the bacteria provide Nitrogen ($N$). These are the most efficient, fixing $50-400\,kg\,N\,ha^{-1}$.   - Associative: These microbes prefer specific microclimates such as the root atmosphere or root exudates (rhizosphere). They are not physically inside the host but associated with it. Example: Azospirillum. They fix $10-200\,kg\,N\,ha^{-1}$.   - Free-Living: These microbes function independently of a host.     - Autotrophs: Example: Cyanobacteria (energy from photosynthesis). They fix $10-80\,kg\,N\,ha^{-1}$.     - Heterotrophs: Examples: Klebsiela (energy from residues), Clostridium. They fix $1-2\,kg\,N\,ha^{-1}$.

Step-by-Step Mechanisms of Symbiotic Nitrogen Fixation

• Host Specificity: The relationship is host-specific; only certain Rhizobium species can form nodules with certain legume species. It is a mutually beneficial (win-win) symbiosis.
• The Signaling Process:   1. The legume plant releases secondary metabolites called Flavonoids into the soil as a signal to attract compatible bacteria.   2. Upon receiving the flavonoid signal, the Rhizobium produces a specific Nod factor (and specific exopolysaccharide).   3. The Nod factor triggers Calcium spiking in the plant cell cytoplasm. Calcium levels, which are normally kept very low, spike rapidly as a signaling mechanism.
• Infection and Nodule Formation:   4. Root Hair Curling: In response to the signal, root hairs curl into a "shepherd's crook" shape, trapping the bacteria inside.   5. Infection Thread: The plant creates a tunnel or bridge called an infection thread, allowing bacteria to travel from the curling site into the root tissue.   6. Cell Division: The plant nucleus travels to the infection site. Cells in the root cortex begin dividing rapidly to form a new growth area (meristem), resulting in the outgrowth known as a nodule.
• Transformation into Bacteroid:   7. Bacteria are released into membrane-bound compartments within the plant cells.   8. The bacteria transform into Bacteroids, which are enlarged, non-dividing states of the bacteria dedicated to synthesizing Ammonium.
• Chemical Reaction of Fixation:   - $N_2 + 8e^- + 8H^+ + 16\,ATP \rightarrow 2NH_3 + 16\,ADP + 16\,P_i$

Comparison of Determinate and Indeterminate Nodules

• Determinate Nodules:   - Shape: Spherical or round.   - Growth Pattern: Meristematic activity is lost shortly after nodule formation; growth occurs primarily through cell expansion (enlargement) rather than division.   - Region: Prevalent in tropical regions and tropical plant species.   - Examples: Soybeans, beans, cowpeas.   - Export Form: Nitrogen is exported as Ureides.   - Energy Efficiency: Higher efficiency; cowpeas spend approximately $3$ units of Carbon to fix $1$ unit of Nitrogen.
• Indeterminate Nodules:   - Shape: Cylindrical or oval.   - Growth Pattern: Possess an active meristem that produces new cells continuously throughout the nodule's lifespan. Growth is due to continuous cell division.   - Region: Prevalent in temperate regions and temperate species.   - Examples: Clover, vetch, pea, lupin.   - Export Form: Nitrogen is exported as Amides (specifically Asparagine and Glutamine).   - Energy Efficiency: Lower efficiency; lupins spend approximately $6$ units of Carbon to fix $1$ unit of Nitrogen. This is because energy is split between continuous cell division and Nitrogen fixation, and amides are less energy-efficient to transport than ureides.

Protection of the Nitrogenase Enzyme

• Nitrogenase Sensitivity: The key enzyme for Nitrogen fixation is Nitrogenase. It is highly sensitive to oxygen ($O_2$) and is destroyed in its presence. Fixation must occur under anaerobic conditions.
• The Aerobic Paradox: Plant roots require oxygen for aerobic respiration (which is preferred because Nitrogen fixation is energy-intensive, requiring $16\,ATP$). Thus, the plant must protect Nitrogenase while still allowing respiration.
• Protection Mechanisms:   - Cyanobacteria: Use specialized compartments called heterocysts that exclude oxygen, keeping Nitrogenase protected.   - Free-Living Anaerobes: Example: Clostridium has evolved to utilize anaerobic respiration exclusively.   - Leguminous Plants (Leghemoglobin): Plants produce Leghemoglobin, a protein similar to mammalian hemoglobin.     - Leghemoglobin binds to free oxygen, maintaining a very low concentration in the nodule to protect Nitrogenase.     - It effectively delivers this trapped oxygen directly to the bacteroid for aerobic respiration without allowing it to come into contact with the enzyme.     - Identification: An active, efficient nodule will release a pinkish liquid when squeezed, indicating the presence of Leghemoglobin.

Factors Affecting Biological Nitrogen Fixation and Mobility

• Soil pH: Nitrogen fixation is significantly improved when the soil pH > 5.5. Legumes generally perform poorly under acidic conditions.
• Nutritional Requirements: Nitrogenase requires an Iron ($Fe$) and Molybdenum ($Mo$) complex to function, along with Sulfur ($S$).
• Excessive Soil Nitrogen: High application of Nitrogen fertilizer reduces nodulation. Because nodulation is carbon-and-energy expensive, the plant will not invest in it if Nitrogen is already readily available.
• Light Levels: Limited light reduces the rate of photosynthesis. Since photosynthesis provides the Carbon/energy required for expensive nodulation, low light eventually limits nodule development.
• Conversion of Ammonia: Excessive Ammonia ($NH_3$) is toxic to plants. After fixation, the plant immediately converts Ammonia into organic forms like Ureides (determinate/tropical) or Amides (indeterminate/temperate) before transporting them through the xylem or phloem.

Nitrogen Use Efficiency and Organic Amendments

• Nitrogen Use Efficiency (NUE): This is the ratio between the total yield obtained and the total Nitrogen input into the field.
• Organic Amendments: Materials such as manure, biochar, compost, and green manures release Nitrogen slowly over time. This slow release allows for more efficient Nitrogen uptake by the plant compared to immediate-release chemical fertilizers.