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Nitrogen

Definition

  • Nitrogen is a naturally occurring element indispensable for growth and reproduction in both plants and animals.

  • It is a key component of amino acids, which form proteins, and is critical for chlorophyll formation.

  • Nitrogen comprises approximately 79% of Earth’s atmosphere as N_2 gas.

  • In soil, nitrogen is primarily found in anionic form (nitrate, NO3^-) and in cationic form (ammonium, NH4^+).

  • Most crops predominantly absorb nitrogen in the nitrate form.

Influence of Nitrogen on Plant Growth and Development

Roles in the Plant

  • Nitrogen is vital for various plant functions, including vigorous growth, reproductive development, and overall productivity. It is a key building block for proteins and chlorophyll.

Deficiency

  • Insufficient nitrogen can lead to stunted growth and poor yields.

Oversupply

  • Excessive nitrogen can result in overabundant vegetative growth at the expense of reproductive development, such as fruit or seed production.

Luxury Consumption

  • Some plants absorb more nitrogen than they immediately need, a phenomenon known as luxury consumption, which can sometimes lead to nutrient imbalances.

Nitrogen Cycle

Key Processes

  1. Nitrogen Mineralization (Ammonification)

    • Converts unavailable organic nitrogen into available inorganic ammonium (NH_4^+) via the action of microorganisms (ammonifiers).

    • This is a crucial step in making nitrogen accessible to plants.

  2. Nitrogen Immobilization

    • The reverse process, where available inorganic nitrogen forms (NH4^+ or NO3^-) are converted into unavailable organic nitrogen compounds by soil microbes.

    • Temporarily removes nitrogen from the available nutrient pool.

  3. Nitrification

    • A two-step biological oxidation of ammonium to nitrate, primarily by aerobic bacteria.

    • Step 1: Ammonium (NH4^+) is converted to nitrite (NO2^-) by Nitrosomonas bacteria.

    • Step 2: Nitrite (NO2^-) is then converted to nitrate (NO3^-) by Nitrobacter bacteria.

    • Nitrate is the nitrogen form most readily absorbed by crops.

  4. Denitrification

    • Converts nitrate (NO3^-) back into gaseous nitrogen compounds (nitrous oxide (N2O) or nitrogen gas (N_2)) under anaerobic conditions.

    • Carried out by facultative anaerobic bacteria like Pseudomonas and Bacillus.

    • Results in nitrogen loss from the soil system to the atmosphere.

Sources and Fates of Nitrogen in the Cycle

  • Inputs (Sources of Nitrogen):

    • Atmospheric deposition (e.g., acid rain, atmospheric fixation by lightning)

    • Biological nitrogen fixation (by various microbes)

    • Industrial fixation (e.g., fertilizer application via the Haber-Bosch process)

    • Addition of organic matter (e.g., manures, crop residues)

  • Outputs/Losses (Fates of Nitrogen):

    • Plant uptake for growth

    • Gaseous losses (denitrification, volatilization of ammonia)

    • Leaching of mobile nitrate (NO_3^-) into groundwater or surface water

    • Erosion (loss of nitrogen contained in organic matter)

Factors Affecting Nitrification

  • Supply of NH_4^+: The availability of ammonium from fertilizers or organic matter directly impacts nitrification rates.

  • Population of Nitrifying Organisms: The diversity and population size of nitrifying bacteria (e.g., Nitrosomonas, Nitrobacter) are crucial.

  • Soil pH: Ideal pH for nitrification ranges between 6.5 to 8.5; activity significantly declines below pH 5.5.

  • Soil Aeration: Aerobic (oxygen-rich) conditions actively favor nitrification; anaerobic conditions inhibit it.

  • Soil Moisture: Moisture levels must be within field capacity for optimal microbial activity; both overly dry and overly saturated soils can minimize nitrification.

  • Soil Temperature: Temperature greatly affects microbial activity; rates may decrease significantly in cold soils.

Ammonium vs Nitrate: Plant Preference

Plant Preferences

  • While plants generally prefer nitrate (NO3^-) as a primary nitrogen source, using ammonium (NH4^+) can conserve energy because nitrate must be reduced to NH4^+ within the plant tissues before assimilation. However, NH4^+ is often rapidly converted to nitrate in the soil.

Nitrate Assimilation Steps

  • Step 1: Nitrate (NO3^-) is reduced to nitrite (NO2^-) in the cytoplasm.

  • Step 2: Nitrite (NO2^-) is further reduced to ammonia (NH3) in the chloroplast.

Challenges with Ammonium

  • High concentrations of ammonium (NH_4^+) can be toxic to plants, impeding growth due to its narrow tolerance limits in plant tissues.

  • The enzyme nitrate reductase is involved in the conversion of nitrate to ammonium inside the plant.

  • Therefore, a recommended ratio of NO3^- to NH4^+ is approximately 75:25, as plants generally tolerate higher levels of nitrate.

Nitrogen Fixation

Definition and Significance

  • Nitrogen fixation is the process of converting atmospheric nitrogen gas (N2), which is biologically inert, into a reactive form like ammonia (NH3) or ammonium (NH_4^+) that plants can use.

  • It's an energy-intensive process due to the strong triple bond in N_2.

  • This process is vital because plants cannot directly use atmospheric N_2 but require fixed nitrogen for synthesizing proteins, nucleic acids, and other essential biomolecules.

Types of Nitrogen Fixation

  1. Atmospheric Fixation: Non-biological fixation by natural phenomena like lightning, which converts N_2 into nitrogen oxides that fall to Earth with rain.

  2. Industrial Fixation (Haber-Bosch Process): A human-engineered process for producing ammonia-based fertilizers from N_2 and hydrogen gas.

  3. Biological Nitrogen Fixation (BNF): Performed by certain microorganisms (bacteria and archaea) that possess the enzyme nitrogenase.

Biological Nitrogen Fixation (BNF)

  • Relies on the enzyme nitrogenase to catalyze the reduction of dinitrogen gas to ammonia:

    N2 + 6e^- + 8H^+ \xrightarrow{\text{nitrogenase}} 2NH3 + H_2

  • BNF can be further categorized:

    • Symbiotic Fixation: Microbes live in close association with plants (e.g., Rhizobium in legume root nodules).

    • Non-symbiotic (Free-living) Fixation: Microbes fix nitrogen independently in soil or water (e.g., Azotobacter, Clostridium).

Symbiotic Nitrogen Fixation: A Closer Look

  • Mechanism: In legumes (e.g., peas, beans, clover), roots form specialized structures called nodules in association with symbiotic bacteria, primarily Rhizobia. Inside these nodules, the bacteria convert N2 to NH4^+ for the host plant.

  • Simplified Reaction within Nodules:

    N2 + 16 ATP + 2H^+ \rightarrow 2NH4^+ + 16 ADP + H_2

  • This partnership is mutually beneficial, with the plant providing carbohydrates to the bacteria, and the bacteria supplying fixed nitrogen to the plant.

Agricultural Importance

  • Inoculation of legumes with appropriate rhizobial strains can significantly enhance nitrogen fixation, leading to improved crop health and yields.

Factors Affecting Nitrogen Fixation

Soil Nutrient Supply

  • Legumes prefer adequate soil nitrogen; low soil N can stimulate fixation potential, whereas excess nitrate can reduce nitrogenase activity.

Soil pH

  • Soil acidity can limit the growth and activity of rhizobial bacteria; acid-tolerant strains can be selected for such soils.

Environmental Conditions

  • Factors affecting photosynthesis (e.g., light intensity, moisture availability, temperature) also influence nitrogen fixation rates, as the host plant supplies energy to the fixers.

Legume Management

  • Management practices that reduce overall legume health or yield will consequently diminish the quantity of N_2 fixation.