U2L4: Biological Components of the Nitrogen Cycle

Introduction to Nitrogen Cycle Transformations

  • Focus on nitrogen transformations impacting plant availability.

    • Key processes include:

    • Mineralization

    • Immobilization

    • Nitrification

Overview of the Nitrogen Cycle

  • Simplified diagram of the nitrogen cycle discussed:

    • Previous focus: Plant uptake of ammonium and nitrate, contributions from inorganic fertilizers (represented by pink arrows).

    • Current focus: Soil organic processes: organic residues recycled into organic matter.

    • Key processes:

      • Mineralization

      • Immobilization

      • Nitrification

      • Denitrification

      • Leaching

      • Plant uptake

Mineralization

  • Definition: Transformation of organic nitrogen (N) into inorganic nitrogen.

    • Step 1: Aminization

    • Process Explanation: Hydrolyzation of proteins into amines and urea.

    • Outcome: Release of energy for microorganisms.

    • Step 2: Ammonification

    • Process Explanation: Degradation of amines and urea into ammonium, carbon dioxide, and alcohols.

    • Outcome: Transfer of energy to microorganisms.

Factors Affecting Nitrogen Mineralization

  • Factors include:

    • C:N ratio of added residue

    • Soil organic matter level

    • Soil temperature (microbes thrive in warmer conditions)

    • Soil moisture (optimal conditions at field capacity)

    • Soil aeration (faster decomposition in tilled soils, slower in waterlogged or no-till soils)

    • Effect of Temperature: Higher temperatures lead to increased N mineralization rates due to biological processes.

    • Typical mineralization rate: 1% to 3% of organic matter (OM) minerialized per year.

    • Example Calculation: For a soil with 4% OM, approximately 80 lb N/A is mineralized into plant-usable forms (nitrate and ammonium).

Nitrification

  • Definition: The process of converting ammonium to nitrate.

  • Classification:

    • Oxidation reaction and energy-requiring reaction.

  • Step 1: Conversion of ammonium to nitrite

    • Bacteria Involved: Nitrosomonas or Nitrosolobus.

    • Byproduct: Hydrogen ions, leading to increased soil acidity.

  • Step 2: Conversion of nitrite to nitrate

    • Catalyst: Bacteria known as Nitrobacter.

    • Note: Both reactions require oxygen, making well-aerated soil conditions essential.

Factors Affecting Nitrification

  • Similar factors to mineralization:

    • Supply of ammonium

    • Population of nitrifying bacteria

    • Soil pH: Optimum at 8.5

    • Soil moisture (optimal at field capacity)

    • Soil aeration

    • Soil temperature: Optimal range 25-35°C

Competition between Reactions

  • Once ammonium and nitrate are produced, they are available for plant uptake.

  • Competing processes include:

    • Immobilization

    • Leaching

    • Denitrification

  • Implication: Increased leaching, denitrification, and immobilization reduce nitrogen availability for plants.

Uptake and Assimilation of Inorganic Nitrogen

  • Both microbes and plants can assimilate inorganic nitrogen into organic compounds.

Carbon:Nitrogen Ratio and its Effects

  • Explanation: The C:N ratio affects the balance between mineralization and immobilization.

    • High C:N ratio residues can induce nitrogen deficiency in plant availability.

    • Example: Chaff left behind in a field has a high C:N ratio.

  • Definition of Immobilization: The reverse of mineralization, it's the incorporation of inorganic nitrogen into amino acids and proteins.

  • Energy Requirement: The process requires energy and can occur in both plants and microorganisms.

  • Rule of Thumb for C:N Ratio:

    • Greater than 30:1 → Net immobilization occurs.

    • Less than 20:1 → Net mineralization occurs.

  • Importance of C:N Ratio in Mineralization: C:N of 30 is a critical breakpoint for inorganic nitrogen release.

Soil Amendments and Plant Residues

  • Common soil amendments and their respective C:N ratios:

    • Green plant materials and manures tend to have low C:N ratios.

    • Cereal residues (corn stalks, wheat straw) tend to have high C:N ratios.

    • Woody materials tend to have high C:N ratios.

Nitrate and Ammonium Concentration Timeline

  • Low C:N residues added to soil:

    • Ammonium concentrations initially increase due to aminization and ammonification.

    • Increased nitrification as ammonium accumulates, resulting in heightened nitrate concentration and decreased ammonium.

  • High C:N residues added to soil:

    • Both ammonium and nitrate concentrations decrease as microorganisms utilize carbon from the residues, drawing upon soil nitrogen to build biomass.

    • Eventually, microbial population dies back, leading to increased soil inorganic nitrogen.

Experimental Insights on Nitrogen Mineralization

  • Two-factor experiments conducted in Oregon soils differing in organic matter content:

    • Concept: Explained in relation to varying soil organic matter levels.

Summary of Nitrogen Cycle Dynamics

  • Understanding terminology and processes discussed is crucial to comprehending the nitrogen cycle.

  • Characteristics of the nitrogen cycle:

    • Dynamic

    • Interactive processes that are sequential, competing, and cyclical

  • Influences on the nitrogen cycle include environmental factors, soil characteristics, and plant variables.

  • Importance: A thorough understanding of the nitrogen cycle is essential for informed nitrogen management decisions.