U2_Lesson 2-4: Biological components of N cycle

The nitrogen cycle incorporates processes by which nitrogen is recycled in the ecosystem.
Importance of inorganic fertilizer N highlighted (pink arrows in the diagram).
Shift to soil organic processes:
- Organic residues (plant leaves, stems, roots) are recycled into organic matter.
- Subject to processes including mineralization, immobilization, nitrification, denitrification, leaching, and back to plant uptake.

Definition: Transformation of organic N into inorganic N.
- Aminization: Hydrolysis of proteins into amines and urea, releasing energy to microorganisms.
- Ammonification: Degradation of amines and urea into ammonium, carbon dioxide, and alcohols (also releasing energy).

Generally, 1-3% of organic matter is mineralized annually.
- Equation: If soil has 4% organic matter, approximately 80 lbs N/A is mineralized into plant-usable forms (nitrate and ammonium).

Definition: Conversion of ammonium to nitrate.
- Is nitrification an oxidation or reduction reaction? It is an oxidation reaction.
- Is it energy-requiring or energy-yielding? It is an energy-yielding reaction.
Stages of nitrification:
- Step 1: Ammonium to nitrite, catalyzed by Nitrosomonas or Nitrosolobus bacteria.
- Step 2: Nitrite to nitrate, catalyzed by Nitrobacter bacteria.
Soil acidity is created in this process due to the production of hydrogen ions.
Both reactions require oxygen, incentivized by well-aerated soil conditions.

Nitrate and ammonium formed through mineralization and nitrification are available for plant uptake.
However, they also experience:
- Immobilization: Inorganic N incorporated back into organic forms.
- Leaching: Loss of N through water movement.
- Denitrification: Conversion of nitrate to gaseous forms, potentially lost to the atmosphere.
These competing reactions can decrease plant N uptake efficiency.

Inorganic N availability for assimilation into organic compounds by:
- Microbes: Convert inorganic forms into organic N.
- Plants: Similar assimilation of inorganic N into biological compounds.

Balance between mineralization and immobilization is influenced by the C:N ratio:
- High C:N ratio organic residues (e.g., chaff behind a combine) can induce a N deficiency in the soil, limiting available N for plant uptake.
- Rule of thumb: C:N ratio greater than 30:1 results in net immobilization; less than 20:1 leads to net mineralization.
C:N ratio of 30 serves as a critical threshold.

Low C:N Residues: Addition leads to initially increased ammonium, which stimulates nitrification, leading to increased nitrate and a decrease in ammonium.
High C:N Residues: Both ammonium and nitrate concentrations decline as microorganisms use carbon from the residues, eventually increasing soil inorganic N after microbial die-back.

Conducted two-factor experiment on N mineralization in Oregon soils differing in organic matter content.
- Focus on understanding the two factors involved in this experiment.

Understanding terminology and processes related to the N cycle is essential.
The N cycle is dynamic and features interactions that are sequential, competing, and cyclical.
A thorough understanding of the N cycle aids in informed N management decisions in agricultural practices.