Immobilization is the microbial conversion of inorganic, plant- available nutrients
(primarily nitrogen
as ammonium NH₄⁺ or nitrate NO₃⁻) into organic forms within microbial
biomass, making them
temporarily unavailable to plants.
The Role of the Carbon-to-Nitrogen Ratio (C/N Ratio)
The C/N ratio of organic material added to soil (e.g., crop residues, manure, compost)
decisively
controls whether net mineralization or net immobilization occurs.
Low C/N Ratio (< 20:1): Organic material is N-rich relative to microbial
needs (e.g., legume
residues, young green manure, animal manure). Microorganisms have enough nitrogen to
decompose the carbon, releasing excess nitrogen as ammonium (net mineralization).
High C/N Ratio (> 30:1): Organic material is C-rich and N- poor (e.g.,
straw, sawdust, mature
cereal stubble). Microorganisms need more nitrogen to build their biomass and decompose the
carbon. They will scavenge mineral N from the soil, leading to net immobilization. This can induce
temporary nitrogen deficiency in crops.
Critical Range (~ 20-30:1): Microbial activity is balanced; nitrogen
is roughly sufficient for
decomposition, resulting in little net change in soil mineral N.
