Immobilization (soil science)
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Immobilization in soil science is the conversion of inorganic compounds to organic compounds by micro-organisms or plants, by which it is prevented from being accessible to plants.[1] Immobilization is the opposite of mineralization.
C:N Ratio
Whether nitrogen is mineralized or immobilized depends on the C/N ratio of the plant residues.[2] In general plant residues entering the soil have too little nitrogen for the soil microbial population to convert all of the carbon into their cells. If the C:N ratio of the decomposing plant material is above about 30:1 the soil microbial population may take nitrogen in mineral form (e.g. nitrate). This mineral nitrogen is said to be immobilized. This may cause nitrogen deficiency in plants growing in the soil.
As carbon dioxide is released via decomposition the C:N ratio of the organic matter decreases, and the microbial demand for mineral nitrogen is decreased. When the C:N ratio falls below about 25:1 further decomposition results in simultaneous mineralization of nitrogen which is in excess to that required by the microbial population.
When decomposition is virtually complete soil mineral nitrogen will be higher than it was initially due to mineralization of the plant residue nitrogen.