EVSC 590.N Cycle.mineralization Immobilization

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    Nitrogen Cycle

    n

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    Nitrogen Mineralization and

    Immobilization

    Introduction;

    Definition: N immobilization is defined as

    the transformation of inorganic compounds(NH4+, NH3, NO3-, NO2-) into the organic

    state.

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    Nitrogen Mineralization and

    Immobilization

    1. Some organisms assimilate inorganic-N

    compounds and transform them to

    organic N constituents of cells, tissues ,and their soil biomass.

    2. NH4+ salts are the most readily

    assimilated nitrogen sources for mostbacteria, fungi and actinomycetes.

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    3. Mineral N decreases in soil due to

    synthesis of microbial biomass

    (protein synthesis).

    4. When C:N ratio > 30 N is taken from

    mineral pool or degradation is slow(immobilization takes place).

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    Nitrogen Mineralization and

    Immobilization

    II. Calculation of Mineralization

    /Immobilization Potential.

    1. Consider decay of typical organicmaterial.

    2. To estimate N needed for cell synthesis,

    data on C assimilated and C:N ratio isrequired

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    3. As a rule for mixed populations, carbon

    assimilated is

    Bacteria 5-10% Fungi 30-40%

    Actinomycetes 15-30%

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    4. C/N ratio of Microbial Tissues are as follows:

    Bacteria 5:1

    Fungi 10:1 Actinomycetes 5:1

    5. Carbon content of microbial biomass is 45-

    50% of dry weight.

    6. % N varies with age. Hyphal cells in old

    cultures have usually less N than young ones

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    Nitrogen Mineralization and

    Immobilization

    Bacteria

    Bacteria Assimilates 5-10% of carbon

    and has C:N ratio of 5:1 100 units of carbon

    5-10 units of carbon is assimilated

    Needs 1-2 units of N regardless of source- If this is not present it will be taken from soil

    solution.

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    Nitrogen Mineralization and

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    Residue is not all carbon, approximately 40-50

    % carbon

    If residue has only 40% carbon Then100 units of residue will give 40 units of

    carbon

    Thus carbon which will be assimilated

    40 X 0.05 = 2 units of carbon

    40 x 0.10 = 4 units of carbon

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    Since C/N ratio of bacterial tissue is 5:1, N neededis

    - 2 x 0.2 = 0.4 units of N

    - 4 x 0.2 = 0.8 units of N

    If residue has 0.5% of N,100 units contains 0.5 unitsof N

    Inefficient bacteria 0.4 units and thus will not havemuch problem.

    Efficient bacteria will need 0.8 units of N and thuswill have to take extra 0.3 units from soil solution.

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    Nitrogen Mineralization and

    Immobilization

    Actinomycetes

    Actinomycetes assimilates 15-30 % of C

    C:N ratio of actinomycetes tissue is 5:1 100 units of residue containing 40%

    carbon and 0.5% N

    Amount of carbon assimilated 0.15 x 40 = 6 units of Carbon

    0.30 x 40 = 12 units of Carbon

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    Nitrogen Mineralization and

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    Fungi

    Fungi assimilates 30-40 % of C

    C:N ratio of fungal tissue is 10:1 100 units of residue containing 40% carbon and

    0.5% N

    Amount of carbon assimilated

    0.3 x 40 = 12 units of Carbon

    0.4 x 40 = 16 units of Carbon

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    Amount of N assimilated

    0.1 x 12 = 1.2 units N

    0.1 x 16 = 1.6 units N

    Thus fungi not great in mineralizing N.

    Amount of N assimilated

    6 x 0.2 = 1.2 units N

    12 x 0.2 = 2.4 units N

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    Nitrogen Mineralization and

    Immobilization Amount of N in residue is 0.5 units

    Therefore N will be taken from soil solution

    i.e. N mineralization will take place. Fungi accumulates a lot of substrate in

    excess of their needs.

    When residue contains 40% C the C:N ratioshould be around 30:1.

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    This is the critical C:N ratio (20-30:1).

    This corresponds to 1.2-1.8% N

    Material with this amount of N tends to bemineralized.

    When ratio is narrow N tends to be released.

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    Nitrogen Mineralization and

    Immobilization

    III. Factors Affecting Immobilization

    1. Availability of organic Molecule.

    a. oxidized CHO is very rapidimmobilization.

    b. Moderate to less suitable materials

    c. Slow with extensively decayed materialsor resistant tissues such as lignin.

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    2. pH

    3. Available P

    4. Temperature

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    IV. Agronomic Significance

    1. Plants are poor competitors with soilmicroflora when inorganic-N is inadequate.

    2. Immobilization follow soil amendment of N-poor plant residue not desirable in growingseason since the nutrient is renderedunavailable.

    3. Reaction may be beneficial by prevention ofleaching.