BIOGEOCHEMICAL CYCLES

SOIL MICROBIOLOGY

BIOL/CSES 4684



The Nitrogen Cycle: Mineralization - Immobilization


MINERALIZATION
In nitrogen mineralization, organic nitrogen from decaying plant and animal residues (proteins, nucleic acids, amino sugars, urea) is converted to ammonia (NH3) and ammonium (NH4+) by the mechanisms listed below. This process is also called ammonification. The resultant ammonia can be converted back to organic N (immobilization) where it is taken up by microbes and plants (assimilated), or nitrified to nitrate (NO3).


Removal of the Amine Group:

1. Direct Removal

2. Deamination (oxidative - aerobic, reductive - anaerobic)

3. Decarboxylation

4. Urea Hydrolysis - Ammonification

Once NH4+ has been formed, there are numerous possible fates:
  1. It can be assimilated (or immobilized) by microbes and plants.
  2. It can be held on exchange complexes in soil.
  3. It can be fixed in the inner layer portions of clays.
  4. It can react with SOM to form quinone-NH2 complexes.
  5. It can be volatilized as NH3 in decaying vegetation or manure.
  6. It can be nitrified by the autotrophic nitrifying bacteria.






The Nitrogen Cycle: Mineralization - Immobilization

IMMOBILIZATION
In nitrogen immobilization, ammonia and nitrate are taken up by microbes and is largely immobilized, or made unavailable to plants, depending on the C:N ratios. When N is abundant, both microbes and plants assimilate ammonia and nitrate (see FATE OF NITRATE). The nitrate undergoes nitrate reduction by a four step process and is converted to ammonia by reactions that are similar to those that occur in denitrification. The ammonia is then incorporated into Kreb's cycle intermediates to form amino acids. For example, pyruvic acid plus ammonia becomes the amino acid alanine.

The carbon:nitrogen ratio (C:N) is used as an indicator of which step in the nitrogen cycle occurs next. From Table N1, ratios less than 20 mean that excess N is present and nitrification proceeds (with a net gain of N). With ratios between 20 and 30, nitrification and immobilization rates are in equilibrium and there is no net gain or loss of N. With a ratio greater than 30, N is limited and net immobilization occurs with uptake (or loss) of N from the active N cycle. When N is limited at high C:N ratios, nitrogen-fixation by free-living nitrogen fixers is stimulated. Table N2 shows the wide range of C:N ratios that occur in some materials that are commonly added to soil.

Table N1. Relationship between the C:N ratios and net immobilization and nitrification.

<20 20-30 >30
Net gain of NH4+ and NO3- Neither gain nor loss Net uptake of NH4+ and NO3-

Material C:N Ratio
Microbial tissues 6-12
Sewage sludge 5-14
Soil humus 10-12
Legume residues and green manures 13-25
Barnyard manure(rotted) 20
Cereal residues and straw 60-80
Oak leaves 65
Pine needles 225
Forest wastes 150-500
Sawdust 400




Table N2. Typical C:N ratios of some organic materials.

Residues that have C:N ratios greater than about 30, equivalent to N contents of about 1.5% or less, result in lowering of mineral N reserves because of net immobilization. This stimulates the process of BIOLOGICAL NITROGEN FIXATION, where nitrogen is added to an N-deficient situation.

Residues with C:N ratios below about 20, or N contents greater than 2.5%, lead to an increase in mineral levels through net mineralization. Excess nitrogen (as NH4+) stimulates the process of NITRIFICATION.

 














Back to Nitrogen Cycle Overview.

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