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NITROGEN CONVERSIONS
NITROGEN REACTIONS
mineralization or ammonification -the conversion of organic nitrogen (in the -NH2 form) to
                                                       inorganic nitrogen (in the NH4 form).
                                                     - the speed of conversion depends on the C:N ratio (see below).

nitrification - a two step process converting ammonium to nitrite, then nitrite to nitrate.
                   - the soil bacterium Nitrosomonas converts ammonium to nitrite
                   - the soil bacterium Nitrobacter converts nitrite to nitrate
                   - this occurs very quickly so little ammonium (which can be toxic if high) and
                     virtually no nitrite (which is highly toxic) accumulates in the soil.

denitrification - the conversion of nitrate in the soil to gaseous nitrogen that escapes into the
                         atmosphere.

nitrogen fixation - the conversion of gaseous nitrogen to ammonia.
                           - only nitrogen fixing microorganisms can cause nitrogen fixation; 
                             some form symbiotic relationships with plants (see table below)
 

CARBON:NITROGEN (C:N) RATIOS IN ORGANIC MATTER
 C:N ratio - proportion  of carbon to nitrogen present in organic matter. 

a) high C:N ratio - wood, sawdust, uncomposted bark 
    - microbes use up all nitrogen in organic matter when consuming carbon, 
    - then the microbes use up the nitrogen in the soil 

b) low C:N ratio - manure, bone meal, fish emulsion, organic fertilizers 
    - microbes consume carbon, 
    - then release excess nitrogen from the organic matter into the soil 
    - thus, low C:N organic matter acts as an organic nitrogen fertilizer

 

MICROORGANISMS THAT CAN FIX NITROGEN
 
Type Microorganism
Nitrogen-Fixing 
Microorganism
Forms Symbiotic
Relationship With
1) Bacteria 
Rhizobium
Legumes
(soybean, clover, mesquite)
2) Bacteria 
Azotobacter 
Clostridium
free living only
3) Filamentous Bacteria
some Actinomycetes
Some woody plants 
(Alnus and Elaeagnus)
4) Blue-green algae 
Anabaena
Azolla and Cycads
(can also be free living)