Nitrogen Cycle
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Atmosphere = 78% gaseous Nitrogen
 N2
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tightly bonded together
Most nitrogen in soil is associated with
organic matter
 Most organic matter is about 5% N
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Inorganic N is only about 1 to 2% of N in soil
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1.
2.
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4.
5.
Necessary for chlorophyll synthesis
Essential part of photosynthesis
Essential part of amino acids
Essential for carbohydrate use.
Stimulates plant productivity
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Taken up by plants as NH4+ or nitrate (NO3)
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Deficiency Symptoms: chlorosis- pale yellow
color occurring in older leaves first, stunted
plants, or spindly plants that mature quickly.
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1. Results in excessive vegetation
2. May cause lodging
3. May delay plant maturity
4. Plants may be more susceptible to
disease.
5. Can cause nitrate poisoning in cattle
grazing forages
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Occurs as microbes decompose organic
matter for energy.
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95-99% of soil N is in organic forms (proteins)
Organic N Pool    Inorganic N Pool
(unavailable)
(available)
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Occurs when crop residues high in carbon and
low in Nitrogen are incorporated into the soil.
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Organic N Pool  Inorganic N Pool
(unavailable)
(available)
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C:N ratio of decomposing tissue determines if
mineralization will occur more than
immobilization.
 Wide ratio (30:1)  favors immobilization
 Narrow ratio (>20:1)  favors mineralization
 20 – 30:1  both occur equally
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Topsoil
Alfalfa
Rotted manure
Cornstalks
Grain straw
Coal
Oak
Spruce
10:1
13:1
20:1
60:1
80:1
124:1
200:1
1000:1
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When immobilization is greater than
mineralization there is little N for plants.
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Applying N fertilizer can shift it to
mineralization.
Adding residue shifts it to immobilization.
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Decomposition of organic matter to
ammonia (NH4)
 Several clay mixtures have the capacity to fix both
ammonia and potassium ions (micas, vermiculite,
and smectites).
 Volatilization – ammonia gas lost to the
atmosphere.
▪ Greater as pH increases
▪ Ammonia gas is produced
▪ As soil dries ammonia volitalizes
▪ Plants can absorb ammonia from the air (cleaning air)
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The bacterial oxidation of ammonium to
nitrite and then to nitrate.
Step 1:
▪ NH4+ + 1½ O2  NO2 + 2H+ + H2O
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Step 2:
▪ NO2 + ½ O2  NO3
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1. pH 4.5 – 10.0
2. Good drainage, adequate O2
3. Temperature 20 – 30o C
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Nitrates are lost to the atmosphere when
they are reduced to Nitrous oxide or
elemental nitrogen.
▪ 2NO3  2NO2  2NO(g)  N2O(g)  N2(g)
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1. Nitrate must be available
2. Decomposable organic compounds
must be available.
3. Soil air should contain less than 10%
oxygen
4. Temperature should be 2-50oC
5. pH above 5.0
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1. Taken up by plants
2. Leached through soil
3. Run-off
4. Lost to atmosphere through
denitrification.
5. Lost to atmosphere through
volatilization.
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Ammonium (NH4) is stable in the soil – held
by the CEC sites.
▪ 1. Ammonium – N does not leach
▪ 2. NH4 is not subject to denitrification
▪ 3. Corn hybrids, wheat, cotton have higher
yields when fertilized with a mixture of NH4 and
NO3
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1. Nitrification inhibitors – deactivate
nitrifying bacteria
▪ Most used on sandy soils
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2. Slow release forms of N
▪ Sulfur coated urea or formaldehyde coated urea.
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1. Symbiotic – bacteria grows with the plant
and fixes N benefiting both the plant and the
bacteria.
▪ Fixes 100 – 500 kg N/ha/year
▪ Bradyrhizobium bacteria and legume roots
▪ Cyanobacteria – non-nodule forming
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2. Non-symbiotic – carried out by free living
bacteria in the soil.
▪ Fixes about 20 lbs N/a/year
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N2(g) + 6H+ + 6e-  2NH3  organic acids
 amino acids  proteins
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Alfalfa group
B. meliloti
▪ Alfalfa
▪ Sweetclover
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Clover Group
B. trifolii
▪ Trifolium spp.
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Soybean
B. japanicum