Transcript Nitrogen
Nitrogen Cycle
Atmosphere = 78% gaseous Nitrogen
N2
tightly bonded together
Most nitrogen in soil is associated with
organic matter
Most organic matter is about 5% N
Inorganic N is only about 1 to 2% of N in soil
1.
2.
3.
4.
5.
Necessary for chlorophyll synthesis
Essential part of photosynthesis
Essential part of amino acids
Essential for carbohydrate use.
Stimulates plant productivity
Taken up by plants as NH4+ or nitrate (NO3)
Deficiency Symptoms: chlorosis- pale yellow
color occurring in older leaves first, stunted
plants, or spindly plants that mature quickly.
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
Occurs as microbes decompose organic
matter for energy.
95-99% of soil N is in organic forms (proteins)
Organic N Pool Inorganic N Pool
(unavailable)
(available)
Occurs when crop residues high in carbon and
low in Nitrogen are incorporated into the soil.
Organic N Pool Inorganic N Pool
(unavailable)
(available)
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
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
When immobilization is greater than
mineralization there is little N for plants.
Applying N fertilizer can shift it to
mineralization.
Adding residue shifts it to immobilization.
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)
The bacterial oxidation of ammonium to
nitrite and then to nitrate.
Step 1:
▪ NH4+ + 1½ O2 NO2 + 2H+ + H2O
Step 2:
▪ NO2 + ½ O2 NO3
1. pH 4.5 – 10.0
2. Good drainage, adequate O2
3. Temperature 20 – 30o C
Nitrates are lost to the atmosphere when
they are reduced to Nitrous oxide or
elemental nitrogen.
▪ 2NO3 2NO2 2NO(g) N2O(g) N2(g)
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
1. Taken up by plants
2. Leached through soil
3. Run-off
4. Lost to atmosphere through
denitrification.
5. Lost to atmosphere through
volatilization.
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
1. Nitrification inhibitors – deactivate
nitrifying bacteria
▪ Most used on sandy soils
2. Slow release forms of N
▪ Sulfur coated urea or formaldehyde coated urea.
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
2. Non-symbiotic – carried out by free living
bacteria in the soil.
▪ Fixes about 20 lbs N/a/year
N2(g) + 6H+ + 6e- 2NH3 organic acids
amino acids proteins
Alfalfa group
B. meliloti
▪ Alfalfa
▪ Sweetclover
Clover Group
B. trifolii
▪ Trifolium spp.
Soybean
B. japanicum