Nitrogen cycle
Forms of inorganic N
 plants need inorganic forms of N to grow
- cannot use organic forms
 ammonium, NH4+ held on clay
 other forms not held by clay particles
- nitrate NO3-, and nitrite NO2- quickly ....
 leach or
 transform into nitrogen gases (N2, NO, or N2O)
and ammonia gas (NH3) & evaporate
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Nitrogen fixation
 mostly (90%) by bacteria : Azotobacter spp., Rhizobium spp. Azospirillum spp.
 and lightning, ultraviolet radiation [can also be
produced by electrical equipment, and artificially by
Haber-Bosch process (N2 + H2 + catalysts + T + P)]
 ammonia (NH3) and nitrates (NO3-) formed
 bacteria become encased in nodules that grow on the
roots of plants
 cyanobacteria (formerly blue-green algae) also fix N
for liverworts, hornworts, cycads, and at least one
genus of flowering plants (Gunnera); also symbiotic
relationship with fungi – combination called lichen
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Azotobacter vinelandii
Rhizobium
leguminosarum
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 non-symbiotic forms may fix N for cereals, e.g:
 Azospirillum spp.
 Klepsiella spp.
 Euterobacter spp
 Acetobacter spp.
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Legume Root Nodules
Over 15,000 species in Leguminosae family, ranging
from forage legumes to grain legumes to trees.
Many of these have organs on their roots called
nodules which are packed full of bacteria called
"rhizobia".
The rhizobia can live either within the nodule or in
the soil, but they can only fix N2 while they are
inside the nodule. Inside the nodule, the rhizobia
are provided with carbon and energy from the plant.
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Nodules on root
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Innoculation
•
fixing of N often can be improved by adding the bacteria to
the soil at the time of planting
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Nitrification
 plants can assimilate NH3 as well as NO3 but plants can
incorporate the nitrates more easily into their tissues
 but most NH3 converted to NO2- and then to NO3 by
aerobic bacteria
 process is called “nitrification”
 bacteria that convert ammonia to nitrites :
(Nitrosomonas, Nitrosospira, Nitrosococcus,
& Nitrosolobus) - family Nitrobacteraceae - use
inorganic chemicals as an energy source
 bacteria that convert nitrites (toxic to plants) to
nitrates : Nitrobacter, Nitrospina, and Nitrococcus
 applying dilute solutions of ammonia results in an
increase in soil nitrates through the action of nitrifying
bacteria
 inorganic nitrogen is also added directly to soil in 8
precipitation, or as fertilisers.
Nitrosomonas
spp.under electron
microscopy
(39,000X).
Nitrobacter sp.
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Leaching
 major loss mechanism - involves two forms of nitrogen
- ammonium (NH4) and nitrate (N03)
 NH4 is held by the clay and OM with only small amounts
in soil solution
 NH4 nitrogen not easily washed out
 but NH4 N broadcast on warm, moist soils rapidly
changed to nitrate & then leached
- reduced by application in bands
 NO3 moves more easily - more subject to leaching
 mainly on sandy soils in high rainfall areas, or under
irrigated conditions
 fine textured clay soils are able to hold more moisture
and allow much less movement of water and nitrate
down through the soil
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Assimilation by plants and animals
 NO3 & NH4 assimilated into tissue of algae and
higher plants
 converted to organic forms, such as amino acids
and proteins
 animals ingest - converting them into own body
compounds
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Ammonification of decaying organic matter
 Organic N exists in materials formed from animal, human,
and plant activities - manures, sewage waste, compost,
and decomposing roots or leaves
 transformed into organic soil material called humus
 process absorbs inorganic nitrogen in a process called
“immobilisation”
 plants cannot use organic N but microbes convert organic
N into inorganic forms – “mineralisation” - that plants
can then use
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 decomposed by micro-organisms such as detritivores in
the process of ammonification
 positive charged NH4 can be adsorbed and fixated on to
the negatively charged soil clay particles or taken up
directly by plants
 although fixation of atmospheric N is essential part of
the N cycle, ammonification and then nitrification are
predominant methods by which organic N is prevented from
returning to the atmosphere
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C: N Ratio
 decomposition of OM requires N and leads to temporary
immobilisation of the nitrogen
 C/N ratio used to indicate speed at which OM decomposes
 low C/N indicates rapid decomposition - nutrients bound up
in the organic fraction released relatively quickly
 as C/N ratio increases, speed of decomposition decreases
 finally, at high C/N ratios, OM organic matter is stable
– decomposes slowly
 adding nutrients - especially N - to soils of high C/N ratio is
likely to result in their being locked up by being bound into
the organic matter until the C/N ratio falls to a value in line
with the local environment
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 plant growth stunted if C:N ratio too high
– good value is 30:1
 in warm areas - speed of cycling is high – OM lower
 C/N ratio in warmer soils higher than that in cooler soils
 allophane (volcanic soils) supports higher OM levels than
other clays, so the C/N ratio in volcanic soils is likely to be
lower, making them relatively better store-houses of
nutrients than other soils in the same climatic zone
 adding compost increases the stable OM fraction
- high C/N ratio gives it slow breakdown rate - helps to
improve soil structure and moisture-holding capacity
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In te rp re ta tio n :
C /N
ra tio
<8
8 -1 0
Level
R a te o f o rg a n ic m a tte r
b re a k d o w n
V e ry lo w ra p id d e co m p o s itio n
Low
ra p id d e co m p o s itio n
11 – 15
M e d iu m
n o rm a l fo r a ra b le so ils
16 - 25
H ig h
slo w d e co m p o s itio n
> 25
V e ry h ig h
little o r n o d e co m p o sitio n
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Denitrification and volatisation
 some N returns to atmosphere as denitrifying bacteria
break down nitrates or nitrites to obtain oxygen & release
gaseous N2 and N2O - escape (volatilise) into the atmosphere
 especially in water-logged, anaerobic and poorly drained soils
 deplete soil fertility and reduce agricultural productivity
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 Thiobacillus denitrificans, Micrococcus denitrificans, &
some species of Serratia, Pseudomonas, and Achromobacter
 Pseudomonas aeruginosa can reduce the amount of fixed
N by 50 percent
 denitrification needed in N cycle or would accumulate
in oceans
 N is lost from plants and soil via other routes such as
erosion, runoff, volatilisation of ammonia, and leaching
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Pseudomonas sp.
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Volatilisation of fertilisers
 N fertilisers containing urea or ammonium may also be lost
through volatilisation as an ammonia gas into the atmosphere
 Ammonium nitrate (34-0-0) and ammonium sulphate (21-0-0)
less subject to ammonium volatilisation than urea [CO(NH2)2]
- (46-0-0)
 losses greater from alkaline than from acid soils
 losses greater from dry rather than wet soils
 losses from sandy soils greater than from heavier soils
 losses greater at higher temperatures than low
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