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REACTIONS OF NITROGEN
Dr. Michael Pfeiffer
Nitrogen is a requisite for plant growth and is
needed in relatively large amounts. If organic or
inorganic forms of nitrogen are added to soil, nitrogen
undergoes reactions which may benefit or reduce plant
growth. The principle reactions of nitrogen that
impact on plant growth are: Mineralization,
Nitrification, Volatilization and Denitrification.
Mineralization:
Conversion of unavailable forms of organic
nitrogen to available forms for plant uptake. If organic
matter such as Milorganite® or manure are added to
soil, nitrogen contained in these materials is unavailable for plant uptake. For plants to utilize the nitrogen
in these materials, it must be converted from the
unavailable organic forms to available inorganic
forms. Microorganism in soils are responsible for this
conversion. Microorganisms break down organic
compounds which contain nitrogen such as proteins to
amino acids. The amino acids are further decomposed
to release ammonia (NH3). Plants will pick up very
small amounts of nitrogen in the form of ammonia.
High amounts of ammonia are toxic. Luckily, when
ammonia contacts water in soil, it forms the
ammonium ion (NH4+). Yes, the same ammonium
that is contained in ammonium sulfate and ammonium
nitrate. Plants may or may not pick up the ammonium.
Yes, if it is dissolved in water in soils. No, if the
ammonium becomes bound to organic matter or clay
in soil. If ammonium is bound to soil particles it must
be displaced off the binding material through cation
exchange before it is in the water in soil for plants to
utilize.
How fast do these mineralization reactions
proceed? Rate of conversion is dependent on the rate
which microorganisms (mainly bacteria and fungi)
degrade the organic material. Factors which impact on
populations of microorganisms impact on the rate of
organic matter degradation. Warm, well aerated soils
with adequate water and a pH above about 5.0
enhance mineralization. Cold, waterlogged, compacted, soils with a pH below about 5.0 inhibit this
process. Fumigation essentially eliminates mineralization for a period of time: time being dependent on
how thorough the fumigation and how long it takes
for organisms from surrounding soil to re-colonize.
The ratio of carbon to nitrogen (C:N) in organic
material can influence rates of mineralization. If the
carbon to nitrogen ratio of added material is low, 15:1,
then decomposition/mineralization proceeds relatively
rapidly. If the C:N ration is high, above 30:1,
mineralization proceeds slowly. The slowdown in the
mineralization process from organic matter with a high
C:N ratio is termed nitrogen immobilization. In order
for microorganisms to decompose organic matter, there
must be sufficient nitrogen available. During
decomposition, if nitrogen run short in the material
being decomposed, microorganisms will remove
nitrogen from surrounding soil. If the availability of
nitrogen in the surrounding soils runs short, then
decomposition comes to a halt. All nitrogen is tied up,
there is none available for further decomposition. If
there are plants growing in soils where immobilization
has occurred, their growth is stopped from lack of
nitrogen. Materials such as sawdust, straw bark mulch
and some planting material have high C:N ratios. I
have seen plant growth of nursery stock come to a halt
after planting in soil because the potting medium they
were grown in at the nursery had a very high C:N ratio.
When they were planted in soil there was insufficient
nitrogen contained/added to decompose the planting
medium: plant growth stopped.
Organic matter has been said to be a slow release
type of fertilizer. It is! It takes time for the microorganisms to degrade the organic matter to release
nutrients which are available for plant uptake: nitrogen,
phosphorus, calcium, magnesium etc. Simplified
reactions for the mineralization process are below.
Organic nitrogen + microbial degradation º amino acids
amino acids + Microbial degradation º Ammonia (NH3)
* NH3 (ammonia) + H2O º NH4+ (ammonium) + OH!
(pH of soil increases slightly from ammonium hydroxide)
* Technically this is not part of the mineralization
reactions but happens readily once ammonia finds
water in soil. This could be ammonia from the degradation of organic matter, from anhydrous ammonia
bubbled into irrigation water or injected into soils or
from material such as urea.
Nitrification:
Processes in soil which convert ammonium
(NH4+) to nitrate (NO3!). Conversion of ammonium to
nitrate is a two step process. Step one, conversion of
ammonium to nitrite (NO2!). Step two, conversion of
nitrite to nitrate. These processes like the
mineralization process are driven by microorganisms
in the soil. While many microorganisms are capable
of conversion of ammonium to nitrite and nitrite to
nitrate, species of the bacteria Nitrosomonas and
Nitrococcus are the primary organisms which carry
out these reactions. Nitrosomonas convert ammonium to nitrite (NO2!) . Nitrococcus bacteria convert
nitrite to nitrate (NO3!) . Reactions for the nitrification
processes are below
Step One Conversion of ammonium to nitrite:
Nitrosomonas + 2NH4+ + 3O2 ÷ 2NO2! + 2H2O + 4H+
Really what is produced is nitrous acid and water.
Ever wonder why ammonium sulfate reduces the pH
of soils? No, it is not the 24% sulfur in the form of
sulfate, it is the above reaction: the production of
nitrous acid. The sulfate has absolutely nothing to do
with the drop in pH from using ammonium sulfate.
Step two Conversion of nitrite to nitrate
Nitrococcus + NO2! + O2 ºNO3!
These reaction proceed most quickly in warm,
well aerated soils. Nobody shows up for work in cold
and/or waterlogged and/or compacted soils. Also note
that oxygen is required. These reactions do not
proceed in the absence of or at low oxygen levels.
Plants pick up most of their nitrogen in the form of
nitrates. There are two reasons for this. The
ammonium ion (NH4+) is positively charge and as
such is held by negatively charged clay particles and
to some degree by organic matter. If ammonium is
displaced off soil particles by cation exchange it is
often grabbed by Nitrosomonas and converted to
nitrite then to nitrate by Nitrococcus. Plants may
rarely see much free ammonium for uptake.
Note the negative charge associated with the
nitrate ion. Negatively charged ions are not held by
negatively charged clay particles. The only material in
soil which has a tendency to hold negatively charged
ions is organic matter which is in short supply in
many soils. Nitrates are subject to leaching. Any
nitrate not picked up by microorganisms and slid past
plant roots is gone: on its' way to groundwater.
There are nitrification inhibiting compounds such
as nitrapyrin and dicyandiamide that will slow the
activity of Nitrosomonas. Reducing the activity of
© PESTICIDE TRAINING RESOURCES 2008 032408
Nitrosomonas reduces conversion of ammonium to
nitrite and ultimately nitrite to ?leachable” nitrate.
Dicyandiamide contains 65% nitrogen by weight so
when added to fertilizer, it slows down the production
of nitrate and acts as a slow release form of nitrogen:
it must be broken down by microorganisms to be
available for plant uptake. Dicyandiamide is effective
at reducing nitrate production for perhaps 4-6 weeks
depending on microbial activity.
Volatilization
When urea (NH2)2CO) is added to the surface
of moist soil, the enzyme urease rapidly hydrolyses
urea to form ammonium carbonate (NH4)2CO3 which
can rapidly break down to form ammonia, carbon
dioxide and water. Since ammonia is a gas, there is
potential for loss of nitrogen from the surface of soils.
Sulfur coating helps reduce loss of ammonia by
reducing the rates of release of the urea. Materials such
as NBPT N(n-butyl) thiophosphorictriamide and PPD
(phenylphosphorodiamidate) when added to urea
sources may reduce loss of nitrogen in the form of
ammonia. They inhibit the activity of urease.
Factors which increase loss of nitrogen from urea
due to volatilization are: warm, wet soils with high pH,
soil with low cation exchange capacity, and materials
applied to the surface of soils and not incorporated
thoroughly. Reactions which result in loss of nitrogen
from urea due to volatilization are given below:
(NH2)2CO (urea) + UREASE º (NH4)2CO3
8+ 2H2O + CO28
(NH4)2CO3 + H2O º 2 NH38
Denitrification:
The process whereby nitrogen is lost because of
microbial conversion of available nitrate (NO3!) to
unavailable forms: nitrite (NO2!), nitrous oxide (N2O),
nitric oxide (NO) and elemental nitrogen (N2). Bacteria
such as species of Bacillus, Micrococcus,
Pseudomonas and Thiobacillus in anaerobic soils
(waterlogged soils containing low or no oxygen) will
convert nitrate nitrogen to other nitrogen containing
compounds which are not available for plant uptake.
Soils prone to nitrogen loss by denitrification are: soils
over-irrigated, compacted, layered soils, any/and all
poorly drained soils. Denitrification reactions are listed
below.
NO3! º NO2! º N2O8
8 º NO8
8 º N28
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