Nitrogen Dynamics of Soils
 Introduction
Forms and Role of Nitrogen in Soils & Plants
Nitrogen Fixation
Distribution of Nitrogen (N-Cycle)
 Nitrogen Transformations
 Fate of Nitrogen in Soils
Nitrogen Fertilizer and Management
Introduction
 Nitrogen is an essential plant nutrient
 It is mainly in organic forms in soil
 In it’s ionic form, it is very mobile in soils
and plants
 It is also responsible for some
environmental problems.
1. Forms of Nitrogen taken by
plants
Nitrogen in the soil solution is in three forms
NO3 NH4+
 SON

NO3- and NH4+ ions behave differently in soils
and are differently preferred by plants.
NO3- exchange with HCO3- and OH- at root surface
–increase pH
 NH4+ exchange with H+ at root surface –reduce pH

2. Role of nitrogen in plants
Component of essential plant compounds
Amino acids ~ building blocks of proteins, enzymes
 nucleic acid ~ hereditary control
 Chlorophyll ~ photosynthesis

Plants respond to good available N by having
deep green color of leaves
 Increased protein content
 increased plumpness of grains
 Increased plant productivity in general

3. Deficiency of nitrogen in
plants
 Nitrogen is quite mobile (easily translocated) within
plants, so available N is sent to newest foliage first
 Deficiency exhibits the following:
Pale yellowish green color (chlorosis) in older leaves
 Have stunted growth and thin stems (low shoot:root
ratio)
 Plants mature more quickly than healthy plants
 Protein content is low and sugar content is high
 There is reduced productivity in general

4. Oversupply of nitrogen in
plants
When too much N is applied, the following
may occur:
Excessive vegetative growth (but weak plant
stems) that lead to lodging with rain or wind
 Delays maturity and cause plants to be prone to
diseases
 General decline of plant product quality
 Environmental aspects (build up of nitrates)

Nitrogen Fixation
 N-Fixation is the process of converting the inert dinitrogen
gas of the atmosphere to nitrogen containing organic
compounds that becomes available to all forms of live
 N-Fixation is accomplished in nature biologically by a)
certain microorganisms, and b) by lightening.
Biological Fixation

Biological fixation which is by far more important
than lightening is catalyzed by enzyme nitrogenase.
N2 + H+ +6e- = 2NH3 + H2

NH3 is formed which combines with organic acids to
form amino acids and ultimately protein.
NH3 + Organic Acids → Amino Acids → Proteins

The two types of biological fixation are:
1.
2.
Symbiotic fixation
Non symbiotic fixation
Symbiotic Fixation (Legumes & Non legumes)
Legumes and bacteria enter into symbiotic
relationship. The bacteria (Rhizobium) infects
the root hair and cortical cells inducing the
formation of root nodules that serve as the site
of nitrogen fixation. Plant provides carbohydrate
Some non legumes have also been observed to
develop nodules that form site of N fixation.
Some nitrogen fixation have even been
described as fixation without nodules.
Nonsymbiotic Nitrogen Fixation
 Certain free living organisms present in soil that
are not directly associated with higher plants are
able to fix nitrogen.
 Because these organisms are not directly
associated with higher plants, the transformation
is referred to as nonsybiotic or free living Nfixation
Some heterotrophs e.g. Azotobacter
 Autotrophs e.g. Photosynthetic bacteria

Distribution of Nitrogen
(The Nitrogen Cycle)
Nitrogen Transformations
Mineralization

Conversion of organic forms of N into inorganic forms
(NO3- and NH4+)
Immobilization

Conversion of inorganic N forms (NO3- and NH4+) to
organic N
Nitrification

Conversion of NH4+ ions into NO3-
Denitrification

Conversion of NO3- to N gas (NO, NO2, N2O)
Volatilization

Transformation of NH4+ ions into ammonia gas
Fate of SON, NH4+ and NO3- in
Soils
a) Soluble organic Nitrogen (SON)
Not much information is available on this N
form
 Comprise of amino sugars and amino acids
 It is taken up directly by plant roots
 OR, it is leached and carried in groundwater to
streams where they cause environmental
problems

b)
NH4+
Like other positively charged ions NH4+ is
attracted to negatively charged soil colloids
 NH4+ can be fixed by 2:1 minerals because of
its unique size
 NH4+ can volatilize into NH3 gas
 NH4+ can also be converted into nitrates (NO3-)
directly (nitrosomonas bacteria)
 OR through an intermediary step (NO2-)
(nitrobacter bacteria)

c)
NO3-
Plants utilize NO3- directly
NO3- is negatively charged and so is not adsorbed
by the negative charges that dominate most soils.
This makes it move down freely with drainage
water causing several environmental problems

(eutrophication and hypoxia).
Nitrate can also be converted to gaseous forms of
nitrogen by series of reduction reactions.

Denitrification
Nitrogen Fertilizer
Fertilizers supply nitrogen in soluble forms such
as nitrate or ammonium, or as urea.
Nitrate or ammonium from fertilizer are taken up
by plants and participate in the N cycle in exactly
the same way as nitrate or ammonium derived
from organic matter mineralization or other
sources.
Fertilizer-N has much more concentration in time
and space than N from other sources.
Management of Soil Nitrogen
Objectives of good N management
1.
2.
3.
Maintain adequate N supply
Regulate the soluble forms of N to ensure enough is
readily available
Minimizing leakage from soil-plant system
Strategies for Achieving the Objectives:




Taking into account N contribution from other
sources so as not to oversupply N
Improving efficiency with which fertilizer is applied
Improving crop response knowledge
Avoiding overly optimistic goals of meeting crop
needs that are higher than possible