Nitrogen Dynamics of Soils

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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

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Introduction

• Nitrogen is an essential plant nutrient
• It is mainly in organic forms in soil
• In its ionic form, it is very mobile in soils
  and plants
• It is also responsible for some environmental
  problems.

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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

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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

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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
  shootroot ratio)
• Plants mature more quickly than healthy plants
• Protein content is low and sugar content is high
• There is reduced productivity in general

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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)

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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
• Symbiotic fixation
• Non symbiotic fixation

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• 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.

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• 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
  N-fixation
• Some heterotrophs e.g. Azotobacter
• Autotrophs e.g. Photosynthetic bacteria

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Distribution of Nitrogen(The Nitrogen Cycle)
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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

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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

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b) NH4

• Like other positively charged ions NH4 is
  attracted to negatively charged soil colloids
• NH4 can be fixed by 21 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)

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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

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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.

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Management of Soil Nitrogen

• Objectives of good N management
• 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