Nitrogen and other Nutrients in Agroecosystems

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Nitrogen and other Nutrients in Agroecosystems
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Nitrogen in Ecosystems

• Nitrogen
• Essential for plant and animal growth.
• Animals gain N by consumption.
• Plants generally obtain N by root uptake.
• Available N tends to be in short supply in
  ecosystems (therefore competition for N).
• Abundant supply of N in air, but available N pool
  is in soil.

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N Supply Critical for Plants
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Forms of N in Soil

• Organic N (originated from living organisms)
• Inorganic N
• NH4 ammonium
• NO2- nitrite
• NO3- nitrate
• N2 gas
• Etc. N2O

Water soluble for uptake by plant roots
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Immobilization Problem

• Organic (immobilized) N not soluble in water so
  not available to plant roots
• Inorganic N in ion form is water soluble and
  available to plant roots.

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Immobilization Problem

• Plants pick up N via water in roots.
• So inorganic N (NH4, NO2 and NO3) forms are
  soluble and can be easily taken up.
• Mineralization (through decomposition) converts
  immobilized N to available N.

mineralization organic N
inorganic N (not available)
(available)
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Immobilization Problem

• Many organisms (plants, bacteria, fungi) seek
  available inorganic N
• When organisms obtain N, it becomes tied up as
  organic N (immobilized)

mineralization organic N
inorganic N (immobilized)
(available) immobilization
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Recycling N in the Soil System

• Mineralization
• organic N ? ammonium.
• by decomposer organisms (fungi and bacteria).
• R - NH2 H2O ? R - OH NH3 energy
• NH3 H2O ? NH4 OH-
• Fate of NH4
• Nitrification by aerobic bacteria.
• Plants use some.
• Some volatilization of NH3 .

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Recycling N in the Soil System

• Nitrification
• 2-step conversion of ammonium to nitrate, by
  nitrifying bacteria or nitrobacteria.
• Relatively rapid conversion depends on aeration,
  temperature, etc.
• 2NH4 3O2 ? 2NO2- 2H2O 4H energy
• 2NO2- O2 ? 2NO3- energy
• Fate of NO3-
• Uptake by plants.
• Leaching.
• Denitrification loss if poor drainage.

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Other Features of N Cycle

• Denitrification
• Loss of N from soil (if poor drainage, low
  aeration).
• Aerobic bacteria use the NO3 as a source of O2
  due to low O2 content.
• 2HNO3 ? N2O (nitrous oxide) ? N2

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Adding N to the Soil System

• Nitrogen fixation
• adds available N to soil system by conversion of
  N2 in pore spaces in soil.
• N2 ? NH4, NH3
• Regulated by soil organisms

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N Fixation regulated by soil organisms

• Symbiotic N fixation - N fixing bacteria (e.g.,
  Rhizobium), form nodules on plant roots (usually
  legumes)
• Nonsymbiotic (asymbiotic) N fixation - organisms
  free in soil, e.g., Azotobacter, blue-green algae
  (Cyanobacteria).

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Rhizobium nodules on legume root
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Legumes useful in N fixation
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Hairy Vetch good winter crop in Southeast to
add N to soil
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Symbiotic N Fixation by Rhizobium

• Plant supplies sugars (nutrients) to bacteria
• Bacteria supplies N to plant
• Inoculant Rhizobium mix may be applied to seed
  with crop

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Adding N to the Soil System

• Inorganic fertilizers - add N at appropriate step
  in soil system
• NH4
• NO3-
• liquid NH3
•  

Organic fertilizers must be mineralized to
inorganic N for uptake
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Side effects of fertilizer use

• Salts
• pH effects
• Energy required in manufacture
• N2 3H2 2NH3

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Summary Adding N to Soil System

• N fixation
• Recycling of organic matter or organic fertilizer
• Rainwater, lightning
• Inorganic fertilizers

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Adding N to Soil System

• Why is N addition needed if there is natural
  recycling in agroecosystems?
• Because material (containing N) is removed at
  harvest !

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Nitrogen Cycle
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Recycling of N in soil environment
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N additions
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N losses
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Essential Elements for Plant Growth
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Iron Deficiency on Kenaf
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Macronutrients and Micronutrients

• Can be immobilized in plant/organic material
• Must be freed by mineralization for plant uptake
• Often immobilized by reaction with inorganic
  compounds, forming insoluble materials

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K (Potassium)

• Abundant in soil, but tied up in inorganic
  compounds, rocks, etc.
• Leaches rapidly
• Soluble K easily removed by crops

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P (Phosphorus)

• Relatively immobile
• Often tied up in soil and not easily available
• Several inorganic forms, dependent on pH
  (complicated soil chemistry)
• Low pH tied up by Fe, Al
• High pH tied up by Ca

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The Phosphorus Problem Immobilization by
Inorganic Compounds
Fe, Al
Ca
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Phosphorus Cycle in Agriculture
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References

• Text, pp. 55-68.
• Brady, 1974. The Nature and Properties of Soils.
  MacMillan Publishing Co., NY.
• Brady and Weil, 1996. The Nature and Properties
  of Soils. Prentice Hall, Upper Saddle River, NJ.
• Carroll et al. 1990.
• Tivy, 1992. Ch. 5.