Chapter5a

Carbon Cycle

All organic matter eventually is oxidized (burned) and converted back to carbon dioxide and water.

Decomposition of Organic Compounds in Aerobic Soils

When organic tissue is added to an aerobic soil, three general reactions take place:

• Carbon compounds are enzymatically oxidized to produce: CO₂, H₂O, energy, and decomposer biomass.

Decomposition of Organic Compounds in Aerobic Soils

When organic tissue is added to an aerobic soil, three general reactions take place:

• The essential nutrient elements, such as: N, P, and S, are released and/or immobilized by a series of specific reactions that are relatively unique for each element.

Decomposition of Organic Compounds in Aerobic Soils

When organic tissue is added to an aerobic soil, three general reactions take place:

• Compounds resistant to microbial action are formed, either: through modification of compounds in the original tissue; or by microbial synthesis.

Decomposition in Anaerobic Soils

• The products of anaerobic decomposition include a wide variety of partially oxidized organic compounds, such as:
  – organic acids
  – alcohols
  – methane gas

Decomposition in Anaerobic Soils

• Anaerobic decomposition releases relatively little energy for the organisms involved.

  – the end products still contain much energy.

Decomposition in Anaerobic Soils

• Issues related to the products of anaerobic decomposition:
  – they produce foul odors/inhibit plant growth.
  – the methane gas produced in wet soils is a major contributor to the greenhouse effect.

Nitrogen Cycle

• Nitrogen in Atmosphere = 78%

• Problem is getting N₂ into a form that plants can use.

Nitrogen Fixation

*Conversion of nitrogen gas (N₂) into NH₃ or RNH₂

• N fixation - the reduction of atmospheric nitrogen gas (N₂) to ammonia (NH₃)

– can only be done biologically by highly specialized group of microorganisms in the presence of the enzyme nitrogenase which catalyzes the reduction of N₂ to NH₃

• The ammonia, in turn, is combined with organic acids to form amino acids and, ultimately, proteins.

Nitrogen Fixation

Biological Fixation

1. Non-Symbiotic (independent organism) - Azotobacter - aerobic & Clostridium - anaerobic

2. Symbiotic - mutually beneficial for host organism and bacteria - complex plant - bacteria interaction

Symbiotic N-Fixation

• Bacteria = Rhizobia
  

• Plant = Legume - mungbeans, soybeans, cowpeas, peanuts, beans

Symbiotic N Fixation

• Bacteria invade host plant root.

• Response of host plant root = to grow a nodule for the bacteria to live in.

• Bacteria takes N₂ from the air and converts it into R-NH₂ which resides in Bacteria in nodule and some is in the form of NH₄₊

• Fate of N Fixed by Rhizobium:

  • used by host plant;

  • leaks out of root to become available to surrounding plants; and

  • as roots and nodules are sloughed-off heterotrophic organisms immobilize the N and it eventually becomes part of the SOM.

Ammonification

• Ammonification = the conversion of organic N (RNH₂) into inorganic ammonia (NH₃)

• R-NH₂ ---> NH₃ + H⁺ ----> NH₄⁺

  heterotrophic organ. (ammonium)

Fates of NH4 +

1. fixed by clay minerals

2. lost by soil erosion

3. used by plants (NH4 +)

4. volatilization

   • NH₄ + ----> NH₃

Nitrification

2-step process

1. NH₄⁺ + 1½O₂ ---> NO₂- + 2H⁺ + H₂0 + 275 kJ E
   • Nitrosomonas

2. NO₂- + ½O₂ --> NO₃ - + 76 kJ E

   • Nitrobacter

Process is acid causing due to release of 4H⁺

Fates of Nitrate

• Immobilization ---> Plant uptake of NO_3-

• NO_3- is not held by soil particles and is easily leached;

  • when ppm NO_3- is > 10 ppm the water is considered to be contaminated.

• Denitrification - stimulated by anaerobic conditions.

Denitrification

• Involves conversion of NO_3- to N₂ gas

  C₆H₁₂O₆ + 4NO_3- --> 6CO₂ + 6H₂O + 2N₂ (gas) + NO + NO₂

• Bacteria = anaerobic

• Through nitrification and denitrification, 10 - 20% of the applied N is lost.

Mineralization & Immobilization

• Mineralization – the conversion of organic N into inorganic forms such as:

  – NH₄₊ and NO_3-

• Immobilization – the conversion of inorganic N back to organic N.

  – available N is used by soil organisms and assimilated into their bodies.

Sulfur Transformations

• Like N, sulfur (S) also undergoes mineralization, immobilization, oxidation, and reduction through microbial activities.

Sulfur Transformations

• S becomes available to plants through decomposition of S-containing compounds like:

  – amino acids cysteine and methionine.

Sulfur Transformations

• Under aerobic environment, S may be oxidized by Thiobacillus thiooxidans to SO_4⁼ with a release of energy:

  -(HS) + 2O₂ —> SO_4⁼ + H⁺ + energy

  Sulfur
  Amino acids

Sulfur Transformations

• Elemental S, thiosulfates (S₂O₃^2-) , and polythionates (S_2xO_3x^2-) are also subject to oxidation by these organisms:

  S + 1 ½ O₂ + H2O —> H₂SO₄

Sulfur Transformations

• Under anaerobic conditions as in paddy soils, sulfate may be reduced by the bacteria Desulfovibrio desulfuricans.

• They use the oxygen in sulfate to oxidize organic compounds.

• The end product is H₂S or, in the presence of Fe, FeS.

• The characteristic offensive odor of flooded soils is primarily emanating from H₂S.

Pesticide Degradation

• Biochemical degradation by soil organisms

  - the single most important method by which pesticides are removed from soils.

Pesticide Degradation

• Certain polar groups on the pesticide molecules, such as —OH, —COO, —NH₂, provide points of attack for the organisms.

Pesticide Degradation

• DDT and other chlorinated hydrocarbons, such as:

  • aldrin, dieldrin, and heptachlor

    - very slowly broken down, persisting in soils for 20 or more years.

Pesticide Degradation

• The organophosphate insecticides, such as parathion, are degraded quite rapidly in soils by a variety of organisms.

Pesticide Degradation

• The most widely used herbicides such as:

  • 2,4-D phenylureas, the aliphatic acids, and the carbamates

    - readily attacked by a host of organisms.

Pesticide Degradation

• Most organic fungicides are also subject to microbial decomposition;

  • the rate of breakdown of some is slow,

    - causing troublesome residue problems.