Soil Carbon Slides

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					       Basic Biological Factors of
Soil Carbon and Soil Organic Matter

 Teaching Team of the Agricultural Microbiology Program,
     Faculty of Agriculture, Gadjah Mada University

 CO2                                  (sugar)

H2 O                                 H2 O

 6 CO2 + 12 H2O +        C6H12O6 + 6 O2 + 6
      Light                    H2O
               The Soil Food Web

                         Animals                                      Birds



  Organic Matter:
 waste, residue, and                                          Worms
metabolites of plants,
    animals, and
              The Soil Food Web
 Burrowing animals, insects, and earthworms mix, help
  form aggregates, and add nutrients to the soil. When
  animals die, they decompose returning nutrients to the
  soil. Insects chop up plant and animal residue which
  increases the surface area available to microorganisms
  for decomposition.

 Soil organisms are responsible for the transformation of
  plant material to humus. Plant and animal residue make
  up a large portion of organic matter (OM) in soil. SOM
  or humus is the glue that helps hold soil into aggregates.
  Plant cover helps stop both wind and water erosion as
  does the aggregation effect of SOM.
                                 Carbon Cycle



                                         Litter        Crop Residues            Animal
Organic                  Soil                                                   Manure
                                      CO3, HCO3
Horizons               Reactions
                                                           Microbial Activity

                                     Carbon Dioxide
      Microbial Activity

                                    Leaching Losses
                  Carbon Cycle
 CO2, through photosynthesis, is converted to plant

 When the crop is harvested and removed from the farm,
  carbon is lost. If livestock consume the crop, the carbon
  may be returned to the soil in the form of manure.

 Crop residue, roots, and manure are a carbon (energy)
  source for microorganisms.

 Converting organic carbon to CO2 is mineralization of
  carbon. When microorganisms respire, CO2 is released
  to the atmosphere.
                  Carbon Cycle
 Short-term SOM is residue that is readily decomposed.
  Short-term SOM is a source of nitrogen, phosphorus,
  and sulfur for plants. Short-term SOM lasts 1 to 3 years.
 Long-term SOM (humus) is the carbon form that resists
  decomposition and may last for greater than 1000 years.
 Soil carbon losses are exacerbated through erosion and,
  to a lesser extent, may be lost through leaching of
  dissolved organic carbon (DOC).
 The basic processes of the carbon cycle are: CO2 in
  through photosynthesis, and CO2 out through
                    Soil Organic Matter

• Organic matter
  encompasses all organic
  components of a soil:
   – Fresh residues
   – Decomposing
     organic matter
   – Stable organic matter
   – Living organisms
               Fresh Residues

• Up to 15% of organic
  matter is fresh residue
• Comprised mainly of
  litter fall
• Much can be recognized
  as plant residue
        Source of Fresh Residues
                 Harvest index (HI)
• HI = grain yield / biological yield
  – Biological yield = grain yield + stover yield

           Corn:        0.50 to 0.55
           Wheat:       0.40
           Soybean: 0.40 to 0.45
           Sorghum: 0.48
            Carbon / Nitrogen Ratio
           High C/N ratio

                                                 Low C/N ratio

            Corn                                              Legume

 Decomposition is slower.            Decomposition is rapid due to higher
                                       nitrogen within the plant.
 Microorganism will deplete soil
  of nitrate and ammonium until       Microorganisms are satisfied with
  they die and release nitrate and     plant N. When microorganisms die,
  ammonium.                            nitrate and ammonia are released,
                                       increasing soil N.
           Carbon / Nitrogen Ratio
 Carbon / Nitrogen (C/N) ratios are important. Plant and
  animal residues that have a C/N of 30:1 and over, have
  too little N to allow for rapid decomposition. Therefore,
  the microorganisms will take ammonium and nitrate out
  of the soil to fuel decomposition. This depletes the soil of
  nitrate and ammonium. Plants and animal residues with
  low C/N ratios (20:1 and less) have sufficient N for the
  microorganisms to decompose the residues without
  taking from the soil.
        Decomposing Organic Matter
• Plant material is
  transformed from one
  organic compound to
  another mainly by
  organisms in the soil
• Organisms create by-
  products, wastes, and cell
• Compounds released as
  waste by one organisms
  can often be used as food
  by another
      Soil Organic Matter =SOM
• SOM is labile -it can decline rapidly if the soil
  environment changes and renewable -it can be
  replenished by inputs of organic material to
  the soil.
• Adequate levels of SOM can be maintained
  with proper fertilization, crop rotations, and
  tillage practices if crop residues are returned
  to the soil.
              Soil Organisms
• A cubic meter of healthy soil =
  home to billions of bacteria and fungi, millions
  of nematodes, tens of thousands of spring tails
  and mites and several hundred earthworms
• These "critters" are nature's recyclers,
  converting plant residue and animal manures
  into usable nutrients and soil organic matter.
              Soil Organisms

• The soil microflora (bacteria, fungi and algae)
  are responsible for 90% of the decomposition of
  organic material.
• Plant residue is the main fuel for sustaining the
  broad spectrum of organisms residing in the soil.
              Soil Organisms
• The microbial population can increase rapidly
 to take advantage of a favorable change in the
 soil environment, doubling in a few hours.
• The size of the microbial population is usually
 controlled    by    soil    moisture,    aeration,
 temperature and their distribution in the soil.
             Active Fraction
• 10 to 30% of the soil organic matter (active
  fraction) is responsible for maintaining soil
• The active fraction of organic matter is most
  susceptible to soil management practices.
  (Inactive = humus)
           Adding Fresh OM
• In a soil which at first has no readily
  decomposable materials, adding fresh tissue
  under favorable conditions:
• 1) immediately starts rapid multiplication of
  bacteria, fungi, and actinomycetes,
• 2) which are soon actively decomposing the
  fresh tissue.
                Fresh SOM
• as most readily available energy sources are
  used up,      microorganisms again become
  relatively inactive, leaving behind a dark
  mixture usually referred to as humus – a stable
  organic compound
          Stable Organic Matter
• Soil organic compounds
  become stabilized and
  resistant to further
  changes by
• Bound inside soil
• Stabilized organic
  matter acts like a sponge
  and can absorb six times
  its weight in water
• Newly-formed humus –
1. combination of resistant materials from the
   original plant tissue
2. compounds synthesized as part of the
   microorganisms' tissue which remain as the
   organisms die (Fluvic and Humic Acid)
• Humus is resistant to further microbial attack-
   N and P are protected from ready solubility.
          Function of Humus
• holds water and nutrients;
• it sticks together & helps establish and
  maintain a strong crumb structure & thus
  reduce soil erosion
• it provides some nutrients (N & P) as it is
  slowly decayed by microbial activity,
• Buffers effects of pesticides
• humus decomposes at the rate of 2.5% per

      SANDY SOIL               CLAY SOIL
    SOM Maintains soil “Tilth”

•   aiding infiltration of air and water
•   promoting water retention
•   reducing erosion
• Measuring SOM is one step in assessing overall
  soil quality or soil health -
• measuring various key attributes of soil
  organic matter quantity and quality will give
  an indication of the health of the soil.
            Can we change it?
• Residue management
  – Removal
  – Additions
     • Manure (animal and green manure crop)
• Tillage
  – Mixing
  – Aeration
             Can we change it?
• Crop selection
    – Species
    – Rotation or crop sequence
    – Crop use (grain, forage, grazing)
   Management practices
       Irrigation
       Fertilization
       Pest control
       Planting time, planting density, etc.
                                               Can you change it?
              organic matter (kg m



                                           Native sod   No-till   Stubble   Bare fallow

(Cambardella and Elliot, 1992)
     Residue Management Study
• Background
  – Response to energy crisis of the 70s
• Objective (question)
  – What is the affect of removing crop residues from
    (and adding residue additional to) the soil surface
     • On soil properties?
     • On crop production?
               Residue Management Study
                                        Over all years
                              Coefficient                               10

                                                        Yield (Mg/ha)
               Intercept            Slope                                        Residue
Y Variable                                                               6
               b0        SE       b1        SE
                                                                         2                   Grain
Residue       4.34       0.31    0.29   0.04     0.86
                                                                             0         4        8          12
  Grain       2.91       0.17    0.13   0.02     0.80                            Applied residue (Mg/ha)

(Wilhelm et al., 1986)
                 Residue Management Study
                                      Soil Water

                                                   Soil water at planting (mm)

                             Coefficient                                         250

 Dependent        Intercept       Slope                                          200
  Variable                                   r2
                   b0    SE      b1    SE                                        150

 Soil water       174    7       6     1    0.84
                                                                                       0         4        8          12
                                                                                           Applied residue (Mg/ha)

(Wilhelm et al., 1986)
                                           Residue Management Study
                                            Soil organic matter (0-30 cm)
      Soil organic matter (g kg -1)

                                            After 8 yr of treatment application (cont. corn, no tillage)




                                               0                  50                 100                   150
                                             Amount of previous crop residue returned (%)
(Maskina et al., 1993)
Without Residue Management Study
            Summary (production)
• Greater plant stress
  – Less available soil water
  – Greater soil temperature
• Decreased yield (50% removal)
  – 13% reduction in grain yield
  – 17% reduction in residue yield
  – Yield reductions more closely related to soil water
    than soil temperature
     Residue Management Study
      Summary (Soil organic matter)
• Soil organic matter
  – Changed linearly with residue application
  – Remove 100%          -5.7%
    Remove 50%           -3.4%
    No removal            -----
    Add 50%              +4.6%
“Throwing rocks is easy!” (Pat Gruber, CargillDo LLC, Sept. 6, 20001)

    A bit of skepticism can be a good thing.
    You can’t have your cake (soil organic matter) and eat
     (burn) it too.
    If it sounds too good to be true, it probably is.
    There are no free lunches.
           Rules in the Real World
• Nature always bats last.
• Consequences of ignoring rules:
  – “Today's solutions become tomorrow's problems.”
    (Tom Franzen, Iowa Farmer)

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