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8/10/2019 E-Soil Organic Matter
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Soil Organic Matter
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Soil ComponentsThe 4 parts of soil
MineralMatter45%
SoilWater
25%
Soil
Air25%
OrganicMatter
5%
About of thesoil volume is
solid particles
About of thesoil volume ispore space
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SOM Improves SoilChemical Properties
Increases Cation
Exchange Capacity so thesoil is better able to storeand supply plant nutrients
Increases pH buffering so
the soil resists changes inpH
Reduces Aluminum, Iron,and Manganese toxicity inacidic soil
Why should you care
about soil organic
matter (SOM)?
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What is Soil Organic
Matter?
All material in soil thatcontains (reduced)
carbon.
SOM is derived from
Plant residue (both litterand roots)
Animal remains and excreta
Living soil microbes
(microbial biomass)
Over time microbestransform fresh organic
material into stable soil
organic matter
Crop residue
Bacteria
FungiActinomycetes
SOM
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Organic matter decompositionEveryone is involved
Earthworms Mix fresh organic materials into
the soil
Brings organic matter into
contact with soil microorganisms
Corn leaf pulled intonightcrawler burrow
Millepede
Ants
Soil insects and other
arthropods Shred fresh organic material
into much smaller particles
Allows soil microbes to accessall parts of the organic residue
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Organic matter decompositionEveryone is involved
Bacteria Population increases rapidly
when organic matter is
added to soil
Quickly degrade simple
compounds - sugars,
proteins, amino acids
Have a harder time
degrading cellulose, lignin,
starch
Cannot get at easily
degradable molecules that
are protected
Bacteria on fungal strands
Spiral bacteria
Rod bacteria
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Organic matter decompositionEveryone is involved
FungiGrow more slowly and
efficiently than bacteria whenorganic matter is added to soil
Able to degrade more complexorganic molecules such as
hemicellulose, starch, andcellulose.
Give other soil microorganismsaccess to simpler moleculesthat were protected by celluloseor other complex compounds.
Soil fungus
Fungus on poplar leaf
Tree trunkrotted by fungi
Fairy ring
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Fungi and Soil Structure Fungal hyphae (threads) help hold soil granules together Fungal exudates (goo) help cement soil particles together
Fungi absent -Soil structure is not maintainedwhen immersed in water
Active Fungi Present Soil structure is maintainedwhen immersed in water
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Organic matter decompositionEveryone is involved
Actinomycetes The cleanup crew
Become dominant in the final
stages of decomposition
Attack the highly complex and
decay resistant compounds Cellulose
Chitin (insect shells)
Lignin
Waxes
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Organic matter decompositionEveryone is involved
Protists and nematodes, thepredators
Feed on the primary
decomposers (bacteria, fungi,
actinomycetes)
Release nutrients (nitrogen)contained in the bodies of the
primary decomposers
Amoeba
Bacteria-feeding nematode
Predatory nematodeRotifer
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Organic matter decompositionCarbon and Nitrogen Cycling
During each cycle of
degradation about 2/3 of
the organic carbon is used
for energy and released as
carbon dioxide (CO2)
Bacteria, FungiSoil organic matter Nematodes, protists, humus
CO2
CO2
Plant litter
During each cycle ofdegradation about 1/3 of
the organic carbon isused to build microbialcells or becomes part ofthe soil organic matter
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All organic matter in soil is not equal
Scientists describe 3 pools of soil organic matter
Passive SOM
500 5000 yrs
C/N ratio 7 10
Active SOM
1 2 yrs
C/N ratio 15 30
Slow SOM
15 100 yrs
C/N ratio 10 25
Recently deposited organic material
Rapid decomposition
1020% of SOM
Intermediate age organic material
Slow decomposition
1020% of SOM
Very stable organic
material
Extremely slow
decomposition
6080% of SOM
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Decomposition
(CO2)
Erosion
Soil Organic Matter
Losses
Inputs
Crop Residues
Crop Roots
Manure
Compost
There is a constant turnover of organic material in soil.
The quantity of SOM depends on the balance between
inputs and losses of organic material
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If losses increase and inputs remain
constant, SOM will decrease
Soil Organic Matter
Decomposition
(CO2)
Erosion
Losses
Inputs
Crop Residues
Crop Roots
Manure
Compost
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If inputs stay constant and losses decrease,
SOM will increase
Soil Organic Matter
Decomp.
(CO2)
Erosion
Losses
Inputs
Crop Residues
Crop Roots
Manure
Compost
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But SOM will not continue to increase or
decrease indefinitely
When inputs or losses are changed, SOM quantity changes to
a different level and a new steady state condition is reached.
SOMlevel
Years of cultivation
SOM in virgin soil
Steady state SOM after
years of continuous
corn cultivation
New steady
state SOM
level
Management
change
imposed
Corn-oats-clover
rotation plus
manure application
1875 1955 2005
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Soil Organic Matter is Dynamic
Rate of decomposition is affected by:
1.
Environm ental Condi t ions
Temperature
Moisture
Aeration (oxygen)
Soil texture
Soil pH
Soil fertility
2.
Qual i ty of added Organic
Material
C/N ratio Composition/Age
Physical properties and
placement
Fresh vs. processed
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Distribution of organic matter in soil under
conventional and no tillage
Soil Organic Carbon (g/kg)
5 10 15 20 25
Depth(cm)
0
5
10
15
20
25
30
35
No-till
ConventionalTillage
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Surface crust impedes infiltration.
Open, granular surface structure enhances
infiltration.
Biopores (earthworm channel)
enhance infiltration and allow water to
rapidly move past the surface horizon.
Conventionally tilled field No-till field
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Managing to Improve Soil Organic MatterTake-home points
Soil Organic Matter is dynamic.
The amount of SOM depends on the balancebetween
inputsof organic material and lossesof SOM from
decomposition and erosion.
Both the quant i tyand qual i tyof organic materialinputs can be managed to increase SOM levels.
Losses of SOM can be reduced by decreasing eros ion
and decreasing t i l lage.
Most change in SOM occurs in the act iveSOM pool.
Many soil quality benefits accrue from the active pool.
Maintaining the size and rapid turnover in the active
pool may be more important for soil quality than
actually increasing the overall SOM level.
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What management changes can be
made to increase i nputof organic
material?Return more crop residues
Add cover crops
Add other sources of
organic material
Diversify crop rotations
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What management changes can be
made to decrease SOM losses?
Decrease erosion
Decrease tillage
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Organic matter decompositionCarbon and Nitrogen Ratio
Average C/N ratioof bacteria and
fungi is 8:1
LitterC/N ratio
around
24:1
CO2
C/Nratio8:1
2/3 of carbonreleased as CO2
Microbial C/N ratio ismaintained at 8:1 with no
uptake or release of N
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Organic matter decompositionCarbon and Nitrogen Ratios
2/3 of carbonreleased as CO2
Average C/N ratioof bacteria and
fungi is 8:1
LitterC/N ratio
around
90:1
CO2
C/Nratio30:1
Immobilization
Soil N
Microbial C/N ratio ismaintained at 8:1 bytaking up N from soil
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Organic matter decompositionCarbon and Nitrogen Ratios
Average C/N ratioof bacteria and
fungi is 8:1
LitterC/N ratio
around
9:1
CO2
C/Nratio3:1
2/3 of carbonreleased as CO2
MineralizationSoil N
Microbial C/N ratio ismaintained at 8:1 byreleasing N to the soil
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