e-soil organic matter

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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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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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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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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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



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


fungi is 8:1

LitterC/N ratio

around

9:1

CO2

C/Nratio3:1


MineralizationSoil N

Microbial C/N ratio ismaintained at 8:1 byreleasing N to the soil

e-soil organic matter

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