Soil Organic Matter and - PowerPoint Presentation

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Slide1

Soil Organic Matter

and

Decomposition

Basic Decomposition Equation

Organic compound + O2 (or other electron acceptor) CO2 + H2O + energy + inorganic nutrients a form of respiration.an oxidation reactionaided by microbial enzymes.

Review of food chain concept:

Trophic levelsAutotrophs: (get C from CO2)Photoautotrophs(Get energy from sun)Chemoautotrophs(Get energy from inorganic chemical reactions):Oxidation of N,S, FeHeterotrophs(get C from organic compounds)

Oxidation

Loss of electronsFe+2 Fe+3

+28

-25

Fe

+3

+28

-26

Fe

+2

e

-

Reduction

Gain of electronsFe+3 Fe+2

+28

-25

Fe

+3

+28

-26

Fe

+2

e

-

Photosynthesis (brought to you by autotrophs)

CO2 + H2O + energy O2 + C6H12O6

Respiration (required by all others)

C6H12O6 + O2 CO2 + H2O + energyEnergy-rich energy-poor Reduced carbon oxidized carbon (no energy available for further reactions)

Decomposition revisited…

Organic compound + O2(or other electron acceptor) CO2 + H2O + energy + inorganic nutrients

1) Organic substrate is oxidized by inorganic oxidizing agent (O, N, S, etc).

2) Nutrient elements are contained in organic substrate too.

These are

mineralized

in respiration.

Decomposition frees nutrients (

N,P,S,etc

).

3) CO2 escapes to atmosphere.

4)

Carbon

cycles through decomposition and photosynthesis, serving as

vehicle of energy flow

among hetero and autotrophic organisms.

Most soil bacteria are heterotrophic and aerobic

Get carbon from organic compoundsGet energy from aerobic respirationUse oxygen as electron acceptor in decomposition

Anaerobic bacteria get energy from:

1. Anaerobic respiration

use nitrate, sulfate (or others) as electron

acceptor

2.

Fermentation

use organic substrate as electron acceptor (instead of oxygen)

reduced to by-product, such as alcohol or organic acid

Oxygen harmful to anaerobes:

In aerobes, when oxygen accepts electrons, and is reduced,

toxic compounds

(e.g., hydrogen peroxide) are produced.

Aerobic organisms have adapted mechanisms (2

enzymes

) to counteract toxins

ANAEROBES LACK THESE ENZYMES

What do microbes need?

Nutrients, Carbon, energy.

Up to 50% of C in decomposed compounds is retained as microbial tissue

Some N,P,S also

If amount of nutrients exceeds amount needed by microbes, they released as inorganic ions

(NH

4

+

, SO

4

-2

, HPO

4

-2

)

organic

compounds mineralization inorganic compounds

immobilization

In

mineralization, nutrients formerly stored in organic form are released for use by living organisms ORGANIC INORGANICIn immobilization, these nutrients are reabsorbed and assimilated by living organisms INORGANIC ORGANIC

Organic matter cycle

Composition of plant residues

1

rapid

to6 slow

1

2

3

4

5

6

6

Humus

“Amorphous, colloidal mixture of complex organic substances, not identifiable as tissue”.

C:N:P:S = 100:10:1:1

Composed of

humic

substances

Resistant, complex polymers

10s to 100s of years

and

nonhumic

substances

Less resistant, less complex

Humus is colloidal

Large surface area per unit volume

Greater than clay

Negatively charged

OH

-

and COOH

-

groups

High

nutrient holding capacity

(high CEC)

High

water-holding capacity

Decomposing microorganisms:

Zymogenous

: opportunists; eat “easy” food; reproduce rapidly (r-strategists)

Autochthonous

: eat very resistant organic compounds; slowly reproducing

(K-strategists)

Notice:

1.CO

2 levels2.Feeding frenzy3.Priming effect4.Arrows: C transfers5.Humus levels

Microbial biomass

Each type of plant residue has a C:N ratio

Decomposing residue is not only a source of energy, but also a source of nutrients for microbial growth.

N

is the element most often lacking in soil/residue to point of limiting

microbial population growth

Limiting factor

Amount of N is more critical than amount of C :

Carbon

usually makes up 45 – 55% of dry weight of tissue

Nitrogen

can vary from < 0.5% - >6.0%

For a residue with:

50% carbon and 0.5% N, C:N ratio would be ?

100:1 (wide/high C:N)

50% carbon and 3.0% N, C:N ratio would be ?

16:1 (narrow/low C:N)

C:N ratio in organic residue

determines

rate at which residue will decay

and whether it will release

(mineralize)

or

immobilize

N after incorporation into soil.

What is cutoff for high and low C:N?

Soil microbe cells need 8 parts C for 1 part N (C:N = 8:1)

only 1/3 of C from food is incorporated into cells

therefore, they need food with a C:N of ?

24:1

If

C:N

ratio > 24:1, intense

competition

among microbes for soil N

If ratio is too wide, N will be used (immobilized) by microbes and plants may suffer N deficiency.

Compost those materials before adding to soil

Organic residues with WIDE C:N ratios:

Comparatively low N Microbes suffer a shortage as they begin decomposing, so have to get N from soil at a cost in energy expenditure and decomposition rateGreater energy expense and release of CO2Higher proportion of C in resistant compounds (cellulose, lignin)slower decomposition

Wide C:N examples:

Sawdust

Newspaper

Wood chips

Straw

Organic residues with NARROW C:N ratios:

Comparatively high N contentMineralized N will be released soon after decay startsSo microbes won’t suffer a shortage as they begin decomposingMore C from residue can be diverted to microbial growthHigher proportion of total C in easily decomposable compoundsFaster decomposition

Narrow C:N examples:

Manure

Cover crop

Household compost (composted)

Add high/wide

C:N residue:

microbial activity, CO2 long nitrate depression final N level

low/narrow C:N:

microbial activity, CO2

no nitrate depression

final N level