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 levels

Autotrophs: (get C from CO2)

Photoautotrophs▪ (Get energy from sun)

Chemoautotrophs▪ (Get energy from inorganic

chemical reactions):▪ Oxidation of N,S, Fe

Heterotrophs (get C from organic

compounds)

Oxidation

Loss of electrons Fe+2 Fe+3

+28

-25

Fe+3

+28

-26

Fe+2

e-

Reduction

Gain of electrons Fe+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 + energy

Energy-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 compounds

Get energy from aerobic respiration Use oxygen as electron acceptor in

decomposition

Anaerobic bacteria get energy from:

1. Anaerobic respirationuse nitrate, sulfate (or others) as electronacceptor

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 ▪ (NH4

+, SO4-2, HPO4

-2)

organic compounds

mineralization

inorganiccompounds

immobilization

In mineralization, nutrients formerly stored in organic form are released for use by living organisms ORGANIC INORGANIC

In immobilization, these nutrients are reabsorbed and assimilated by living organisms

INORGANIC ORGANIC

Organic matter cycle

Composition of plant residues

1 rapidto6 slow

123

45

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.CO2 levels

2.Feeding frenzy

3.Priming effect

4.Arrows: C transfers

5.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

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 rate

Greater energy expense and release of CO2

Higher 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 content Mineralized N will be released soon

after decay starts So microbes won’t suffer a shortage as

they begin decomposing More C from residue can be diverted

to microbial growth Higher proportion of total C in easily

decomposable compounds Faster decomposition

Narrow C:N examples:

Manure Cover crop Household compost

(composted)

1. Add high/wide C:N residue:microbial activity, CO2

long nitrate depressionfinal N level

2. low/narrow C:N: microbial activity, CO2 no nitrate depression final N level