Soil Organic Matter
and
Decomposition
Organic compound + O2
(or other electron acceptor)
CO2 + H2O + energy + inorganic nutrients
a form of respiration.
an oxidation reaction
aided by microbial enzymes.

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)


Loss of electrons
Fe+2
Fe+3
-26
+28
Fe+2
e-
-25
+28
Fe+3


Gain of electrons
Fe+3
Fe+2
-26
+28
Fe+2
e-
-25
+28
Fe+3
CO2 + H2O + energy
O2 + C6H12O6
C6H12O6 + O2
Energy-rich
Reduced carbon
CO2 + H2O + energy
energy-poor
oxidized carbon
(no energy available for further reactions)
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.

Get carbon from organic compounds

Get energy from aerobic respiration
 Use oxygen as electron acceptor in decomposition
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

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
 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
immobilization
inorganic
compounds

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
1 rapid
to
6 slow
4
5
2
3
6
6
1



“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

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

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

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)

determines rate at which residue will decay
and whether it will release (mineralize) or
immobilize N after incorporation into soil.
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





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




Sawdust
Newspaper
Wood chips
Straw


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



Manure
Cover crop
Household compost (composted)
1.
Add high/wide
C:N residue:
microbial activity, CO2
long nitrate depression
final N level
2.
low/narrow C:N:
microbial activity, CO2
no nitrate depression
final N level