1. Soil Organic Matter
and
Decomposition

2. Basic Decomposition Equation
Organic compound + O2
(or other electron acceptor)
CO H2O + energy + inorganic nutrients
a form of respiration.
an oxidation reaction
aided by microbial enzymes.

3. 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)

4. Oxidation
Loss of electrons
Fe Fe+3 e-
-26
-25
+28
+28
Fe+2
Fe+3

5. Reduction
Gain of electrons
Fe Fe+2 e-
-26
-25
+28
+28
Fe+2
Fe+3

6. Photosynthesis (brought to you by autotrophs)
CO2 + H2O + energy O2 + C6H12O6

7. Respiration (required by all others)
C6H12O6 + O CO H2O + energy
Energy-rich energy-poor
Reduced carbon oxidized carbon
(no energy available for further reactions)

8. Decomposition revisited…
Organic compound + O2(or other electron acceptor) CO2 + H2O + energy + inorganic nutrients
1) Organic substrate (dead plants and animals) 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 soil atmosphere.
4) Carbon cycles through decomposition and photosynthesis, serving as vehicle of energy flow among hetero and autotrophic organisms.

9. Most soil bacteria are heterotrophic and aerobic
Get carbon from organic compounds
Get energy from aerobic respiration
Use oxygen as electron acceptor in decomposition

10. 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 or nitrate or sulfate,etc)
reduced to by-product, such as alcohol or organic acid

11. 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

12. 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)

13. organic compounds mineralization inorganic
immobilization

14. 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

15. Organic matter cycle

16. Composition of plant residues
1 rapid to
6 slow 4 5 1 2 3 6 6

17. 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

18. 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

19. Decomposing microorganisms:
Zymogenous: opportunists; eat “easy” food; reproduce rapidly (r-strategists)
Autochthonous: eat very resistant organic compounds; slowly reproducing
(K-strategists)

20. Notice:
1.CO2 levels
2.Feeding frenzy
3.Priming effect
4.Arrows: C transfers
5.Humus levels
Microbial biomass

21. 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

22. 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)

23. 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.

24. 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

25. 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

26. 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

27. Wide C:N examples:
Sawdust
Newspaper
Wood chips
Straw

28. 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

29. Narrow C:N examples:
Manure
Cover crop
Household compost (composted)

31. long nitrate depression final N level
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

32. Calculating C:N of SAP compost pile
5(100:1) + 7 (45:1) + 28 (80:1) =
= = 76 : 1