Decomposition Basic Decomposition Equation Organic compound O 2 or other electron acceptor CO 2 H 2 O energy inorganic nutrients a form of ID: 775852
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Slide1
Soil Organic Matter
and
Decomposition
Slide2Basic Decomposition Equation
Organic compound + O2 (or other electron acceptor) CO2 + H2O + energy + inorganic nutrients a form of respiration.an oxidation reactionaided by microbial enzymes.
Slide3Review 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)
Slide4Oxidation
Loss of electronsFe+2 Fe+3
+28
-25
Fe
+3
+28
-26
Fe
+2
e
-
Slide5Reduction
Gain of electronsFe+3 Fe+2
+28
-25
Fe
+3
+28
-26
Fe
+2
e
-
Slide6Photosynthesis (brought to you by autotrophs)
CO2 + H2O + energy O2 + C6H12O6
Slide7Respiration (required by all others)
C6H12O6 + O2 CO2 + H2O + energyEnergy-rich energy-poor Reduced carbon oxidized carbon (no energy available for further reactions)
Slide8Decomposition 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.
Slide9Most soil bacteria are heterotrophic and aerobic
Get carbon from organic compoundsGet energy from aerobic respirationUse oxygen as electron acceptor in decomposition
Slide10Anaerobic 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
Slide11Oxygen 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
Slide12What 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
)
Slide13organic
compounds mineralization inorganic compounds
immobilization
Slide14In
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
Slide15Organic matter cycle
Slide16Composition of plant residues
1
rapid
to6 slow
1
2
3
4
5
6
6
Slide17Humus
“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
Slide18Humus 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
Slide19Decomposing microorganisms:
Zymogenous
: opportunists; eat “easy” food; reproduce rapidly (r-strategists)
Autochthonous
: eat very resistant organic compounds; slowly reproducing
(K-strategists)
Slide20Notice:
1.CO
2 levels2.Feeding frenzy3.Priming effect4.Arrows: C transfers5.Humus levels
Microbial biomass
Slide21Each 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
Slide22Amount 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.
Slide24What 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
Slide25If
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
Slide26Organic 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
Slide27Wide C:N examples:
Sawdust
Newspaper
Wood chips
Straw
Slide28Organic 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
Slide29Narrow C:N examples:
Manure
Cover crop
Household compost (composted)
Slide30Slide31Add 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