soil organic matter and decomposition. organic compound + o 2 (or other electron acceptor) co 2 + h...
Post on 19-Dec-2015
222 views
TRANSCRIPT
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)
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)
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
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
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