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Soil Organic Matter
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Review - Soil organisms
Bacteria Most numerous, smallest Aerobic and anaerobic
Actinomycetes Share characteristics of bacteria and fungi Active in degradation of resistant compounds
Fungi
Aerobic only, filamentous Active in degradation of resistant compounds
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Major Soil Organisms
Bacteria 10 8/gram
Actinomycetes10 7/gram
Fungi 10 6/gram
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Soil Microorganisms
Can be classified according to C and energysources and their oxygen requirement: photoautotrophs
Energy from sunlight & C from CO 2 Some bacteria and algae only
chemoautotrophs Energy from oxidizing inorganic material, C from CO 2 Some bacteria only
chemoheterotrophs Energy and C from oxidation of organic materials Most bacteria, all fungi and actinomycetes
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Soil Microorganisms Oxygen requirement
aerobic Require free O 2 for respiration All fungi and actinomycetes, most bacteria
anaerobic Must use alternative electron acceptors instead of O 2 NO 3 -, SO 4 2-, Fe 3+ , CO 2
Some bacteria are anaerobic
facultative Can be aerobic or anaerobic. Some bacteria
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Decomposition of Plant Residues
(Under aerobic conditions)
Plant
Residues
CO 2
NH 4+, SO 42-, etc. (inorganic waste)
Humus (organic waste)
+DeadMicroorganisms
More microbial biomass
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Soil Organic Matter
Soil organic matter: all organic matterin the soil, including humus, microbialbiomass, and plant and animal residuesin various stages of decomposition. Composed of a wide range of organic
materials, from highly decomposable to
resistant to decomposition.
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Roles of Soil Organic Matter
Microbial substrate Nutrient reserve (esp. N, P, S)
CEC Water-Holding capacity Soil structure
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Humus
The stable portion of soil organic matter thatresults from microbial degradation ofresidues.
Dark colored About 58% C, 5% N Complex chemical structure, aromatic plus
aliphatic functional groups
Difficult to break down because of structure high CEC
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Humus
The major organic waste by -product ofOM degradation.
The percentage of a residue that willbecome humus is approx. proportional toits lignin content.
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Lignin
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Humus
CarbonHydrogenOxygenNitrogen
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Decomposition of OrganicMatter
Organic materials are decomposed byheterotrophic microorganisms. The
organic matter is a source of _______, __________, and _____________ tothese organisms.
carbonenergy nutrients
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Humus and Nutrients
Humus contains about 58% C, 5%N, 0.6%P, and 0.6% S
How much humus in soils?
How much OM does this represent? An Aridisol with 0.5% SOM in the top 30 cm will contain3000 m 3/ha x 1500 kg/m 3 x 0.005 = 22,500 kg/ha (top 30 cm)
A Mollisol with 5.0% SOM in the top 30 cm will contain3000 m 3/ha x 1500 kg/m 3 x 0.05 = 225,000 kg/ha (top 30 cm)
An Aridisol might contain 0.5% SOM by weight, a Mollisol 3-5% by weight
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Decomposition of Humus The rate of decomposition of humus is most strongly
affected by soil moisture and temperature (5%/yr).
Humus is chemically complex and has a C:N ratio ofabout 11:1
High soil temperatures, abundant (but not excessive)moisture encourages rapid humus breakdown
In soils where OM content is not decreasing, synthesis
of new humus approximately equals decomposition ofold humus.
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Decomposition (Mineralization) ofHumus
Releases N as NH 4+ , available for plants
If 2.5% of the N in SOM is mineralized each year,how much N would be released for plant uptake?
Aridisol (from previous example) 22,500 kg SOM/ha x 0.05 kg N/kg SOM x 0.025 (% min)
= 28 kg N/ha
Mollisol (from previous example)
225,000 kg SOM/ha x 0.05 kg N/kg SOM x 0.025 (%min) = 280 kg N/ha
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Decomposition of Plant Residues
(Under aerobic conditions)
Plant
Residues
CO 2
NH4+, SO 42- , etc. (inorganic waste)
Humus (organic waste)
+DeadMicroorganisms
More microbial biomass
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What Happens to Residues?
CO2
Biomass
Waste
CO2
Biomass
Waste
Chemically simpleresidues
Chemically complexresidues
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Decomposition of Plant Material
The rate of decomposition of plantresidues is governed mostly by: Chemical makeup of the residue C:N ratio Available soil N Temperature, moisture, oxygen, and other
environmental conditions that affect microbialgrowth
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Chemical Composition of Plant Residues
Sugars Complex proteins Hemicellulose Cellulose LigninSimple proteins WaxesStarchs
Increasing chemical complexity
Increasing rate of decomposition
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C:N Ratio
Why is the C:N ratio important? Microorganisms need C and N in fixed ratios,
because C and N are used to synthesize proteins,
nucleic acids, etc. Bacterial cell C:N is 5:1 to 8:1. Since about 50% of
the C in an organic material is converted to CO 2,they need roughly a C:N of 10:1 to 16:1 in the
residue they consume. Fungi need a C:N of about 40:1 in their diet
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decomposition
C:N Ratio
50 g C20 g as CO 2
20 g as biomass
Microbial biomass has an averageC:N of 10:1, therefore how much Nis needed to balance the new biomassC?
10 g as waste
2 g
Therefore, if the residuecontaining 50 g of Ccontains < 2 g of N
(C:N>25:1), it will haveinsufficient N for microbialneeds. What about>2 g N (C:N
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C:N Ratios
High C:N material: Woody Grain crop residue
Mature plant tissues
Low C:N material: Green Young plant tissues
Legume residues Composts Manures
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C:N Ratio and Residue Mgmt.
What are the implications of the C:N ratioof crop residues for nutrient management?
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Immobilization
The conversion of inorganic (available) N (NH 4+ , NO3-)to microbial biomass N. Results from...
N H
4 + a n
d N O
3 - )
Time
C O 2 r e l e
a s e
C:N ratio of residues
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Mineralization
The conversion of organic (unavailable) N to NH 4+ .Results from...
N H
4 +
Time
C O 2 r e l e
a s e
C:N ratio of residues