5-soil organic matter

7/26/2019 5-Soil Organic Matter

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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

5-soil organic matter

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