Transcript Soil Organic Matter - AAMU Myspace Login

Soil Organic Matter
Carbon Cycle
 Importance of Organic Matter to Soils
Components of organic matter
 Decomposition of organic matter in Soils
 Factors that Affect Decomposition
Classification of Soil Organic Matter
 Management of Soil Organic Matter
What is Organic Matter
Organic Matter is a complex and varied
mixture of organic substances.
All organic substances by definition
contain carbon.
On the average, C comprises about
50% of the mass of organic matter in
soils.
Organic Matter in Soils
 Soil is a large reservoir of organic matter
and therefore C

Since Carbon comprises about 50% of SOM
 Soil organic matter has 3 times more
carbon than C in all the world’s
vegetation.
 Therefore soil organic matter plays an
important role in global C balance.
 See The Carbon Cycle
The Carbon Cycle
Importance of Organic
Matter to Soils
Soil physical properties
1.
2.
3.
a)
Enhances aggregate stability (helps form soil structure, reduce
erosion)
b)
Reduces stickiness of clayey soils (easy to manipulate)
c)
Lowers bulk density of the soil (easy for roots and organisms to live)
d)
Improves soil capacity to retain water
Soil chemical properties
a)
Holds nutrient cations (Ca, K, Mg, etc) in easily
exchangeable form
b)
Provides pH buffering
c)
Complexes toxic ions and make them unavailable
Biological effects
a)
b)
Provides food for the community of soil organisms
Affects the rates of decomposition
Composition of Plant Residues
Components of Organic Matter
1.
•
•
Carbohydrates (CH2O)x
usually the most prominent compound found in plants
range from simple carbohydrates (sugars and starch) to
more complex carbohydrates (cellulose and
hemicellulose).
Proteins
2.


contain nitrogen in addition to C, H, & O
are also primary sources of other essential plant
elements such as sulfur, manganese, copper and iron.
Lignins and Polyphenols
3.




complex compounds with ring-type (phenol) structures
Component of plant cell walls
found more in woody plant tissues as plants get older
very resistant to decomposition
Fats, Waxes, and Oils
4.
•
•
Found in seed and leaf coatings
More resistant than carbohydrates, but less resistant than
lignins and polyphenols
Organic Matter Decomposition
1. Organic matter decomposition depends on
the components of the Organic Matter
Rates of decomposition (in descending order)
–
–
–
–
–
–
Sugars, starches, and simple proteins Rapid decomposition
Crude proteins
Hemicelluloses
Cellulose
Fats, waxes, etc.
Very slow decomposition
Lignins
2. Organic matter decomposition also depends
Physical and environmental conditions in the
soil
 Decomposition of organic matter
depends on presence of microorganisms
 Presence of microorganisms depends on
the soil aeration.
 Decomposition of organic matter would
be different for aerobic and anaerobic
soils
Aerobic Soils
 Breakdown of Carbohydrates
oxidized to CO2, and H2O
release of energy, and essential nutrient elements if present e.g.,
Nitrogen, Phosphorus, and Sulfur
 Breakdown of Proteins
Yields amino acids in addition to CO2, and H2O; contain N, and S
So amino acids are further broken down to NH4+, NO3-, and SO42-.

Breakdown of Lignin
Molecules are large and complex interlinked structures most of
which are phenolic ring units
Only few microorganisms can breakdown lignin
Thus decomposition of lignin proceeds very slowly with the help of
the physical activities of fauna to yield phenolic units (stable soil
organic matter).
Anaerobic Soils
• In anaerobic soils, anaerobic or facultative
organisms are dominant.
• Decomposition is much slower, so these soils
tend to accumulate large amounts of organic
matter.
• Decomposition end products include organic
acids, alcohols, and methane gas.
• These end products contain a lot of energy
because anaerobic decomposition in the first
place releases little energy to organisms.
Other Factors Controlling Rates of
Decomposition and Mineralization
a) Other Physical and environmental
conditions in the soil
pH –a near neutral pH
Temperature –warm tempt (25-35 °C)
Moisture –sufficient moisture
b) Quality of the added residue as a food
source for for organisms
Physical placement of the residue
Surface residues, incorporated residues, particle size
C/N ratio of organic matter is important for the
following:
Competition among soil organisms for available soil N
Determines rate of decomposition and nitrogen release
Classification of Soil Organic Matter
Humus
Humus is a heterogenous mixture of complex
organic compounds resulting from
biochemical degradation and synthesis.
The group of substances called humus can no
longer be identified as tissues
Humus is made up of a humic substances
group and a nonhumic substances
group.
Humic Substances
•
60-80% of soil humus
•
Characterized by variable structures formed by decomposition,
synthesis and polymerization
•
Variable rather than specific in composition, dark in color
•
very high molecular weight, from a few thousand to several
thousand g/mol
•
fulvic acids - lowest mol. wt. & lightest in color, decomposes
in 10-50 years, soluble in acid & alkali, and most susceptible to
microbial attack.
•
humic acids - medium in mol. wt. & color, intermediate in
decomposition, soluble in alkali.
•
humin - highest mol. wt., darkest in color, most resistant to
microbial attack, insoluble in acid & alkali.
Nonhumic Substances
•
•
•
•
•
20-30% of soil humus
This is a group of identifiable biomolecules
that are mainly produced by microbial action.
They have less complex structures with
molecules of defined physical and chemical
properties.
less resistant to microbial attack than humic
group
many polysaccharides, organic acids &
protein-like materials, polyuronides,
polyphenols are examples of compounds
produced by microbes.
Management of Amount and
Quality of Soil Organic Matter
1. Add continuous supply of organic materials
(plant residues, manures, compost, cover
crops)
2. Don’t try to maintain SOM at a higher level
than indicated by the natural conditions (ie
dictated by climate and the other factors)
3. Maintain nitrogen at appropriate levels by
planting legumes in crop rotations and the
judicious use of N fertilizers
Management of Soil Organic
Matter (contd.)
4. Maximize plant growth to increase residue
5.
6.
7.
8.
inputs. May require addition of lime and
nutrients depending on the soil type.
Use conservation tillage.
Maintain perennial natural vegetation
wherever possible.
Boost productivity of existing cropland so as
to limit the need for claiming natural land for
farming.
Allow exhausted and marginal cropland to
return to natural ecosystems.