Download ORGANIC MATTER DECOMPOSITION

ORGANIC MATTER
DECOMPOSITION
CHAPTER 6
OBJECTIVES
Process of organic matter decomposition
Factors affecting organic matter
decomposition
Relate the activities of microorganism
with soil fertility
SOIL
There are three major properties of soil.
Physical – soil structure and texture
Chemical – chemical components; pH, nutrients
Biological – micro and macro fauna/flora
Soil organic matter (OM) is any material produced
originally by living organisms (plant or animal)
that is returned to the soil and goes through the
decomposition process
Contain five major groups of
microorganisms
Bacteria
Actinomycetes
Fungi
Algae
Protozoa
All these microorganisms participate in the
various activities that take place in the soil.
Among the activities are;
Decomposition of organic matter
Nutrient Cycling
Nutrients transport/flow
Protection
ESSENTIAL PLANT NUTRIENTS
There are at least 16 essential chemical elements for plant
growth
Plant must have these nutrients to performance the
various physiological functions
C, H and oxygen (O), (from air & water)
N, phosphorus (P), potassium (K), Ca, Mg, sulfur (S), iron
(Fe), manganese (Mn), Zn, copper (Cu), boron (B),
molybdenum (Mo), and chlorine (Cl) (from soil)
Sodium (Na), silicon (Si), and nickel (Ni) Cobalt (Co)
(required by certain plants)
SOURCES OF PLANT NUTRIENTS
IN THE SOIL
1) weathering of soil minerals
2) decomposition of plant residues, animal
remains, and soil microorganisms
3) application of fertilizers and liming materials,
4) application of manures, composts, biosolids
(sewage sludge) and other organic amendments
5) N-fixation by legumes
6) ground rock powders or dusts including
greensand, basalt, and rock phosphate
7) inorganic industrial byproducts
8) atmospheric deposition, such as N and S from
acid rain or N-fixation by lightning discharges,
9) deposition of nutrient-rich sediment from
erosion and flooding
Basic Plant Nutrient Cycle
The basic nutrient cycle highlights the central role
of soil organic matter and microorganisms.
Cycling of many plant nutrients, especially N, P, S,
and micronutrients, closely follows the Carbon
Cycle.
Plant residues and manure from animals that are
fed forage, grain, and other plant-derived foods are
returned to the soil.
This OM pool of carbon compounds becomes food
for bacteria, fungi, and other decomposers.
As OM is broken down to simpler compounds,
plant nutrients are released in available forms for
root uptake and the cycle begins again.
Plant-available nutrients such as K, Ca, Mg, P, and
trace metal micronutrients are also released when
soil minerals dissolve.
DECOMPOSITION OF OM
Definitions:
Breakdown of dead plant and animal material
and release of inorganic nutrients
Decomposition is a biological process that
includes the physical breakdown and
biochemical transformation of complex organic
molecules of dead material into simpler organic
and inorganic molecules (Juma, 1998).
SOURCE OF ORGANIC MATTER
Plant remains
Animal tissues and excretory products
Cells of microorganisms
However, plant is the main contributor to
OM
ORGANIC CONSTITUENTS OF
PLANTS
1. Cellulose, most abundant 15 to 60% of dry
weight
2. Hemicelluloses, 10 to 30%
3. Lignin, 5 to 30%
4. Water soluble fraction include simple sugar,
amino acids, and aliphatic acids, 5 to 30% of
tisue weight
5. Ether and alcohol-soluble constituents; fats, oils,
waxes, resins and a number of pigments
6. Proteins
WHY MICROORGANISMS
DECOMPOSE OM
Supplying energy for growth
Supplying carbon for new cell synthesis
The cells of most microorganisms commonly
contain approximately 50% carbon. This is
derived mainly from the substrates.
WHY DO WE CARE ABOUT
DECOMPOSITION?
Decomposition is important in releasing
nutrients tied up in dead organic matter
and return it back to the soil.
WHO ARE THE DECOMPOSERS?
A. Soil fauna (e.g., earthworms, arthropods):
physical
fragmentation (cominution) increases surface area,
distributes OM within soil profile, doesn’t alter litter
chemistry
B. Soil microorganisms: heterotrophic bacteria and
fungi
derive energy, carbon, and nutrients from dead OM;
in the process they release CO2 through respiration;
RESPONSIBLE FOR BULK OF DECOMPOSITION!!
DECOMPOSITION PROCESSES
There are three main processes
1. Assimilation = conversion of substrate materials into
protoplasmic materials. Eg. OM carbon to microbial
carbon. Protein to microbial protein.
2. Mineralization = conversion of organic substance to
inorganic form. Eg. Protein from the OM will be
converted to inorganic nitrogen (NH4, NO3) in the soil.
3. Immobilization = conversion of inorganic form into
organic form. Eg. Inorganic nitrogen from the soil
converted into microbial protein.
FACTORS AFFECTING RATE
OF DECOMPOSITION
1. Temperature
Microbial activity responds exponentially to increased
temperature until enzymes denature, etc.
2. Moisture
Microbial activity has optimum moisture
Low moisture = desiccation, slow diffusion
High moisture = low O2 availability; no lignin
degradation
3. pH
Most microbes exhibit optimum activity near pH 7.
Fungi most active in acid soil and bacteria in moderate
soil pH.
Substrate Quality: Carbon
Different carbon compounds are decomposed
at different rates.
Cellulose faster
Lignin slower decomposition as compared to
cellulose.
C:N of the OM determine the rate:
high slower, this is due to insufficient of N for
microorganisms to assimilate carbon;
low faster, nitrogen is sufficient for rapid
assimilation of carbon.
WHAT IS THE C:N RATIO?
The carbon-to-nitrogen (C:N) ratio is often used
as an indication of whether mineralization or
immobilization will occur.
The C:N ratio is the total concentration of C
divided by the total concentration of N.
C:N RATION RANGE
Because there is a suite of microorganism and OM
quality, generally we can predict whether
mineralization or immobilization will take place base
on the C:N ration range
When surface of soil layer have a:
C:N > 30:1 => immobilization highly likely to occur
C:N < 20:1 => mineralization is likely to occur
C:N between 20-30:1 => both processes may occur
but will generally in balance
C:N OF SOME ORGANIC MATTER
Organic Matter
C:N Ratio
legumes
13 – 25:1
manure
20 - 30:1
straw
80:1
sawdust
400 – 600:1
microorganisms
5 – 10:1
The C:N ratio is the most commonly used in soils
because N is the most limiting element.
A microbe with a C:N ratio of 8:1 would require OM
with a C:N ratio of 24:1.
The C:N ratio in lower in microorganisms and = 8.
Since microbes incorporate only about 1/3 of the C
metabolized into biomass, the substrate material must
have C:N ratio = 24 to satisfy the N requirement of
microbes.
If the C:N ratio > 24, available soil N is consumed by
microbes and plant available N decrease.