Transcript organic matter decomposition

ORGANIC MATTER
DECOMPOSITION
CHAPTER 7
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 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 microorganmisms.
 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 organic matter pool of carbon compounds
becomes food for bacteria, fungi, and other
decomposers.
 As organic matter 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 ORGANIC MATTER
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
1. Plant remains
2. Animal tissues and excretory products
3. Cells of microorganisms
However, plant is the main contributor
to organic matter
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 ORGANIC MATTER
1.
Supplying energy for growth
2.
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 (comminution) increases surface area,
distributes organic matter within soil profile, doesn’t
alter litter chemistry
B. Soil microorganisms: heterotrophic bacteria
and fungi
derive energy, carbon, and nutrients from dead
organic
matter; 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. Organic matter carbon to
microbial carbon. Protein to microbial protein.
2. Mineralization = conversion of organic substance to
inorganic form. Eg. Protein from the organic matter will
be converted to inorganic nitrogen in the soil.
3. Immobilization = conversion of inorganic form into
organic form. Eg. Inorganic nitrogen NH4 from the soil
converted into microbial protein.
FACTORS AFFECTING RATE OF
DECOMPOSITION
Environmental Factors
Temperature
Microbial activity responds exponentially to increased
temperature until enzymes denature, etc.
Moisture
Microbial activity has optimum moisture
Low moisture = dessication, slow diffusion
High moisture = low O2 availability; no lignin
degradation
pH
Most microbes exhibit optimum acitivty 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 organic matter 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.
The C:N ratio is the most commonly used in
soils because N is the most limiting element.
Knowing the impact of litter decomposition on
available N is therefore very important.
Microbes can easily out-compete plants for
available N
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
• In general, for every gram carbon used for microbial
biomass, another 2 gram will be respired as CO2.
A microbe with a C:N ratio of 8:1 would require organic
matter with a C:N ratio of 24:1.
• Because there is a suite of microorganism and organic
matter quality, generally we can predict whether
mineralization or immobilization will take place base on
the C:N ration range
1. C:N > 30 = Nett N immobilization
2. C:N > 20 but < 30 N immobilization = N
mineralization
3. C:N < 20 = Nett N mineralizarion