Transcript Organic Matter

Soil Organic Matter
Chapter 12
Soils and the greenhouse effect
C cycle
Decomposition of organic matter
C / N ratio
Humus
Factors affecting soil organic matter
Managing soil organic matter
Other related stuff
Histosols
Composts
Soils and the Greenhouse Effect
Increasing concentration of certain gases
in atmosphere increases retention of solar
energy
Size of wedges indicate
relative effect, not absolute
concentrations.
Anaerobic
organisms
Wetlands bad?
Produced by soil microorganisms
Carbon Cycle
The C cycle is complex, with many different
pools and rates of transformation from one to
another. However, in essence, it can be
condensed into cycling between C in organic
combinations and C in inorganic forms, like
CO2, HCO3-, etc.
What if this rate
is larger?
Well, you get a build-up of
inorganic C and perhaps
anthropogenic global
climate change.
Deforestation,
drainage and
tillage increase
decomposition
of soil organic
matter
According this accounting,
annual net loss of C from
the soil (62 – 60) as CO2
is > 1/3 of emissions due
to burning fossil fuels –a
major contribution. Thus,
we can slow on-set of climate change by reversing this situation, i.e., sequestration
of atmospheric C (CO2) into soil organic C.
Decomposition of Organic Matter
Releases CO2 + H2O
N, P and S released as inorganic ions
mineralization
This is microbially-mediated.
And at same time
N, P and S reincorporated into organics
immobilization
Taken up by soil bugs and plants, and incorporated into their biomass.
Thus, these nutrients are immobilized.
Decomposition rate depends on
Composition of residue
Aeration
C / N ratio
Composition should make sense. More complex
molecular structures like lignin are less susceptible
to decomposition than, say, starch. As for aeration,
biological processes are faster under aerobic
conditions. The last factor, C / N, should also make
sense. If a substrate contains other essential elements,
like N, P, S, etc., then the bugs decomposing it may
obtain these nutrients from it and the more of these
in the substrate the better for the bugs. N is often the
limiting nutrient, thus, the focus on C / N ratio.
C / N Ratio
Relatively constant in soils = 12
Variable in plants
Increases as plants mature
Decomposition faster if C / N low
That is, if N is present in a relatively
high concentration.
Compare rye and vetch, N-fixing plant (via
association in the actual atmospheric N-fixers,
certain microbes). The vetch has more N, therefore,
is decomposed faster than the rye. The less mature
rye, killed 4 / 8,
has a higher N
concentration
than the older
rye.
C / N of fresh organic matter also affects
availability of N to plants
This is a matter of competition between the microbes decomposing the
residue and plants. If the organic matter has little N (high C / N), then
the bugs use available soil N to satisfy the demand of their increasing
population (growth stimulated by added organic matter). The next couple
of slides shows the arithmetic of the matter.
C / N in microorganisms = 8 / 1
Microbes incorporate about 1/3 of C
metabolized into biomass
So OM must have C/N ≤
24 / 1 to meet
N needs of microbes
(24 – 2/3 x 24) / 1 = 8 / 1, no?
What happens if C / N is > 24 / 1?
What if C / N of residue = 36 / 1
2 / 3 of C respired and 1 / 3 makes biomass
So C / N = 12 / 1
To get to C / N = 12 / 1½ = 8 / 1
The ½ temporarily depletes the available soil N.
Microbes use available soil N and
plant- available N decreases
Contrast the two data sets.
Top: Addition of high C / N
organic matter does stimulate
microbial growth but to build
microbial biomass, N in addition to what is in the organic
matter is needed. Thus, the
level of available N (stated
here as NO3-, but NH4+ also)
is depleted. Eventually, the
N in microbial biomass is
released and the level of
available N in the soil is
higher than initially, but there
is this temporary (could be
long) depletion.
Bottom: High N / C in organic
matter results in increasing
concentration of available N
from the time the organic
material was added.
C / N Ratio Problem
Wheat straw is incorporated in soil during the spring and to avoid
N-depression supplemental N is also added. Assume the wheat straw
contains 48 % C and 0.50 % N by weight (dry matter), the C / N ratio of
microbial biomass is 8 and soil microorganisms assimilate 1/3 of the
carbon in the wheat straw.
1. What is the C / N ratio of the wheat straw?
2. How much C from the wheat straw is assimilated into microbial
biomass (kg C per 100 kg straw)?
3. How much supplemental N (kg) must be added for every 100 kg of
wheat straw in order to supply all the N required by microorganisms?
Addition of N will prevent the temporary reduction in available inorganic
N that otherwise would occur as the growing microbial population
satisfies its demand for N.
The stable end-product of organic
residue decomposition in soil is called
A) humus
B) humorous
C) humongous
Humus
Soil OM includes
So, soil organic matter is everything
and humus is just an important part
of soil organic matter.
Biomass
Partially decomposed residue
Decomposed and colloidal residue
humus
Humus includes
About 80% of humus
is humic substances and
the balance is slowly
decomposed biomolecules.
Humic substances
But compared to the humic
substances, these slowly
decomposed biomolecules
are rapidly decomposed.
Slowly degraded polymers
Less resistant biomolecules
Non-humic substances
Humic substances
In lab you extracted fulvic and humic
acid from the organic soil using base.
When you added acid, the humic acid
began to come out of solution and you
reversed this by adding base again.
Substance MW
Decomposition
Fulvic acid lowest
slow
Humic acid higher
very slow
Humin
extremely slow
highest
The ½ life of fulvic acid (or acids, there is no set structure to these things, just
commonalities among what we call fulvic acid and humic acid) is maybe +1 year.
That for humic acid, +10 years and for humin, +100 years. If curious about structure,
look at the earlier figure for lignin –proposed structures are somewhat similar.
Adsorption on clay increases resistance of
humus to microbial decomposition
Does this help explain why
there is more organic matter in
heavier texture soils?
Yes, and this ought to make sense.
Properties of humus
High surface area / mass > silicate clays
High CEC (pH-dependent or no?)
High water holding capacity
Of course it’s pH-dependent. The concept of permanent
charge due to isomorphic substitution in an organic structure
is nonsense. Please recall the charge on organics is due
to ionization –COOH, Ar-OH, etc., giving – sites, or protonation
of C-NH2, etc., giving + sites, mostly -, though.
Effects of OM
on
Plants, Soil and Water Quality
Effects on plants
Greater water holding
Supplies nutrients
Better aeration
Stimulates microbes
Growth + / - compounds
Effects mostly indirect. The better
aeration comes from the effect
of organic matter on increasing
soil aggregation, thus, interaggregate porosity (large pores).
To a much lesser extent, certain
organics may directly affect
plant growth, as with allelopathic
substances.
Effects on soil
Color
Aggregation
Water holding capacity
CEC
Effects on water quality
Greater infiltration
Reduced storm flow, soil erosion and
surface transport of contaminants
Greater chemical adsorption
Higher soil fertility
So, need less in fertilizers. You know that
soluble N and P may degrade water quality
in certain circumstances.
So, less mobility of chemicals
in the soil or loading into runoff.
Factors Affecting Soil OM
Climate
Type vegetation
Texture
Drainage
Natural conditions
Cropping
Tillage
Rotations
Fertilizer
Managed conditions
Climate
Temperature and
Moisture
Aerated
If you examine these curves,
you should see maximal
accumulation of organic matter
where not only relative wet but
also cool.
Wet
Vegetation
Grasslands > forests
Texture and drainage
Clay > sand
Poorly drained > well drained
Forest
Prairie
Farmed
Dry
Wet
Native
Weird and Wet Forest
Typical distribution of organic matter (or organic C) with depth. More organic C
where wet, in grassland than forest, and less where a soil initially high in organic
C has been farmed from a long time. So, what is the subsurface bulge in organic C
in the Spodosol called?
Repetitious but good –more where wetter and more in prairie.
Cropping and tillage
Virgin land >
No-till
>
cropped land
conventional-till
Rotations and fertilizers
?
?
?
Take a good look at the next slide for answers. The data are from an old
field plot study from the U. of Illinois. Famous in the agricultural domain.
What a drop in organic C with continued farming!
The unfertilized rotation was not as bad as always
corn. The fertilizer with rotation treatment was best
but results are probably biased because manure
was included. What do you think?
Established in 1876, the
Morrow Plots (University of
Illinois) are the oldest
agronomic experiment fields
in the United States. They
include the longest-term
continuous corn plot in the world.
Designated a National Historic
Landmark in 1968.
Management Guidelines
Plant growth
Reduced tillage
Adding OM
good
good
good
These make sense, right?
Regardless, the level of organic
C (proportional to organic matter)
is set by a balance between rates
of organic C addition to the soil
and its decomposition. Both
depend on how the soil is
managed.
Know limitations –a ceiling exists
Is N also required for OM accumulation?
If there is something of a equilibrium between N and C (C / N ratio ~ 12), can you
expect the addition of 100 tons of organic C in sawdust to result in a persistent
level of organic C ≥ 10% (assuming an acre of soil weighs 1,000 tons)?
Histosols
Peat partially decomposed residue
Muck highly decomposed residue
General properties
Low bulk density
0.2 - 0.4 g cm-3
High water holding
capacity
2 - 3 x mass
Importance
Role in C cycle
20 % of soil OM
Agricultural use
oxidation and
wind erosion
problems
Potting media
Composts and Composting
Create humus-like matter outside soil
Of course, this is a misnomer because it ain’t.
Aerobic process in which
Nutrients
conserved and
concentrated
This makes it a whole
lot cheaper to transport
on a per mass of
nutrient basis. Good.
But you sure wouldn’t want to use
the stuff unless you were confident
it didn’t contain weed seeds and
pathogens. Good thing the composting
process generates really high temperatures.
High internal
temperature
kills weed seeds
and pathogens
Mulch
Potting mix
Slow-release fertilizer
Possible Lignin Structure
Possible HS Structure (Part)
Biochemistry of Humic Substance Formation
Formation of HS not understood but thought to involve 4 stages
1
2
3
4
Decomposition of biomolecules into simpler structures
Microbial metabolism of the simpler structures
Cycling of C, H, N, and O between soil OM and microbial biomass
Microbially-mediated polymerization of the cycled materials
Lignin (lignin-protein) theory
(Waxman, 1932)
Lignin incompletely used by microbes and residual part makes up HS
Lignin
Microorganisms Use Part
Residual Unused Part
Demethylation, oxidation and
condensation with N compounds
Humic acids
Fulvic acids
Polyphenol theory
These from either from lignin decomposition or derived by
microbes from other sources such as cellulose
Oxidation of polyphenols to quinones leads to ready addition
of amino compounds and development of structurally large
condensation products
Sugar-amine condensation theory
Simple reactants derived from microbial decomposition
undergo polymerization
All may occur but relative importance is site-specific