Soil organic carbon plays a key role in soil health and fertility. It is an important component of soil organic matter, comprising 5% of average soil composition. Soil organic matter improves soil structure, increases the soil's water holding capacity, and serves as a "nutrient fund" by regulating the release of nutrients for plant uptake. Maintaining or increasing soil organic carbon levels is important for sustaining agricultural productivity and mitigating climate change, as soils can sequester atmospheric carbon through conservation practices that promote the buildup of soil organic matter over time.

1. Soil Organic Carbon
All availed amenities by industrialized
societies are based on fossil fuel
derived energy. Thus, the modern
civilization can be appropriately termed
“the Carbon Civilization” or the C-Era
(Rattan Lal,2007),
The modern civilization is dependent on C-based
energy sources. It is literally hooked on carbon, and
in need of a big-time rehabilitation.

2. N
C
P
K
Ca
Mg
S
Zn
Cl
Mn
Carbon is a
“keystone”
Bo
Building of agriculture

3. Nutrient cycling requires carbon!
Carbon is the “Lord of the Rings”
C
H2O
K
P
S
Cl
Mg
Ca
Zn
N
Bo
Mn
Mo
Cu
Fe
Na

7. ORGANIC MATTER
The living, the dead and the very dead
Roots, micorrhizae
and bacteria
Humus
Crop residues, dead
roots, microbial biomass stabilized OM

8. AVERAGE SOIL COMPOSITION
25% Water
{
Pore
space
50%
25% Air
45% Inorganic
(mineral materials)
}
Solids
50%
5% Organic Matter
Mechanical Strength = f (Bulk density, Aggregation, Water content)

9. Soil Organic Matter Composition
Soil organic matter
1-6% of total soil mass
Soil
Mineral particles
Readily
decomposable
7-21%
Soil microbial biomass
3-9% of total SOM mass
Fauna
10%
Bacteria &
actinomycetes
30%
Stable (humus)
70-90%
SOM as a “revolving nutrient fund”
Fungi
50%
Yeast,
algea,
protozoa,
nematodes
10%

10. Relative C content (g C m-2)
Soil Organic C Dynamics
P = net primary production
Conversion to
cultivated
agriculture
Original
accumulation
D = decomposition
Adoption of
conservation
practices
loss
sequestration
P>D
P<D
P>D
Time
prairie agroecosystem
(Janzen et al., 1998)

11. Role of soil Organic matter

12. Effect of organic matter on available soil water
Organic matter is another soil property that has a large influence on plant available water. As we saw with clay,
increasing organic matter increases the amount of water held in soil both at wilting point and at field capacity.
Since the increase in field capacity is greater than the increase in wilting point, plant available water increases as
organic matter increases. This is because, like clay, a small amount of stable soil organic matter has extremely
high surface area. Soil organic matter behaves much like a sponge as it soaks up large amounts of water that
roots can squeeze back out again. As we will see soon, organic matter is also important for other aspects of soil

15. SOM: What is it?
SOIL
ORGANIC
MATTER
Living
Organisms:
BIOMASS
Dead Oi
tissues
and wastes:
DETRITUS
Oa
Non-living,
non-tissue:
HUMUS

17. Humic
substances
Solubility
Colour Degree of
polymerisation
Molecular
Weight
Carbon
(%)
Oxygen
(%)
Fulvic acid
Alkali and
acid soluble
Yello low
w
brown
Low
45
48
Humic acid
Alkali soluble
acid insoluble
Dark moderate
brown
Moderate
50
40
Humin
Alkali and
acid insoluble
black
High
62
30
high
Humus is a complex and rather resistant mixture of brown
amorphous and colloidal susbstances modified from the
original tissues or synthesised by the various soil
organisms

19. When Organic tissue is added to aerobic soil, 3 general
reactions takes place
1.The bulk of the material undergoes enzymatic oxidation with CO2,
water , and heat as the major products. And also decomposer
biomass is produced.
2. The Nutrient elements, N, P, and S etc. are released and /or
immobilized by element specific reactions.
3. Compounds resistant to microbial action are formed (lignin) by
degradation and/or synthesis reactions
Decomposition: An oxidative process:
In aerobic decomposition , a major portion of all these compounds
undergoes essentially a “burning” or oxidation process.
Organic matter is a potential energy source; A soil containing 4% of
O.M. Carries 150-180 million kilocalories of potential energy/acrefurrow slice. This is equivalent in heat value perhaps to 20-25 tons
of anthracite Coal.

20. How does anaerobic differ from aerobic decomposition?
Decomposition proceeds most rapidly with O2 as the
electron acceptor
Anaerobic decomposition releases relatively little
energy
Products of anaerobic decomposition are partially
oxidized organic compounds (organic acids),
alcohols, CO2, and methane (high energy products)
Methanogenic bacteria and Archaea
(Archaea)

21. mposition of Organic Matter
Form
Formula
Cellulose
Decomposition
(C6H10O5)n
Composition
rapid *
15-50%
Hemicellulose
glucose
5-35%
C6H12O6
moderate-slow
C5H10O5
moderate-slow
galactose
mannose
xylose
Lignin(phenyl-propane)
slow
Crude Protein
rapid
RCHNH2COOH**
15-35%
1-10%
Polysaccharides
Chitin
(C6H9O4.NHCOCH3)n
rapid
Starch
glucose chain
rapid
Pectins
galacturonic acid
rapid
Inulin
fructose units
- decomposition more rapid in the presence of N
** - amino acid glycine (one of many building blocks for proteins)

22. Cellulose Structure
• Simple, repeating structure
– Polymer of Glucose units
– “Easy” to decompose

23. Lignin structure
• Complex, nonrepeating
structure
– Phenyl rings
– Harder to
decompose
– Need lots of
enzymes to do it
Only a few microbes
can break them (e.g.,
white-rot fungi)

24. Factors affecting decomposition of organic matter

26. K-strategists (high
affinity constants
for specific
resistant
compounds) have
an advantage
when soil is poor
in easily digested
compounds .
With residue input
r-strategists
(opportunistic) will
rapidly multiply .
Intense microbial
activity can
stimulate humus
breakdown
(priming effect) .
As easy residue is
lost r-strategist die
and bodies are

27. Most of the carbon released during the initial rapid breakdown of the
residues is converted to CO2, but smaller amounts of Carbon are
converted into microbial biomass (and synthesis products) and,
eventually, into soil humus. The peak level of microbial activity appears
to accelerate the decay of the original humus, a phenomenon known as
priming effect.

28. Nitrogen mineralisation process
Protein & allied Compound undergoes mineralization in three steps,
viz., Aminization, Ammonification, Nitrification
Aminization : (Protein → Proteose → Peptone → Peptide →
Amino acid compd)
Proteins
R- NH2 + CO2 + energy + other products
Ammonification : (R-NH2 + H2O → R – OH + NH3 + E
by enzymatic hydrolysis)
H 2O
NH4+ + OHThe relesaed (NH4+) is subject to following changes:
Nitrification:
(i) 2NH+4 +3O2 → 2NO2 +2 H2O + 4H+ + 66 KCal (enzymatic oxdn)
Nitrosomonas europae
2NO2- + O2→ 2NO3- + 18 KCal (enzymatic oxdn)
Nitrobacter winogradskii
(ii) It (NH4+) may be absorbed directly by plants
(iii) It (NH4+) may be fixed by lattice of expanding type clay mineral

30. Significance of C:N ratio
C:N ratio in arable soil is 10:1 whereas ratio in plant
material is variable, ranging from 20:1 to 30:1 (legumes,
Farm manure) to as high as 100:1(straw), microbes 10:1
The C:N ratio in SOM is important for two major reasons;
a) keen competition for available N results when residues
having a high C:N ratio are added to soils, and
b)because this ratio is relatively constant in soil, the
maintenance of Carbon-and hence soil organic matter-is
dependent on the soil Nitrogen level.

31. 80
60
Net Immobilization
C:N
40
Net Mineralization
20
0
4 to 8 Weeks
NO
3
-
CO Evolution
2
New NO Level 3-
Amount
CO 2
Time
Cultivation and addition of straw, N immobilization & mineralization of N, evolution of CO 2

32. Practical example:
Assume that a representative cultivated soil in a condition favouring
vigorous nitrification is examined. Nitrates are present in relatively large
amounts and the C:N ratio is narrow . The general purpose decay organisms
are at a low level of activity, as evidenced by low carbon-di-oxide production.
Now, suppose that the large quantities of organic residues with a wide C:N
ratio (50:1) are incorporated in the soil under conditions supporting vigorous
digestion. A change quickly occurs. The heterotrophic flora-bacteria, fungi,
and actinomyctes - become active and multiply rapidly, yielding CO2 in large
quantities. Under these conditions, nitrate nitrogen practically disappears
from the soil because of the insistent microbial demand for this element to
build their tissues. And for the time being, little or no N , is in a form available
to higher plants. As decay occurs, the C/N ratio of the plant material decreases
since C is being lost and N conserved.
This condition persists until the activities of the decay organisms gradually
subside due to lack of easily oxidisable Carbon. Their number dercrease, CO2
formation drops off, N ceases to be at a premium and nitrification can
proceed. Nitrates again appear in quantity and the original conditions again
prevail except that, for the time being, the soil is somewhat richer both in
nitrogen and humus.

33. High CN added
Immobilization of
N. Nitrate
depression until
all easy C is gone
and activity drops
and
microbes die.
N mineralization
Low CN added
N is present to
meet microbial
needs; thus,
do not immobilize
N

34. Effect of C:N on rate of decomposition

35. Holding carbon in the soil!

37. Soil Carbon “C”
: easy come, easy go!
Deep plowing of organic
matter might increase
Carbon storage for the
upper foot of soil.
Gaining Carbon
Conservation tillage and cover crops
may result in net carbon sequestration.
Losing Carbon
Intensive tillage results in carbon loss.

42. Ten Options of Sustainable
Management of Soils
1. Retain crop residue as mulch.
2. Adopt no-till farming.
3. Include leguminous cover crops in
the rotation cycle.
4. Maintain a positive nutrient balance
INM (e.g., manure, compost).
5. Use precision farming/site specific
management.

43. Ten Options (continued)
6. Conserve water through sub/drip
irrigation and water harvesting.
7. Restore marginal/degraded/desertified
soils.
8. Grow improved/GM plants along with
agroforestry measures.
9. Integrate principles of watershed
management.
10. Restore wetlands.

54. Sustainability of a Land Use System
S1 =
CNPP
n
(Σ Ci)
i=1
S1
= Sustainability index of a land use system
CNPP
= C output as net primary productivity
Ci
= C input from all factors of production

55. Soil is meant to be covered.
Manage soil carbon - make the world a better place.

56. A Precious Resource
Irrespective of the climate debate,
soil quality and its organic matter
content must be restored, enhanced
and improved.

57. Soil and the Life-Cycle of Civilizations
How long would it take to erode 1 m thick soil?
Thickness of soil divided by the difference between
Rate of soil production and erosion.
1m
1mm - .01 mm
≈ 1000 years
This is about the life-span of most major
civilizations...

58. A nation that destroys its soils, destroys itself.
– President Franklin D. Roosevelt, Feb. 26, 1937.
National Archives: 114 SC 5089

59. Let the Hundred flowers bloom
Prepare yourself thoroughly

61. Fossil carbon cycle.
Biological carbon cycle.
Atmospheric Carbon as CO2
CO2
Energy from
fossil fuels
CO2
Energy from
bio-fuels
CO2
C
Plant biomass and
roots left on or in
the soil contribute
to Soil Carbon or
Soil Organic Matter
and all associated
environmental and
production benefits.
Nonrenewable
Renewable