Read slide contents. Carbon is “key” to nutrient cycling in soil systems. All forms of carbon are important, even humic acids, and all provide numerous functions important to agriculture and the environment.
Conceptual model of soil organic C dynamics in a prairie agroecosystem (from Janzen et al., 1998). During soil formation, net primary production (P) exceeds decomposition (D), resulting in the accumulation of soil C until P and D again converge. Upon conversion of the land to arable agriculture, D initially exceeds P, resulting in the loss of soil carbon until a new steady state is approached. Adoption of C-retentive cropping practices (e.g., no-till) reduces D relative to P, resulting in a gain in C until D and P again converge.
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 water relations.
Soil organic matter pkm
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
The modern civilization is dependent on C-based
energy sources. It is literally hooked on carbon, and
in need of a big-time rehabilitation.
Carbon is a
Building of agriculture
Nutrient cycling requires carbon!
Carbon is the “Lord of the Rings”
The living, the dead and the very dead
Crop residues, dead
roots, microbial biomass stabilized OM
AVERAGE SOIL COMPOSITION
5% Organic Matter
Mechanical Strength = f (Bulk density, Aggregation, Water content)
Soil Organic Matter Composition
Soil organic matter
1-6% of total soil mass
Soil microbial biomass
3-9% of total SOM mass
SOM as a “revolving nutrient fund”
Relative C content (g C m-2)
Soil Organic C Dynamics
P = net primary production
D = decomposition
(Janzen et al., 1998)
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
SOM: What is it?
Colour Degree of
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
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.
How does anaerobic differ from aerobic decomposition?
Decomposition proceeds most rapidly with O2 as the
Anaerobic decomposition releases relatively little
Products of anaerobic decomposition are partially
oxidized organic compounds (organic acids),
alcohols, CO2, and methane (high energy products)
Methanogenic bacteria and Archaea
mposition of Organic Matter
- decomposition more rapid in the presence of N
** - amino acid glycine (one of many building blocks for proteins)
• Simple, repeating structure
– Polymer of Glucose units
– “Easy” to decompose
• Complex, nonrepeating
– Phenyl rings
– Harder to
– Need lots of
enzymes to do it
Only a few microbes
can break them (e.g.,
Factors affecting decomposition of organic matter
when soil is poor
in easily digested
With residue input
rapidly multiply .
(priming effect) .
As easy residue is
lost r-strategist die
and bodies are
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
Nitrogen mineralisation process
Protein & allied Compound undergoes mineralization in three steps,
viz., Aminization, Ammonification, Nitrification
Aminization : (Protein → Proteose → Peptone → Peptide →
Amino acid compd)
R- NH2 + CO2 + energy + other products
Ammonification : (R-NH2 + H2O → R – OH + NH3 + E
by enzymatic hydrolysis)
NH4+ + OHThe relesaed (NH4+) is subject to following changes:
(i) 2NH+4 +3O2 → 2NO2 +2 H2O + 4H+ + 66 KCal (enzymatic oxdn)
2NO2- + O2→ 2NO3- + 18 KCal (enzymatic oxdn)
(ii) It (NH4+) may be absorbed directly by plants
(iii) It (NH4+) may be fixed by lattice of expanding type clay mineral
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.
4 to 8 Weeks
New NO Level 3-
Cultivation and addition of straw, N immobilization & mineralization of N, evolution of CO 2
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.
High CN added
all easy C is gone
and activity drops
Low CN added
N is present to
do not immobilize
Soil Carbon “C”
: easy come, easy go!
Deep plowing of organic
matter might increase
Carbon storage for the
upper foot of soil.
Conservation tillage and cover crops
may result in net carbon sequestration.
Intensive tillage results in carbon loss.
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
Ten Options (continued)
6. Conserve water through sub/drip
irrigation and water harvesting.
7. Restore marginal/degraded/desertified
8. Grow improved/GM plants along with
9. Integrate principles of watershed
10. Restore wetlands.
Sustainability of a Land Use System
= Sustainability index of a land use system
= C output as net primary productivity
= C input from all factors of production
Soil is meant to be covered.
Manage soil carbon - make the world a better place.
A Precious Resource
Irrespective of the climate debate,
soil quality and its organic matter
content must be restored, enhanced
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.
1mm - .01 mm
≈ 1000 years
This is about the life-span of most major
A nation that destroys its soils, destroys itself.
– President Franklin D. Roosevelt, Feb. 26, 1937.
National Archives: 114 SC 5089
Let the Hundred flowers bloom
Prepare yourself thoroughly
Fossil carbon cycle.
Biological carbon cycle.
Atmospheric Carbon as CO2
Plant biomass and
roots left on or in
the soil contribute
to Soil Carbon or
Soil Organic Matter
and all associated
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