soil organic matter pkm
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Soil organic matter, genesis, classification and options for carbon sequestration in soilTRANSCRIPT
Soil Organic CarbonAll 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.
C
Carbon is a “keystone”
N PK Ca
Mg S
Zn Mn
ClBo
Building of agriculture
H2O N
Carbon is the “Lord of the Rings”
KP Ca Mg
Bo
Cu
S Cl Zn
Mn
Mo Fe Na
C
Nutrient cycling requires carbon!
ORGANIC MATTER
The living, the dead and the very dead
Roots, micorrhizaeand bacteria
Crop residues, dead roots, microbial biomass
Humusstabilized OM
AVERAGE SOIL COMPOSITION
{ }Pore spac
e 50%
Solids 50%
25% Water
25% Air 5% Organic Matter
45% Inorganic (mineral
materials)
Mechanical Strength = f (Bulk density, Aggregation, Water content)
Soil Organic Matter Composition
Soil
Soil organic matter1-6% of total soil mass
Soil microbial biomass
3-9% of total SOM mass
Mineral particles
Stable (humus)70-90%
Readilydecomposable
7-21%
Fungi50%
Bacteria & actinomycetes
30%
Yeast, algea,
protozoa, nematodes
10%
Fauna10%
SOM as a “revolving nutrient fund”
Soil Organic C Dynamics
Time
Rel
ativ
e C
con
tent
(g
C m
-2)
P > D P > DP < D
P = net primary production D = decomposition
(Janzen et al., 1998)
loss
sequestration
Originalaccumulation
Conversion tocultivated agriculture
Adoption ofconservation
practices
prairie agroecosystem
Role of soil Organic matter
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 water relations.
SOIL ORGANICMATTER
LivingOrganisms:BIOMASS
Dead tissues
and wastes:DETRITUS
Non-living, non-tissue:
HUMUS
SOM: What is it?
Oi Oa
Humic substances
Solubility Colour Degree of polymerisation
Molecular Weight
Carbon(%)
Oxygen (%)
Fulvic acid Alkali and acid soluble
Yellow brown
low Low 45 48
Humic acid Alkali soluble acid insoluble
Dark brown
moderate Moderate 50 40
Humin Alkali and acid insoluble
black high High 62 30
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
In aerobic decomposition , a major portion of all these compounds undergoes essentially a “burning” or oxidation process.
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
Organic matter is a potential energy source; A soil containing 4% of O.M. Carries 150-180 million kilocalories of potential energy/acre-furrow slice. This is equivalent in heat value perhaps to 20-25 tons of anthracite Coal.
Decomposition: An oxidative process:
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)
Form Formula Decomposition CompositionCellulose (C6H10O5)n rapid * 15-50%
Hemicellulose 5-35%
glucose C6H12O6 moderate-slow
galactose
mannose
xylose C5H10O5 moderate-slow
Lignin(phenyl-propane) slow 15-35%
Crude Protein RCHNH2COOH** rapid 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)
Composition of Organic Matter
Cellulose Structure• Simple, repeating structure
– Polymer of Glucose units– “Easy” to decompose
Lignin structure
• Complex, non-repeating 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)
Factors affecting decomposition of organic matter
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 mineralized
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.
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) H2O
NH4+ + OH-
The 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
Nitrogen mineralisation process
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.
60
40
20
0
Net Mineralization
C:N
Time
CO Evolution2NO 3
-
CO 2
80
Net Immobilization
3-New NO Level
Amount
4 to 8 Weeks
Cultivation and addition of straw, N immobilization & mineralization of N, evolution of CO2
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.
High CN added
Immobilization of N. Nitrate depression untilall 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
Effect of C:N on rate of decomposition
Holding carbon in the soil!
Gaining Carbon Losing Carbon
Conservation tillage and cover crops may result in net carbon sequestration.
Soil Carbon “C” : easy come, easy go!
Intensive tillage results in carbon loss.
Deep plowing of organic matter might increaseCarbon storage for the upper foot of soil.
Ten Options of Sustainable Ten Options of Sustainable Management of SoilsManagement 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.
Ten Options (continued)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.
Sustainability of a Land Use System
S1 =
CNPPn
(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
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 and improved.
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.
1 m ≈ 1000 years 1mm - .01 mm
This is about the life-span of most major civilizations...
National Archives: 114 SC 5089
A nation that destroys its soils, destroys itself.
– President Franklin D. Roosevelt, Feb. 26, 1937.
Let the Hundred flowers bloom
Prepare yourself thoroughly
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 environmental and production benefits.
Energy from bio-fuels
CO2 CO2
Biological carbon cycle.Fossil carbon cycle.
CO2
CEnergy from fossil fuels
RenewableNonrenewable