soil organic matter pkm

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