Process of humus formation

educational purpose

1. Process Of Humus Formation & Its Role In Nutrient Cycling Submitted by- Sunil Kumar Meher M.Sc.(Ag) Dept of Soil Science Guided By - Dr. R.N. Singh Prof. Dept of Soil Science IGKVV, Raipur Soil Biology & Biochemistry SOILS- 506 (2+1)

2. Outlines What Is HUMUS ?? Importance Of Soil Humus Process Of Humification The Formation Of Humic Substances The Lignin- Protein Theory The Polyphenol Theory Sugar-amine Condensation Fractionation Of Organic Matter Role Of Humus In Plant Nutrient Cycling Basic Plant Nutrient Cycle Interaction Of Plant-soil-atmosphere In Plant Nutrient Cycling References

3. What is HUMUS ?? Humus is defined as a complex and rather resistant mixture of brown or dark brown amorphous and colloidal substances modified from the original tissues or synthesized by the various soil organisms” . It is a natural body having variable heterogeneous mass. Soil Humus is also known as “Soil skin/Flesh”. The formation of humus is strictly a biochemical process and rather complicated one. Humification of organic residues incorporating the soil depends upon their chemical composition and soil conditions influencing the activity of soil micro-organisms. On reaching the soil, organic residues of plant and animal origin undergo diverse changes brought about by various factors.

4. Importance Of Soil Humus The process that converts raw organic matter into humus feeds the soil population of microorganisms and other creatures, thus maintains high and healthy levels of soil life. Improve soil physical condition by lowering Bulk Density of soil and increasing soil WHC, Infiltration Rate, improved structure, aggregates etc.. Help in Plant Nutrition as its CEC is 150-300 cmol/ kg of Humus, Highest among any soil colloids it store cations on its negatively charged surface. Effective humus and stable humus are further sources of nutrients to microbes, the former provides a readily available supply, and the latter acts as a longer-term storage reservoir. Decomposition of dead plant material causes complex organic compounds to be slowly oxidized (lignin-like humus) or to break down into simpler forms (sugars and amino sugars,  aliphatics , and phenolics organic acids), which are further transformed into microbial biomass (microbial humus) or are reorganized, and further oxidized, into humic assemblages (fulvic and humic acids), which bind to clay minerals and metal hydroxides.

5. Continue.. There has been a long debate about the ability of plants to uptake humic substances from their root systems and to metabolize them. There is now a consensus about how humus plays a hormonal role rather than simply a nutritional role in plant physiology . Humus is a colloidal substance, and increases the soil's cation exchange capacity, hence its ability to store nutrients by  chelation . While these nutrient cations are accessible to plants, they are held in the soil safe from being leached by rain or irrigation. Humus can hold the equivalent of 80–90% of its weight in moisture, and therefore increases the soil's capacity to withstand drought conditions. The biochemical structure of humus enables it to moderate or buffer excessive acid or alkaline soil conditions.

6. Process of Humification Some of these changes take place without the participation of microorganisms which are as follows: (a) Destructive changes due to the physical action of natural factors (rainfall, wind etc.) and to the action of man (soil cultivation). (b) Changes in the chemical nature of organic residues under the direct action of water, light, air and reaction of the medium, e.g. the oxidation of fats and resins in light, the oxidation of aromatic compounds under alkaline soil conditions etc. (c) Changes resulting from the effect of tissue enzymes, whose action, in dead cells, has a uni -directional, pre-dominantly oxidative character (e.g. oxidation of tannins, polyphenols and other aromatic amino acids etc.) forming complex dark- coloured condensation products.

7. Humus formation from the residues of plant and animal is two-stage process:- 1. The decomposition of the original components of tissues and their conversion by micro-organisms into simpler chemical compounds and partially to products of complete mineralization (CO , NO , NO , NH , CH , H O etc.). 2. The synthesis of organic compounds with the formation of high molecular-weight humus substances of specific nature. Micro-organisms participate mainly in the first stage of humus formation the decomposition of the fresh organic materials and the synthetic activity is limited to the re-synthesis of bacterial plasma. In the second stage of humus formation i.e. the condensation reactions ( physico -chemical reactions) can take place without the participation of microorganisms. Humus or soil organic matter consists of two major types of compounds- Un- humified and H umified residues of plant and animal. The un- humified substances are represented by certain organic compounds like, carbohydrates, fats, waxes and proteins. The humified fractions are considered as the most active part of humus which consists of a series of highly acidic, yellow to black- coloured , high molecular weight polyelectrolytes known as humic acid, fulvic acid and so on.

8. The Formation Of Humic Substances The formation of humic substances is one of the least understood aspects of humus chemistry and one of the most intriguing. Studies on this subject are of long-standing and continued research can be justified on theoretical and practical grounds.  Several pathways exist for the formation of humic substances during the decay of plantand animal remains in soil, Classical theory, popularized by Waksman, is that humic substances represent modified lignins  (pathway 1)  but the majority of present-day investigators favor a mechanism involving quinones  (pathway 2 and 3). In practice all four pathways must be considered as likely mechanisms for the synthesis of humic and fulvic acids in nature, including sugar-amine condensation  (pathway 4). This four pathways may operate in all soils, but not to the same extent or in the same order of importance. A lignin pathway may predominate in poorly drained soils and wet sediments (swamps, etc.) whereas synthesis from polyphenols may be of considerable importance in certain forest soils. The frequent and sharp fluctuations in temperature, moisture and irradiation in terrestrial surface soils under a harsh continental climate may favor humus synthesis by sugar-amine condensation.

9. Pathway 1 - The lignin- protein theory ( Walksmann Classical Theory- 1932) For many years it was thought that humic substances were derived from lignin(pathway1).  According to this theory, lignin is incompletely utilized by microorganism and the residuum becomes part of the soil humus. Modification in lignin include loss of methoxyl (OCH 3 ) groups with the generation of ortho-hydroxyphenols and oxidation of aliphatic side chains to form COOH groups. The modified material is subject to further unknown changes to yield first humic acids and then fulvic acids. This pathway, be of considerable importance in certain forest soils.

10. In pathway 3 lignin still plays an important role in humus synthesis, but in a different way.In this case phenolic aldehydes and acids released from lignin during microbiological attack undergo enzymatic conversion to quinones , which polymerize in the presence or absence of amino compounds to form humic like macromolecules. Pathway 2 is somewhat similar to pathway 3 except that the polyphenols are synthesized by microbes from non-lignin C sources ( e.g.celulose ). The polyphenols are then enzymatically oxidized to quinones and converted to humic substances. As noted earlier, the classical theory of Waksman is now considered obsolete by many investigators. According to current concepts quinones of lignin origin, together with those synthesized by microorganisms, are the major building blocks from which humic substances are formed. Pathway 2 And 3 - The Polyphenol Theory ( Flaig & Kononova Concepts- 1964)

11. The concept of humus formation as described by Flaig is given below: 1. Lignin, freed of its linkage with cellulose during decomposition of plant residues, is subjected to oxidative splitting with formation of primary structural units (derivatives of phenyl-propane). 2. The side-chains of the lignin-building units are oxidised , de- methylation occurs, and the resulting polyphenols are converted to quinones by poly-phenol- oxidase enzymes. 3. Quinones arising from lignin (as well as from other sources) react with N-containing compounds to form dark- coloured polymers. A. Flaig’s Concept of Humus Formation :

12. There are three stages leading to the formation of humic substances which are as follows: Stage 1: Fungi attack simple carbohydrates and parts of the protein and cellulose in the medullary rays, cortex of plant residues. Stage 2: Cellulose of the xylem is decomposed by aerobic myxobacteria . Polyphenols synthesized by the myxobacteria are oxidised to quinones by poly-phenol- oxidase enzymes, and the quinones subsequently react with N compounds to form brown humic substances.  Stage 3: Lignin is decomposed. Phenols released during decay also serve as source materials for humus synthesis. B. Kononova’s Concept of Humus Formation :

13. The concept that humus is formed from sugars dates back to the early days of humus chemistry. According to this concept reducing sugars and amino acids, formed as by-products of microbial metabolism, undergo non-enzymatic polymerization to form brown nitrogenous polymers of the type produced during dehydration of certain food products at moderate temperatures. A major objection to this theory is that the reaction proceeds rather slowly at the temperatures found under normal soil conditions. However, drastic and frequent changes in the soil environment (freezing and thawing, wetting and drying), together with the intermixing of reactants with mineral material having catalytic properties, may facilitate condensation. An attractive feature of the theory is that the reactants (sugars, amino acids etc.) are produced in abundance through the activities of microorganisms. Pathway 4 - Sugar-amine condensation ( Maillard Concept- 1911)

14. Fractionation of Organic Matter

15. Role of Humus in Plant Nutrient Cycling The basic nutrient cycle usually describes the outstanding role of soil organic matter. Cycling of many plant nutrients, especially N, P, S, and micronutrients, are similar to the Carbon Cycle. Plant residues, grain green manure, farmyard manure and other substances are incorporated to the soil. This organic matter pool of carbon compounds serve as food for bacteria, fungi, and other decomposers. As organic matter is decomposed to simpler compounds, plant nutrients are released in available forms for root uptake and the cycle begins again. Plant-available macronutrients such as N, P, K, Ca, Mg, S and micronutrients are also released when soil minerals dissolve. As soil organic matter contains large number of microbes(Bacteria, Fungi, Algae, Protozoa etc) these organisms decompose all incorporated organic matter readily so generally soil organic matter constitute about- 95% of total soil N 25-70% of total soil P 70-85% of total soil S

16. Atmosphere Interaction of Plant-Soil-Atmosphere in Plant Nutrient Cycling ;-

17. References- Books- Fundamentals of Soil Science, ISSS Introductory Soil Science ( Dr D.K. Das) Internet- rosealchemy.org scirp.org tankonyvtar.hu/en ( Nutrient management ) researchgate.net karnet.up.wroc.pl/

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