Lecture 19: Soil Organic Matter

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Matthew Hodge
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1 Lecture 19: Soil Organic Matter

3 Effects of OM Properties on Soil Property Dark color Excellent water retention Binds to clay minerals Metal chelation Low water solubility ph buffering High CEC Nutrient source Binds organic chemicals Effect on Soil May facilitate soil heating Prevents drying and shrinking; improves soil water retention, especially for sandy soils Stabilizes soil, promotes aggregation, increases soil permeability and porosity Transports Fe 3+ and Al 3+ into subsurface horizons; enhances micronutrient availability Little OM lost by leaching Maintains a uniform soil environment Increases CEC of a soil, even at small OM contents NH 4+, NO 3-, PO 4 3-, and SO 4 2-, all important nutrients, are released during soil OM decomposition Traps contaminants and affects pesticide function

4 Sources of OM: Plant Residues

5 Organic Compounds in Plant Residues Cellulose: Polysaccharide, structural component of cell walls Hemicellulose: Polysaccharide, weaker than cellulose, binds together cellulose and lignin Sugars and starches Lignin: Aromatic polymer high in phenols, provides mechanical strength, associated with cell walls, source of woody material Other polyphenols: Tannins and flavonoids, abundant in leaves, typically brown in color (e.g., tea) Lipids: Fats, waxes, and oils Proteins: Contains ~16% N, minor amounts of other elements

6 Typical Composition of Green Plant Material Cellulose, 45% Hemicellulose, 18% Sugars and Starches, 5% Lignin, 20% Other Polyphenols, 2% Fats and Waxes, 2% Protein, 8%

other, making enzymatic breakdown difficult Hemicellulose: Shorter

7 Cellulose A polysaccharide: linear chain of glucose units 100 s to 10,000 s of units long Multiple strands are strongly bonded to eat other, making enzymatic breakdown difficult Hemicellulose: Shorter chains (100 s of units), branched, contains multiple types of sugar units

8 Organic Compounds in Cell Walls from: Smith (2001) Nature Reviews Molecular Cell Biology 2, 33-39

9 POSSIBLE lignin structure Lignin Large, complex molecules consisting of hundred of interlinked phenolic ring units, most with methoxyl (-OCH 3 ) groups Image from: Glazer, A. W., and Nikaido, H. (1995). Microbial Biotechnology: fundamentals of applied microbiology

10 Decomposition (Where organic matter comes from)

11 OM Decomposition Plant tissue and debris added to soil is broken down into smaller compounds to gain energy Two general outcomes (not mutually exclusive): Oxidation of organic carbon to produce CO 2, H 2 O, & energy Release and/or immobilization of N, P, and S A fraction of plant matter persists because it is physically or chemically protected Rates of OM decomposition vary among compounds Sugars, starches, simple proteins Complex proteins Hemicellulose Cellulose Lipids, Fats, and Waxes Lignins and phenolic compounds Rapid decomposition Very slow decomposition

12 Breakdown of Specific Compounds Cellulose: Broken down into short chains, then into individual sugars Requires the cellulase enzyme, produced primarily by fungi, bacteria, and protozoa; some termites make cellulase Proteins: Decomposed by microbes into CO 2, H 2 O, and amino acids, eventually yielding NH 4+, NO 3-, SO 4 2- Lignin: Strong and varied linkages among phenolic units makes decomposition difficult Only a few microorganisms (e.g., white rot fungi) can breakdown lignin Much may end up in soil organic matter, but this is a matter of renewed debate

13 Mineralization Process of releasing inorganic compounds through the decomposition of organic matter Geologists hate this term! Production of CO 2 from organic matter is called carbon mineralization Important for N and S: Proteins contain amide (R-NH 2 ) and thiol (R-SH) groups These are released first as NH 4+ and HS -, and are then typically oxidized to NO 3- and SO 2-4

14 Example Decay Processes when Plant Residue is Added to Soil r-strategists quickly degrade sugars, etc., if available Produces large, transient increase in microbial biomass K-strategists slowly degrade cellulose and lignin In mature ecosystems, humus addition offset by steady K-decomposition throughout year

Anaerobic Decomposition Anaerobic (O 2 -free) conditions exist in wetlands, flooded soils, and the interiors of peds and aggregates OM decomposition is much

15 Anaerobic Decomposition Anaerobic (O 2 -free) conditions exist in wetlands, flooded soils, and the interiors of peds and aggregates OM decomposition is much slower under anaerobic conditions Little energy released OM accumulates in wet, anaerobic soils Methanogenesis is an important process in anaerobic decomposition

16 Factors Affecting Decomposition

17 Factors Controlling the Rates of Decomposition and Mineralization Physical location and form: Deposition on soil surface results in slower decomposition Intact materials degrade slower than particulates Carbon:Nitrogen ratio: High ratios often slow microbial decomposition of OM Soil ecology: Lack of organisms that feed on bacteria and fungi prevent N cycling and slow decomposition Lignin and polyphenol content: Lignin and other polyphenols are slow to degrade Polyphenols may bind proteins, limiting N availability

18 C:N Ratios in Soil Components Woody Material = Newspaper = 120 Wheat straw = 80 Oak leaves = Corn stover = 57 Maple leaves = 34 Bluegrass (lawn) = 20 Alfalfa = Hairy vetch = 11 Bacteria = 5 Actinobacteria = 6 Nematode = 6 Fungi = 10 OM in Spodosol O = 90 Forest O = 45 Forest A = 20 Mollisol A = 11 Typical B = 9

19 C:N Ratios in Soil Systems Microbial decomposers require 1 g of N for every 24 g of carbon in their food If the N content is too low (C:N >24) then they obtain N from soil This removes available N from the soil and may cause nutrient deficiencies in plants C:N ratios of plant debris and OM play a dominant role in controlling decomposition and nutrient availability in soils C:N <24 GOOD C:N >24 BAD

20 Addition of High C:N Material Causes Nitrate Depression Microbes take N from existing pool of soil OM N does not build up in soil until C:N of residue drops below 24

21 Low C:N Material Immediately Increases Available N N content of added material meets the needs of microbes N released as they are consumed by higher organisms

22 Key Concepts on the Degradation of Plant Residues Decomposition releases CO 2, energy, and may release or immobilize nutrients Decomposition rates vary among plant residue compounds: Sugars/starches > proteins > cellulose > fats/waxes > lignin Organisms must take different routes to breakdown lignin and cellulose; lignin much more difficult to breakdown

23 Key Concepts on the Degradation of Plant Residues When residues added to soil two distinct decay processes occur: r-strategists and k-strategists Under anaerobic conditions, decomposition is much slower and performed exclusively by bacteria Mineralization rate is affected primarily by C:N and lignin and polyphenol content Low C:N plant residues are good for soils (more N) The breakdown of residues differs between low and high C:N materials C:N and lignin content can combine to affect the rate and extent of OM decomposition

24 Nature of Soil OM

25 Major components are: Biomass, detritus, humus, and char (from natural and anthropogenic fires) A subset is rapidly degraded (labile) because it is either fresh biomass or identifiable small molecules Humus is the residual material that degrades slowly Char is partially-burned remnants of plant matter

26 Compound Structure May Not Control Long-Term Decomposition Rates

27 Traditional View of Soil Organic Matter was Based on Harsh Chemical Extractions

28 Modern View Shows that Compounds Obtained by Extractions are Artifacts: OM in Soils is Largely Identifiable Molecules

Soil Organic Matter. Unit 2. Forms of Soil Organic Matter: OM OC x (assumes 30% C) (1.72 typically used as a conversion factor)

Soil Organic Matter. Unit 2. Forms of Soil Organic Matter: OM OC x (assumes 30% C) (1.72 typically used as a conversion factor) Unit 2 Soil Organic Matter OM OC x 1.7-2.0 (assumes 30% C) (1.72 typically used as a conversion factor) Histosol Alfisol Spodosol Forms of Soil Organic Matter: - dissolved (soil solution): DOM Nonliving