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ESRM 409 DECOMPOSITION AND CARBON CYCLING 1. Introduction Reading: Coleman et al. Chapter 5

ESRM 409 DECOMPOSITION AND CARBON CYCLING 1. Introduction Reading: Coleman et al. Chapter 5 Edmonds, 1991 Berg et al. 1995. LECTURE TOPICS Definition of decomposition Why do we study decomposition?

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ESRM 409 DECOMPOSITION AND CARBON CYCLING 1. Introduction Reading: Coleman et al. Chapter 5

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  1. ESRM 409 DECOMPOSITION AND CARBON CYCLING 1. Introduction Reading: Coleman et al. Chapter 5 Edmonds, 1991 Berg et al. 1995

  2. LECTURE TOPICS • Definition of decomposition • Why do we study decomposition? • 3. Factors controlling decomposition • 4. Determining decomposition rates • 5. Decomposition rates • 6. Modeling of decomposition and carbon cycling

  3. 1. What is decomposition? • Breakdown of organic material by microbes and microfauna and physical processes to carbon dioxide, and other gases (methane in anaerobic environments) water, nutrients, and soil humus. Faster in aerobic than anaerobic environments. • Microbially mediated extraction of potential chemical energy from organic molecules. • Recycling of stored carbon and nutrients.

  4. 2. Why study decomposition? • Important soil forming process • Important in carbon budgets and cycling • Influenced by global warming; more carbon dioxide • in the atmosphere • d. Related to cycling of nutrients; N, P, and S especially • e. Indicator of microbial activity • Related to plant productivity • Important for phytoremediation and other restoration activities • Important for breakdown of toxic compounds in soil (xenobiotics). • Affected by acid rain and excess N deposition • Dollars – influences crop and fiber production; • decomposition of wooden structures results in millions • of dollars in damage each year.

  5. Changes in nutrients with time in decomposing western hemlock log

  6. 3. FACTORS CONTROLLING DECOMPOSITION • Physical, chemical and biological factors • Physical factors • Temperature • Moisture • Actual evapotranspiration (AET) • Size – surface to volume ratios • Substrate location – buried, forest edges, clearcuts, etc. • -reflects temperature and moisture • Oxygen and CO2 • Fire • Weathering

  7. Mackensen ewt al. (2003)

  8. b. Chemical factors – substrate chemistry (substrates are made up of cellulose, hemicellulose, lignin, extractives (phenolics), waxes, lipids, sugars, amino acids, nutrients) Nitrogen concentration – High N/high decomposition Lignin concentration – High concentration; slow decomposition Lignin/N ratio – High ratio; low decomposition Lignocellulose index Extractives - high extractives; slow decomposition C/N ratio – high C/N ratio slow decomposition

  9. Cellulase attack Cellulose is broken down at oxygen linkages by fungi Producing extracellular enzymes (cellulases) into component glucose molecules. Bacteria are not efficient at breaking down cellulose

  10. LIGNIN Difficult for fungi to breakdown – Peroxidases are enzymes involved What is lignin? Even lignin chemists have trouble agreeing to an answer. There is general agreement, however, that the term lignin refers to a group of phenolic polymers that confer strength and rigidity to the woody cell wall of plants. These compounds make up the second most abundant class of chemicals found in wood. Cellulose is first.

  11. Species chemistry

  12. Biological factors • Microbes – fungi, bacteria • Soil animals – especially earthworms and insects • Plant species – reflects substrate chemistry

  13. 4. Determination of decomposition rates a. Respiration b. Substrate disappearance - mass loss with time (litter bags and changes in specific gravity of woody substrates) - determination of decomposition constants (k values) c. Soil enzymes

  14. From Sylvia et al. 2005.

  15. Most decomposition rates are determined from litter bags and mass loss with time; logs from changes in specific gravity with time. k values or decomposition constants (yr-1) are then calculated X = Xo e-kt X = mass at time t Xo = initial mass k = decomposition constant t = time - years

  16. 5. DECOMPOSITION RATES OF DIFFERENT SUBSTRATES

  17. TYPES OF ORGANIC MATTER (SUBSTRATES) There are many different substrates Fine litter - needles, leaves, cones, lichens, catkins, fine roots Woody litter – twigs, branches, logs, snags, large roots Humus and well decomposed substrates

  18. Edmonds 1991

  19. Commonly k values are determined after 1 or 2 years decomposition. k values, however, get smaller with time. In temperate ecosystems mass loss tends to level off near 20-40 % mass remaining. Why?

  20. Edmonds 1991

  21. Berg et al. 1995

  22. Leveling off of decomposition (decomposition limit) • is a result of: • Initial loss of easily decomposing substrates • leaving difficult to decompose substrates like lignin • (b) Lack of soil invertebrates • (c) Initial high N concentration – e.g., long-term • decomposition rates of red alder leaves is slower than • Douglas-fir needles

  23. 6. Predicting decomposition rates Meentemeyer 1978

  24. Doug Sprugel – Use of k values in a Douglas-fir C model

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