Organic matter decomposition (W4)

Question 1

Sources of soil organic matter

Answer 1

1. Plant tissue:roots, leaves & stems of vegetation

2. Soil fauna and micro-organisms

Question 2

what is Decomposition?

Answer 2

• physical and chemical breakdown of organic material
• Leaching, fragmentation and chemical alteration of dead plant material (litter) produces CO2 and mineral nutrients and complex organic macro-molecules

Question 3

what 2 process of physical decomposition are there?

Answer 3

leaching and fragmentation.

Question 4

chemical decomposition

Answer 4

mineralisation eg ammonification.
Chemical alteration is performed by soil microbes:
- Fungi
- Bacteria
accounting for 80-90% of the total decomposer biomass and respiration
and by soil animals (meso and microfauna)

Question 5

what are the phases of decomposition?

Answer 5

Phases of decomposition Phase 1: mainly leaching of cell
soluble compounds
Phase 2: fragmentation & chemical alteration
Phase 3: chemical alteration & leaching of breakdown products

Question 6

what is leaching of leaf litter?

Answer 6

Leaching is the physical process by which these water-soluble organic compounds and the mineral ions dissolve in water and move through the soil

Question 7

what is fragmentation of leaf litter?

Answer 7

Fragmentation pierces the plant protective barriers, the cell walls.

Agents of fragmentation = animals (mammals mixing the soil) and soil invertebrates.
Fragmentation:
(i) Creates fresh surfaces for microbial colonization; and
(ii) Increases the proportion of litter mass accessible to microbial attack

Question 8

name the main decomposers

Answer 8

fungi, bacteria and soil animals

Question 9

Describe fungi

Answer 9

• Fungi are the main initial decomposers of terrestrial dead plant material
• Fungi have networks, and acquire carbon in one place and nitrogen in another

Question 10

Describe Bacteria

Answer 10

-Bacterial communities are present as biofilms coating aggregates – a microbial community embedded in a matrix of polysaccharides secreted by the bacteria.
- Bacteria have
• Small size
• Large surface to volume ratio

Question 11

Describe soil animals

Answer 11

Microfauna, Mesofauna and Macrofauna

Question 12

Microfauna

Answer 12

(< 0.1 mm)
Protozoa consume bacteria
Nematodes specialize on bacteria, fungi,roots or soil animals

Question 13

Mesofauna

Answer 13

(0.1 – 2mm)
Animals with the greatest effect on Decompositio - fragment and ingest litter creating fecal matter which produces favourable environment for decomposition

Question 14

Macrofauna

Answer 14

(> 2mm)
Ecosystem engineers that modify the physical Properties of soils and litter. Burrow and ingest soil enhancing decomposition

Question 15

how does the Method of measuring decomposition by litter bags work?

Answer 15

• Decomposition can be measured as mass loss from dead leaves, roots or twigs placed in a mesh litter bag and measured periodically.
• The holes in the mesh allow micro-organisms to enter and consume the litter and the bag prevents the litter being lost so you can measure the change in weight.

Question 16

what do the decomposition rates in the tea bag experiment depend on?

Answer 16

1. Environmental conditions:
   humidity, acidity, the amount of nutrients in the soil, temperature
2. The chemical properties of the material in the litter bag Branch, stem, leaf
3. Which decomposers are present
   microfauna, mesofauna etc all break down material at different speeds

Question 17

what are the factors controlling decomposition?

Answer 17

• Physical environment
• Quantity and quality of substrate available to
decomposers
• Characteristics of the microbial community

Question 18

How does temperature affect microbial respiration?

Answer 18

Temperature affects decomposition directly by promoting microbial activity Temperature causes an exponential increase in microbial respiration
Over longer timescales an increased temperature will alter microbial community

Question 19

How does moisture affect microbial respiration?

Answer 19

Decomposers are most productive under warm moist conditions providing sufficient oxygen is available

Question 20

how does ph affect decomposition?

Answer 20

pH: decomposition occurs more rapidly in neutral (pH=7) than acidic soils where fungi predominate