Organic matter decomposition (W4) Flashcards
(20 cards)
Sources of soil organic matter
- Plant tissue:roots, leaves & stems of vegetation
2. Soil fauna and micro-organisms
what is Decomposition?
- 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
what 2 process of physical decomposition are there?
leaching and fragmentation.
chemical decomposition
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)
what are the phases of decomposition?
Phases of decomposition Phase 1: mainly leaching of cell
soluble compounds
Phase 2: fragmentation & chemical alteration
Phase 3: chemical alteration & leaching of breakdown products
what is leaching of leaf litter?
Leaching is the physical process by which these water-soluble organic compounds and the mineral ions dissolve in water and move through the soil
what is fragmentation of leaf litter?
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
name the main decomposers
fungi, bacteria and soil animals
Describe fungi
- Fungi are the main initial decomposers of terrestrial dead plant material
- Fungi have networks, and acquire carbon in one place and nitrogen in another
Describe Bacteria
-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
Describe soil animals
Microfauna, Mesofauna and Macrofauna
Microfauna
(< 0.1 mm)
Protozoa consume bacteria
Nematodes specialize on bacteria, fungi,roots or soil animals
Mesofauna
(0.1 – 2mm)
Animals with the greatest effect on Decompositio - fragment and ingest litter creating fecal matter which produces favourable environment for decomposition
Macrofauna
(> 2mm)
Ecosystem engineers that modify the physical Properties of soils and litter. Burrow and ingest soil enhancing decomposition
how does the Method of measuring decomposition by litter bags work?
- 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.
what do the decomposition rates in the tea bag experiment depend on?
- Environmental conditions:
humidity, acidity, the amount of nutrients in the soil, temperature - The chemical properties of the material in the litter bag Branch, stem, leaf
- Which decomposers are present
microfauna, mesofauna etc all break down material at different speeds
what are the factors controlling decomposition?
• Physical environment
• Quantity and quality of substrate available to
decomposers
• Characteristics of the microbial community
How does temperature affect microbial respiration?
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
How does moisture affect microbial respiration?
Decomposers are most productive under warm moist conditions providing sufficient oxygen is available
how does ph affect decomposition?
pH: decomposition occurs more rapidly in neutral (pH=7) than acidic soils where fungi predominate
