Plant Microbiology 1 (Caroline)

Question 1

State and describe the four soil forming processes

Answer 1

Additions: Materials added to the soil, such as decomposing vegetation and organisms (organic matter), or new mineral materials deposited by wind or water.

Losses: Through the movement of wind or water, or uptake by plants, soil particles or chemical compounds can be eroded, leached, or harvested from the soil, altering the chemical and physical makeup of the soil.

Transformations: The chemical weathering of sand and formation of clay minerals, transformation of coarse organic matter into decay resistant organic compounds (humus).

Translocations: Movement of soil constituents within the profile and/or between horizons. Over time, this process is one of the more visibly noticeable as alterations in colour, texture, and structure become apparent.

Through the interactions of these processes, the soil constituents are reorganised into visibly, chemically, and/or physically distinct layers referred to as horizons.

Question 2

State the six soil horizons and what they are composed of (top to bottom)

Answer 2

O horizon - loose and partly decayed organic matter.
A horizon - mineral matter mixed with some humus (mineral top soil).
E horizon - zone of eluviation and and leaching (mineral subsurface).
B horizon - accumulation of clay, iron, and aluminium from above.
C horizon - partially altered parent material.
R horizon - unweathered parent material.

Soil = O + A + E + B
Regolith = soil + C
Bedrock = R

Question 3

Name the three main soil minerals

Answer 3

Clay
Sand
Silt

Question 4

Describe the role of microbes in soil formation

Answer 4

• Lichen is the association of ascomycete fungus and green algae and/or cyanobacteria.
• Fungus obtains carbon and oxygen from its partner while the fungus provides a firm substrate, water, minerals, and protection from high light intensity.
• Lichen play important roles in soil formation:
  1) Deteriorate rock via excretion of acids.
  2) Physically penetrate rock between mineral grains.
  3) Nitrogen-fixation by cyanobacteria.
• Lichen are used as an indicator species as they are sensitive to environmental changes.
• Play play key roles in the nitrogen and phosphorous cycles which are major plant nutrients.

Question 5

Describe how bacteria promote plant growth

Answer 5

• Increase nutrient supply by solubilisation of otherwise poorly soluble nutrients via secretion of acidic organic compounds.
• Produce phytohormones which stimulate root development.
• Nitrogen-fixing bacteria such as Rhizobium fix atmospheric nitrogen to ammonium compounds which increases nitrogen intake by plants.
• Plants compete with pathogens for nutrients which leads to induced systemic resistance and the production of antibiotics and antifungal metabolites by the plant.
• Bacteria act as a biofertiliser by increasing soil fertility as a result of their decay.

Question 6

Describe the two types of mycorrhizal associations (plant growth promoting fungi)

Answer 6

Ectomycorrhizae

• Fungi that remain extracellular.
• Formed by Ascomycete and Basidiomycete fungi.
• Colonise almost all trees in cooler climates.
• Aggregation of the fungal hyphae results in production of rhizomorphs and a meshwork of hyphae forming a Hartig net.
• The fungi transfer essential nutrients such as nitrogen and phosphorous to the root, and the fungi receive carbohydrates from the plant.

Endomycorrhizae/Arbuscular mycorrhizal fungi (AMF)

• All belong to the phylum Glomeromycotina.
• Can be found associated with grasses, crop, and tropical plants.
• They have tree-like hyphal structures called arbuscules that develop with plasma membrane.
• Transfer phosphorous and possibly nitrogen to plant roots, and the plant provides the fungus with carbohydrates.
• Provide plant host with protection from disease, drought, and nematodes.