9. Biogeochemistry And The Phosphate Cycle

Question 1

List two examples of organic and inorganic phosphates

Answer 1

Organic phosphates
- Phytate (my-inositol hexaphosphate) is produced by plants as a PO4 store
- Phytate is released into the soil by decaying plant matter where some bacteria and fungus are able to cleave phosphate groups from the carbon ring

Inorganic phosphates
- The weathering of inorganic minerals such as apatite can release inorganic phosphates that are labile.
- Due to its highly polar structure PO3 does not exist in soil in an unbound form
- It readily binds with inorganic minerals to form phosphate precipitates such as FePO3 and AIPO4

Question 2

What is the primary form of organic phosphate in terrestrial ecosystems with abundant flora and how do microbes access it?

Answer 2

The primary form of organic phosphates is that of organic biomatter, Phytate and phosphate esters

Some soil microbes can ‘mine’ phosphate releasing available PO4 for plants and for their own growth
- phosphate solubilising bacteria are able to cleave PO4 from organic and inorganic phosphates using different mechanisms

Question 3

What are two important nutrients in soil? How could removal of topsoil drive a system to nutrient limitations?

Answer 3

There are three essential macronutrients required for microbial life
- carbon - decaying plant and animal matter
- nitrogen - decaying plant and animal matter and nitrogen compounds produced via N-fixation
- Phosphorous - decaying plant and animal matter and organic and inorganic phosphates

If one or more of these macronutrients are unavailable it will severely limit plant and microbial growth, if a soil is lacking one or more of these it is said to be nutrient limited. The top soil is the richest zone of nutrients and organic matter, abundant in microbes.

Question 4

List each main layer of the soil horizon and give a brief description of composition of each

Answer 4

O horizon = often called humid layer, comprising of decaying organic matter
A horizon = a mixture of sand and decayed organic matter
B horizon = comprising mostly of sand
C horizon = comprised of weathered parent material mostly in the form of clay
Bedrock = hardened inorganic materials and minerals

Question 5

Bacteria can glue soil together how can they accomplish this?

Answer 5

Bacteria is not only good for phosphate solubilisation but many bacteria also improve soil health by increasing the organic carbon pool in soil. This is achieved by the secretion of exopolysaccharides (EPS); a mucilage that forms soil particle aggregates and improves water retention

Question 6

List 4 types of bacteria that you would expect to find from a sample of rich soil

Answer 6

• Proteobacteria and acidobacteria are the most abundant
• three types of soil bacteria that fix nitrogen without a plant host and live freely in the soil
  -> azotobacter, azospirillum and clostridium