Soil Biology

Why is decomposition important?

• main biological process where co2 goes from land back to the atmosphere

what is decomposition ?

complex molecules broken down into simpler molecules

What are the main key physical and chemical changes of decomposition?

• soluble materials are leached out

• UV radiation causes photo oxidation

• animals fragment the litter

• animals and microorganisms metabolise ( biochemically degrade & digest ) the litter leading to mass loss

What does this graph show?

• this graph shows a mass loss curve

1st phase

• Microbes quickly use up the easy sugars and other soluble compounds.

• A lot of mass is lost as CO₂ (respiration), and some soluble material is leached away by water.

2nd phase

• Cellulose is harder to break down due to its structure 

• Ruminants have bacteria that break down the cellulose

3rd phase

• Utilisation of lignin is low

• Begins to flatten

• There is an increase in cellular material which gets colonised by fungi as they produce enzymes that can break down the lignin

what is the difference between colonisation in invertebrates in cold regions vs tropical regions?

• cold regions = earthworms are the most important invertebrate

• tropical regions = termites and ants

What are the main key points to remember about earthworms?

• has a complex relationship with SOM

• spatially re distribute SOM and nutrients ( vertical and horizontal transfer of material)

• fascilitates soil C storage through the formation of aggregates due to the release of mucus they produce

• Accelerates litter decay ( C loss and respiration )

• Has bioturbation and aggregate effects

How do earthworms improve water infiltration?

• burrows create channels ( macropores) that allow water to enter soil more easily

How do earthworm burrows affect water movement?

• they provide pathways for water to move deeper through the soil profile

Why is burrowing so important for soil fertility?

• create channels that allow air to penetrate deeper into the soil

• promotes nutrient cycling

• soil aeration = contributes to root respiration

what is the link between bioturbation and soil aggregation?

• earthworms mix soil and organic matter helping to form and stabilise soil aggregates

How do earthworms help form soil aggregates?

• release of mucus that helps bind soil particles together

• they excrete nutrient rich casts contain compacted mineral particles and organic matter that form stable aggregates

How do worms contribute to improved soil structure?

improve soil structure by burrowing, aerating, and mixing organic matter with minerals, creating a fertile, well-drained, and nutrient-rich environment for plants.

How do worms help with water retention?

• Burrowing: Earthworms create channels that allow water to penetrate deeper into the soil, reducing runoff and erosion.

• Organic Matter Decomposition: As earthworms digest organic material, they produce castings that enrich the soil with nutrients, enhancing its ability to hold water.

• Soil Aggregation: The mucus secreted by earthworms helps bind soil particles together, forming stable aggregates that improve soil porosity and moisture retention.

How do worms improve hydrology/ water infiltration?

• burrowing which helps creates macro pores / channels that allow water to penetrate deeper and more evenly

What are the key points for termites being soil superstars!

• accelerates OM decay

• sensitive OM quality = research

• there are two types: wood feeding and soil feeding

• Bioturbation, nest building and cultivation of fungal gardens = spatial distrubution of C, nutrients and hydrology

what is the difference between wood feeding termites and soil feeding termites?

• wood feeding = form mounds above ground and consume cellulose

• soil feeding = mounds underground and break down dead wood rich in cellulose

How are the soil termites mounds made and the benefit of them?

• Formed by mixing soil particles with saliva and fecal matter, serve as localized "hotspots" of fertility,

• contain concentrations of organic matter, clay particles, and essential nutrients.

How do worms make OM more stable/ resistant to decay?

• brings ingested material in close proximity like OM and minerals which enables a chemical reaction to occur

Are fungi the main initial decomposers?

Yes

What is an example of fungi beginning with H?

• Hyphae which are filaments and allow growth into new substrates and over surfaces

What are the roles of Hyphae fungi?

• physically penetrate leaf cuticles and plant cell walls to gain access to soluble cellular material

• secrete extracellular enzymes that facilitate decay of complex polymers

• some also decomposes lignin and other complex structural molecules

Is there a low relative abundance of fungi?

• yes because they require o2 to function

What is another example of fungi?

Mycorrhiza forming fungi

What are the key points to know about bacteria and archaea?

• small size, unicellular, large surface area to volume ratio

• short lifespan

• sessile = adsorbed to surfaces

• absorb soluble resources rapidly

• concentrated in the rhizosphere

• secrete enzymes to break down complex molecules but challenged by lignin

are bacteria only able to exploit local environments?

• yes

What do bacteria form in the soil?

form microbial biofilms

What is a biofilm?

a structured community of bacterial cells enclosed in a self-produced polymeric matrix and adherent to an inert or living surface

What are the benefits of biofilms in the soil?

• enables a high concentration of enzymes in one spot speeding up chemical breakdown

• Bioclogging of soil pores caused by biofilms = positive impact on the water permeability.

• Exopolysaccharides produced by soil biofilms fortifies the internal cohesion among soil microaggregates

• bacterial biofilms developed on roots of the rhizosphere plants inhibit plant pathogens from colonizing by limiting the nutrient availability

What is a consortium?

• a multi species community

• each un related bacteria has a specific niche and produce enzymes needed to decompose complex molecules

What are the two broad functional categories of bacteria?

• Copiotrophic

• Oligotrophic

What are the functions of Copiotrophic bacteria?

• fast growing ( r selected)

• high nutrient uptake capacity

• use labile or simple c

• dominate in the rhizosphere

• gram negative

what are two examples of copiotrophic?

• B proteobacteria

• bacteroidetes

What are the key functions of Oligotrophic bacteria?

• slow growing ( K selected )

• low nutrient uptake capacity

• prefer complex (recalitrant) C

• dominate outside the rhizosphere

• gram positive bacteria

what is an example of oligotrophic bacteria?

• acidobacteria