10b - soil biological engine

rhizosphere

* organic matter inputs from living root (rhizodeposits)

* higher microbial biomass and activity

* lower microbial diversity

* fast biomass turnover; high rates of organic matter flow

* increased predation

detritusphere

* organic matter inputs from dead litter

* higher microbial biomass and activity

* higher prevalence of saprotrophic fungi

* high rates of organic matter flow

bulk soil

* lower microbial biomass and activity

* higher microbial diversity

* slower biomass turnover and rates of organic matter flow

How does pore size affect soil organic matter (SOM)?

Small pores: protect SOM by trapping organic matter and limiting microbes/O₂.

Medium pores (~30-150 µm): highest microbial + enzyme activity.

Large pores: more aeration/root growth, faster SOM decomposition.

Key idea: Small pores store carbon; larger pores promote decomposition.

soil microorganisms biotic interactions

Antagonistic, resource competition, mutualism, predation

More activity near soil surface & near roots where there is more organic matter and oxygen

microbial death pathways

Grazing predation, viral lysis, desiccation, chemical warfare, bacterial predation, osmotic shock

How is stable soil organic matter formed?

Plant material is decomposed and assimilated by microbes. When microbes die, their necromass binds to minerals, forming stable SOM. Some carbon is lost as CO₂ during mineralisation.

Key idea: Stable SOM mainly comes from microbial residues (necromass), not undecomposed plant material.

How do environmental factors affect soil bacteria vs fungi?

Environmental conditions (e.g. pH, moisture, nutrients, climate) have a stronger effect on bacterial communities and functions than on fungal communities. Fungi are generally more resistant/stable.

Key idea: Bacteria are more environmentally sensitive; fungi are more resilient.

How are soil porosity and SOC related?

Higher total porosity is strongly linked to greater SOC stocks

How does soil texture affect response to organic inputs?

texture controls pore connectivity, influencing how carbon and water move through soil.

What positive feedback can occur with increased carbon inputs?

More C input → greater connected porosity + water-holding capacity → increased SOC storage.

How does disturbance affect microbial communities in arable soils?

High disturbance disrupts dominant organisms, often increasing microbial diversity.

Why are microbes important in soil carbon cycling?

Microbes have high energy demands, driving decomposition and carbon turnover.