Sustainable Soil Management - The Soil Biological Engine

How does microbial activity vary throughout soil?

Rhizosphere

. Large OM inputs from the living root (rhizodeposits)

. Higher microbial biomass & activity

. Lower microbial diversity

. Fast biomass turnover/OM flow

. Increased predation

Detritusphere (soil surrounding dead roots)

. OM inputs from dead litter

. Higher microbial biomass & activity

. Higher microbial diversity

. High rates of OM flow

Bulk soil

. Lower quality OM available for microbial activity (so slower biomass turnover/OM flow)

. Lower microbial biomass & activity

. Higher microbial diversity.

How can pore size affect SOC/microbial activity?

Increased pore size = increased water and gas fluxes

Pores with 10-few hundred um diameters accommodate fine plant roots/root hairs - they are locations of maximal rhizodeposition

- Optimal for microorganisms due to plant inputs, optimal water and O2 availability

- Any smaller = too small for bacterial colonies, larger = occupied by predators which decrease microbial density.

Optimal pore size for carbon protection = smaller than a few um. In a process called entombing (spatial exclusion from microbial & enzymatic activities).

How do necromasses form? What types?

Microbial death pathways. E.g.

1) Grazing predation

2) Viral lysis

3) Desiccation

4) Chemical warfare

5) Bacterial predation

6) Osmotic shock

Dead microbe bodies produce SOC.

How can SOC be increased in the field?

Increasing crop diversity in the field. This spreads out the growth period, lengthening duration of OM inputs

How are soil pores and SOC related?

There is a strong linear relationship between total porosity/connectivity and SOC stocks! (more OM = more pores)

- Texture determines this - e.g. in clay, porosity exponentially increases pore connectivity but in sand, increased porosity makes no difference in connectivity.

- Pore number is different to conductivity. E.g. if you have many dead end pores, if they are blocked then the root is stumped. If connected, it can quickly diverge into another channel.

How does pore space connectivity benefit soils?

1) Increased water holding capacity

2) Increased carbon sequestration (more root growth into soils and thus organic deposits)

3) Increased nutrient use efficiency (increased root foraging and oxygen availability).