We generally think of soil from the ground up Soil as a medium for plant growth We eat plants or eat things that eat plants We need soil to eat

However, there is a parallel universe below us The health and functioning of this universe is critical to the functioning of the world we see at the soil surface

Functioning of soil biota underground Is critical for the ability of soils to support plant life This soil- biota- plant- predator continuum is integral to ecosystem functioning Life underground is also the most poorly understood component of this continuum

Value of ecosystem services value ($trillion) soil 50 clean water 6 food etc. 2 genetic resources 2 Total ($60T) is about twice global GNP Single most valuable ecosystem Boumans et al. 2002 Ecol. Econom. 41

Architecture & Biology of soil Iain M Young

~It is the habitat of all soil biota Q] What is the structure of soil? Key characteristics: ~ It is the framework in and through which ALL soil process occur. ~ It exists in three dimensions. ~ It’s heterogeneous in space and time. ~It is the habitat of all soil biota

Life in Inner Space

The soil-plant-microbe interface: There literally can be billions of individual organisms in a relatively small area. 1g of soil = c. 20m2 In 1 g of soil we have: c. 10,000 protozoa  7 x 10-6 m2 c. 107 bacteria  4 x 10-6 m2 c. 5 km fungi  1 x 10-4 m2 5.5 x 10-4% of the total space is covered

World underground is likely as heterogeneous as the world at the surface With similar population density

Soil as a microbe sees it

‘Inner Space’.

Naked amoebae accessing small pores - shape changing 20 mm

Fungal hyphae Bridging soil Pores, Rhizotonia solani seeking roots

SELF-ORGANISATION OF THE SOIL SYSTEM FEED-FORWARD FEED-BACK BIOTIC ACTIVITY SOIL ARCHITECTURE “We can no more manufacture soil with a tank of chemicals than we can invent a rain forest or create a single bird” Worster 1993

Soil water and microbes CO2 will move 1cm in 1 day in this system. Pores full of water Pores emptying Pores empty CO2 will move 1cm in less than 1 hour in this system. Complexity of pore-scale structure allows air and water can coexist in soil – a vital fact for sustaining life The relative proportion determines most key rate processes in soil Only a tiny fraction of the soil pore space will contain active microbes.

Impact of biota on moisture release curve Matric potential (- cm) Volumetric moisture content (%) R.solani Sterile Root surfactant

Field-scale interactions between soil water and soil microbes

Fine-scale porosity measurements Zero day Unplanted Outer planted Inner planted 5.29 4.64 7.55 11.67 Inner planted: Greater geostatistical range: ~ larger porosities ~ aggregate (non-random) structures: correlation length increases in presence of biota. Simulation model: Origin & Loss of fractal scaling: Crawford, Verral & Young (1998)

Soil microbes live to eat Energy from organic substrates is primary source of food Aerobic and anaerobic reactions Can also get energy from chemical reactions Look at N cycle

Nitrogen basics In atmosphere N2 gas (77% of atmosphere) N is the most limiting plant nutrient N can also be a primary contaminant in water systems

The N cycle

How does it go from atmosphere to soil? Lightning N2 NO3 Bacterial associations

All transformations of N in soil are microbially mediated Eating is the first step This is what happens when microbes eat SOM, they need N to use the C Mineral N to organic N Immobilization Organic N to NH4+ or NO3 - Mineralization

Energy, respiration reactions NH4 + to NO3 - - nitrosimonas, nitrobacter Nitrification NO3- to gaseous N Denitrification Occurs in anaerobic environments Wetlands, soil microsites

Soil water and microbial processes Nitrogen transformations are dependent significantly on the soil’s ability to hold water and allow gaseous transport Young & Ritz 2000. Soil & Till. Res.

For Nitrogen transformations All are microbially mediated Which occur depend on local environmental conditions Local environmental conditions are also somewhat dependent on microbial populations

In general: If you want your soil to work for you High microbial population is required for highly functional soil Food source- primarily organic matter- is requirement for high microbial population Microbes are ubiquitous- no need to seed, just feed