Guideline Index

Chapter 5: Understanding and Managing Soil Biology

5.3 Components of the soil biological community

Soil biology may be described by size, taxonomic group and the functions they perform (Figure 5.2). The smallest – the microflora and microfauna – are microscopic in size, that is, they can only be seen using high powered magnification. The meso- and macrofauna may be seen with lower level magnification, a hand lens, or the naked eye.

Figure 5.2  Elements and functions of soil biological communities (from  Source: Gupta http://www.csiro.au/Outcomes/Food-and-Agriculture/SoilOrganismsPoster.aspx
Figure 5.2 Elements and functions of soil biological communities
Source: Gupta http://www.csiro.au/Outcomes/Food-and-Agriculture/SoilOrganismsPoster.aspx

5.3.1 Functions of biota in natural soil processes

There are three levels of participation by biota in natural soil processes (Figure 5.3):

 

Figure 5.3  Roles and relationships; soil micro-, meso- and macro-organisms (redrawn from Wardle, 2002)
Figure 5.3 Roles and relationships; soil micro-, meso- and macro-organisms (redrawn from Wardle, 2002)
5.3.1.1 Ecosystem engineers

Ecosystem engineers alter the physical structure of soil by the action of earthworms, beetles and ants physically rearranging and pushing soil particles together. They also move organic materials from the surface of soils below ground. In addition, the movement of macrofauna up and down the soil profile creates semi-permanent channels through which air, water and roots can penetrate deeply. Improved aeration and infiltration, together with the deposition of organic materials below ground, create additional sites for soil microbes to chemically break down organic compounds, thereby releasing nutrients for plant uptake – a process known as mineralisation. In this way, the ‘ecosystem engineers’ have a strong influence over rates of nutrient cycling and energy flow in soils.

5.3.1.2 Litter transformers

Litter transformers are those organisms which are capable of shredding and ‘pre-digesting’ organic materials. Collembola (springtails) and oribatid mites are potentially numerous in productive agricultural soils. Through the action of their chewing mouthparts, plant litter is fragmented and its surface area expanded to improve availability of nutrients to microbes (Franzluebbers, 2004).

5.3.1.3 Micro-food webs.

In contrast to the larger organisms, bacteria, archaea and fungi decompose plant litter through enzyme action. Enzymes are released outside the cell and break down litter and any other suitable substrate, thereby releasing nutrients for uptake by other microbes, and plant roots. In their turn, bacteria, archaea and fungi provide a food source for predatory protozoa, nematodes and many of the arthropods such as collembola and mites (Franzluebbers, 2004). Predatory organisms regulate soil populations maintaining stability within a community. They may also control numbers of disease causing organisms – a phenomenon referred to as ‘disease suppression’. Suppressive soils are identified has having potentially damaging levels of pests or disease, yet the pest or disease is either expressed at a sub-economic level, or not at all, due to the presence of certain beneficial organisms.

5.3.2 Where are soil organisms located in soils?

Soil organisms are not uniformly distributed. The factors that influence their distribution are principally access to suitable substrate, air and moisture. Typically, larger populations of soil organisms are found around decomposing residue and in the rhizosphere (root zone) of plants where readily available food sources are located (Schmidt et al, 2011).

Soil organisms may be more evenly distributed in cultivated soils that are thoroughly mixed, particularly with successive cultivations. In dairy soils, which can remain undisturbed for many years, populations can be discontinuous and centred around food resources such as pasture roots. However, given the high earthworm populations commonly found in many dairy soils, the degree of bioturbation (the mixing of soils through the action of the ‘ecosystem engineers’) in the topsoil can result in a higher degree of soil mixing than might otherwise be found, thereby increasing the distribution of soil organisms throughout the topsoil.