The effect of microbial activity on soil structural genesis : a fractal approach
The relationship between soil structure and microorganisms is a unique two way interaction. Whilst soil structure influences microbial transport, activity and predation, microorganisms affect soil structure by playing a major role in its genesis and stabilisation. Microorganisms are largely thought to influence structural genesis and stabilisation through the exudation of polysaccharides, which act to bind soil particles together as well as microorganisms to soil particles. Filamentous microorganisms are also thought to influence structural genesis and stabilisation by enmeshing soil particles together. A laboratory-based experimental system was developed whereby the structural heterogeneity of soil cracking patterns, generated from homogeneous soil slurries, was quantified using fractal concepts. A fractal approach was used because it provided a theoretical framework in which structural heterogeneity could be quantified and linked to soil processes such as gaseous diffusion, water movement and microbial population dynamics. This study demonstrated the potential of fractal geometry to functionally quantify soil structure. A greater understanding was gained of some of the processes acting to direct the formation of particular structural geometries. The latter is particularly important since it is the geometry of soil structure which will regulate processes such as gaseous diffusion and water movement. Microbial activity (mainly through the production of polysaccharides) was shown to have a significant effect on the generation of soil structure as well as on its water retention properties. The nutritional management of the microbial population was also shown to be a key factor regulating its contribution to structural genesis. Microorganisms can play a significant role in structural genesis ultimately impacting upon all aspects of the soil-plant-microbe ecosystem. It will be essential to understand how to effectively manage the indigenous microbial population if their potential as soil conditioners is to be fully reached.