Understanding soil structure, in particular the void spaces through which water, gases and solutes flow and in which organisms exist, is vital to a sustainable future on earth. The investigation of the structural behaviour of soil under different influences is fundamental to understanding and protecting the soil.
This study has investigated the impact of bacteria on the biophysics of water retention and flow, aiming to elucidate the effect of three key components produced by the model organism, Pseudomonas fluorescens SBW25. Cellulose is an extracellular polysaccharide involved in the formation of the matrix of the bacterial biofilm, lipopolysaccharide is a cell membrane component required for bacterial attachment, and viscosin is a biosurfactant released from the bacteria. Four isogenic strains mutated so as to heighten or suppress production of one of these key components were used in addition to the wild-type strain.
Labfield sandy loam soil was sieved and packed into replicate experimental cores which were incubated with different bacterial treatments. Following sterilisation, the gravimetric water content (u g g-1) of the soil was determined at equilibrated matric potentials from -1 cm to -100 cm during two wet-dry cycles. Sorptivity (S, mm s-1/2) of the soil, indicative of water repellency, was determined using a mini-infiltrometer setup and has been reported as the rate of infiltration of water into the soil. Bacteria have been shown to increase water repellency of soil, decrease the total water content at saturation and increase the water retaining ability of the soil as it drains (p < 0.05). Three-dimensional analysis of core scale structure was carried out using micro X-ray computed tomography (µXCT) and of aggregate scale structure using synchrotron-µXCT. Volumetric analyses of the 3D structures has shown decreased pore connectivity and destabilisation of aggregates in soil systems treated with bacteria deficient in the production of a key extracellular component, cellulose, LPS or viscosin (p < 0.05). Analyses of cracking patterns in two types of sandy loam soil, Labfield and Bullionfield has highlighted the importance of taking into account the soil type and its composition when studying soils, as even within soil classification groups different behaviours are observed. This study has provided clear evidence of the ability of bacteria and their extracellular components to impact upon (i) the hydrodynamics of water retention and flow in soil and (ii) the structural organisation, aggregation and stabilisation of soil.
|Date of Award||May 2012|
|Supervisor||Andrew Spiers (Supervisor), Iain Young (Supervisor), John Crawford (Supervisor) & Phillip Collier (Supervisor)|