An experiment was set up to investigate the effects of soil loosening on the drainage efficiency of a structurally unstable silty clay loam in Devon. Such soils are common to the Palaeozoic slates of South West England, and in many parts of Wales and Scotland. The dominance of the fine earth fraction these soils by silt sized particles and the associated low clay fractions ( 10-35%) gives rise to weak structures that are unsuitable f or mole drainage. Six drains were laid with permeable fill at a 20 m spacing on a gently sloping site at Seale-Hayne College Farm. A year after installation half the site was loosened to a depth of 0.4 m. The whole site was then cultivated and winter wheat sown. The study concentrated on the modification of the soil physical environment and water transmission routes following loosening operations for autumn re-wetting, the winter period and spring drying. Data collection involved the analysis of drainflow hydrographs and the measurement of soil water physical parameters. Results from an intensive twelve month monitoring period indicate that the modification of subsoil to improve drainage does not necessarily give the results expected . In this study it has been shown that loosening of an unstable silty clay loam did not enhance drainage, in fact the reverse was true which can have dire consequences in terms of trafficability and plant root growth. Loosening lead to an increased porosity which resulted in a greater, albeit temporary, retention of water within the disturbed zone. This means that loosened soils were wetter prior to a rainstorm and remained so for a longer period after rainfall. This response, in turn lead to significant reductions in soil shear strength, which has important implications for successful crop husbandry in the autumn and spring periods when field operations are necessary. Further work is required to assess the temporal persistence of soil loosening on soil physical conditions and crop responses.

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The rhizosphere supports greater bacterial densities than root-free soil. Rhizosphere bacteria (RB) can affect plant health and nutrition however, attempts to manipulate introduced and/or indigenous communities to benefit plants are unreliable. Current evidence indicates that habitat factors strongly influence bacterial communities. In the rhizosphere many processes give rise to a high degree of habitat heterogeneity therefore, to understand how RB interact with their environment their ecology should be studied at the micro-spatial-scale. The objectives of this research were to develop a method for sampling RB at the microhabitat-scale, and to investigate techniques that can link these samples with key factors, such as substrate availability and pH. A novel method enabling non-destructive, micro-scale sampling of bacteria was developed. Its efficiency for removing bacteria from the root surface was similar to that of existing methods but offered greater accuracy in estimating RB densities. The novel method revealed that RB density was inversely proportional to distance from the apex of Brassica napus roots and that the composition of RB communities was highly variable at the micro-scale. Imaging of 14C-labelled root exudates revealed that RB density was not reflective of exudate availability but attempts to link RB with available C were unsuccessful. A key outcome of this work was the development of a strategy to combine micro sampling with microelectrode measurements. Microelectrodes revealed that pH at the root surface was highly variable at the micro-scale which, combined with similar observations for RB density/diversity, highlights the appropriateness of this scale for linking RB communities with their environment. This thesis considers the link between RB community structure, habitat and function and provides a detailed description of micro-sample analyses as well as a set of methods that will enable for the first time, the interactions between any surface-associated bacteria and their environment to be investigated at a microhabitat-scale.

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