Soil is a complex heterogeneous system comprising of highly variable and dynamic micro-habitats that have significant impacts on the growth and activity of resident microbiota. A question addressed in this research is how soil structure affects the temporal dynamics and spatial distribution of bacteria. Using repacked microcosms, the effect of bulk-density, aggregate sizes and water content on growth and distribution of introduced Pseudomonas fluorescens and Bacillus subtilis bacteria was determined. Soil bulk-density and aggregate sizes were altered to manipulate the characteristics of the pore volume where bacteria reside and through which distribution of solutes and nutrients is controlled. X-ray CT was used to characterise the pore geometry of repacked soil microcosms. Soil porosity, connectivity and soil-pore interface area declined with increasing bulk-density. In samples that differ in pore geometry, its effect on growth and extent of spread of introduced bacteria was investigated. The growth rate of bacteria reduced with increasing bulk-density, consistent with a significant difference in pore geometry. To measure the ability of bacteria to spread thorough soil, placement experiments were developed. Bacteria were capable of spreading several cm’s through soil. The extent of spread of bacteria was faster and further in soil with larger and better connected pore volumes. To study the spatial distribution in detail, a methodology was developed where a combination of X-ray microtopography, to characterize the soil structure, and fluorescence microscopy, to visualize and quantify bacteria in soil sections was used. The influence of pore characteristics on distribution of bacteria was analysed at macro- and microscales. Soil porosity, connectivity and soil-pore interface influenced bacterial distribution only at the macroscale. The method developed was applied to investigate the effect of soil pore characteristics on the extent of spread of bacteria introduced locally towards a C source in soil. Soil-pore interface influenced spread of bacteria and colonization, therefore higher bacterial densities were found in soil with higher pore volumes. Therefore the results in this showed that pore geometry affects the growth and spread of bacteria in soil. The method developed showed showed how thin sectioning technique can be combined with 3D X-ray CT to visualize bacterial colonization of a 3D pore volume. This novel combination of methods is a significant step towards a full mechanistic understanding of microbial dynamics in structured soils.

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The effect of the time and rate of application of nitrogen on the yield of high yielding varieties of wheat was studied under the semiarid conditions of Northern Tunisia. Analysis of the experimental data showed that nitrogen increased the yield of wheat in most locations which were chosen for this experiment. In high rainfall areas, late application of nitrogen (tillering, jointing stages) was more effective than early applications. The optimum yield was obtained by applying 90 kilograms of nitrogen per hectare. In the medium to low rainfall area, early applications (seeding time) were more effective than the late ones. The optimum yield was obtained by applying 67 kilograms of nitrogen per hectare. The yields varied from one location to another. This variability was found to be very dependent on the amount and distribution of the rainfall, thus moisture in the soil and the residual nitrogen.

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xvii, 275 leaves : ill., tables, photos ; 26 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Agricultural Biochemistry and Soil Science, 1976

Two greenhouse studies were undertaken: (1) To assess the effects of prescribed fire frequency on AM spore abundance, species variety, and AM percent root colonization of Sudan grass Sorghum sudanese L., between an annually burned site, and a seven-year burned site on a loblolly pine, Pinus taeda L. stand, at Schenck Forest, Wake County, NC., (2) To determine the effects of two levels of fire intensity of pile burning (343oC- 371oC and >470oC), at two depths (0-2cm and 3-6cm), by year and season, on AM fungal spore abundance, AM species variety, and AM percent root colonization of Sudan grass Sorghum sudanese L. at James Goodwin Forest, Moore County, NC. All soil samples were air-dried at room temperature (23oC), stored at 4oC prior to use as inoculum in a greenhouse trap culture. At Schenck Forest, repeated fire was found to impact AM spore abundance though less affecting the species variety. The seven-year burned site had higher number of spores overall. The percent root colonization study revealed non-significant effects of repeated fire between the annually burned and the seven-year burned sites. The percent AM fungal root colonization in spring was always significantly higher than in summer at the annually burned site, but always higher in summer than in spring at the seven-year burned site. Summer had significantly more spores than spring. At James Goodwin Forest, fire disturbance coupled with mild soil surface erosion induced a highly significant difference in AM percent colonization between the control and the disturbed sites pre-burn and burn. Fire disturbance significantly affected AM root colonization by depth compared with unburned. The response of AM root colonization to disturbance is very significantly site specific. The effect of fire intensity is significantly affected by vertical distribution of the propagules. Species variety at both Schenk Forest and James Goodwin Forest non-significantly decreased, propagules survived from high intensity fire but spore numbers were significantly reduced.

Sediment pollution from construction sites has been of increasing concern since the impacts on nearby streams can be severe. Controlling erosion is the most effective approach to reducing sediment loads, but construction sites typically have large areas of exposed soil during the active phase of clearing and grading. As a result, sediment traps and basins are required to capture eroded sediment on most of these sites. The purpose of this research was to determine the trapping efficiencies of sediment basins of various designs installed on active construction sites. Five traps and one basin were monitored in the Piedmont of North Carolina, all on highway construction sites except one trap on a private development. Automatic samplers were installed to measure flow and to obtain representative samples during storm events. The basins were surveyed after storms to determine the change in volume after repeated surveys. Trapping efficiency was calculated from the sediment accumulation within the traps or basin and the amount of sediment discharged, the sum of which was the total sediment entering the device. Particle size distribution in the sediment deposits was also determined. Two standard traps with rock outlets were found to have 37% and 46% trapping efficiencies after three storm events. A standard trap with silt fence baffles was found to have 45% and 36% efficiency rates during two time periods. Two additional traps, which had been sized for a 25-year storm event, instead of the standard 10-year event, had retention efficiencies of 96% and 99%. A sediment basin with porous baffles and a skimmer outlet had a retention efficiency of 99.8%. One standard trap had particle size distributions for sand, silt, and clay of 34%, 36%, and 30% while a standard trap with a permanent pool had particle size distributions of 55%, 25%, and 20%. The standard trap with silt fence baffles had a distribution of 36%, 50%, and 14%. The 25-year traps had distributions of 75%, 18%, and 7%; and 55%, 20%, and 25%, respectively. The skimmer basin had a distribution of 62%, 28%, and 10%. The higher proportion of sand in the more efficient devices suggests that the less efficient traps are releasing significant amount of sand-size sediment. Larger basins and surface outlets clearly provide greater sediment trapping on construction sites.