Thesis (M.S.- Hydrology)--University of Arizona. / Includes bibliographical references (leaves 190-198).

Thesis (M.S.)--Ohio State University, 1955. / Includes bibliographical references (leaves 41-42). Available online via OhioLINK's ETD Center

Thesis (M.S. - Soil and Water Science)--University of Arizona, 1984. / Includes bibliographical references (leaves 63-66).

Thesis (M.S.)--Virginia Polytechnic Institute, 1951. / Vita. Includes bibliographical references (leaves 56-58). Also available via the Internet.

Thesis (Ph. D.)--University of Minnesota, 1924. / Biographical sketch. "Literature cited": leaves 85-88.

Thesis (Ph. D.)--University of Minnesota, 1924. / Biographical sketch. "Literature cited": leaves 85-88.

No description available.

Soil is the most widely used material in the construction of various civil infrastructure. Various types of soils are extensively used in its natural or compacted form in the construction of dams, canals, road and railway subgrades, and waste containment structures such as soil covers and liners. These infrastructure and foundation soils are exposed to the influence of environmental factors. In the permafrost and seasonally frozen regions, soils can be in different states (e.g., saturated or unsaturated, frozen or thawed, or combinations of them) due to the variations in moisture content and temperature. The soil-water characteristic curve (SWCC), which is the relationship between soil water content and suction, is used in the interpretation and prediction of unsaturated soils behavior. Similarly, the soil-freezing characteristic curve (SFCC), which is the relationship between unfrozen water content and subzero temperature, is used in the prediction and interpretation of frozen soils behavior. In this thesis, the SWCC and SFCC of two Canadian soils (i.e. Toronto silty clay (TSC) and Toronto lean clay (TLC)) were extensively investigated for better understanding the fundamental relationship between SWCC and SFCC. The soil resilient modulus (MR) is a key material property used in the rational design of pavements. Experimental investigations were undertaken to determine the MR of five Canadian soils (i.e., TSC, TLC, Kincardine lean clay (KLC), Ottawa Leda clay (OLC), and Indian Head till (IHT)), considering the influence of moisture and temperature, with the aid of an advanced triaxial testing equipment. Two simple models were proposed for estimating the MR of frozen soils, in this thesis. In addition, an artificial neural network (ANN) model was developed for estimating the MR of the five Canadian soils considering various influencing factors. The conclusions from the various studies in this thesis are succinctly summarized below. (1) Four expressions (i.e. power relationship, exponential relationship, van Genuchten equation, and Fredlund and Xing equation) that are widely used for representing the SFCC were selected for providing comparisons between the measured and fitted SFCCs for different soils. The results suggest that the exponential relationship and van Genuchten equation are suitable for sandy soils. The power relationship reasonably fits the SFCC for soils with different particle sizes, but not for saline silts. The Fredlund and Xing equation is flexible and provides good fits for all the soils. (2) The SFCC and SWCC of TSC and TLC were experimentally determined, analyzed, and compared. Many factors influence the reliable measurement of SFCC, which include sensors’ resolution and stability, sensor calibration for each soil, and thermodynamic equilibrium condition. The hysteresis of SFCC for the two soils is mainly attributed to the supercooling of pore water. The quantitative dissimilarity in the measured SFCC and SWCC may be attributed to specimen structure variations during compaction and saturation, and during freezing / thawing processes, and cracks formation due to sensors insertion. In addition, some fundamental differences may exist between the drying / wetting and freezing / thawing processes, resulting in dissimilarity. (3) Two novel models were proposed for the estimation of MR of frozen soils. The semi-empirical model extends the mechanics of unsaturated soils and employs SFCC for prediction. Several coarse- and fine-grained saturated soils were used to validate this model. The empirical hyperbolic model was proposed considering that the frozen MR versus subzero temperature relationship resembles hyperbola. This model was validated on coarse- and fine-grained soils under saturated / unsaturated conditions. The hyperbolic model has wider application since it can be used for both saturated and unsaturated frozen soils. Both the models are simple and promising. (4) The MR of five Canadian soils subjected to wetting and freezing was determined by using the GDS ELDyn triaxial testing system. A freezing system was established for controlling the desired testing temperatures within the soil specimens. The results suggest: (i) The effect of subzero temperature on the MR is significant. (ii) For TLC, KLC, OLC, and IHT, the frozen MR versus subzero temperature relationship of the saturated specimen typically has steeper slope than specimen at the optimum water content, for the temperature range from 0 to -5 °C. (iii) The effect of stress levels on the frozen MR depends on soil type, water content, and subzero temperature. Lastly, (iv) Loading frequency does not show a significant influence on the frozen MR. (5) The MR of the five Canadian soils was determined considering wetting and freeze-thaw (F-T) conditions. The results suggest: (i) The F-T cycles result in weak soil structure due to reduction in suction, particles movement, loss of cohesion, and formation of cracks / channels. (ii) The critical numbers of F-T cycles were determined as 1, 1, 2, and 1 for TLC, KLC, OLC, and IHT at the optimum water content, respectively. (iii) The percentage of reduction in MR after the critical number of F-T cycles was strongly related to the plasticity index for specimens tested at the optimum water content. (iv) The wetting process results in the decrease in suction and enlargement of soil pores. Consequently, relatively low MR values were measured at high water contents, and the effect of F-T cycles becomes insignificant. Finally, (v) The effect of stress levels on the MR was dependent on the initial water content of the specimen and soil type.

A major uncertainty in soil carbon studies is how inputs of fresh plant-derived carbon affect the turnover of existing soil organic matter (SOM) by so-called priming effects. Priming may occur directly as a result of nutrient mining by existing microbial communities, or indirectly via microbial population adjustments. Soil type and conditions may also influence the intensity and direction of priming effects. However the mechanisms are poorly understood. The objectives of this study were (1) to investigate how additions of labile C4 substrate affected SOM turnover in two contrasting unplanted C3 soils (clayey fertile from Temple Balsall, Warwickshire (TB) and sandy acid from Shuttleworth, Bedfordshire (SH) using13 C isotope shifts; (2) to investigate the influence of rhizodeposition from plant roots on SOM turnover in the same two soils planted with a C4 grass; (3) to assess an automated field system for measuring soil temperature, moisture and photosynthesis sensitivities of SOM turnover in the same two soils over diurnal to seasonal time scales. I used a combination of laboratory incubation, glasshouse and field experiments. In the soil incubation experiment, I made daily applications of either a maize root extract or sucrose to soil microcosms at rates simulating grassland rhizodeposition, and followed soil respiration (Rs) and its δ13 C over 19 days. I inferred the extent of priming from the δ13 C of Rs and the δ13 C of substrate and soil end-members. There were positive priming effects in both soils in response to the two substrates. In the SH soil there were no differences in priming effects between the substrates. However in the TB soil, sucrose produced greater priming effects than maize root extract, and priming effects with sucrose increased over time whereas with maize root extract declined after the first week. I explain these effects in terms of the greater fertility of the TB soil and resulting greater microbial nitrogen mineralization induced by priming. Because the maize root extract contained some nitrogen, over time microbial nitrogen requirements were satisfied without priming whereas with sucrose the nitrogen demand increased over time. In the glasshouse experiment, I planted C4 Kikuyu grass (Pennisetum clandestinum) in pots with the same two soils. The extent of rhizodeposition by the plants was altered by intermittently clipping the grass in half the pots (there were also unplanted controls) and priming effects were inferred from the δ13 C of Rs and the δ13 C of plant and soil end-members. Unclipped plants in both soils generated positive priming effects, while clipping reduced priming in TB soil and produced negligible PEs in SH soil. Microbial nutrient mining of SOM again explained the observed PEs in this experiment. Photosynthesis was a major driver of priming effects in the planted systems. In the third experiment, I found that the tested automated chamber system provided reliable measurements of Rs and net ecosystem exchange (NEE), and it was possible to draw relations for the dependency of Rs and NEE on key environmental drivers. Collectively, the results contribute to a better understanding of the mechanisms of priming effects and highlight possibilities for further research. The methods developed here will allow high temporal and spatial resolution measurements of Rs and NEE under field conditions, using stable isotope methods to separate fluxes into plant- and soil-derived components. Keywords: Soil respiration, soil moisture, soil temperature, Isotope ratio, maize root, flux chamber, climate change, organic matter, rhizodeposition.

Research Doctorate - Doctor of Philosophy (PhD) / Amidst growing concerns about global warming, efforts to reduce atmospheric CO2 concentrations (i.e. C sequestration) have received widespread attention. One approach to C sequestration is to increase the amount of C stored in terrestrial ecosystems, through improved land management. Terrestrial ecosystems represent a critical element of the C interchange system, however a lack of understanding of the C cycle at regional and sub-regional scales means that they represent a source of primary uncertainty in the overall C budget. This thesis aims to address this deficiency by developing an understanding of catchment-scale processes critical for accurate quantification of C in the landscape. An investigation into the spatial and temporal dynamics of soil organic carbon (SOC) was conducted for a 150ha temperate grassland catchment in the Upper Hunter Valley, New South Wales, Australia. The major factors controlling the movement, storage, and loss of SOC were investigated, including climate, vegetation cover, soil redistribution processes, topography, land use, and soil type. This study falls into four broad areas. In the first part of this study the spatio-temporal dynamics of soil moisture and temperature at the catchment scale are assessed for a range of soil depths. Data recorded from a network of monitoring sites located throughout the study catchment was compared with independently derived soil moisture and temperature data sets. The data indicates that soil moisture and temperature in surface soil layers were highly dynamic, in their response to rainfall and incoming solar radiation, respectively. Deeper soil layers however were less dynamic, with longer lag times observed with increasing soil depth, as topography, soil type, and landscape position were the dominant controlling factors. Climate related variables are important factors affecting plant growth and net primary productivity. The second part of the study quantified spatial and temporal vegetation patterns using both field-based measurements of above-ground biomass and remotely sensed vegetation indices from the MODIS and Landsat TM 5 platforms. A strong and statistically significant relationship was found between climate variables and MODIS derived NDVI, leading to the development of a predictive vegetation cover model using ground-based soil moisture, soil temperature, and sunshine hours data. The ability of remotely sensed data to capture vegetation spatial patterns was found to be limited, while it was found to be a good predictor of temporal above-ground biomass trends, enabling net primary productivity to be quantified over the three-year study period. In the third part of the thesis soil redistribution patterns and erosion rates were quantified using the caesium-137 method and empirical and physically-based modelling approaches. The impact of soil redistribution processes on SOC distribution was investigated, and the amount of erosion derived SOC loss quantified. A significant proportion of SOC stored within the catchment was found below a soil depth of 0.30m, which is the depth of sampling set out in the IPCC and Australian Greenhouse Office guidelines for carbon accounting. Soil depth was identified as a key factor controlling the spatial distribution of SOC, which is in turn determined by position in the landscape (i.e. topography). The fourth and final part of the study describes how data on erosion derived SOC loss were used in conjunction with net primary productivity estimates, to establish a SOC balance. This involved mapping the spatial distribution of SOC using a high resolution digital elevation model of the catchment, in conjunction with soil depth measurements, and quantifying the total SOC store of the catchment. It was observed that temporal changes in SOC were minimal over the limited three-year study period, however, the continuity of catchment management practices over the previous decades suggest that steady-state conditions have perhaps been reached. The study concludes that the key to increasing the amount of SOC and enhancing carbon sequestration in the soil, is to increase the amount of SOC stored at depth within the soil profile, where factors such as soil moisture and temperature, which control decomposition rates, are less dynamic in space and time, and where SOC concentrations will be less vulnerable to changes occurring at the surface in response to global warming and climate change.