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Environmental or human-induced disturbances, such as compaction and drought commonly caused by modern agricultural machinery and extreme climate events, have drawn great attention towards their impacts on soil microbial activities and associated declines in soil health and crop yield. To be specific, soil compaction combined with local extreme weather events, such as intensive rainfall or prolonged drought, may cause a significant decrease in soil microbial activity and threaten sustainable crop production. The use of organic amendment and crop rotation are commonly adopted as improved field management strategies to mitigate the disturbance-induced (compaction and drought) declines in crop yield, soil carbon, and soil health. Although widespread interest exists in the assessment of the detrimental impacts of environmental disturbances on agricultural systems, the response of belowground nutrient turnover processes to compaction and/or drought and how such responses are mediated by the microbial communities are less explored. In addition, there is a paucity of information on soil microbial responses to compaction and drought stresses and their recovery from the stressed conditions, particularly the differences in soils with conventional and improved management history. Furthermore, a robust, cheap, and easy-to-understand index needs to be developed in monitoring soil microbial community responses to compaction and drought stresses as current mainstream soil health indicators are expensive and complicated for end-users.This thesis aimed to investigate microbial responses to soil constraints as a measure of soil health in sugarcane and grain systems. The specific obj ctives of this thesis were to a) determine the mechanisms responsible for soils’ microbial functional response and its thresholds to environmental disturbance (compaction and drought); b) adopt this mechanism of the responses of soil microbial community following soil compaction (or drought) stress into both conventional and improved soil management practices contexts; c) generate soil resistance and resilience index based on the responses of soil microbes to compaction and/or drought ; d) enrich the knowledge of soil microbial activity and nutrient pools responses to different practices of removing compaction stress, such as deep trenching mill-mud application and shallow furrow application, in fields condition. Three incubation experiments and a field trial were designed to test following hypotheses: 1) Soil biological functions such as respiration and microbial biomass and activity are reliable indices for assessment of soil health and resilience to compaction and moisture (waterlogging or drought) stresses; 2) Soils with improved management practice history have higher resistance against compaction stress and higher resilience following removal of the stress; 3) Response pattern of soil microbial community to drought stress is highly related to the applied stress levels rather than the history of field management practices; 4) Application of organic amendments (e.g., crop residues, mill-mud) and removal of soil compaction stress would increase the size of soil microbial community and microbial activity (indicating soil resistance and resilience), which further improves soil health condition and increases crop yield. In the first incubation experiment, two contrasting sugarcane soils (Planosols at Rocky Point and Nitisols at Ingham) were exposed to a combination of compaction and drought stresses, and microbial functions were investigated going into the stress as well as their response coming out of the stress. We artificially applied a gradient of bulk densities (0.9 - 1.5 g cm-3) and water fill pore space (WFPS; 21% to 100%). Under high water content (i.e., WFPS of 47% to 100%), low compaction levels (1.1 and 1.2 g cm-3) increased soil cumulative microbial respiration in Planosols treatments by 18% in comparison to nil compaction treatment (0.9 g cm-3), while high compaction levels (1.3 - 1.5 g cm-3) decreased soil cumulative microbial respiration by 25% with increasing of compaction stress in 74 days incubation experiments. In contrast, in Nitisols with high water content, the highest compaction treatment (1.5 g cm-3) significantly increased soil cumulative microbial respiration by 12% in comparison to nil treatment (0.9 g cm-3). Our data revealed that the Nitisols had a higher resistance index of microbial C use efficiency, while the Planosols had a higher resilience index of microbial C use efficiency to compaction and moisture stresses. This could be attributed to greater aggregation potential (associated with more finely textured particles), and higher diversity of the microbial community in the Nitisols, which provide higher functional stability to resist environmental disturbance, while fast drainage and higher adaptation of microbial communities in the Planosols would provide a faster recovery from the applied stresses. This also indicates that soil microbial responses to compaction stress are governed by soil texture and moisture status. In the second incubation experiment, Nitisols with conventional and improved management history from adjacent sugarcane farms at Foresthome, North Queensland, Australia, were exposed to compaction stress (1.4 g cm-3), and microbial functions were investigated during compaction and ploughing cycle as well as their response to surface and mixing plant residue application methods over 70 days. The improved management block was managed with minimum tillage, mound planting, and legume plantation in comparison to conventional furrow management. Compaction was applied at the commencement of the experiment and removed at day 28 via plant residue application combined with ploughing. Overall, the concentration of labile C was 42% higher in treatments with improved management history. Within treatments with the same management history, the treatments with ploughing practice following compaction stress generated the highest cumulative and net cumulative CO2 emissions, followed by compaction-only and ploughing-only treatments. Interestingly, we found that legume residue incorporation along with improved land management minimized fluctuation of net cumulative CO2 emissions between compacted and non-compacted treatments by the end of experiment; however, we did not find the same pattern in treatments with conventional land management. Our results revealed that different field management practices might not alter soil microbial response patterns to compaction stress as microbial activity is mainly governed by water content and water fill pore space. Improved field management practices can improve soil resistance and resilience to compaction stress, which is shown as a faster stabilization of microbial properties after compaction stress and its removal due to their higher organic matter content and complexity of microbial community. The present study confirmed that application of legume residue increased the supply of organic C and N for soil microbial community, enhanced the nutrient cycling processes, and improved soil health status. In the third incubation experiment, Planosols collected from Wickepin, Western Australia (one soil with conventional management history and one soil with improved management history) were exposed to severe, moderate and nil drought stress and microbial functions were investigated going into stress as well as their response coming out of stress via rewetting and plant residue input. In general, treatments with improved management history had higher (15% - 53%) cumulative CO2 respiration in comparison to treatments with conventional management throughout the incubation period without plant residue amendment. In addition, drought stress significantly decreased cumulative CO2 respiration regardless of soil management history. We found that the differences of cumulative CO2 emissions decreased at day 56 but increased at the end of experiment (day 70) in both drought stressed treatments with conventional management history. The hot water extractable organic C pool was highly related (r = 0.56 - 0.78) to the umulative CO2 respiration and microbial activity. The results showed that different field management practices may not change soil microbial response patterns to drought, as microbial activity is mainly governed by soil texture when water content is limited. Improved field management could help to build soil resilience to drought, which is shown as the tolerance to moderate drought and resistance to severe drought due to their high organic matter content and complexity of microbial community. However, soil under conventional field management would have a lower resistance, but higher recovery to drought stress. The plant residue application increased the concentration of microbial biomass and enzyme activity via increasing labile C and nutrients contents regardless of soil management history. A field trial was conducted at Burdekin, Australia, to investigate the effects of different decompaction field managements on soil nutrient cycling, associated biological activities, and sugarcane yield. This experiment included four treatments comprising: control (CK, without mill-mud), mill-mud shallow furrow (MS), deep trench without mill-mud (DT), and deep trench mill-mud application (MD). Overall, the application of mill-mud significantly improved soil organic matter and nutrient content. Surface millmud application increased the concentration of soil Colwell P six-fold in comparison to the control. Deep trench application of mill-mud increased concentrations of hot water extractable organic C by 30% - 70% and hot water extractable total N by 30% - 90% at the application depth (ca. 20 cm depth). Soil microbial biomass C and N were also higher in mill-mud applied layers (ca. 20 cm depth). As expected, in comparison to the control, mill-mud applied treatments increased plant cane yield by 7% (MS treatment) and 14% (MD treatment). The deep trench without mill-mud (DT) practice also increased the plant cane yield by 11% compared to the control, which may be due to the release of native organic C and improved soil health. Deep trench combined with the mill-mud application (MD) increased the supply of organic C and N and nutrients to the microbial community within the entire soil profile, enhanced nutrient cycling processes and soil health for sugarcane growth, and thus increased sugarcane productivity. In the presented thesis, soil microbial responses to two main soil constraints (compaction and drought) under different soil textures, vegetation type and field management history were investigated. Based on the presented result, soil texture and WFPS played vital roles in regulating microbial responses to compaction and drought. The finer soil texture provided a better and stable microbial habitat and WFPS governed the diffusion of soil labile C and nutrients for microbial growth. In addition, field managements such as organic matter amendment and/or ploughing would stimulate microbial responses to soil constraints as organic amendment would directly increase soil organic matter content and ploughing would adjust soil WFPS. Also, these two factors are intensively linked to crop yield as increased sugarcane yield was found in organic matter applied and ploughed treatments, particularly in ploughed only treatment. Additionally, the presented study found that the soil microbial community had higher tolerance to soil disturbances under improved field management history than conventional management history, which could be attributed to the complexity of microbial community structure established through long term improved field management. However, microbial community structure and key functional gene analysis are needed to confirm this statement. In conclusion, application of molecular biology approaches would further increase the potential of using microbial responses to soil constraints as a measure of soil health and generate a comprehensive index in monitoring soil health in the long run.

When constructing wells ranging from simple land wells to complex deepwater wells, incompatibility between oil-based and synthetic-based muds (OBM / SBM) and Portland cements can lead to poor cementation and loss of cement integrity, which in turn may compromise zonal isolation. An alternative cementitious material based on geopolymers has been developed with improved OBM / SBM compatibility for primary cementing and lost circulation control as well as well abandonment. Benefits of using geopolymers go beyond mere OBM / SBM compatibility: it is in fact possible to solidify non-aqueous drilling fluids (NAF) such as SBM and OBM using geopolymer formulations. This also means that such NAFs can be disposed of in a more cost-effective way, which presents a viable option for environmentally acceptable on-site or off-site disposal of drilling muds and cuttings. In the following, focus will be primarily on the compatibility between SBM and geopolymers, with the understanding that the results obtained for SBM can generally be extrapolated to OBM as well. Geopolymer is a type of alkali-activated material that forms when an aluminosilicate precursor powder (such as fly ash) is mixed with an alkaline-activating solution (such as sodium hydroxide). A novel SBM solidification method was developed by blending varied amounts of geopolymer and SBM. The consolidated mud was named a “geopolymer hybrid cement”. In an effort to develop the geopolymer hybrid system as a novel well cementing material, the solidification method was comprehensively studied with various sources of precursor powders, activators, as well as SBM and OBM formulations. Fresh state properties, such as slurry rheology and thickening time, and hardened state mechanical properties, such as compressive strength (under both uniaxial and triaxial confinement conditions), as well as the self-healing capabilities of the geopolymer hybrid cement were evaluated. Strength testing results showed that geopolymer cement can solidify up to a 60/40 geopolymer/SBM ratio by volume. The incorporation of SBM greatly improved the rheological properties of the geopolymer hybrid, allowing for the otherwise non-pumpable slurry to become pumpable for well cementation and lost circulation control purposes. The laboratory evaluations showed that the geopolymer hybrid cement could meet typical requirements as a well cementing slurry. By changing the amount of geopolymer and SBM in the slurry, the geopolymer hybrid can be deliberately designed with high compressive strength for primary cementation, or with lower compressive strength for lost circulation control. Moreover, geopolymer and geopolymer hybrid cements reveal true self-healing capability, which means that they can recover and even increase their strength after prior yielding. This ability would possibly allow such cements to better adapt to subsurface stress changes acting on abandoned wells, making them better suited for use in permanent barriers in plug and abandonment operations.

Sugarcane, as a worldwide cultivated commercial crop, has drawn a great attention towards its nitrogen (N) use efficiency as well as potential effects on associated watersheds. Particularly, there is an increasing concern about potential loss of carbon (C) and nitrogen from sugarcane fields and its environmental impacts on the Australian Great Barrier Reef catchments. Sugarcane field management practices play a vital role in refining soil C and N cycling and improving plant N use efficiency. The common management practices in sugarcane fields such as liming, legume intercropping, green harvest and burning have different effects on soil C and nutrients dynamic. To be specific, liming is widely used as an effective amendment to remediate soil acidity. Moreover, green harvest, which is known as trash blanket practice, is promoted in intensive cropping regions as decomposition of sugarcane trash can sequester C and N under the trash blanket layer. Although, previous studies had investigated the effects of liming and trash blanket on cropping fields, but the understandings of how liming and trash blanket affect Australian sugarcane cropping system, is largely unknown. Additionally, some studies have suggested that sugarcane compost may provide significant amounts of bioavailable C and N to sugarcane cropping system. However, different composition of compost have different influence on soil properties such as soil pH, soil C and N contents. The effects of compost on soil properties may also vary with its application methods as well as soil and vegetation type. Unfortunately, the scientific knowledge regarding the effects of composted sugarcane residue on Australia sugarcane fields remain uncertain. Although several studies have reported the relationship between the soil nutrients cycle and field managements, the mechanistic understanding of how sugarcane field managements affect the N dynamics remained unclear. The objectives of this thesis were to determine: 1) the impacts of liming on the N availability and dynamics in tropical acidic sugarcane soils; and 2) the dynamics of soil C and N in sugarcane cropping fields after application of subsoil organic amendments to sugarcane soil. Two field experiments were designed to achieve these major goals, including: a) Impacts of liming on soil N dynamics in sugarcane fields in Bundaberg; b) Impact of subsoil organic amendments to sugarcane cropping system on long term soil physical, chemical and biological properties in Maryborough, Australia. It was hypothesized that: 1) liming practice in acidic soils would alter soil N dynamics as well as the abundance and activity of soil microorganisms; and 2) subsoil amendment of compost would contribute substantially to the labile C and N supply for soil microbial activity as well as plant uptake and consequently increase sugarcane yield. In a field trial at Bundaberg, Australia, the adjacent sugarcane treatments of 26 months after liming (26ML), 14 months after liming (14ML) and no lime amendment (CK) were selected to investigate the effect of liming practice on soil N bioavailability and microbial activity in a long term subtropical sugarcane cropping system. Liming in both 14ML and 26ML treatments significantly increased soil pH (by 1.2-1.4 units) and exchangeable Ca2+ (> 2 folds) content compared with the CK treatment. The lower concentrations of hot water extractable organic C, total N and NH4+-N in the 14ML treatment compared to the CK and 26ML treatments, was attributed to the absence of trash blanket placement in the former and enhanced N immobilization, by soil microbial community, due to improved soil pH. Liming practice (14ML and 26ML) increased soil microbial biomass C and N contents, particularly in the presence of the trash blanket (26 ML treatment), while decreased soil respiration and qCO2, indicating that acidic stress conditions were relieved in the liming treatments. Soil pH value was shown to be the main factor governing soil enzyme activities, with an overall decrease in all enzyme activities in response to liming practice. In a field trial at Yerra, Maryborough, Australia, Four subsoil amendment treatments, including no amendment as control (CK), gypsum subsoil application as calcium enriched treatment (GP), compost subsoil application as organic ameliorant treatment (CP) and mineral fertilizer subsoil application as fertilizer treatment (FE), were designed to investigate the effect of subsoil amendments on sugarcane soil health. The ameliorants were applied to the subsoil two weeks before sugarcane planting. At the end of the third ratoon harvest, the compost treatment showed a slightly higher soil pH compared to other three treatments. The concentration of NH4+-N in the CP treatment, at the 0-10 cm soil depth, was significantly higher than gypsum (GP) and mineral fertilizer (FE) amended treatments. This may be due to the higher N uptake at the sugarcane root zone (10-25 cm depth) in this treatment and consequently transport of higher amount of N to the topsoil through trash blanket decomposition after sugarcane harvest. The highest concentrations of hot water extractable organic C and N (HWEOC; HWETN) in the CP treatment (10-25 cm depth), can be attributed to the organic matter input and its decomposition within the compost application layer. Soil MBC and MBN contents (0-10 cm depth) generally tend to be higher in the CP treatment in comparison with the other treatments. This may be due to leaching of soluble organic C and N in this treatment, which provide an available energy source for utilization of soil microbial community. The CO2 respiration in the CP and FE treatments, were significantly higher than CK and GP treatments, while GP treatment showed the lowest CO2 respiration in the 10-25 cm depth. At soil depths below 25 cm, soil labile C becomes a limiting factor for soil CO2 respiration, which is supported by significant positive correlation found between HWEOC and soil CO2 respiration. Significant increase in sugarcane yield, over 4 years, has been observed in the CP compared to other treatments, which may be due to the increased labile C and N pools provided by the slow decomposition of subsoil-applied compost.

ABSTRACTPolymer blend thin films are of great importance for many industrial applications, such as microelectronic circuits, optoelectronics, anti-reflection coatings and photoresists, etc. Controlling the phase-separated morphologies of polymer blend thin films can adjust the performance of those applications. However, polymer blend films normally phase separate isotopically resulting in disordered, uncontrolled surface morphologies. Here we demonstrated a facile fabricating strategy to obtain well-controlled, highly ordered, phase-separated morphology in polymer blend thin films utilizing capillary force lithography (CFL) and preferential wetting. The influence of blend ratio and annealing time on the phase-separated morphology of patterned PS/PMMA blend films on UVO-treated silicon substrate was studied. We observed that with increasing annealing time, phase-separated domains were coarsened and diffuse into specific area (PS towards elevated region and PMMA towards base region) due to entropic force and preferential wetting. The well-controlled, anisotropic phase-separated morphology was achieved when the blend ratio was larger than the volume ratio of elevated region to base region of the patterned film. Our work may open new avenues to fabricate polymer blend thin films with well-controlled, anisotropic phase-separated morphology.

The incompatibility of cement and shale and the subsequent failure of primary cementing jobs is a very significant concern in the oil & gas industry. On wells ranging from hydraulically fractured shale land wells to deepwater wells, this incompatibility leads to an increased risk in failing to isolate zones, which could possibly present a well control hazard and can lead to sustained casing pressure. The cement-shale interface presents a weak link that often becomes compromised by the loads incurred either during drilling, completion/stimulation or production phases. To formulate cements for effective zonal isolation, it is crucial to evaluate the bond strength of the cement-shale interface. Although several studies have focused on the interactions between cement and sandstone, very few studies have addressed the bonding behavior of cement with shale. The conventional push-out test protocol used to measure cement-to-sandstone shear bond strength has proven to be difficult to apply on shale due to its laminated or brittle nature that complicates sample preparation and can lead to shale or cement matrix failure instead of failure at the interface. In this paper, we present a novel, simple and versatile laboratory test procedure to measure the shear bond strength between cement and shale. The new procedure was used to develop cement formulations to improve the cement-to-shale bond. Two different design approaches were investigated. One involves introducing Gilsonite into cement to maintain shale integrity. The second design involves using surfactant to improve cement interfacial sealing property. Our results indicate that bond strength of cement with shale can be enhanced significantly incorporating surfactant in cement slurries.