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Greenhouse Accounting PhD Thesis Topics: 2002-2005
More info about how to apply...1. Role of Vegetation Type in Soil Organic Matter AccumulationCRC Program A: Measurement
& Spatial Estimation of Carbon Pools and their Turnover A preponderance of world literature on soil carbon-stock change ofter the conversion of pastures to forest indicates that pine trees tend to reduce soil organic matter stocks despite the much greater carbon stocks aboveground and in surface litter and the lack of vegetation removal by grazing. It is not known why. Of several hypotheses that seek to explain the observation above, one is that pine roots may foster more rapid soil organic matter decomposition, relative to pasture. This might come about via material excluded from pine roots, from the character of the soil microbial populations supported by pines compared with pastures, or the effect of pine roots compared with the effect on pasture roots on soil structural stability and hence organic matter stability. Another hypothesis is that the removal of N-fixing legumes present in pastures when pines establish, causes loss of ecosystems C-stocks and consequentially, aloss of soil carbon. It has been further hypothesised that the reduction in organic matter is primarily due to the reduction of the active fast-turnover fraction of the soil orgainic carbon. The Cooperative Research Centre (CRC) for Greenhouse Accounting is setting up a 2 to 3 year field study in the Australian Capital Territory (ACT) where pine forest and improved pasture sites are located immediately adjacent to native pasture from which they both developed. The study will evaluate up to ten hypotheses relating to the change in soil C stocks following land cover change that includes the above hypotheses. The context of this work is to assist the development of methodologies to calculate expected CO2 emission or uptake by the soil folllowing land use change for the natinal effort to comply with the intent of the Kyoto Protocol. The
CRC invites PhD proposals that develop one or more of the hypotheses
above 2. Changes in Tree Carbon Allocation in response to Water AvailabilityCRC
Program A: Measurement
& Spatial Estimation of Carbon Pools and their Turnover Water
limitation on plant growth is ubiquitous in the Australian landscape.
Studies in this area will quantify the impact of water availability
on root:shoot ratios. Water availability to trees can vary for a number of reasons. These include absolute rainfall, the season in which it falls, the vapour pressure deficit during water use by the tree, the ability of the soil to store water (soil depth and texture) and additional water sources (water tables and lateral flow of water) will all affect the availability and use of water by the tree. A simple aridity index may be calculated as the ratio of rainfall to potential evaporation. Such a ratio only accounts for some of the variation in water availability. To account for the all source ratio requires an integrated assessment of the water availability as seenn by the tree. Isotope discrimination (carbon and oxygen) measurements provide a long term averaged measure of the soil - atmosphere water balance. Potentially such measurements may account for many of the finer scale variations outlined above. 3. Topography-fuel Modelling for Carbon AccountingCRC
Program B: Projecting
Changes in the Carbon Cycle Fire models are currently constrained by the lack of data about spatial variability in fuel across landscapes. Fine scale topographic controls on soil water and surface radiation results in landscape heterogeneity in litter production and decomposition rates, and consequently fire regimes. Thus different parts of the landscape can experience variable fire regimes, in terms of fire intensity and the mean fire interval, which is an important factor in Net Biome Productivity. 4. Hydrological Flux of CarbonCRC
Program B: Projecting
Changes in the Carbon Cycle There is very little known about the lateral loss of carbon from catchments as a result of hydrological flows. The little that is known suggests that it is possible for landscape ecosystems to continue functioning as carbon sinks (due to the hydrological flux of carbon), even when biomass and soil carbon have reached equilibrium. 5. Climatic Variability and Net Biome ProductivityCRC
Program B:
Projecting Changes in the
Carbon Cycle Regionally-scaled global circulation models (such as the CSIRO regional model, DARLAM) do not currently have the capacity to capture mesoscale climatic regimes at a resolution commensurate with landscape processes. However, these models can generate useful scenarios indicating broad climatic trends. Scenarios can also be derived using downscaling techniques applied to Global Climate Model output data. The challenge is to develop a quantitative understanding of past and current climatic variability, in order to determine the potential significance of the projected climatic trends for Net Biome Productivity. 6. Modelling Net Ecosystem Carbon Exchange of Woodland Systems in NE AustraliaCRC
Program C:
Systems Development and Support The project involves using a physiologically-based modelling approach to model the growth, death and decomposition of woodland systems under normally varying climatic conditions. The project would use the TRAPS data set (developed by CRC Member, Dr Bill Burrows, Qld DPI) as the primary data set for testing. As part of this project, one aim is to extrapolate findings from the individual patch scale to the Australian continent. 7. Decomposition of Residues from native Australian Vegetation: a Quantification of Chemical Changes and Soil InputsCRC
Program A: Measurement
and Spatial Estimation of Carbon Pools and their Turnover Soil organic carbon contents are a function of the balance between inputs and mineralisation. Inputs are dependent on the productive capability of the site as defined by vegetation type, climate, and fertility, which all have the capacity to regulate the amount of biomass entering the soil system. The mineralisation of these inputs depends on their chemical composition and any protection offered by the soil matrix. In the proposed work, the carbon balance and chemical changes (structural chemistry and isotopic composition) associated with the decomposition of residues derived from Australian native vegetation will be studied. The proportion of residue carbon that finds its way into the soil and the subsequent biological stability and position of the carbon within the soil matrix will be quantified. In addition, the isotopic changes associated with the changes in chemical composition will be examined to further our understanding of carbon balance calculations based on C3-C4 transitions. It is important to define the mechanisms that enhance or protect residue-derived carbon from mineralisation, in order to assess the impacts of land use change on soil carbon stocks. 8. Impacts of Salinity on Soil Carbon Stocks and FluxesCRC
Program A: Measurement
and Spatial Estimation of Carbon Pools and their Turnover The behaviour of carbon in saline soils is not very well understood worldwide. Nearly 11 million hectares of land throughout the world is being converted to saline land each year. The basic task in this project is to investigate the underlying processes (chemical, physical and biological) involved in the breakdown of organic matter to form a part of soil, particularly in saline environments. Some of the key questions that the student will investigate are:
The behaviour of soil organic carbon is affected by climatic conditions; hence the student would be required to investigate these issues in Queensland and in Western Australia. 9. Carbon Dioxide and Nitrous Oxide Emissions from Soil following the Ploughing of Legume and Legume-grass LeysImproved ley pastures are the salient features of sustainable farming in Australia. The ley pasture systems (pasture legume/crop or pasture legume + grasses/crop of variable duration) occupy over 21 million hectares of Australia, varying in climate from "Mediterranean" to "tropical". Although the tropical regions had not used ley systems widely, adoption is steadily increasing on fertility-depleted lands, mainly due to a highly variable climate and thus uncertain responses to fertilisers, but also due to increasing fertiliser nitrogen costs. It has been estimated that improved ley pastures could store 10.8 million tonnes CO2-equivalent per year during the ley phase. Since atmospheric N2 is also usually fixed as organic N, there is a potential for nitrous oxide emissions following ploughing of legume and legume-grass leys in the tropics. However, there is very limited information available on carbon dioxide and nitrous oxide emissions following the ploughing of legume and legume-grass leys. We require field studies in the tropical environment to quantify carbon dioxide and nitrous oxide emissions from different soil depths at the termination of the ley phase at different times of the year. These studies need to be linked to controlled environment studies in the laboratory to quantify the effects of temperature, moisture, carbon and nitrogen substrates (various carbon pools), and aeration on carbon dioxide and nitrous oxide emissions from these soils. This study will provide an important input to the Australian Greenhouse Gas Inventory on the emissions of nitrous oxides, which may counterbalance the benefits of enhanced carbon sinks by ley pastures. The study will also provide a better understanding of below-ground carbon dynamics in ley systems in the tropics. CRC
Program A:
Measurement and Spatial Estimation
of Carbon Pools and their Turnover 10. Potential of Zero Tillage Practices to Sequester Carbon in Soils under Continuous CroppingCRC
Program A: Measurement
and Spatial Estimation of Carbon Pools and their Turnover There is an age-old perception that tillage enhances the decomposition of organic matter in soil by changes in temperature and moisture fluxes, aeration, exposure of new surfaces through aggregate disruption, increasing plant residue-microbial contact, reduced organic material additions, and frequently leads to increased soil erosion. Therefore, by reducing or eliminating tillage we can theoretically sequester more carbon in soil. Many countries, including the USA, thus advocate the inclusion of zero-till (no-till) practices in Additional Activities, Article 3.4 of the Kyoto Protocol for the consideration of soil carbon sinks. A number of studies in the USA have shown that soil under no-till practices retains about 28% more carbon that that under conventional tillage. On the other hand, results to date in Australia are conflicting. They show increases, decreases, nil or at the most very small effects on soil carbon storage under zero-till systems. We are not entirely certain about the processes involved in litter (or plant residue) decomposition as a function of placement, moisture, temperature, soil type, and litter quantity and quality under zero-till and conventional till systems. We require specific studies to quantify the effect of these variables on carbon retention so that we are in a position to address the effect of zero-till on carbon storage, since Australian farmers are increasingly using no-till practices and the zero-till area will continue to increase. These studies will require the measurement of various carbon pools, and also direct measurement of carbon dioxide and nitrous oxide emissions from soil.
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