School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemPhytocaps as biotic systems to mitigate landfill methane emissions( 2013)Landfill gas is a significant source of anthropogenic methane emissions and accounts for more than half of greenhouse gas emissions from waste sectors. While harvesting landfill gas for energy is the best mitigation option, methane oxidation by landfill cover soils is considered an important secondary measure to reduce landfill methane emissions. In recent years, regulatory control has evolved to allow consideration of alternative options for final covers. An evapotranspiration cover, also commonly known as phytocap in Australia, is one of the alternative cover options that has been widely considered and investigated. A phytocap presents a soil-plant alternative to the traditional barrier cap approach. It relies on the capacity of a porous layer of soil to store water, and the combination of evaporation and transpiration of vegetation to control the percolation of water into a landfill. When planted with native vegetation, it also improves the ecology and sustainability of a closed landfill. While the hydrological performance of phytocap has been investigated by a number of studies resulting in positive outcomes, its ability to serve also as a “biocover” for effective methane oxidation to mitigate emissions has received little attention. The main aim of this thesis was to assess phytocap performance in terms of enhancing methane oxidation activity in the cover soil and mitigating methane emissions. The research methodology included a full-scale field comparison between phytocaps and conventional compacted clay covers in terms of methane oxidation and emissions. A supplementary glasshouse experiment with both blank and planted soil columns was also conducted to investigate vegetation-methane interactions, and to identify plant influenced soil properties that would affect methane oxidation and emissions. This research forms a part of the 5-year Australian Alternative Cover Assessment Project (A-ACAP), co-funded by the Australian Research Council and Waste Management Association of Australia. In the full-scale field comparison, trial sites located at five landfills under a broad range of Australian climatic conditions have been monitored. The 5 A-ACAP trial sites with side-by-side phytocap and conventional cover test pads were built directly on top of active landfills with an aim to study their hydrological performance as well as methane mitigation efficiency. This thesis related to the methane mitigation component focused on the trial site located in Melbourne where more frequent monitoring campaigns have been conducted. The results of the field trial indicated that phytocaps could mitigate methane emissions more effectively compared to conventional covers. Emission rates detected from the Melbourne phytocap averaged at 1.45 gCH4/m2/day (out of the 17% measurements that resulted in significant positive fluxes), compared to the conventional cover which averaged at 5.57 gCH4/m2/day (out of the 65% measurements that resulted in significant positive fluxes). This positive finding is supported by the gas concentration profile data obtained from both types of covers. The field trial also concluded that the effectiveness of methane oxidation in phytocaps can be significantly enhanced with methane emission reduced to a negligible level when used in combination with gas extraction systems. In contrast, only a marginal gas extraction influence was observed on conventional covers. In addition to the overall reduction in emissions, phytocaps can also significantly reduce the amount of hot spots in surface emissions. For the glasshouse experiment, at both high and low gas influx rates, the planted soil columns showed high oxidation fractions (mostly higher than 0.5), which are comparable to the performance of some biocovers reported in the literature. Rather unexpectedly, the blank soil columns exhibited an even higher average CH4 oxidation fraction (average 0.89 under 36.5-73 gCH4/m2/day load) compared to the planted soil columns (average 0.67 under 36.5-73 gCH4/m2/day load). This finding appeared to be contradictory to the positive methane oxidation enhancement effects of vegetation in soil covers commonly reported in previous studies. With a closer examination, it was observed that the plant roots brought in a significant increase in soil gas diffusivity of the planted columns, which significantly shortened the methane retention time in the soil and subsequently reduced the methane oxidation capacity of the planted columns. The high oxidation fraction of the blank columns was attributed to the organic rich soil. Combining the research of this thesis with the findings of a concurrent A-ACAP hydrological study, it can be concluded that phytocaps provide an economical and sustainable option for new and old landfills to minimise water percolation and to mitigate methane emissions. As a result of achieving the objective of minimising percolation, the soil moisture profile of a phytocap may not be at its optimum for methane oxidation during certain periods of the year. Maintaining a balance between minimising water infiltration and promoting methane oxidation has to be addressed in a phytocap design in order to achieve optimum performance in both functionalities.
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ItemThe impact of fire disturbance and simulated climate change conditions on soil methane exchange in eucalypt forests of south-eastern Australia( 2013)Soils in temperate forest ecosystems globally act as sources of the greenhouse gas carbon dioxide, and both sinks and sources of the greenhouse gases nitrous oxide and methane (CH4), with well-drained aerated soils being one of the most important sinks for atmospheric CH4. Soil CH4 uptake is driven by aerobic CH4 oxidation through methanotrophic bacteria that oxidize CH4 at atmospheric to sub atmospheric concentrations with soil gas diffusivity being one of the key regulators of soil CH4 uptake in these systems. Climate change predictions for south-eastern Australia indicate a high probability of increasing temperatures, lower average rainfall and an increase in the frequency and severity of droughts and extreme weather events. As a further consequence of climate change in south-eastern Australia, there is a predicted increase in days with high fire risk weather and an increased probability of severe wildfires. In response to these predictions, the use of planned burning as a management strategy within Australian temperate forests and woodlands has increased significantly in an attempt to mitigate this risk of uncontrolled wildfire. Changes in soil moisture regimes, temperature regimes and soil disturbance have the potential to alter soil CH4 uptake, however this has generally been studied in the deciduous and coniferous forests of the northern hemisphere. Currently there is a lack of knowledge regarding temporal and spatial regulators of soil CH4 uptake in temperate Australian forest systems and results from northern hemisphere studies cannot be confidently applied to the eucalyptus dominated Australian forests. Consequently, it is difficult to assess how climate change might affect this important soil based CH4 sink, resulting in significant uncertainty around the magnitude and future trends of the CH4 sink strength of forest soils in south-eastern Australia. To help address this uncertainty, this study investigated both the seasonal drivers of soil CH4 uptake and the sensitivity of soil CH4 uptake to altered soil conditions caused by wildfire, planned burning or simulated climate change scenarios in south-eastern Australian temperate eucalypt forests. This thesis encompasses four field studies: (i) To investigate the possible impacts of the predicted decrease in average rainfall and increase in temperature on soil CH4 uptake we measured soil CH4 flux for 18 months (October 2010 – April 2012) after installing a passive rainfall reduction system to intercept approximately 40% of canopy throughfall (as compared to control plots) in a temperate dry-sclerophyll eucalypt forest in south-eastern Australia. Throughfall reduction caused an average reduction of 15.1 ± 6.4 (SE) % in soil volumetric water content, a reduction of 19.8 ± 6.9 (SE) % in water soil filled pore space (WFPS) and a 20.1 ± 6.8 (SE) % increase in soil air filled porosity (φair ). In response to these changes, soil CH4 uptake increased by 54.7 ± 19.8 (SE) %. Increased temperatures using open top chambers had a negligible effect on CH4 uptake. Relative changes in CH4 uptake related more to relative changes in φair than to relative changes in WFPS indicating a close relationship between φair and soil gas diffusivity. Our data indicated that soil moisture was the dominant regulating factor of seasonality in soil CH4 uptake explaining up to 80% of the seasonal variability and accounting for the observed throughfall reduction treatment effect. This was confirmed by additional soil diffusivity measurements and passive soil warming treatments. We further investigated non-linear functions to describe the relationship between soil moisture and soil CH4 uptake and a log-normal function provided best curve fit. Accordingly, soil CH4 uptake was predicted to be highest at a WFPS of 15%. This is lower than in many other ecosystems, which might reflect a drought tolerant local methanotrophic community. However, the applicability of the log-normal function to model CH4 uptake should be evaluated on global datasets. Soil moisture during our study period rarely fell below 15% WFPS and the observed mean was approximately 40% WFPS. It is therefore likely that soil CH4 uptake will increase if rainfall reduces in the dry-sclerophyll forest zone of Australia as a consequence of climate change. (ii) Planned burning is a management strategy applied in south-eastern Australia that aims to reduce fuel loads and therefore mitigate the risk of large, uncontrolled wildfires. Recent government policy changes have led to a significant increase in the total area of public land subject to planned burning activities within the region. To investigate the impact of fire frequency (as a result of planned burning) on soil CH4 uptake, soil methanotrophic activity and soil CO2 fluxes we measured these three variables in six campaigns across all seasons (March 2009 – February 2011) in a dry sclerophyll eucalypt forest in the Wombat State Forest, Victoria. Three different fire frequency treatments had been applied since 1985: planned burning in autumn i) every 3 years, ii) every 10 years, and iii) not burned since before 1985. Mean soil CO2 emissions were significantly higher in the planned burn treatments compared to the unburnt treatments. In contrast, soil CH4 oxidation did not show the same response to planned burning. Our data indicate that differences in soil CO2 fluxes in response to planned burning might be driven by increased autotrophic root respiration most likely related to decreased nutrient and water availability to overstorey plants. This theory contrasts with alternative explanations that focus on post fire changes in soil nitrogen dynamics, increased heterotrophic respiration and increase soils surface temperatures. Given the long-term nature of the applied burning treatments (implemented for over 25 years) it is therefore unlikely that increases in planned burning will have an impact on the CH4 uptake capacity of these fire resistant eucalypt forests. (iii) Wildfire is the most important disturbance event that alters composition and stand age distribution in forest ecosystems in south-eastern Australia. Wildfire impacts often alter environmental conditions that influence CH4 uptake of forest soils. The impact of wildfire on the CH4 uptake capacity of forest soils is currently unknown. In 2010/2011 we measured soil atmosphere CH4 exchange along a chronosequence in a Tasmanian wet sclerophyll eucalypt forest where the time since the last stand-replacing disturbance ranged between 11 years and approximately 200 years and was due to either wildfire or wildfire emulating harvest operations. Our results indicate an initial increase in soil atmosphere CH4 uptake from the most recently disturbed sites (11 years post-disturbance) to ‘mature’ sites (46 and 78 years post-disturbance). This initial increase was followed by a time-since-last-disturbance (TSLD) related decrease in soil atmosphere CH4 uptake. Our data indicate the initial increase in CH4 uptake is related to a decrease in soil bulk density and an associated increase in soil gas diffusivity. However, the subsequent decline in CH4 uptake with increasing TSLD (from 78 to 200 years) was more likely driven by an increase in soil moisture status and a decrease in soil gas diffusivity. We hypothesize that the observed increase in soil moisture status for the stands aged 78 years and older was driven by forest succession related changes in soil organic matter quality/quantity, an increase in throughfall and an overall decrease in stand water use as demonstrated for tall mixed wet sclerophyll eucalyptus forests elsewhere. (iv) In order to gain a better understanding of seasonal and inter-annual variation in soil CH4 exchange for temperate eucalypt forests in south-eastern Australia, we measured soil CH4 exchange in high temporal resolution (every 4 hours or less) over two consecutive years (March 2010 – March 2012) in the Wombat State Forest, Victoria and over one year (October 2010 – February 2012) at the Warra, Tasmania. These two sites are both temperate Eucalyptus obliqua (L. Her) dominated forest systems however they have contrasting annual precipitations (Victoria Site= 870 mm yr-1, Tasmania Site = 1700 mm yr-1). Both systems were continuous CH4 sinks with the Victorian site having a sink strength of -1.79 kg CH4 ha-1 yr-1 and the Tasmanian site having a sink strength of -3.83 kg CH4 ha-1 yr-1 in 2011. Our results show that CH4 uptake was strongly regulated by soil moisture with uptake rates increasing when soil moisture decreased, which explained up to 90% of the temporal variability in CH4 uptake at both sites. Furthermore, when soil moisture was expressed as soil air filled porosity (φair) we were able to predict the CH4 uptake of one site by the linear regression between φair and CH4 uptake from the other site, indicating a generic relationship. Soil temperature only had an apparent control over seasonal variation in CH4 uptake during periods when soil moisture and soil temperature were closely correlated. The natural fluctuation in generally low soil nitrogen levels did not influence soil CH4 uptake at either site. Comparing our measured site data to modelled data utilising a process based methane uptake model (Curry 2007), our two sites showed reasonable agreement providing scaling factors used to account for soil temperature (rT) response and moisture response (rSM) of methane oxidation rate (k) were forced to unity. Under these conditions CH4 uptake was primarily regulated by diffusivity in the model, indicating that observed seasonal variability in soil CH4 uptake at both sites was primarily regulated by soil moisture related changes in soil gas diffusivity. This study filled some important knowledge gaps with regards to information about magnitude and controls of temporal variability but also with regards to climate changes sensitivity of soil CH4 uptake in temperate eucalypt forests in south-eastern Australia and provides important datasets that will enable better predictive modelling of changes in soil CH4 uptake across the temperate forest landscape in south-eastern Australia. The results indicate it is likely that soil CH4 uptake will increase if rainfall reduces in the dry-sclerophyll forests of Australia as a consequence of climate change. Our findings on the impact of wildfire on soil CH4 exchange highlight the potentially large spatial variability in CH4 uptake across the landscape within the same forest and soil type, a factor that would need to be accounted for in global CH4 uptake models. This issue could be partially addressed for tall wet temperate eucalypt forests in case the here theorized relationship between forest succession and CH4 uptake can be verified in further studies.The finding that low intensity planned burning does not have an effect on soil CH4 uptake suggests that fire may need to be of a particular severity before changes in soil properties and the associated changes in soil CH4 uptake can be observed. Our long term monitoring results further highlight the importance of long-term field measurements in establishing relationships between soil environmental drivers and soil CH4 uptake and are therefore useful for the calibration of models that calculate the global CH4 sink distribution and magnitude.
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ItemGreenhouse gas emissions from Australian beef feedlots( 2011)Emissions of the greenhouse gases, methane (CH4) and nitrous oxide (N2O) and the indirect greenhouse gas ammonia (NH3) play an increasing role in public concern about the environmental impact of concentrated animal feeding operations, including feedlots. However, there is a lack of emissions measurements under typical commercial conditions and there is high uncertainty in the estimation. The lack of accurate measurements and baseline emissions also makes it difficult to evaluate efficiency of current mangemange practices and identify the potential reductions under mitigation options. The objective of this study was to achieve increased understanding of greenhouse gas emissions from Australian beef feedlots, elucidating the biophysical factors controlling emissions from feedlot systems. Specifically, the study utilises measurements of greenhouse gas emissions undertaken at commercial feedlots in Australia using micrometeorological methods and integrates data collected from the feedlot operators into empirical models with the aim to identify and quantify the sources of variation in measured emissions between sites and seasons; test the validity the modelling approach used specifically for feedlots and quantify the link between animal behaviour and diurnal emissions patterns. This study comprised two detailed modelling exercises. The first utilising the results of published studies to validate a range of equations for predicting enteric methane emissions and for predicting emissions of methane, nitrous oxide and ammonia from manure. The second modelling exercise utilised the results of measurements undertaken in two commercial Australian feedlots to evaluate a range of models under commercial conditions. Finally, the diurnal variation in micrometeorological measurements of CH4 and NH3 were examined in the context of animal feeding behaviour in order to examine implications for measurement accuracy and examine correlations between fluxes and behaviour. This thesis indicates that the current Australian Inventory methodology for estimating greenhouse gas emissions from feedlots (enteric CH4, manure CH4, N2O and NH3) suffers from considerable inaccuracies. Although more accurate estimates of CH4 emissions appear to be associated with utilising an equation based on ration composition, particularly carbohydrate fractions the current approach over estimates emissions considerably. Inaccuracies in prediction of emissions of N2O and NH3 are related primarily to the use of single “emissions factors” which do not adequately reflect the changes in potential emissions associated with changing environmental conditions. This thesis also explored the contribution of CH4, N2O and NH3 using IPCC default factor of 1.25% deposited NH3 is lost as N2O to total feedlot emissions, represented as CO2-e. Initial estimates suggest that feedlot emissions were dominated by CH4, with minor contributions of direct and indirect N2O. However, based on the measurements nitrogenous greenhouse gases are predicted to contribute up to 52% of total CO2-e. These results indicate that mitigation options to reduce feedlot emissions need to be applied to both enteric CH4 and nitrogenous gas emission, particularly NH3.
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ItemMethane and carbon dioxide exchange in the tropical savannas of northern Australia: the role of termites( 2011)Termites are one of the most uncertain components of global CH4 budget mainly because of the lack of long-term field based studies from different biogeographical regions. This thesis investigated the exchange of CH4 and CO2 between termites and atmosphere, and between soil and atmosphere in the tropical savannas of northern Australia. Diurnal variations in CH4 fluxes were measured from mounds of Microcerotermes nervosus, Microcerotermes serratus and Tumulitermes pastinator every four hours over a 24 hour period. There was large diurnal variation in mound CH4 fluxes caused by diurnal temperature patterns. Mound CH4 fluxes measured between 10:00 and 12:00 hours best represented the mean daily flux. Seasonal measurements of mound CH4 fluxes were up to 25-fold greater in the wet season than the dry season and always greater in the wet season for all investigated species. Detailed studies in M. nervosus revealed that these differences were not associated with changes in environmental pattern but seasonal changes in termite mound population size. The magnitude of diurnal and seasonal variations in mound CH4 fluxes measured in this study suggest that estimates of global CH4 emissions from termites that do not account for such variations will contain larger errors and uncertainty. The contribution of mound-building, hypogeal and wood-nesting termites to the CH4 balance was estimated for a savanna woodland at Howard Springs near Darwin. Methane fluxes were measured from termite mounds and from the soil - from which CH4 fluxes from hypogeal termites were estimated. Methane fluxes from wood-nesting termites were estimated based on known species abundance. Termites were an annual CH4 source of +0.24 kg CH4-C ha-1 y-1 and soils a CH4 sink of -1.14 kg CH4-C ha-1 y-1. Thus, termites offset 21% of CH4 consumed by soil methanotrophs, but overall the savanna ecosystem was a sink for CH4 of -0.90 kg CH4-C ha-1 y-1. Two indirect methods were tested to predict CH4 and CO2 fluxes from termite mounds. The first predicted mound CH4 fluxes from ‘easier-to-measure’ mound CO2 fluxes. The second predicted CH4 and CO2 fluxes from termite mounds based on the relationship between internal mound concentrations and external mound flux. For both indirect methods the prediction errors were small when calculated separately for each species, whereas, a generic relationship or predictions between species resulted in large errors, probably associated with different mound structures for different species. This study shows that CO2 emissions from termite mounds are up to two orders of magnitude greater than CH4 emissions, when expressed in CO2-equivalents. There was large variation in both CH4 and CO2 fluxes from termite mounds and soil among different sites which suggests caution when scaling up fluxes from the plot or site scale to a regional or greater scale. This study filled important knowledge gaps in the ecosystem ecology of termites and Australian savannas. This study establishes North Australian savannas as one of the few biogeographical regions where the contribution of termites to ecosystem CH4 exchange has been investigated. The study highlights the difficulties associated with predicting CH4 flux from termites on a biome scale, which are caused by the high temporal and species-specific variability in flux. Future studies will have to consider these issues in order to reduce the uncertainty of the role of termites in the global CH4 budget.