School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemEvaluation of fat, nitrate and 3-nitrooxypropanol for reducing enteric methane emissions from ruminants( 2019)Enteric methane produced by ruminants is a dietary inefficiency that contributes to global warming. Feeding diets containing starch to ruminants has been reported to decrease enteric methane emissions. The feeding of methane mitigating agents such as supplemental fat, nitrate and 3-nitrooxypropanol (3- NOP) to ruminants has also been reported to decrease enteric methane emissions, but the degree of methane mitigation may depend upon the basal diet offered to the animal. Furthermore, data are lacking on the net impact of 3-NOP and nitrate on whole farm greenhouse gas (GHG) emissions across different production systems. The aim of this thesis was to quantify the productivity and mitigation potential of these methane mitigating agents when they were included in different diets, and to model the effects of feeding 3-NOP and nitrate on whole farm GHG emissions and on the economics of dairy and beef farms. Initially, the methane mitigating agents were tested in vitro, they decreased methane production (MP) and their methane mitigating efficacies were not affected by basal diet. In the following in vivo experiment feeding supplemental fat to dairy cows decreased methane yield in cows fed either wheat or corn based diets. A modelling study predicted that both 3-NOP and nitrate would decrease whole farm GHG emissions; however, 3-NOP had a greater effect on enteric methane, and thus whole farm emissions, than nitrate. The research of this thesis indicates that the antimethanogenic responses to fat, nitrate and 3-NOP are not affected by substrate type, suggesting that these compounds could be effective in ruminants fed wheat grain, which is routinely fed to ruminants in Australia. Feeding 3-NOP could be economical for beef and dairy farms, depending on the cost of 3-NOP. It is concluded that 3-NOP could make an important contribution to reducing whole farm GHG emissions; however, either a production benefit would have to be demonstrated or a carbon offset method would have to be in place to incentivize its use in the livestock industries.
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ItemEnhancement of methane generation by reducing nitrogen concentration during anaerobic digestion of swine manure( 2017)Anaerobic digestion of swine manure for the production of biogas principally CH4 may add value to manure through the production of energy. However, the large nutrient load, from total ammoniacal nitrogen (TAN – ammonium (NH4+) + ammonia (NH3(l))), in the manure and digestate is a problem during anaerobic digestion of swine manure. Low carbon:nitrogen (C:N) ratios and related NH3 inhibition are major issues affecting methane (CH4) generation during anaerobic digestion of swine manure. The clay mineral zeolite adsorbs NH4+ and may be added to slurry or digestate in the anaerobic digester to reduce NH4+ concentrations and thus mitigate inhibition by ammonia. However, the effects of the physico-chemical properties of zeolite in reducing TAN in digestate, and mechanisms for improving biogas production have not been examined. This information is vital for developing the applications of zeolite to anaerobic bioreactors to reduce TAN so that it can be used at industrial scale. In addition, the recovery of nitrogen (N) from swine manure is an option for reusing N in agriculture and reducing the amount released into the environment. The primary objectives of this study were to develop a methodology for effective recovery of N during anaerobic digestion of swine manure, to investigate impacts of N recovery on CH4 generation, and to identify the feasibility of modifying digester by zeolite technology. To achieve the above objectives the study comprised a series of laboratory adsorption/desorption experiments and swine manure anaerobic digestion experiments with different rates of Australian zeolite. The bulk commercial Australian natural zeolite obtained from Zeolite Australia Pty Limited, Werris Creek, NSW, Australia, and zeolite that had been treated with sodium chloride (NaCl) (hereafter referred to as sodium zeolite) were used in this study. The NH4+ adsorption characteristics of Australian natural zeolite at aqueous solutions with high NH4+ concentrations were investigated under a range of experimental conditions. The results showed that initial NH4+ concentration, temperature, reaction time, and pH of the solution had significant effects on the NH4+ adsorption capacity of Australian zeolite. Increased retention time and temperature generally had a positive impact on adsorption. The NaCl treatment increased the NH4+ adsorption capacity of sodium zeolite by 25% at 1000 mg-NH4+-N in solution compared to the natural Australian zeolite. The maximum adsorption capacities of both natural Australian zeolite and sodium zeolite were estimated as 9.48 and 11.83 mg-N/g, respectively at 1000 mg-NH4+-N in solution. However, NH4+ was adsorbed 44% and 57% less by natural and sodium zeolites respectively when zeolites were added to the solutions which consisted of other ions that resembled the ionic composition of digested manure. Maximum anaerobic digestion of swine manure from selected hydraulic retention times (HRTs) was obtained at 16 days HRT when comparing CH4 production at 8, 12 and 16 days HRT. The digestion process appeared unstable at HRTs shorter than 12 days. The C:N ratio of swine manure was 8.6:1 which was lower than optimum values for digestion and this was the reason for low CH4 production at all HRTs. Direct application of different rates of Australian natural and sodium zeolites into the digesters proved that all digesters with added zeolite produced more biogas and CH4 – over 10% more than the control nil-zeolite digesters. The digesters which had natural zeolite at a dose of 40g/L resulted in the largest increase (29%) in total CH4 yield from swine manure compared to the control. The lag phase of digestion was shorter with increasing zeolites doses up to 100g/L. NH4+ adsorption by zeolites increased linearly with increasing zeolite doses in the digesters. Both Australian natural and sodium zeolites adsorbed approximately similar amounts of NH4+ during the anaerobic digestion of swine manure. Natural and sodium zeolites at a dose of 100g/L reduced the TAN concentration by 50% and 52% of NH4+ (P = 0.034 compared to the control respectively. However, the increases in CH4 yield of digesters that had natural and sodium zeolites at a dose of 100g/L were only 10% and 12% (P=0.013). Although NH4+ adsorption by zeolites showed a linear relationship with increasing zeolite doses, biogas, and CH4 production increased linearly only up to 40g/L for natural zeolite. Variations of cations concentration inside the digesters as a result of adsorption-desorption caused by adding zeolites at different rates might be another reason for variations of CH4 production from different rates of zeolites in digesters. Both natural and sodium Australian zeolites increased CH4 production significantly when cellulose was added to adjust the swine manure C:N ratio to that optimal for CH4 production. The addition of zeolite, which adsorbs NH4+, affects not only the toxicity of NH3 and the C:N ratio, but also the regulation of pH and this might be the reason for enhancing CH4 yield. Therefore, adding Australian zeolite is the best solution for the problems associated with swine manure co-digestion with different carbon sources, resulting in enhanced CH4 production rates in C:N adjusted swine manure while adsorbing substantial quantities of NH4+ from the medium. Australian natural zeolite was shown to be a potential sorbent for the removal of NH4+, K+ and P ions during anaerobic digestion of swine manure. However, higher concentrations of natural zeolite at higher pH might not be appropriate for anaerobic digestion as zeolite enhances P- precipitation by releasing more Ca2+ to the medium. Different designs of digesters with zeolite treatment were developed and digesters with an external zeolite column (Ex-Zeo digester), with an inside zeolite bed (In-Zeo digester), and a digester without addition of zeolite (No-Zeo digester) were examined. The increases in CH4 yields during the 40 day digestion period, compared to No-Zeo digester, were 5% for the EX-Zeo digester and 15% for the In-Zeo digesters. The results indicated that treatment of swine manure with 7% zeolite during anaerobic digestion has the potential to improve biodegradation of organic material in swine manure and consequently to produce more CH4. Adsorption of NH4+ was lower in Ex-Zeo digesters compared to the In-Zeo digesters, and this resulted in the lower CH4 production. The research reported in this thesis contributed a number of significant findings to improve the understanding of the adsorption-desorption properties of Australian zeolite, CH4 enhancement mechanisms by zeolite, effects of the addition of zeolite during the anaerobic digestion of swine manure, and their optimum rates of enhancing CH4 production while mitigating N concentration inside the digesters. The experiment with an external zeolite column connected to the digester showed the feasibility of applying zeolite as an external column to the digesters. This knowledge will enable the use of Australian zeolite during the anaerobic digestion of swine manure to enhance CH4 production while mitigating NH4+ concentration inside the digesters.
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ItemLivestock performance in a varied landscape and climate( 2016)The sheep and beef industries are significant contributors to Victoria’s economy. Despite competition for land and water resources and an increasingly variable climate, these industries need to continue to increase productivity to remain profitable. As major contributors of greenhouse gases, particularly methane, reducing emissions is also a challenge for these enterprises when considering alternative food and fibre sources. Financial benchmarking, modelling and emission intensity studies seldom recognise that these enterprises are often grazed on diverse landscapes and in the most variable climates, where options for alternative food production are limited. The co-production of meat and fibre from most sheep breeds is often not well recognised and consistently attributed in greenhouse gas emission allocation. This study aimed to investigate whether the relative profitability of a range of sheep and beef enterprises, change across land class and climate. It also aimed to develop measures of and to provide insight into drivers of flexibility, efficiency and profitability leading to guidance on better placed enterprise choice for environment. Four farm case studies were selected to represent climate and land class variability and modelled in GrassGro™. Each farm had one to three land classes enabling eleven ‘farms’ differing in land class over four locations to be simulated. Diversity of land class in this study represented differences in soil type, depth and fertility, slope of land and pasture species. A range of sheep and beef enterprises were tested across all farms to enable comparisons of profitability, flexibility and enteric methane emissions over a forty year period (1970-2010). Enterprise profitability was estimated as net margin per hectare, to allow for labour requirement differences between the enterprises. Flexibility was explored by considering enterprise profit variability and profit response in the top and bottom 10% of years. Strategies that reduced the effects of poor years (higher profit) and increased or did not reduce profitability in good years were considered to have greater flexibility. Enteric methane emissions were calculated in GrassGro™ and an estimation of total end-product made to address the differences in processing stages of red meat and wool, and provide a measure of emission intensity. To further consider the effects of droughts, studies were conducted to quantify commodity price changes in droughts and early warning indicators. A model using total soil water and the Southern Oscillation Index was tested to investigate the potential for predicting droughts before the onset of spring and so provide opportunity for early decision making to mitigate some of the financial impacts of droughts. Wool contributed most to enterprise profitability when pasture production was lowest, as on unimproved pastures, poor soil types and/or in low rainfall years. As pasture production increased across sites and was more reliable, live weight was a stronger driver of profit and systems that had a high proportion of immature animals, tended to perform the best. The specialist meat sheep enterprises were the most profitable when pasture production was highest, but had the highest sensitivity to climate variability. The spring calving enterprise tested in this study was consistently more profitable than the autumn calving enterprise, although the difference was less when pasture production was lowest. These results support, and help to explain, farm financial analyses that have reported enterprise profitability changes between rainfall or regional zones. Where these results did not reflect modelling studies, it was considered to be due to the range in pasture production at the sites tested in this study, as most of the differences were evident at the extremes in pasture production. Most modelling studies in Victoria use sites with high and reliable rainfall, and/or highly productive pastures. Whilst wool production provided a buffer to the susceptibility to droughts, the ability to increase meat production from the system increased flexibility and enterprise profit. For the prime lamb enterprises, a first cross or self-replacing meat enterprise suited to the climate and land class were equally profitable and able to provide options for increased flexibility by feeding and selling lambs early, joining ewes as lambs and running less ewes. Total pasture production, seasonal pasture supply curves as well as replacement ewe turn over price contributed to the most profitable and flexible strategies. The results help to explain some of the inconsistency in the literature on the contribution of strategies to enterprise profit and add to the discussion on the value of increasing reproduction rates. The method for measuring flexibility in this study provides a more quantifiable measure of the term, and addresses factors other than average profitability, of particular relevance to highly variable climates. As with profitability, relative differences in emissions between enterprises were less and/or changed at the sites with the lowest pasture production. As most modelled studies make estimations on fully improved pastures at high rainfall sites, they may overestimate the relative efficiencies of meat specialist systems over wool or dual purpose systems in poorer pasture and land class environments. Research has identified strategies that contribute to lowering emission intensity, such as increased fecundity, improving the feedbase and/or genotype and systems that have a higher proportion of immature stock associated with higher feed efficiency. Consistent with research, these strategies reduced emission intensity but were also most profitable when flexibility rather than profit maximisation was addressed. Therefore the most efficient systems that were also highly profitable tended to be those that maximised returns in good years and reduced the susceptibility to droughts, compared to those that focused on profitability alone. Strategies to do this were not always the same across sites. Analysis of feed and stock prices in recent droughts indicated that steeper price changes occurred from July onwards, compared to other years. Incorporating proportional price changes in drought years in programs like GrassGro™ would allow more realistic analyses of the potential financial implications of drought. Early warning of droughts could provide the opportunity to mitigate losses by using tactical strategies such as selling surplus stock before prices fall and through early purchasing of feed. An explorative study tested triggers of soil moisture and the Southern Oscillation Index which provided reliable indicators of low decile pasture producing springs, with limited risk of above average springs. Further studies are required to test and validate this across more sites and explore the useability for farmers to make informed decisions. The changes in relative profitability, flexibility and emission intensity across landscapes contribute insight into the variability in performance of farm enterprises, within regions and/or when measured as per unit of rainfall. Hence the ability for some farms to attain enterprise profitability achieved by the top 20% of farms based on profitability per mm of rainfall, may not be realistic or achievable. Whilst the industry is currently pushing to increase reproduction rates in sheep enterprises, this study indicates that strategies to do so may vary across enterprise and land class and may not be the most appropriate strategy or profit driver across all farms. More in depth work is needed to identify profit drivers for sheep meat production across land class and environment, particularly in the less reliable pasture production sites. With predictions of increased climate variability, some areas may need to reconsider the suitability of the enterprise for the location and land capability. Similarly, modelling studies that use only sites with high rainfall and improved pastures may not be able to confidently extrapolate results across the wider Victorian environment and may be underestimating emission efficiencies.
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ItemApplications of the eddy-covariance micrometeorological technique for estimating methane emissions from grazing cattle( 2017)Methane, produced through enteric fermentation in the foregut of cattle and sheep, represents the largest source of greenhouse gas emissions from Australian agriculture. Yet quantifying emissions under typical management conditions, particularly from grazing cattle remains a challenge. In Australia, where roughly half of the national cattle inventory is grazed on extensive rangelands, the need for quantification of these emissions is central to future mitigation efforts. Eddy covariance is well suited to long term flux monitoring and the recent development of fast-response sensors for methane has led to an increasing number of studies reporting methane fluxes from a variety of environments. Employing eddy covariance in a cattle-grazed landscape however creates a significant interpretation problem. This work reports on the development, validation and use of an adapted Lagrangian stochastic dispersion model to interpret eddy covariance methane fluxes in terms of animal emissions. Two controlled gas release studies, using fixed release points, demonstrated that the new method was able to deliver estimates within 15% of the true emission rate which is in the range of accuracy typical of other micrometeorological approaches. Model performance was best under near-neutral atmospheric stability conditions which typically prevailed in the mid-afternoon to early evening. Accuracy was also found to be dependent upon the source to sensor distance and the measurement height. Use of this technique in a grazing environment required time-lapse camera systems and an image analysis procedure to provide the necessary animal position information. Emission estimates for animals on the landscape were best achieved by positioning measurements near a central water point where cattle tended to congregate during the day. An attractant on the landscape in the form of a feed supplement was also found to be effective in retaining animals within the measurement footprint of a pasture situated eddy covariance system. The attractiveness of the eddy covariance technique lies in its small instrumentation footprint and its suitability for remote monitoring. Instrumentation is confined on a single tower with power requirements low enough to be met through solar panels and batteries. The installation of optically based sensors in harsh, dusty environments will of necessity require a base level of human intervention for general maintenance but this study has demonstrated that with a degree of care and oversight, eddy covariance instrumentation can prove useful for investigating long-term emissions from cattle-grazed landscapes.
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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.