School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemExamining the effects of ‘urban biochar’ on physical and chemical properties of plant growing media and soil( 2016)Management of municipal waste, such as biosolids and greenwaste, is becoming increasingly problematic with the increase and spread of urban populations. Traditionally such urban resources were viewed as wastes and were often discarded to landfill. More recently these waste resources have been combined, composted and returned to land as a soil conditioner. However, several issues including land availability primarily due to concerns of odour as well as the emission of gases such as ammonia, nitrous oxide and methane limits large scale disposal on land. An alternative way to manage these wastes is to convert them to biochar at a centrally located urban location and use it as a growing media or as a soil amendment. Biochar is a carbon rich porous solid material, produced as a by-product during thermo-chemical conversion of biomass under anaerobic conditions, at temperatures above 300 °C. Biochar resembles charcoal but is produced from a wider range of feedstocks such as biosolids and manure and unlike charcoal biochar is not aimed for energy applications. Properties of biochar mainly depend on the pyrolysis temperature and choice of feedstock. The biochar used in this study was slow pyrolysis high temperature (650°C) biochar produced using biosolids and greenwaste in a 2:1 ratio on dry mass basis, called as urban biochar (UB)Although many studies have been conducted to investigate the short term effects of biochar in soils, long term impact of biochar on soil properties remains under studied. Furthermore, the studies conducted to date lack a whole system approach, as they have not investigated the soil - plant system, so do not consider major nutrient loss and use pathways such as: leachate, gaseous loss, storage by soil and plant uptake. Considering, the high cost of biochar production, commercial application of biochar is feasible only if the biochar is used for high value production systems, such as the growing media industry. However, there are very limited studies investigating the effects of biochar on physical and chemical properties of growing media. To fill this knowledge gap, in our study, I performed detailed characterization of UB and compared with published values for standard growing media to assess the potential of UB to be used as a growing media substrate. Growing media was then formulated according to the standard industry practice using different rates of UB. To understand the effect of UB on physical and chemical properties of growing media, two laboratory incubation and two glasshouse experiments were conducted. Some key physical and chemical properties tested in the incubation studies were water retention capacity at different matric potential under wetting - drying cycles, air filled porosity, change in bulk density over time, physical breakdown of media particles and nutrient release from UB amended unfertilized growing media mixes. Silverbeet plants were grown in the glasshouse experiment where major nitrogen and phosphorus loss pathways were measured using custom made chambers. Co-composting of UB with food waste was performed to artificially age UB in order to predict long term changes in UB. Suites of chemical and surface analysis were performed to identify the difference between the fresh and aged UB. The fresh and aged UB were then applied to a sandy acidic horticultural soil (Semiaquic Podosol) where important chemical and physical properties were tested to understand nutrient use efficiency and plant growth. The results of comprehensive characterization of urban biochar and comparison of those results with the published data for growing media substrates indicated that urban biochar has potential to be used as growing media substrate. Incubation studies using different rates of UB in growing media showed UB can completely replace peat from growing media and can be used up to 60% on a volume basis. UB amended media performed better than industry standard media in terms of pH, capacity to supply nutrients, particle size distribution and greater water holding capacity particularly at lower suction when media dries out. Furthermore, UB amended media proved to be more stable than industry standard media in terms of physical and chemical stability even after being exposed to periodic wetting and drying cycles. UB also reduced the loss of both nitrate and phosphate from growing media. For example 60% UB amended media had 95% less loss of nitrate and 51% less loss of phosphate than the media without UB. Co-composting biochar with food waste showed that co-composting can be used as a tool for artificial ageing of UB as it increases biochar surface oxidation (CEC of UB increased by 37%) and increases the nutrient load of biochar. Addition of 10% UB in composting also accelerated the composting process and improved the germination index. However, application of co-composted/aged UB to soil resulted in lower plant growth (plant growth in fresh UB amended soil was 74% higher than co-composted UB amended soil), lower nutrient use efficiency, greater N2O emission and lower water holding capacity as compared to soil amended with fresh UB. In conclusion, the present research suggests that i) UB can replace peat and can be used as a growing media substrate when used up to 60% on volume basis; ii) UB improves physical and chemical properties of growing media; iii) Plants grown in UB amended growing media have better growth and greater nutrient use efficiency than industry standard growing media; iv) Co-composting UB with food waste ages the UB, however, it does not have agronomical benefit when applied to sandy acidic horticultural soils.
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ItemHabitat complexity impacts soil biodiversity and ecological processes in urban ecosystems( 2015)In urban ecosystems human management alters the structural complexity of vegetation, litter and soil creating novel habitat combinations not observed in natural and semi-natural ecosystems. This provides a unique opportunity to investigate how habitat complexity affects soil biodiversity, soil biogeochemical and hydrological processes using novel field experimental settings. In this PhD thesis, I tested whether urban habitat complexity operates as a trait-based environmental filter able to shape the diversity of ant assemblages. I found a higher habitat complexity to be detrimental for ant species richness. Nonetheless, habitat complexity did not operate as an environmental filter for ant species acting upon their morphological traits, suggesting the presence of other more complex ecological mechanisms structuring urban ant assemblages. The second aim of the project was to investigate the effects of urban habitat complexity upon macrofauna detritivores, litter and soil microbes, relating their diversity and community composition to organic matter comminution and decomposition processes. A strong positive relationship between habitat complexity and decomposition and comminution processes was found. Organic matter comminution was significantly enhanced by macrofauna detritivore species richness and abundance. Nonetheless, while habitat complexity did not affect soil and litter microbial functional diversity, it did affect litter microbial community composition. Litter microbial community composition was also correlated to the decomposition status of organic matter, but not microbial functional diversity. A final component of this study evaluated effects of the complexity of the habitat components, vegetation, litter and soil upon urban hydrological processes. Canopy stormwater interception, litter water storage and soil water infiltration were higher under more complex urban habitats. This research suggests that urban habitat complexity can exert significant effects on soil biodiversity, biogeochemical and hydrological processes as previously observed in natural and semi-natural ecosystems. Management practices targeting urban habitat complexity through the modification of vegetation, litter and soil components can be designed to promote soil biodiversity and key ecological processes sustained by soil organisms.
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ItemEcological benefits of termite soil interaction and microbial symbiosis in the soil ecosystem in two climatic regions of Australia( 2015)Termite soil interaction is a multidimensional process, the interphase between the surface and subsurface being the most prominent location termitaria and other termite structures usually occupy. Genetic and environmental conditions, including soil type and moisture content, in different climatic regions affect this interaction. There is scant information on termite preferences, foraging behavior within these conditions and impact on soil profile and associated symbiont microorganisms. Foraging activity of termites (Coptotermes frenchi), depth and changes in soil profile with layers of top soil, fine sand, coarse sand and gravel, was studied using a test tank in a laboratory. Termite activities were intensive in only the longest foraging galleries via which they reached and foraged up to the edge of the tank. Wood stakes inserted vertically at three different depth level intervals (0-100, 100-200, and 200-300 mm), visual observations of soil profile samples taken using auger and excavated cross sections of the soil profile all confirmed presence of termite activity, transport and mixing of soil up to the lowest horizon in the otherwise uniform sandy or gravely lower horizons. However, termite activity did not result in complete mixing of soil horizons within the study period. Termites (Coptotermes acinaciformis) were tested for their preference topsoil, fine sand, potting mix and peat, in a laboratory condition at soil moisture contents of 0, 5, 10, 15 and 20% for 30 days. The experimental apparatus involved termite colonies foraging from nesting jars connected to four sets of standing perspex tubes filled with each soil type and moisture content combination attached to the jar lid on top. Soil type had a significant effect on termite preference whereas soil moisture content did not. At lower moisture levels of 0 and 5%, termites preferred fine sand while topsoil was preferred at 10, 15 and 20%. Soil heterogeneity and textural variability with respect to particle size distribution due to termite activity was investigated in two climatic regions of Australia. Mound and surrounding soils of Coptotermes lacteus in Boola Boola State Forest, Victoria, and Amitermes laurensis and Nasutitermes eucalypti in Gove, Northern Territory were studied. The residual effects on bacteria and fungi counts were also investigated in the former. For C. lacteus and A. laurensis mounds the very fine particles sizes (< 0.045 mm) were significantly higher than that of the surrounding soil while the reverse was true for the 2 - 1 mm particle size ranges. For the Nasutitermes mound, however, they recorded significantly higher 2 - 1 mm particle sizes and significantly lower < 0.045 mm particle size ranges than the surrounding soils. For the other particle size ranges in both sites no significant difference was observed between the mound and surrounding soils except for the 0.5 – 0.2 and 0.20.063 mm ranges in the A. laurensis mound which were significantly higher than surrounding soil. Average moisture content of the surrounding soils was significantly higher than that of the mound surfaces which could have resulted in the higher bacteria and fungi counts (cfu/ml) in the surrounding soils.
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ItemMethane oxidation in soil from the Bogong High Plains, Victoria: controlling factors and fire effects( 2014)Fire rapidly changes terrestrial ecosystems by removing vegetation and exposing mineral soil. Methane (CH4) is an important greenhouse gas but little is known about the effects of fire on its production or consumption. Aerobic soils, such as those found in temperate forests and woodlands are important sinks of CH4, consuming 15-45 Tg y-1 of global atmospheric CH4. The composition and activity of the methanotrophic bacteria responsible for this sink are affected by disturbances such as fire. In this study, sub-alpine sites in the Bogong High Plains, Victoria, were used to examine interacting effects of climate and abiotic soil properties on CH4 consumption. Two predominant vegetation types, Alpine Ash (Eucalyptus delegatensis) forest and Snow Gum (E. pauciflora) woodland, were selected to encompass a range of recent fire histories. Methane oxidation followed first-order enzyme kinetics in Alpine Ash soil so that rates were exponentially proportional to the concentration of available CH4. Methane concentrations decreased exponentially down the soil profile indicating that the sole source of CH4 for oxidation was diffusion from the atmosphere. Soil at depths 5 to 20 cm had the greatest capacity to oxidise CH4. The Michaelis-Menten model was used to describe the kinetics of the enzyme rate reaction of CH4 oxidation, and the model parameters indicated that the bacteria responsible were high-affinity methanotrophs (Type II). Soils were confirmed as CH4 sinks with oxidation rates averaging 76.9 ± 5.3 µg CH4 m-2 h-1 in Alpine Ash forest, and 84.2 ± 4.9 µg CH4 m-2 h-1 in Snow Gum woodland. The effects of key abiotic properties such as CH4 diffusion, soil moisture, temperature, bulk density, inorganic nitrogen and pH were examined in relation to potential variation in CH4 oxidation associated with climate change and fire. Soil moisture influenced CH4 oxidation such that at high moisture content, CH4 oxidation was limited by diffusion, and at low soil moisture content methanotroph activity was limited by physiological water stress. Temperature had a relatively small effect with marginal linear increases in CH4 oxidation in the temperature range of 5 to 30 C°C. Methane oxidation was exponentially limited by increasing soil ammonium concentrations, which had a greater effect on CH4 oxidation than temperature and pH. This indicated a potential impact of fire, as soil ammonium concentrations increased in the first year after fire. The relationship between soil pH and CH4 oxidation was quadratic, with CH4 oxidation decreasing with changes in pH either side of an optimum. Soil pH increased after fire although sites that were more recently or severely burnt were still below the optimum pH for CH4 oxidation. This study provided an understanding of the variation of CH4 oxidation in soil from sub-alpine vegetation in the Bogong High Plains. Further research is required to estimate the magnitude of the soil CH4 sink in the Bogong High Plains, and to predict net changes in CH4 fluxes under future fire and climate scenarios, including improved understanding of the composition of methanotroph populations responsible for this important CH4 sink.
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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.