Soil salinity is a widespread problem that affects crop productivity. We expect that saline soils also have altered microbial community structure, soil food webs and related soil properties. To test this, we sampled field soils across four farms in eastern North Dakota that host salinity gradients. We evaluated microbial biomass carbon, phospholipid fatty acid analysis and nematode counts in moderately saline and low saline soils. Additionally, we measured soil properties that represent potential food sources and habitat characteristics that influence microbial communities. We found higher microbial group abundance in moderately saline soils than in the lower saline soils. In contrast, we found lower nematode abundances in the moderately saline soils. We also observed increased labile carbon, nitrogen, phosphorus, and water content in the moderately saline soils. Based on our results, saline soils appear to have unique soil biological characteristics, which have implications for overall soil function along salinity gradients.

The structure and function of microbial communities are critical to the maintenance and sustainability of terrestrial ecosystem processes. Consequently, there is substantial interest in assessing how microbial communities respond to various land management practices, and if alterations to the characteristics of microbial communities has the potential to disrupt ecosystem processes. This thesis was conducted to identify the long term effects of fertilisation and different levels of post-harvest organic matter removal on the characteristics of the FH litter and soil microbial communities in six, second rotation Pinus radiata plantation forests located around New Zealand. The six sites, established between 1986 and 1994, were sampled in 2002 and 2003. Various physical and chemical properties of the sites were measured, and litterfall production was determined. The microbial biomass in the FH litter layer and soil was determined by chloroform fumigation-extraction, and Biolog plates were used to assess the relative differences in microbial community diversity, based on patterns of substrate utilisation. Fertilisation substantially altered the physical and chemical properties of the forest floor, including FH litter moisture content, mass, carbon content, nitrogen content and carbon: nitrogen ratio and soil pH, nitrogen content and carbon: nitrogen ratio. The same range of FH litter and soil properties were also significantly changed by different levels of organic matter removal. The biomass and diversity of the FH litter and soil microbial communities were significantly altered by fertilisation and organic matter removal, and the differences in the microbial community characteristics were significantly correlated to the effects of the fertilisation and organic matter removal treatments on the physical and chemical environment in the majority of cases. The physical and chemical properties of the sites were significantly correlated to estimates of wood production, and it was also found that the characteristics of the microbial community were strongly related to productivity at several sites. The results demonstrated that fertilisation and organic matter removal regimes have had long term effects on the microbial communities at the sites. The persistence of the effects of the organic matter removal treatments were particularly noteworthy, as these treatments were applied at site establishment, and despite no subsequent reinforcement over the life of the trials, were still substantially influencing the physical, chemical and microbiological properties of the FH litter and soil up to 17 years later. The results of this thesis also emphasised the value of long-term experiments in assessing the effects of disturbance on the physical, chemical and microbiological characteristics of forest ecosystems. Further research into the specific nature of the relationship between site productivity and microbial community characteristics was suggested as an important focus for future studies.

The structure and function of microbial communities are critical to the maintenance and sustainability of terrestrial ecosystem processes. Consequently, there is substantial interest in assessing how microbial communities respond to various land management practices, and if alterations to the characteristics of microbial communities has the potential to disrupt ecosystem processes. This thesis was conducted to identify the long term effects of fertilisation and different levels of post-harvest organic matter removal on the characteristics of the FH litter and soil microbial communities in six, second rotation Pinus radiata plantation forests located around New Zealand. The six sites, established between 1986 and 1994, were sampled in 2002 and 2003. Various physical and chemical properties of the sites were measured, and litterfall production was determined. The microbial biomass in the FH litter layer and soil was determined by chloroform fumigation-extraction, and Biolog plates were used to assess the relative differences in microbial community diversity, based on patterns of substrate utilisation. Fertilisation substantially altered the physical and chemical properties of the forest floor, including FH litter moisture content, mass, carbon content, nitrogen content and carbon: nitrogen ratio and soil pH, nitrogen content and carbon: nitrogen ratio. The same range of FH litter and soil properties were also significantly changed by different levels of organic matter removal. The biomass and diversity of the FH litter and soil microbial communities were significantly altered by fertilisation and organic matter removal, and the differences in the microbial community characteristics were significantly correlated to the effects of the fertilisation and organic matter removal treatments on the physical and chemical environment in the majority of cases. The physical and chemical properties of the sites were significantly correlated to estimates of wood production, and it was also found that the characteristics of the microbial community were strongly related to productivity at several sites. The results demonstrated that fertilisation and organic matter removal regimes have had long term effects on the microbial communities at the sites. The persistence of the effects of the organic matter removal treatments were particularly noteworthy, as these treatments were applied at site establishment, and despite no subsequent reinforcement over the life of the trials, were still substantially influencing the physical, chemical and microbiological properties of the FH litter and soil up to 17 years later. The results of this thesis also emphasised the value of long-term experiments in assessing the effects of disturbance on the physical, chemical and microbiological characteristics of forest ecosystems. Further research into the specific nature of the relationship between site productivity and microbial community characteristics was suggested as an important focus for future studies.

The PLFA profiling method was adopted and used to determine changes in microbial community structure and abundance along natural and human-induced environmental gradients. The presented studies were based on field sampling campaigns combined with targeted laboratory experiments. According to the aims of particular studies, microbial PLFA fingerprinting was combined with the auxiliary below- and aboveground ecosystem characteristics to identify the drivers of microbial responses to environmental changes or with 13C-labelling and metagenomics to obtain more complex information about running processes and involved microorganisms.

No description available.

Increased amounts of pesticide production and application of pesticides for agriculture, plant protection and animal health has resulted in soil, water and air pollution, consequently relating a serious risk to the environment and also to human health. Pesticides include several groups of compounds, herbicides, insecticides, rodenticides and fumigants consisting of several hundred individual chemicals. Herbicides are an integral pan of modem agriculture and for industries requiring total vegetation control. Most herbicides are soil applied and more and more concern is raised that herbicides not only affect target organisms but also the microbial community present in soil. The ESKOM sub-station Zeus, in Mpumalanga (South Africa) used to apply an industrial weed control program for the eradication of vegetation, which led to the contamination of soil by several herbicides. These herbicides consisted of Bromacil, Tebuthiuron and Ethidimuron which are all photosynthesis inhibitors, more specifically, they disrupt the plastoquinone protein during electron transport at photosystem I1 (PSII). In this study the effect of biostimulation and bio-augmentation of a specific bioremediation agent (B350) as prescribed by ESKOM, on residual herbicides, Bromacil, Tebuthiuron and Ethidimuron was evaluated by monitoring the soil physical and chemical properties, microbial attributes, including potential microbial activity and community structure, as well as the physiological effect experienced by plants (Cynodoh dactylon and Zea mays). Results from soil physical and chemical analyses were correlated with results obtained for the functional and structural diversity of microbial communities. All results were investigated through statistical and multivariate analysis and the most prominent soil physical and chemical parameters that influence the biological and biochemical properties of the soil were identified. Results obtained from this study indicated that there were no significant difference (p < 0.05) between the treatments, with bioremediation agent, irradiated agent and without the agent based on results obtained from soil microbial properties and plant physiology. Before the trial started the uncontaminated soil showed an active microbial function, characterised by dehydrogenase, urease and arylsulphatase activity, but community structure was not very diverse. The contaminated soil, irradiated contaminated soil and silica sand showed less enzymatic function and was characterised by phospholipid fatty acid groups, mid-branched saturated fatty acids, terminally branched saturated fatty acids, normal saturated fatty acids and monosaturated fatty acids which are indicative of microorganisms that survive better in harsh environments. Three weeks after the addition of the specific bioremediation took place, the uncontaminated soil showed an increase in P-glucosidase activity and percentage organic carbon (%C), which could be a result of the presence of available plant material. Furthermore, an increase in major PLFA groups were seen, suggesting that an increase in diversity within the soil community occurred. The contaminated soil, irradiated contaminated soil and silica sand once again was characterised by a low microbial function and diversity, showing no improvement. Fluorescence data clearly show a decline in PS 11 function that result in the decline of the rate of photosynthesis, which was seen from COz gas exchange rates. Furthermore, the decrease in photosynthetic activity after three weeks was too severe to supply additional information about the mechanism within photosynthesis or the photoprotective mechanisms. A detailed study was conducted in which a 3: 1 dilution of contaminated soil with silica sand, was also monitored for changes within plant physiology. Results revealed that inhibition of PS I1 function already takes place within a few days time and the decline in photosynthesis is as a result of electron transport that does not supply adenosine triphosphate (ATP) and P-nicotinamide adenine dinucleotide (NADPH) to the Calvin cycle (or Reductive Pentose Phosphate pathway). It does not appear that rubulose-1,sbisphosphate carboxylase-oxygenase (Rubisco) is affected within the Calvin cycle. As a result of PS I1 function failure, reaction centres are damaged by the production of harmful singlet oxygen and photoprotective mechanisms (xanthophyll cycle) can not be activated. Thus, except for dealing with ineffective electron transport, additional damage is caused to physiological functions. After six weeks a decrease in the estimated viable biomass for all growth mediums was found. Results of the of trans- to cis- monoenoic fatty acids and cyclopropyl fatty acids to their monoenoic precursors ratios indicated that the soil microbial community for the contaminated growth mediums, all experienced nutritional stress throughout this trail. The specific bioremediation agent (B350) used, seemed to have no effect on the microbial function and community structure within soil and as agent had no effect on the residual herbicides or the plant physiology which experienced an extreme decline in major metabolic functions. / Thesis (M. Environmental Science)--North-West University, Potchefstroom Campus, 2

Mining activities cause severe disturbance to the soil environment in terms of soil quality and productivity and are of serious concern worldwide. Under South African legislation, developers are required to ecologically rehabilitate damaged environments. The application of agronomic approaches for the rehabilitation of coal discard sites has failed dismally in the arid areas of southern Africa. It is obvious that compliance with mitigation and rehabilitation requirements cannot be enforced without a thorough understanding of the ecological principles that ensure ecological stability and subsequent sustainability of soil ecosystems. Soil micro organisms are crucial role-players in the processes that make energy and nutrients available for recycling in the soil ecosystem. Poor management practices and other negative impacts on soil ecosystems affect both the physical and chemical properties of soil, as well as the functional and structural properties of soil microbial communities. Disturbances of soil ecosystems that impact on the normal functioning of microbial communities are potentially detrimental to soil formation, energy transfers, nutrient cycling, plant reestablishment and long-term stability. In this regard, an extensive overview of soil properties and processes indicated that the use of microbiological and biochemical soil properties, such as microbial biomass, enzymatic activity and the analysis of microbial community structure by the quantification of specific signature lipid biomarkers are useful as indicators of soil ecological stress or restoration properties because they are more responsive to small changes than physical and chemical characteristics. In this study, the relationship between the physical and chemical characteristics and different biological indicators of soil quality in the topsoil covers of seven coal discard sites under rehabilitation in South Africa, as well as three reference sites was investigated. Through the assimilation of basic quantitative data and the assessment of certain physical, chemical and biological properties of the topsoil covers obtained from the various coal discard sites as well as the reference sites, the relative success or progress of rehabilitation and the possible correlation between the biological indicators of soil quality and the establishment of self sustaining vegetation covers was determined. Results from soil physical and chemical analyses and percentage vegetation cover were correlated with the results obtained for the functional and structural diversity of microbial communities at the various sites. All results were investigated through statistical and multivariate analysis and the most prominent physical and chemical parameters that influence the biological and biochemical properties of the soil and possibly the establishment of self-sustainable vegetation cover on these mine-tailing sites were identified. Results obtained from this study indicated no significant difference (p>0.05) between the various discard sites based on conventional microbiological enumeration techniques. However, significant differences (p<0.05) could be observed between the three reference sites. All enzymatic activities assayed for the rehabilitation sites, with the exception of urease and alkaline phosphatase displayed a strong, positive association with the organic carbon content (%C). Ammonium concentration had a weak association with all the enzymes studied and pH only showed a negative association with acid phosphatase activity. A positive association was observed between the viable microbial biomass, vegetation cover and the organic carbon content, ammonium, nitrate and phosphorus concentrations of the soil. The various rehabilitation and reference sites could be differentiated based on the microbial community structure as determined by phospholipid fatty acid (PLFA) analysis. It is hypothesised that the microbial community structure of the Hendrina site is not sustainable when classified along an r-K gradient and that the high percentage of vegetation cover and high levels of estimated viable microbial biomass are an artificial reflection of the current management practices being employed at this site. Results obtained during this study, suggest that an absence or low percentage of vegetation cover and associated lower organic matter content of the soil have a significant negative impact on soil biochemical properties (enzymatic activity) as well as microbial population size. Furthermore, prevailing environmental physico-chemical and management characteristics significantly influences the vegetation cover and subsequently the microbial community structure. The results indicate that the microbial ecosystems in the coal discard sites could become more stable and ecologically self-regulating, provided effective management to enhance the organic carbon content of the soil. This could enhance nutrient cycling, resulting in changes of soil structure and eventually an improved soil quality which could facilitate the establishment of self sustaining vegetation cover. Results obtained during this study suggest that a polyphasic assessment of physical and chemical properties; microbial activities by enzymatic analysis; the characterisation of microbial community structure by analysis of phospholipid fatty acids; and the multifactorial analysis of the data obtained can be used as complementary assessment criteria for the evaluation of the trend of rehabilitation of mine tailings and discard sites. Strategic management criteria are recommended based on the soil quality environmental sustainability indices to facilitate the establishment of self sustainable vegetation covers. The contribution of this research to soil ecology is significant with regards to the intensive investigation and explanation of characteristics and processes that drive ecological rehabilitation and determine the quality of the soil environment. The multidisciplinary approach that is proposed could, furthermore, assist in the successful rehabilitation and establishment of self-sustaining vegetation covers at industrially disturbed areas, as well as assist in improving degraded soil quality associated with both intensive and informal agriculture. Additionally, this approach could negate the negative social and environmental impacts frequently associated with these activities. / Thesis (M. Environmental Science)--North-West University, Potchefstroom Campus, 2004.

Increased amounts of pesticide production and application of pesticides for agriculture, plant protection and animal health has resulted in soil, water and air pollution, consequently relating a serious risk to the environment and also to human health. Pesticides include several groups of compounds, herbicides, insecticides, rodenticides and fumigants consisting of several hundred individual chemicals. Herbicides are an integral pan of modem agriculture and for industries requiring total vegetation control. Most herbicides are soil applied and more and more concern is raised that herbicides not only affect target organisms but also the microbial community present in soil. The ESKOM sub-station Zeus, in Mpumalanga (South Africa) used to apply an industrial weed control program for the eradication of vegetation, which led to the contamination of soil by several herbicides. These herbicides consisted of Bromacil, Tebuthiuron and Ethidimuron which are all photosynthesis inhibitors, more specifically, they disrupt the plastoquinone protein during electron transport at photosystem I1 (PSII). In this study the effect of biostimulation and bio-augmentation of a specific bioremediation agent (B350) as prescribed by ESKOM, on residual herbicides, Bromacil, Tebuthiuron and Ethidimuron was evaluated by monitoring the soil physical and chemical properties, microbial attributes, including potential microbial activity and community structure, as well as the physiological effect experienced by plants (Cynodoh dactylon and Zea mays). Results from soil physical and chemical analyses were correlated with results obtained for the functional and structural diversity of microbial communities. All results were investigated through statistical and multivariate analysis and the most prominent soil physical and chemical parameters that influence the biological and biochemical properties of the soil were identified. Results obtained from this study indicated that there were no significant difference (p < 0.05) between the treatments, with bioremediation agent, irradiated agent and without the agent based on results obtained from soil microbial properties and plant physiology. Before the trial started the uncontaminated soil showed an active microbial function, characterised by dehydrogenase, urease and arylsulphatase activity, but community structure was not very diverse. The contaminated soil, irradiated contaminated soil and silica sand showed less enzymatic function and was characterised by phospholipid fatty acid groups, mid-branched saturated fatty acids, terminally branched saturated fatty acids, normal saturated fatty acids and monosaturated fatty acids which are indicative of microorganisms that survive better in harsh environments. Three weeks after the addition of the specific bioremediation took place, the uncontaminated soil showed an increase in P-glucosidase activity and percentage organic carbon (%C), which could be a result of the presence of available plant material. Furthermore, an increase in major PLFA groups were seen, suggesting that an increase in diversity within the soil community occurred. The contaminated soil, irradiated contaminated soil and silica sand once again was characterised by a low microbial function and diversity, showing no improvement. Fluorescence data clearly show a decline in PS 11 function that result in the decline of the rate of photosynthesis, which was seen from COz gas exchange rates. Furthermore, the decrease in photosynthetic activity after three weeks was too severe to supply additional information about the mechanism within photosynthesis or the photoprotective mechanisms. A detailed study was conducted in which a 3: 1 dilution of contaminated soil with silica sand, was also monitored for changes within plant physiology. Results revealed that inhibition of PS I1 function already takes place within a few days time and the decline in photosynthesis is as a result of electron transport that does not supply adenosine triphosphate (ATP) and P-nicotinamide adenine dinucleotide (NADPH) to the Calvin cycle (or Reductive Pentose Phosphate pathway). It does not appear that rubulose-1,sbisphosphate carboxylase-oxygenase (Rubisco) is affected within the Calvin cycle. As a result of PS I1 function failure, reaction centres are damaged by the production of harmful singlet oxygen and photoprotective mechanisms (xanthophyll cycle) can not be activated. Thus, except for dealing with ineffective electron transport, additional damage is caused to physiological functions. After six weeks a decrease in the estimated viable biomass for all growth mediums was found. Results of the of trans- to cis- monoenoic fatty acids and cyclopropyl fatty acids to their monoenoic precursors ratios indicated that the soil microbial community for the contaminated growth mediums, all experienced nutritional stress throughout this trail. The specific bioremediation agent (B350) used, seemed to have no effect on the microbial function and community structure within soil and as agent had no effect on the residual herbicides or the plant physiology which experienced an extreme decline in major metabolic functions. / Thesis (M. Environmental Science)--North-West University, Potchefstroom Campus, 2

Mining activities cause severe disturbance to the soil environment in terms of soil quality and productivity and are of serious concern worldwide. Under South African legislation, developers are required to ecologically rehabilitate damaged environments. The application of agronomic approaches for the rehabilitation of coal discard sites has failed dismally in the arid areas of southern Africa. It is obvious that compliance with mitigation and rehabilitation requirements cannot be enforced without a thorough understanding of the ecological principles that ensure ecological stability and subsequent sustainability of soil ecosystems. Soil micro organisms are crucial role-players in the processes that make energy and nutrients available for recycling in the soil ecosystem. Poor management practices and other negative impacts on soil ecosystems affect both the physical and chemical properties of soil, as well as the functional and structural properties of soil microbial communities. Disturbances of soil ecosystems that impact on the normal functioning of microbial communities are potentially detrimental to soil formation, energy transfers, nutrient cycling, plant reestablishment and long-term stability. In this regard, an extensive overview of soil properties and processes indicated that the use of microbiological and biochemical soil properties, such as microbial biomass, enzymatic activity and the analysis of microbial community structure by the quantification of specific signature lipid biomarkers are useful as indicators of soil ecological stress or restoration properties because they are more responsive to small changes than physical and chemical characteristics. In this study, the relationship between the physical and chemical characteristics and different biological indicators of soil quality in the topsoil covers of seven coal discard sites under rehabilitation in South Africa, as well as three reference sites was investigated. Through the assimilation of basic quantitative data and the assessment of certain physical, chemical and biological properties of the topsoil covers obtained from the various coal discard sites as well as the reference sites, the relative success or progress of rehabilitation and the possible correlation between the biological indicators of soil quality and the establishment of self sustaining vegetation covers was determined. Results from soil physical and chemical analyses and percentage vegetation cover were correlated with the results obtained for the functional and structural diversity of microbial communities at the various sites. All results were investigated through statistical and multivariate analysis and the most prominent physical and chemical parameters that influence the biological and biochemical properties of the soil and possibly the establishment of self-sustainable vegetation cover on these mine-tailing sites were identified. Results obtained from this study indicated no significant difference (p>0.05) between the various discard sites based on conventional microbiological enumeration techniques. However, significant differences (p<0.05) could be observed between the three reference sites. All enzymatic activities assayed for the rehabilitation sites, with the exception of urease and alkaline phosphatase displayed a strong, positive association with the organic carbon content (%C). Ammonium concentration had a weak association with all the enzymes studied and pH only showed a negative association with acid phosphatase activity. A positive association was observed between the viable microbial biomass, vegetation cover and the organic carbon content, ammonium, nitrate and phosphorus concentrations of the soil. The various rehabilitation and reference sites could be differentiated based on the microbial community structure as determined by phospholipid fatty acid (PLFA) analysis. It is hypothesised that the microbial community structure of the Hendrina site is not sustainable when classified along an r-K gradient and that the high percentage of vegetation cover and high levels of estimated viable microbial biomass are an artificial reflection of the current management practices being employed at this site. Results obtained during this study, suggest that an absence or low percentage of vegetation cover and associated lower organic matter content of the soil have a significant negative impact on soil biochemical properties (enzymatic activity) as well as microbial population size. Furthermore, prevailing environmental physico-chemical and management characteristics significantly influences the vegetation cover and subsequently the microbial community structure. The results indicate that the microbial ecosystems in the coal discard sites could become more stable and ecologically self-regulating, provided effective management to enhance the organic carbon content of the soil. This could enhance nutrient cycling, resulting in changes of soil structure and eventually an improved soil quality which could facilitate the establishment of self sustaining vegetation cover. Results obtained during this study suggest that a polyphasic assessment of physical and chemical properties; microbial activities by enzymatic analysis; the characterisation of microbial community structure by analysis of phospholipid fatty acids; and the multifactorial analysis of the data obtained can be used as complementary assessment criteria for the evaluation of the trend of rehabilitation of mine tailings and discard sites. Strategic management criteria are recommended based on the soil quality environmental sustainability indices to facilitate the establishment of self sustainable vegetation covers. The contribution of this research to soil ecology is significant with regards to the intensive investigation and explanation of characteristics and processes that drive ecological rehabilitation and determine the quality of the soil environment. The multidisciplinary approach that is proposed could, furthermore, assist in the successful rehabilitation and establishment of self-sustaining vegetation covers at industrially disturbed areas, as well as assist in improving degraded soil quality associated with both intensive and informal agriculture. Additionally, this approach could negate the negative social and environmental impacts frequently associated with these activities. / Thesis (M. Environmental Science)--North-West University, Potchefstroom Campus, 2004.

Plant residues added to soil are a source of nutrients for plants and soil organisms and increase soil organic matter which has an important role in improving soil structure and fertility, hence maintaining soil quality for sustainable agriculture. In order to utilize plant residues for increasing soil organic matter more effectively, it is necessary to understand the mechanisms of plant residue decomposition. Soil organic matter decomposition is influenced by several factors such as plant residue quality, temperature, water availability, soil structure and soil texture, particularly clay content. The interaction of clay and decomposition of organic matter has been studied in the past. Nevertheless, many studies investigated this interaction in natural soil or under field conditions over long periods of time. Variation in environmental factors may influence the interaction of clay and decomposition of organic matter, thus in most previous studies their effect cannot be separated from the direct effect of clay on decomposition. To study the direct effect of clay on organic matter decomposition, four experiments with different objectives were carried out using isolated natural clay, under controlled conditions (e.g. temperature and organic matter input) and a short incubation period (approximately one month). All experiments were carried out using a sand matrix to which different clay types, clay fractions (natural or with iron oxide partially removed) or clay concentrations were added together with mature wheat straw (C/N 122 in most experiments, except Experiment 2 where the wheat straw had a C/N of 18) and a microbial inoculum. To investigate the effect of clay type, two clay types were added. They were isolated from Wiesenboden (W) and Red Brown Earth (RBE) soil. Clay types from both soils contained kaolinite and illite, but smectite only occurred in W clay. Iron oxide is thought to be important for the binding of organic matter to clay, therefore two clay fractions were used, the clay with native iron oxide (natural clay) and clay from which iron oxide was partially removed by citrate-dithionite-bicarbonate treatment (citrate-dithionite clay, CD clay). The following parameters were measured: pH, water loss, respiration rate, microbial community structure using phospholipid fatty acid analysis and, in some experiments, particulate organic matter. In all experiments, the water content of the substrate mixes was adjusted only at the start; water loss was greatest in the control and decreased with increasing clay content. The aim of the first experiment was to study the effect of the concentration of W clay on decomposition of wheat residues. Respiration (i.e. decomposition of the wheat straw) was affected by clay in two ways (i) decreased decomposition, thus protection of organic matter, in the initial phase at all concentrations (5, 10, 20 and 40%) and throughout the incubation period at ≤ 20% clay, and (ii) greater water retention at higher clay concentration particularly 40% clay that allowed maintenance of higher respiration rates towards the end of incubation. Generally, clay concentration had an effect on microbial community structure but not on microbial biomass. The effect of clay concentration was also investigated in the second experiment, but using RBE clay and a narrower range of concentrations (0, 2.5, 5, 10 and 20% clay) than in the first experiment with W clay. The wheat residue used in this experiment had a lower C/N ratio compared to the other three experiments (C/N 18 compared to 122). In contrast to the first experiment, cumulative respiration of the clay treatments was greater than that of control throughout the incubation, thus clay increased rather than decreased decomposition. This may be due to the properties of the wheat residue used in this experiment which contained more water-soluble compounds, the diffusion of which would be enhanced in treatments with clay compared to the control due to their higher water availability. However, considering only the treatments with added clay, cumulative respiration followed the same pattern as in the first experiment, with highest cumulative respiration at 20% clay. In general, microbial community structure, microbial biomass and microbial groups (i.e. bacterial and fungal fatty acids) were affected by the presence of clay and sampling time, but there was no clear relationship between these factors and the richness and diversity of the microbial community. The aim of the third experiment was to determine the effect of clay concentration (5 and 40% of W clay) and fraction (natural or citrate-dithionite clay) on decomposition of wheat straw and microbial community structure. Clay fraction and concentration strongly affected the respiration rate and microbial community structure as well as microbial biomass but not the concentration of particulate organic matter (POM). Compared to the control, partial removal of iron oxide strongly increased decomposition at both concentrations whereas clay with iron oxides reduced the decomposition. Microbial community structure was affected by clay fractions, particularly at 40% clay. The aim of the fourth experiment was to determine the effect of clay fraction (natural and citrate-dithionite clay) and clay type (W clay or RBE clay) at 5% clay on decomposition of wheat straw and microbial community structure. Clay type and the partial removal of iron oxide had a significant effect on the decomposition rate but did not affect POM concentration. As in the third experiment, partial removal of iron oxide increased respiration rate, the effect was less pronounced in RBE clay than in W clay. Clay type and fraction strongly affected microbial community structure. In conclusion, the experiments showed that native clay generally reduces organic matter decomposition by binding and occlusion. The importance of iron oxide for the protective effect of clay on organic matter decomposition was shown by the fact that partial removal of iron oxide strongly increased decomposition rate compared to the native clay. The two clay types differed in their effect. The W clay containing smectite protects organic matter to a greater extent than RBE clay with predominantly illite and kaolinite due to its higher surface area and CEC that lead to binding and or occlusion. The results also showed that although clay reduces organic matter decomposition under optimal water availability, this effect can be reversed as the substrates dry out because the greater water retention of substrates with clay concentrations > 10% compared to the pure sand matrix allows maintenance of a greater microbial activity. Clay type, fraction and concentration affected microbial community structure via their effect on organic matter and water availability. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1521949 / Thesis (M.Ag.Sc.) -- University of Adelaide, School of Earth and Environment Science, 2010