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We investigated the influence of multiple disturbances on ecosystem structure and function in a headwater stream adjacent to an abandoned arsenic mine using an upstream (reference) and downstream (mine-influenced) comparative reach approach. In this study, floods were addressed as a pulse disturbance, and the abandoned arsenic mine was characterized as a press disturbance. Chronically elevated levels of arsenic were specifically addressed as a ramp disturbance. Stream ecosystem structure and biogeochemical functioning were characterized monthly over a period from July to December 2004 by determining benthic organic matter standing stocks, ecosystem metabolism, and by using solute additions to examine differences in phosphorus uptake and hydrology over the monitoring period. Influences of the press disturbance were evident in the mine-influenced reach where arsenic concentrations (254 ± 39 µg/L) were >30 higher than in the reference reach (8 ± 1 µg/L). However, in almost all cases the presence of the abandoned arsenic mine appeared to exert little influence on reach-scale measures of ecosystem structure and function (e.g., organic matter standing crops, phosphorus uptake). Conversely, floods (i.e., pulse disturbances) influenced organic matter standing stocks and hydrologic interactions between the stream and transient storage zones in both study reaches. Interactions between press and pulse disturbances were evident in several cases and illustrated by phosphorus uptake responses. Phosphorus uptake was best predicted by coarse particulate organic matter standing stocks in the reference reach. However, in the reach exposed to the press disturbance (i.e., mine-influenced reach), both coarse particulate organic matter standing stocks and characteristics of the pulse disturbance regime (i.e., number of days post-flood) were significant predictors of phosphorus uptake. Within the mine-influenced reach, arsenic concentrations increased from 16–600 µg/L and were addressed as a ramp disturbance. Analysis of phosphorus uptake in the mine-influenced reach across a gradient of arsenic concentrations correlated with Michaelis-Menton models of enzyme kinetics in the presence of a competitive inhibitor. These results suggest that arsenic appears to competitively inhibit phosphorus uptake by microbial assemblages in the mine-influenced reach. Results from this study highlight the fact that ecotoxilogical studies at the ecosystem scale should consider not only contaminant influences, but rather place its implications within the extant disturbance regime generated from both natural and anthropogenic sources. / Master of Science

There are no seeding rates established for organic production of field pea and lentil in Saskatchewan and organic producers must rely upon rates recommended for conventional production of these crops. These seeding rates may not be suitable for organic production as the two systems differ in the use of inputs and in pest management. The objectives of this study were to determine an optimal seeding rate for organic production of field pea and lentil in Saskatchewan considering a number of factors, including yield, weed suppression, soil nitrogen (N) and phosphorus (P) concentrations, soil water storage, colonization of crop roots by arbuscular mycorrhizal fungi (AMF), plant P uptake, and profitability. A field experiment was conducted to determine the optimal seeding rates of field pea and lentil. Field pea seeding rates were 10, 25, 62, 156 and 250 plants m-2 and lentil seeding rates were 15, 38, 94, 235 and 375 plants m-2. Sites were established at Vonda, Vanscoy and Delisle, SK using a randomized complete block designs with summerfallow and green manure treatments included for each crop. Seed yield increased with increasing seeding rate for both crops, up to 1725 kg ha-1 for field pea and 1290 kg ha-1 for lentil. Weed biomass at physiological maturity decreased with increasing seeding rate for both crops. In field pea, weeds were reduced in weight by 68%, while lentil reduced weed biomass by 59% between the lowest and highest seeding rates. <p>Post-harvest soil phosphate-P levels did not change consistently between treatments, indicating that there was no trend in soil P concentration with seeding rate. Post-harvest soil inorganic N, however, was higher for the summerfallow and green manure treatments than for the seeding rate treatments in both crops. Inorganic N was higher at some sites for the highest two seeding rates in field pea. Soil water storage following harvest was not affected by treatment.<p>Colonization of crop roots by AMF increased for lentil with increasing seeding rate, but the same trend was not observed in field pea. A growth chamber experiment to study the rate of colonization of field pea between 10 and 50 d after emergence did not show any differences in AMF colonization between seeding rates. Colonization levels were high (70 to 85%) for both crops in both the field and growth chamber. Arbuscular mycorrhizal fungi colonization and seeding rate had no effect on plant P concentration for either field pea or lentil. Both crops became increasingly profitable as seeding rate increased. Field pea reached a maximum return at 200 plants m-2 and lentil return increased to the highest seeding rate of 375 plants m-2. Organic farmers should increase seeding rates of these crops to increase returns and provide better weed suppression.

There are no seeding rates established for organic production of field pea and lentil in Saskatchewan and organic producers must rely upon rates recommended for conventional production of these crops. These seeding rates may not be suitable for organic production as the two systems differ in the use of inputs and in pest management. The objectives of this study were to determine an optimal seeding rate for organic production of field pea and lentil in Saskatchewan considering a number of factors, including yield, weed suppression, soil nitrogen (N) and phosphorus (P) concentrations, soil water storage, colonization of crop roots by arbuscular mycorrhizal fungi (AMF), plant P uptake, and profitability. A field experiment was conducted to determine the optimal seeding rates of field pea and lentil. Field pea seeding rates were 10, 25, 62, 156 and 250 plants m-2 and lentil seeding rates were 15, 38, 94, 235 and 375 plants m-2. Sites were established at Vonda, Vanscoy and Delisle, SK using a randomized complete block designs with summerfallow and green manure treatments included for each crop. Seed yield increased with increasing seeding rate for both crops, up to 1725 kg ha-1 for field pea and 1290 kg ha-1 for lentil. Weed biomass at physiological maturity decreased with increasing seeding rate for both crops. In field pea, weeds were reduced in weight by 68%, while lentil reduced weed biomass by 59% between the lowest and highest seeding rates. <p>Post-harvest soil phosphate-P levels did not change consistently between treatments, indicating that there was no trend in soil P concentration with seeding rate. Post-harvest soil inorganic N, however, was higher for the summerfallow and green manure treatments than for the seeding rate treatments in both crops. Inorganic N was higher at some sites for the highest two seeding rates in field pea. Soil water storage following harvest was not affected by treatment.<p>Colonization of crop roots by AMF increased for lentil with increasing seeding rate, but the same trend was not observed in field pea. A growth chamber experiment to study the rate of colonization of field pea between 10 and 50 d after emergence did not show any differences in AMF colonization between seeding rates. Colonization levels were high (70 to 85%) for both crops in both the field and growth chamber. Arbuscular mycorrhizal fungi colonization and seeding rate had no effect on plant P concentration for either field pea or lentil. Both crops became increasingly profitable as seeding rate increased. Field pea reached a maximum return at 200 plants m-2 and lentil return increased to the highest seeding rate of 375 plants m-2. Organic farmers should increase seeding rates of these crops to increase returns and provide better weed suppression.

The laboratory-scale University of Cape Town (UCT) process was designed to investigate the effects of changing COD/TP ratios on the performance of biological nutrient removal (BNR) processes. Specific objectives of the research were to investigate the effects of COD/TP ratio on the rates of phosphorus removal, COD removal, nitrogen removal, PHB utilization and oxygen uptake. The system was fed with municipal wastewater and operated at 20° C. The influent COD concentration was held approximately constant while the phosphorus concentration was varied to obtained the desired COD/TP ratio. Once robust enhanced biological phosphorus removal (EBPR) has been established, the COD/TP ratios of 20, 30, 40 and 60 were investigated. The COD/TP ratio of the influent wastewater was observed to have a substantial effect upon the performance of the UCT BNR system. The amount of phosphorus removed by the system and the percent phosphorus in the aerobic zone MLVSS decreased as the COD/TP ratio increased. In addition, the amount of phosphorus released in the anaerobic zone per unit of COD removed in the anaerobic zone increased as the COD/TP ratio decreased. From this research, the amount of anaerobic COD removal required to remove 1 mg/L of phosphorus in the aerobic zone approached a minimum value as the COD/TP ratio decreased. It was also shown that PHB production increased as the COD/TP ratio increased. The highest specific oxygen uptake rate was always observed in the second aerobic reactor and tended to increase as the COD/TP ratio increased. However, the changes in the COD/TP ratio did not significant affect COD removal, nitrogen removal and the observed yield coefficient, but did strongly affect the MLSS concentration. The MLSS concentration at the COD/TP ratio of 60 was only 55% of that at the COD/TP ratio of 20. A high level of anaerobic COD removal, an elevated percent phosphorus in the waste activated sludge (WAS) and a high soluble effluent phosphorus concentration can be used as indicators that the system is operating under COD limiting conditions. Several phenomena were also observed during this research. Firstly, the performance of the UCT BNR system for EBPR was greatly enhanced by reducing the aerobic volume. Secondly, the correlation between non-oxic phosphorus release and the aerobic phosphorus uptake improved when anoxic phosphorus release was taking into consideration. This indicated that the anoxic phosphorus release was not secondary release once the aerobic zone volume was reduced. Finally, no denitrification was observed in the aerobic zone from this study, based on the assumption that 12% of nitrogen was required for bacterial growth. / Master of Science

The lack of forage resources in the Sahelian region of Mali is a major constraint to food production and food sufficiency. Madiama commune is located in northern Mali, in the Niger Delta region. Three separate experiments were conducted to investigate ways to improve pasture resources and productivity. The first experiment (2003) was designed to investigate the influence of sheep grazing tethered at two different residual heights on botanical composition, forage biomass and animal performance. Young sheep weighing approximately 18-24 kg were tethered for a certain period of time depending on residual canopy height. Two treatments 3 or 6 cm residual height were each replicated 4 times. Animals were rotated based on canopy height and each tethered animal followed an 8 paddock rotation. Measurements included forage biomass, plant diversity, animal performance, and botanical composition. The forage species found on these pastures were primarily Schoenfeldia gracilis, Panicum laetum, Setaria palludefusca, Eragrostis turgida, Eragrostis tremula, Zornia glauchidiata, Tephrosia pedicellata, and Cynodon spp. Accumulated seasonal forage biomass increased while forage quality declined as the growing season progressed. Treatment had only a slight effect on animal weight gains (1 to 3kg season-1). These results suggest that residual height may not affect livestock gain. The second experiment was designed to investigate the potential of Cassia tora (C. tora) which is an invasive weed in the region as a supplemental feed for livestock. Cassia tora was harvested within the Madiama commune and ensiled with or without additives (water and or honey/sugar) for 60 or 90 days. Harvest occurred at the vegetative stage in year 1 and mature growth stage in year 2. Prior to placing the chopped material in the bags for ensiling, sub-samples of fresh C. tora were obtained for dry matter (DM) and chemical analysis (NDF, ADF, CP, IVDMD and TDN). In year 1, the ensiled material/fresh material across treatments and locations had NDF varying from 48 to 56 %/ 56 to 57%, ADF from 34 to 41 %/40 to 42%, CP from 9 to 10 %/9 to 23%, and IVDMD from 53 to 64 %/52 to 54%. In year 2, CP averaged twice as much as year 1 with significantly less fiber probably due to the fact that harvest occurred at the vegetative stage. Addition of water or sugar/honey improved the nutritive values of the ensiled material. These results suggest that C. tora can be a reliable feed source during the dry season. A greenhouse experiment was conducted using various P sources (Tilemsi phosphate rock (TPR), North Carolina phosphate rock (NCPR), Aluminum phosphate (AlP), Iron phosphate (FeP), and Triple superphosphate (TSP) and rates (0, 20, 40, 60, and 80 mg P kg-1 soil). Plants were grown for 10 wks, harvested and separated into above and below ground plant parts. The root and plant material were dried, ground and analyzed for elemental P. The result showed variable P solubility and uptake by the plant. Overall, addition of P resulted in an increase in above ground biomass as well as root mass compared with the untreated control. Field and greenhouse experiments showed that in the Sahel region of Africa where feed resources are scarce 8 out of 12 months a year, anything we can do to increase pasture resources and animal productivity while maintaining a healthy ecosystem, could improve the quality of life in the community. / Ph. D.

Cultivares de arroz variam quanto à capacidade de absorver e utilizar fósforo em solos deficientes neste nutriente. O melhoramento genético é realizado em condições ótimas de fertilidade, sendo possível que alguns dos materiais genéticos modernos apresentem perdas ou diminuição na capacidade de absorver P. A eficiência agronômica das fontes é influenciada por diversos fatores, dentre eles a espécie vegetal. Cultivares de arroz tradicionais podem apresentar mecanismos que possibilitam maior eficiência das fontes de menor solubilidade. O presente trabalho constou de dois estudos, ambos realizados em casa-de-vegetação do Departamento de Solos e Nutrição de Plantas da ESALQ/USP: (i) avaliação da eficiência de absorção e utilização de fósforo por cultivares de arroz de terras altas tradicionais e modernos, no qual nove cultivares de arroz de terras altas foram utilizados, sendo cinco tradicionais (Caiapó, Carajás, IAC 47, Bico Ganga e Guarani) e quatro modernos (Bonança, Curinga, Vencedora e Primavera), e (ii) avaliação da eficiência agronômica de fontes de fósforo com solubilidade variável em água em cultivares de arroz, no qual quatro cultivares de arroz, sendo dois tradicionais (Bico Ganga e Guarani) e dois modernos (Bonança e Primavera) interagiram com quatro fontes de P (fosfato monocálcico - FMC, termofosfato - Termo, multifosfato magnesiano - MFM e fosfato natural de Arad - FN). Em ambos os estudos foram utilizadas amostras de um Latossolo Vermelho Amarelo, sendo aplicadas duas doses de P, 25 e 100 mg kg-1, mais o controle. Cada tratamento foi repetido três vezes em um delineamento de blocos ao acaso. No estudo 1 o objetivo foi avaliar cultivares de arroz de terras altas tradicionais e modernos quanto à eficiência de absorção e utilização de fósforo, tendo como hipótese que o processo de melhoramento não conduz necessariamente a materiais genéticos de menor eficiência de utilização de P. Os resultados obtidos com o cultivar moderno Curinga para eficiência de absorção e utilização de fósforo, bem como pelo quadrante de eficiência, permite concluir que o processo de melhoramento genético não afeta necessariamente os mecanismos para eficiência e reposta a aplicação de fósforo. No estudo 2 o objetivo foi avaliar a eficiência agronômica relativa de fontes de fósforo com solubilidade variável em água em cultivares de arroz tradicionais e modernos, tendo como hipótese que fertilizantes fosfatados de baixa solubilidade apresentam elevada eficiência agronômica relativa para cultivares de arroz tradicionais, quando comparados aos modernos. Os resultados obtidos indicaram que considerando as doses aplicadas, a EAR das fontes de baixa solubilidade varia de acordo com os cultivares, independentemente de ser tradicional ou moderno. / Cultivars of upland rice may vary in terms of absorption and utilization of phosphorus in soils deficient in this nutrient. Breeding is generally performed in optimal conditions of soil fertility leading to the possibility that new cultivars decrease their condition to absorb and utilize P. The agronomic effectiveness of P sources is related to many aspects, among those the crop specie. Cultivars of traditional upland rice may present mechanisms leading to higher effectiveness of P sources containing low water solubility. This study consisted of two greenhouse experiments conducted at the Department of Soil and Plant Nutrition, ESALQ/USP: (i) evaluation of the absorption and utilization of P by nine cultivars of upland rice (five traditional cultivars - Caiapó, Carajás, IAC 47, Bico Ganga e Guarani - and four modern cultivars - Bonança, Curinga, Vencedora e Primavera), and (ii) evaluation of the agronomic effectiveness of P sources varying in water solubility for traditional and modern cultivars of upland rice, in which four cultivars (two traditional - Bico Ganga and Guarani - and two modern - Bonança and Primavera) interacted with four P sources (monocalcium phosphate - MCP, termophosphate - Termo, magnesium multiphosphate - MMP and Arad phosphate rock - PR). Samples of a Red Yellow Latosol, with the addition of 25 and 100 mg P kg-1 plus the control, were utilized in both experiments. Each treatment was repeated three times with the experimental design as blocks. In experiment 1 the objective was to evaluate traditional and modern cultivars of upland rice as related to their effectiveness in terms of absorption and utilization of P, with the hypothesis that breeding not necessarily lead to materials with lower effectiveness. The results obtained for the modern cultivar Curinga confirm the hypothesis once its effectiveness was higher as compared to traditional cultivars. In experiment 2 the objective was to evaluate the relative agronomic effectiveness (RAE) of P sources varying in water solubility for traditional and modern cultivars, with the hypothesis that P sources with low water solubility may present higher agronomic effectiveness when applied to traditional as compared to modern cultivars. The results showed that the RAE of the P sources with low water solubility varied among cultivars but not related to the fact if traditional or modern, thus denying belief in literature.

Cultivares de arroz variam quanto à capacidade de absorver e utilizar fósforo em solos deficientes neste nutriente. O melhoramento genético é realizado em condições ótimas de fertilidade, sendo possível que alguns dos materiais genéticos modernos apresentem perdas ou diminuição na capacidade de absorver P. A eficiência agronômica das fontes é influenciada por diversos fatores, dentre eles a espécie vegetal. Cultivares de arroz tradicionais podem apresentar mecanismos que possibilitam maior eficiência das fontes de menor solubilidade. O presente trabalho constou de dois estudos, ambos realizados em casa-de-vegetação do Departamento de Solos e Nutrição de Plantas da ESALQ/USP: (i) avaliação da eficiência de absorção e utilização de fósforo por cultivares de arroz de terras altas tradicionais e modernos, no qual nove cultivares de arroz de terras altas foram utilizados, sendo cinco tradicionais (Caiapó, Carajás, IAC 47, Bico Ganga e Guarani) e quatro modernos (Bonança, Curinga, Vencedora e Primavera), e (ii) avaliação da eficiência agronômica de fontes de fósforo com solubilidade variável em água em cultivares de arroz, no qual quatro cultivares de arroz, sendo dois tradicionais (Bico Ganga e Guarani) e dois modernos (Bonança e Primavera) interagiram com quatro fontes de P (fosfato monocálcico - FMC, termofosfato - Termo, multifosfato magnesiano - MFM e fosfato natural de Arad - FN). Em ambos os estudos foram utilizadas amostras de um Latossolo Vermelho Amarelo, sendo aplicadas duas doses de P, 25 e 100 mg kg-1, mais o controle. Cada tratamento foi repetido três vezes em um delineamento de blocos ao acaso. No estudo 1 o objetivo foi avaliar cultivares de arroz de terras altas tradicionais e modernos quanto à eficiência de absorção e utilização de fósforo, tendo como hipótese que o processo de melhoramento não conduz necessariamente a materiais genéticos de menor eficiência de utilização de P. Os resultados obtidos com o cultivar moderno Curinga para eficiência de absorção e utilização de fósforo, bem como pelo quadrante de eficiência, permite concluir que o processo de melhoramento genético não afeta necessariamente os mecanismos para eficiência e reposta a aplicação de fósforo. No estudo 2 o objetivo foi avaliar a eficiência agronômica relativa de fontes de fósforo com solubilidade variável em água em cultivares de arroz tradicionais e modernos, tendo como hipótese que fertilizantes fosfatados de baixa solubilidade apresentam elevada eficiência agronômica relativa para cultivares de arroz tradicionais, quando comparados aos modernos. Os resultados obtidos indicaram que considerando as doses aplicadas, a EAR das fontes de baixa solubilidade varia de acordo com os cultivares, independentemente de ser tradicional ou moderno. / Cultivars of upland rice may vary in terms of absorption and utilization of phosphorus in soils deficient in this nutrient. Breeding is generally performed in optimal conditions of soil fertility leading to the possibility that new cultivars decrease their condition to absorb and utilize P. The agronomic effectiveness of P sources is related to many aspects, among those the crop specie. Cultivars of traditional upland rice may present mechanisms leading to higher effectiveness of P sources containing low water solubility. This study consisted of two greenhouse experiments conducted at the Department of Soil and Plant Nutrition, ESALQ/USP: (i) evaluation of the absorption and utilization of P by nine cultivars of upland rice (five traditional cultivars - Caiapó, Carajás, IAC 47, Bico Ganga e Guarani - and four modern cultivars - Bonança, Curinga, Vencedora e Primavera), and (ii) evaluation of the agronomic effectiveness of P sources varying in water solubility for traditional and modern cultivars of upland rice, in which four cultivars (two traditional - Bico Ganga and Guarani - and two modern - Bonança and Primavera) interacted with four P sources (monocalcium phosphate - MCP, termophosphate - Termo, magnesium multiphosphate - MMP and Arad phosphate rock - PR). Samples of a Red Yellow Latosol, with the addition of 25 and 100 mg P kg-1 plus the control, were utilized in both experiments. Each treatment was repeated three times with the experimental design as blocks. In experiment 1 the objective was to evaluate traditional and modern cultivars of upland rice as related to their effectiveness in terms of absorption and utilization of P, with the hypothesis that breeding not necessarily lead to materials with lower effectiveness. The results obtained for the modern cultivar Curinga confirm the hypothesis once its effectiveness was higher as compared to traditional cultivars. In experiment 2 the objective was to evaluate the relative agronomic effectiveness (RAE) of P sources varying in water solubility for traditional and modern cultivars, with the hypothesis that P sources with low water solubility may present higher agronomic effectiveness when applied to traditional as compared to modern cultivars. The results showed that the RAE of the P sources with low water solubility varied among cultivars but not related to the fact if traditional or modern, thus denying belief in literature.

Phosphorus (P) is an essential plant nutrient that may limit plant growth and agricultural productivity if not available for crop plant uptake in sufficient quantities at the time required. Many Australian soils are deficient in available P, despite a long history of P fertilizer application, and this is due to fertilizer P rapidly becoming unavailable largely through biochemical fixation. The resulting low P fertilizer efficiency, coupled with rapidly rising cost of fertilizers, has increased interest in biological cycling of P from sources such as crop residues. However, to date, much of the Australian research has focussed on soils with relatively high organic matter content (> 2%) and relatively heavy texture i.e. medium to high clay content. Furthermore, although there is information on pasture residue decomposition and P release for sandy soils with low organic matter in Australia, a recent shift to continuous cropping systems means that information for a range of crop residues is required but is not currently available. Therefore the aims of the work described in this thesis were to (i) increase the efficiency of P use when crop residue P are applied to crops and (ii) determine the effect of crop residue biochemical quality on decomposition rates, soil P dynamics and plant P uptake in light textured sandy soils with low organic matter which are typical of a large proportion of the southern Australian wheat growing area. A further aim was to investigate the effects of combined additions of plant residue and P fertilizer on P cycling in these soils, a scenario highly relevant to farming systems. A series of soil incubation and plant growth experiments were undertaken to characterize P dynamics in soil following addition of a wide range of crop residues (total 15) collected from agricultural sites throughout South Australia. The residues, differing in age and biochemical quality, were young shoots of canola, lupin, pea, lucerne and lentil; mature shoot residues of canola, lupin, pea and wheat and mature root residues of wheat, canola and lupin. The concentration of total and water soluble P, C, and N in the residues was measured using standard wet chemical analyses and the carbon chemistry was determined by NMR spectroscopy. Decomposition of crop residues was continuously monitored over a period of up to 140 days by measuring soil respiration. Available P and microbial biomass P and C were also assessed at different times during the incubations. The total P in residues ranged from 0.16% to 0.32% and 0.05% to 0.08% in young and mature shoots, respectively. Water-soluble P was related to residue total P and ranged from 29% to 81% and 13% to 29% of total P in young and mature shoots, respectively. The C: P ratio ranged from 133: 1 to 253: 1 and 504: 1 to 858: 1 in young and mature shoots, respectively. Phosphorus availability and microbial P uptake differed between soils amended with crop residues and soluble P fertilizer as triple super phosphate (TSP). Soil respiration rates were significantly higher in soils amended with crop residues than in the soils amended with TSP or the unamended control in the first 58 days of incubation. In an experiment in which residues and TSP were added at a rate of 10 mg P kg⁻¹, available P was greater for TSP than residue-amended soil, whereas microbial P showed the opposite trend. Respiration rate and microbial P were positively correlated with C addition rate, which was highest in mature wheat residue because it had the lowest P concentration. In order to assess when P released from the residues is available for plants, wheat was grown over three consecutive crop periods with each period lasting for 4 weeks. Young residues with high content of water soluble P, C, N and amide and low lignin and phenolic content decomposed faster than mature residues. The C type and amount added with residues controlled the dynamics of P availability. Surprisingly, canola mature root increased available P and plant growth as much as young shoot residues while root residues of wheat and lupin resulted in P immobilization and low plant growth. Compared to canola young shoot, canola mature root has a higher total P concentration and a lower C: P ratio. Plant P uptake was positively correlated with residue total and water-soluble P content and negatively correlated with residue C: P and C: N ratio and amount of C added with the residues. In another experiment where residue was added at 2.5 g C kg⁻¹ soil and compared with TSP (4 and 10 mg P kg⁻¹ soil), available P and plant P uptake decreased in the following order: TSP-10P > canola root ≥ young shoot ≥ TSP-4P > control > mature shoot. Microbial P was greater with residue addition than with TSP and in the control. Residues with low total P and high C: P ratio resulted in P immobilisation in the microbial biomass. Therefore, residues with high total P and low C: P ratio can be an important source of P for plants. Net P immobilisation of mature wheat residues (0.07% P) was significantly reduced by combining wheat residue (C: P ratio 615: 1) with TSP leading to a C: P ratio of 155: 1 to 310: 1. Furthermore, the combination of wheat residue with TSP increased available P in residue and TSP-amended soils by 3.0 mg P kg⁻¹ soil, which was shown to be sufficient to support wheat growth in the early stages of development in the other experiments. Although water-soluble P fertilizers provide plants with immediately available P, a large proportion becomes unavailable over time. Addition of low C: P residues on the other hand, may not result in high amounts of immediately available P, but the P supply is more sustained due to P release from decomposing residues and turnover of microbial biomass P. Phosphorus immobilization after addition of residues which have high C: P ratio (615: 1) may be offset when residue is applied together with inorganic P fertilizer if the resulting C: P ratio is 300: 1 or less. Overall, this study has highlighted the potential role that crop residues, either alone or in combination with inorganic P, can play in increasing P availability in the light textured, low organic matter, P-limited soils typical of many southern Australian farming systems. The results provide important quantitative information on the potential of a wide range of crop residues to supply wheat with P, and how additions of inorganic P interact with residue decomposition and influence available P supply. This quantitative information will be valuable for the construction or validation of mechanistic models of residue decomposition relevant to low organc matter light textured soils in farming systems of southern Australia, and will ultimately assist in the development of economic management strategies for minimizing P fertilizer inputs and maximizing the benefits of biological cycling of P. / Thesis (Ph.D.) -- University of Adelaide, School of Earth and Environmental Sciences, 2009