>Magister Scientiae - MSc / The heterogeneous photocatalytic oxidation of dinitrogen to nitrate, NO3 and/or nitrite, no2 using peroxy species of titanium (IV) in aqueous suspensions has been investigated. The photocatalysts used were titanium peroxide and Degussa p25 TiO2 pretreated with H2O2. These photocatalysts were investigated by SEM and FTIR spectroscopy.

Doctor of Philosophy / Department of Agronomy / Ignacio Ciampitti / Potential yield (PY) is defined by the yield limited by temperature, radiation, and genetics – under no limitation on nutrients or water. The difference between PY and actual yield (AY) is defined as yield gap (YG). Management practices such as planting date, row spacing, seeding rate, fertilization program, pest, and disease control can help producers to intensify the productivity of the farming systems and consequently, close the YGs. To evaluate the impact of different management system (MS, specific combination of management practices) on closing the YG the following objectives were established: i) conduct a historical synthesis analysis to characterize shifts in soybean yields, biomass and nutrient uptake and partitioning dissecting the main physiological component related to nutrient use efficiency, seed nutrient composition and nutrient stoichiometry; ii) study the contribution of five MS for intensifying maize-soybean production systems; iii) quantify the nitrogen (N) contribution from the biological N fixation (BNF) process for soybeans under two contrasting MSs (low vs. high inputs); and iv) utilize the same contrasting input treatments to calibrate the Agricultural Production System Simulator (APSIM) for modeling a maize – soybean rotation and apply the parametrized model to estimate a long-term (1980-2016) simulation. For the first objective, main findings indicate that soybean yield increase over time was driven by an increase in biomass with a relatively small variation in harvest index, and with modern varieties producing more yield per unit of N uptake. For the second objective, field experiments demonstrated that intensification practices (narrow row spacing, increasing seeding rate and implementation of a balanced nutrition program) increased yields in both soybeans and maize under rainfed and irrigated conditions. For the third objective, to better understand the soybean N status, BNF measurements were collected during the 2015 growing season and also investigated in a greenhouse setting. The B value, N fixation when plants are fully relying on atmospheric N, changed among varieties, growth stages and plant fractions. Overall B value at R7 (beginning of maturity) was -1.97 contrasting with the -1.70 value reported as mode according to a literature review. For the range of fixation measured in this research (average of 45-57%), utilization of a B value obtained from the scientific literature or measured in field conditions will have a reduced impact on BNF estimations. Lastly, for the last and fourth objective, the APSIM performed well in estimating yield, biomass production and total N uptake with a high model efficiency and low relative root mean square error (RRMSE). The long-term simulation helped characterize the YG for each crop and MS according to different weather patterns. The modeling approach increased the value of data collected in field experiments. Overall, this research project provided an approach to quantifying and understanding YGs in a maize-soybean rotation and the impact of different MSs on intensifying productivity. Future work can be conducted to model specific MSs to advise producers on the best management systems (BMSs) for sustainably intensifying productivity while minimizing the environmental footprint of current farming systems.

The projected exponential growth of the human population necessitates a concomitant increase in food supplies, and by implication an increase in fixed nitrogen for crops and pastures. This can to a large extent be supplied by biological nitrogen fixation (BNF). However, to achieve this goal improved effectivity of the legume-rhizobium symbiosis is required, implicating improvement in the macro- as well as the micro symbiont. Therefore the search for more effective microsymbionts is a sine qua non to provide better matching and tolerance to stress conditions. The aim of this study was to investigate the range of rhizobia associated with the exotic Australian Acacia species (A. meamsii, A. dealbata and A. decurrens) in South Africa and to determine whether these species could be useful to provide rhizobial strains for application in the South African inoculant industry in order to improve local existing biological nitrogen-fixing systems. Although these Acacia species are geographically widespread throughout South Africa, their root nodule bacteria have never been investigated in depth. Their widespread occurrence and presumed promiscuity suggested that they might form nitrogen-fixing symbioses with a wide range of indigenous rhizobial strains with different ecological adaptations. In this study nodulated plants of the three Acacia spp. were collected from diverse geographic areas with diverse climatic conditions and different soil pH's. Isolates were obtained from root nodules, purified and the putative rhizobial isolates characterized with sodium dodecyl-sulphate polyacrylamide gel electrophoresis (SDS-PAGE), supplemented at the genomic level with 16S rDNA sequence data of selected isolates. The majority of the isolates investigated were members of the genus Bradyrhizobium, whilst some isolates showed close relationships to the genera Agrobacterium, Mesorhizobium, Rhizobium and Sinorhizobium. As a result of their predominant association with the slow-growing strains of the genus Bradyrhizobium, the legume spp. A. meamsii, A. dealbata and A. decurrens as trap plants would not playa significant role as a source of diverse rhizobia for application in the South African inoculant industry. / Dissertation (MSc (Microbiology))--University of Pretoria, 2006. / Microbiology and Plant Pathology / unrestricted

Forage growth in the subarctic is sub-optimal due to low soil nutrient levels. Forage crops in the Yukon Territory consistently require nitrogen (N) and phosphorus fertilization to meet plant requirements. Fertilization is expensive due to transportation costs and potentially harmful to the environment so alternative, more sustainable, sources of nutrients are being sought. Alfalfa is an alternative, but there is limited knowledge in the Yukon of the benefits and management of this crop as a replacement for fertilizer N. Experiments were carried out in south central Yukon during the 2005 and 2006 field seasons to examine the potential of co-inoculation of alfalfa with N-fixing Ensifer meliloti and phosphate-solubilizing Penicillium bilaii to increase the dry matter yield and N fixation of monoculture alfalfa (Medicago sativa) cv Peace and binary mixed alfalfa with smooth bromegrass (Bromus inermis) cv Carlton or timothy (Phleum pratense) cv Climax forage swards. Interactions between alfalfa inoculation and N fertilization and late season harvest treatments were assessed. The TagTeam® inoculant from Philom Bios was used as the rhizobium source which contains both Ensifer meliloti isolate NRG-34 and Penicillium bilaii isolate PB-50. Nitrogen fixation was determined using the total plant N difference method. Alfalfa growth and nodulation was successful in the trials. Inoculation had a positive impact on N fixation, whereas urea fertilizer at 25 kg N/ha had a negative impact in most cases. In the mixed alfalfa and smooth bromegrass stand there was a positive contribution from the alfalfa in both the establishment and second year with N fixation rates of up to 14 kg/ha. In the mixed timothy and alfalfa stand the N fixation reached 35 kg/ha in the establishment year and 102 kg/ha in the second year. In the establishment year the dry matter yield and N fixation of the TagTeam® inoculated, monoculture alfalfa plots were 3.1 t/ha and 77 kg N/ha. In the second year, the unharvested inoculated alfalfa treatment yielded 3.4 t/ha with N fixation of 66 kg/ha compared to the late harvest treatment which yielded only 1.5 t/ha and an N fixation rate of 20 kg/ha. The effects of the late season harvest are startling and reflect the importance of removing grazing animals during the fall to allow plant energy reserves to accumulate in the roots. Fertilizer N replacement is possible with the seeding of alfalfa into existing hay stands or in monoculture. / Land and Food Systems, Faculty of / Graduate

For millions of years, the nutrient exchange symbiosis between corals and their endosymbiotic algae has formed the foundation of the ecological success of coral reefs. Yet, in recent decades anthropogenic climate change is increasingly destabilizing this symbiosis, and thus the reefs that rely on it. High-temperature anomalies have caused mass mortality of corals due to repeated coral bleaching, the expulsion or digestion of symbionts by the host during stress. Hence, in-depth knowledge of the cellular processes of bleaching is required to conceive strategies to maintain the ecological functioning of coral reefs. In this thesis, we investigated the role of symbiotic nutrient cycling in the bleaching response of corals. For this, we examined the mechanisms that underlie the functioning of the symbiosis in a stable state and how heat stress affects these metabolic interactions during coral bleaching. Our findings reveal that the functioning of the coral – algae symbiosis depends on the resource competition between host and symbionts. In a stable state, symbiotic competition for ammonium limits nitrogen availability for the algal symbiont, thereby ensuring symbiotic carbon translocation and recycling. During heat stress, however, increased metabolic energy demand shifts host metabolism from amino acid synthesis to degradation. The resulting net release of ammonium by the host, coupled with the stimulated activity of associated nitrogen-fixing microbes, substantially increases nitrogen availability for algal symbionts. Subsequently, stimulated algal growth causes selfish retention of carbon, thereby further reducing energy availability for the host. This positive feedback loop disturbs symbiotic nutrient recycling, eventually causing the collapse of carbon translocation by the symbiont. Hence, heat stress causes shifts in metabolic interactions, which directly and indirectly destabilizes the symbiosis, and ultimately undermines the ecological benefits of hosting algal symbionts for corals. In summary, this thesis shows that integrating symbiotic nutrient cycling into our conceptual understanding of coral bleaching is likely to improve our ability to predict coral bleaching in light of environmental conditions and may ultimately help to conceive new strategies to preserve coral reef functioning.

Drought causes considerable reduction of legume productivity and significantly threatens the food security, and this situation is expected to be aggravated due to climate change. In soybean and common bean, water resource capturing through plant root architectural plasticity and the role of symbiotic nitrogen fixation have not been investigated in greater detail yet. This study was therefore conducted to identify and apply useful morphological and physiological performance markers (traits) for selection of drought-tolerant common bean and soybean cultivars under both controlled phytotron and field conditions that might be applicable as markers in future legume breeding programs. In soybean, traits related to above ground performance, such as photosynthesis, biomasses, and stomatal conductance, were related to parameters for nitrogen acquisition in nodules. The ability to maintain vigorous shoot growth under drought-induced nitrogen limitation was identified as an important trait that can be used to select for improved drought tolerance. Further, experiments carried out growing different common bean inbred lines under controlled phytotron conditions revealed the importance of growth and gas exchange parameters as well as nitrogen fixing ability as performance markers to select superior performing bean lines for growth under drought. As a further result, the strong association of symbiotic nitrogen fixation with CO2 assimilation and stomatal conductance was also ascertained. In field experiments the effective use of water through enhanced lateral root development and maintaining the water status of the plant was found to be crucial for enhanced productivity under drought, with root morphology traits (root length, area and volume) as well as root architectural traits (first whorl angle, basal root number and adventitious root branching density) significantly related to seed yield. Measurement of these traits might be added to future bean varietal improvement programs. Further, a direct relationship between both water use efficiency (WUE) estimated using carbon isotope discrimination (CID) and nitrogen fixation (15N abundance) with root morphological and architectural traits (root length, area and volume, basal root number, 1st as well as 2nd whorl angles) was identified. CID (WUE) and 15N abundance (SNF ability) had a direct relationship with each other and also with productivity traits (seed yield and pod harvest index). Soybean field experiments verified the importance of root system architecture and morphology for providing drought tolerance with root architectural traits, tap and lateral roots (diameter and branching density) and morphological traits (root length, surface area and volume) contributing to better performance under drought. Moreover, the strong association of CID (WUE) with ä15N (SNF), root traits as well as seed yield in soybean exposed to drought was ascertained. Findings suggested that higher performance in CID under drought stress may be due to higher CO2 assimilation and better N2 fixation resulting in better root system architecture and morphology of the drought-tolerant cultivar through maintenance of the water status of the plant for efficient biological activity. Overall the study has generated new knowledge about the use of physiological markers (traits) that can be used widely for legume evaluation under drought suitable for both phytotron and field studies. / Thesis (PhD)--University of Pretoria, 2012. / Plant Science / unrestricted

Mangrove plants are a productive ecosystem that provide several benefits for marine organisms and industry. They are considered to be a food source and habitat for many organisms. However, mangrove growth is limited by nutrient availability. According to some recent studies, the dwarfism of the mangrove plants is due to the limitation of nitrogen in the environment. Biological nitrogen fixation is the process by which atmospheric nitrogen is fixed into ammonium. Then, this fixed nitrogen can be uptaken by plants. Hence, biological nitrogen fixation increases the input of nitrogen in the mangrove ecosystem. In this project, we focus on measuring the rates of nitrogen fixation on Red Sea mangrove (Avicennia marina) located at Thuwal, Saudi Arabia. The nitrogen fixation rates are calculated by the acetylene reduction assay. The experimental setup will allow us to analyze the effect of crab bioturbation on nitrogen fixing rates. This study will help to better understand the nitrogen dynamics in mangrove ecosystems in Saudi Arabia. Furthermore, this study points out the importance of the sediment microbial community in mangrove trees development. Finally, the role of nitrogen fixing bacteria should be taken in account for future restoration activities.

Nitrogen is a critical nutrient for growth and reproduction in living organisms. Although the Earth’s atmosphere is composed of ~80% nitrogen gas (N2), it is inaccessible to most living organisms in that form. Biological nitrogen fixation, however, can be performed by microbes that harbor the enzyme nitrogenase. This enzyme converts N2 into bioavailable ammonia (NH3) and accounts for at least half of the “fixed”nitrogen on the planet. The other major contributor to ammonia production is the industrial Haber-Bosch process. While the Haber-Bosch process has made significant advances in sustaining the global food supply through the generation of fertilizer, it requires high temperature and pressure and fossil fuels. This makes nitrogenase an ideal system for study, as it is capable of performing this challenging chemistry under ambient conditions and without fossil fuels. Nitrogenase requires energy and electrons to convert N2into NH3. The work presented here examined how the enzyme receives electrons to perform the reaction. It was discovered that some microbes employ a novel mechanism that adjusts the energy state of the electrons so that nitrogenase can accept them. Further, the slowest step that takes place in nitrogenase once the electrons are taken up was identified. Finally, by capitalizing on fundamental knowledge, a biohybrid system was designed to grow nitrogen-fixing bacteria in association with electrodes for light-driven production of fixed nitrogen that has potential to be used as a fertilizer for plant growth. Gaining an in-depth understanding of nitrogenase provides insight into one of the most challenging biological reactions, and the newfound knowledge may be a catalyst in developing more efficient systems for sustainable ammonia production.

A greenhouse study was conducted involving interactive effects on bean yield of three levels of irrigation water salinity, three frequencies of irrigation water app li cation, two level s of nitrogen, and seven levels of phosphorus fertilization to determine if the effects of adverse saline conditions or water stress could be overcome by fertility and / or saline water management. Dry matter and bean yields were reduced with decreasing irrigation frequency (2 days to B days} and increasing salinity of irrigation water (0.5 mmho/cm to 8.0 mmho/cm). At low salinity levels (0 .5 mmho/cm) dry matter production and the number of pods per pot were increased with the addition of nitrogen in the irrigation water (at the rate of 112 kg N/ha). The application of banded fertilizer phosphorus helped beans overcome the effects of saline conditions and water stress and resulted in increased yield. The application of liquid 10-34-0 fertilizer produced a statistically significant increase in all yield parameters at every level of salinity, and every irrigation frequency studied. Nitrogen fixation was found to decrease as irrigation water salinity increased. The application of 0-50-40 as a fertilizer band treatment increased nitrogen fixation in t he presence of added nitrogen, due probably to the presence of potassium.

A greehhouse study was conducted to assess the interactive effects of three irrigation water salinity levels (1 . 0, 3. 0, and 9 . 0 mmho/cm) and three quantities of water app lied per irrigation (120 , 240, 360 ml) on plant growth and nitrogen fixation by alfalfa (Medicago sativa L. cv. Resistador). Harvest dates corresponded t o 10, 30, and 50 days after the initiation of salt and water treatments which were started after nodulation had been established in young plants. Alfalfa top growth was limited by both salt and water s tresses. Irrigation water s alinity had a grea t er effec t on top growt h than root grm;th while root distribution was unaffected by either the quant i ty of water app l ied or by water salinity. The effects of salinity on pl ant growth were reduced in the presence of l imi ting moisture. The specific nodul e act ivi t y (mmol c2H4/hr/g) of water s tressed alfalfa pl ants was enhanced by increasing the quantity of wat er applied a t each irriga t ion and was adversely effected by increased irrigation water salinity . In contrast, both nodulation and nodule growth were insensitive t o salt stress and sensi tive only to severe mo i s ture s tress . Alfalfa plants continued to exhibit acetylene reducing capacity at the third harvest even under severe moisture and salt stress. The species apparently continues to fix nitrogen even though environmental stress is quite substantial.