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This thesis reports a study carried out to investigate the involvement of VA mycorrhizas in interplant 15N transfer in a model pasture system and in a model agroforestry system. Two pot experiments were designed to investigate the effect of VAM inoculation on 15N transfer from clover to rye grass in sterile (in the first experiment the sterilisation was by autoclaving and in the second, by gamma-irradiation) and fresh soil. A third pot experiment was designed to investigate the effect of VAM inoculation on 15N transfer from grass and clover to wild cherry seedlings in fresh soil. For these pot experiments donor plant seedlings were labelled with 15N by growing in Hoaglands solution containing K 15NO3 (5 atom % 15N). To study 15 N transfer and its possible mechanisms, plants and soil samples were analysed for 15N, total N (14N + 15N) and P concentrations. Rates of soil nitrogen mineralisation and nitrification, as well as 15N enrichment of available N were also determined to investigate N transfer through soil. There was transfer of 15N from donor to receiver in the simulated pasture system as well as in simulated agroforestry system. The transfer of 15N was increased by VAM inoculation. The transfer of 15N was small in relation to plant nutrition and was not associated with an increase in total N in the receiver plants although P concentrations were generally increased. There was no increase in the soil N fluxes of mineralisation and nitrification associated with enhanced 15N transfer from donor to receiver in the VAM inoculated system. In addition, there was often no increase in the 15N concentration in the soil available N pool of VAM inoculated systems. There was no evidence, therefore, of increased transfer of 15N through the soil in VAM inoculated systems with enhanced 15N interplant transfer, suggesting the likely involvement of VAM fungal hyphae. There was no marked effect on plant growth due to VAM inoculation. The growth of cherry, however, was reduced when grown with grass and this may have been due to N competition between the root systems. The growth of cherry seedlings was reduced to a lesser extent when grown with clover compared to when grown with grass. The findings of this study suggest that VA mycorrhizal hyphae play a part in interplant transfer of nitrogen. Although the study did not demonstrate high rates of 15N transfer in VAM inoculated system, circumstances are discussed where VAM interplant transfer of nutrients may become significant at the single plant, community and ecosystem level. The benefits to man of VAM mediated N transfer may be best realised in land use systems such as pastures having legumes and non-legumes, in mixed cropping of legumes and non-legumes and in an agroforestry system with a legume component, particularly when the soil is deficient in nutrients such as nitrogen and phosphorus.

Dissolved organic nitrogen (DON) represents a major pool of fixed, reactive nitrogen in marine systems. It is now recognized that this pool can support primary production and the ability of some algal species to exploit DON compounds as sources of Nitrogen (N) may indicate that specific DON components can exert selective pressure on the composition of the phytoplankton community. In this study the ability of monocultures of ecologically-relevant algal species from the English Channel (Emiliania huxleyi, Micromonas pusilla, Alexandrium minutum and Chaetoceros peruvianus) to grow with DON as the only N source was examined using different artificial media. Among the two tested artificial seawater recipes, Aquil* was preferred as it contained lower micronutrient concentrations, and gave better growth results for all used species. In order to constrain the DON uptake to algae alone, a method for bacterial removal was tested using antibiotic additions. Both Slocombe antibiotic mixture (Cefotaxime-Carbenicillin-Kanamycin-AugmentinTM) and Penicillin-Streptomycin-Neomycin used were effective and not toxic to the algae. Incubation with the antibiotic up to 48 hours and a transfer period into antibiotic-free medium after 72 hours proved to be effective. However, the treatment removed bacteria in A. minutum cultures only; further treatment would be required for the other species to be cultured axenically. The ability to use DON was tested for the above mentioned species using the amino acid L-Arginine (ARG) as the sole N source, and growth was compared with nitrate-containing cultures of the same species. All the selected species grew in both NOᴈ‾ and in ARG, reaching lower final densities when incubated with ARG, although these were not significant. This study has shown that E. huxleyi, A. minutum, M. pusilla and C. peruvianus can grow on organic N, either by direct or indirect uptake, and develop comparable biomasses to species using inorganic N. Both C. peruvianus and M. pusilla cultures contained dissolved ammonium at the end of the experimental period, indicating potential indirect use by the algae of organic N converted to inorganic N by bacteria. A. minutum grew in the presence of ARG along with the cosmopolitan E. huxleyi; N-demand estimates, based on the molar concentration of N-ARG consumed, correlated with the final cell density, indicating that the species did not develop on inorganic N produced from ARG mineralisation, but directly on the ON substrate. Since A. minimum has been linked to harmful algal blooms, and E. huxleyi contributes significantly to oceanic CaCOᴈ deposition, their ability to utilise DON has environmental consequences in addition to the oceanic N-budget. Climate change scenarios predict both episodic conditions of elevated rainfall and extended periods of dry conditions leading to variable riverine inputs to coastal areas, altered nitrogen to phosphorus (N:P) ratios, and changes in the inorganic to organic balance of the nutrient pools. Organic N can constitute up to 69 % of the total N pools, respectively, making it crucial, to understand the cycling of this fraction in coastal waters, and how changes in the composition of nutrient pools could impact on marine ecosystem function and health.

Nitrogen (N) fertilization due to atmospheric deposition (NDEP ) may explain some of the net carbon (C) sink (0.6-0.7 Pg y-1) in temperate forests, but estimates of the additional C uptake due to atmospheric N additions (∆C/∆N) can vary by over an order of magnitude (5 to 200 ∆C/∆N). High estimates from several recent studies [e.g. Magnani (2007), Nature 447 848-850], deriving ∆C/∆N from regional correlations between NDEP and measures of C uptake (such as eddy covariance -derived net ecosystem production, or forest inventory data) contradict estimates from other studies, particularly those involving 15N tracer applications added as fertilizer to the forest floor. A strong ∆C/∆N effect requires nitrogen to be efficiently acquired by trees and allocated to high C:N, long-lived woody tissues, but these isotope experiments typically report relatively little (~ 20 %) of 15N added is found above-ground, with less than 5 % of the total 15N applied found in wood. Consequently the high correlation-derived ∆C/∆N estimates are often attributed to co-variation with other factors across the range of sites investigated. However, 15N-fertilization treatments often impose considerably higher total N loads than ambient NDEP , while almost all exclusively only apply mineral 15N treatments to the soil, often in a limited number of treatment events over relatively short periods of time. Excessive N deposition loads can induce negative physiological effects and limit the resulting ∆C/∆N observed, and applying treatments to the soil ignores canopy nitrogen uptake, which has been demonstrated in numerous studies. As canopies can directly take up nitrogen, the chronic, (relatively) low levels of ambient NDEP inputs from pollution may be acquired without some of the effects of heavy N loads, with trees obtaining this N before it reaches the soil, allowing canopies to substitute for, or supplement, edaphic N nutrition. The strength of this effect depends on how much N uptake can occur across the canopy under field conditions, and if this extra N supplies growth in woody tissues such as the stem, as well as the canopy. Similarly, such mineral fertilizer isotope trace experiments are also unable to trace N in the decomposing litter and humus layers of the soil, which even under heavy NDEP loading contribute most of the N utilised for forest growth. Recent literature suggests that some organic (early decomposition) forms of N may be taken up by roots. If this litter N is not retained or distributed in the same way as mineral fertilizers, its contribution to plant nutrition and ∆C/∆N may need to be reassessed under nitrogen deposition. We tested some of these assumptions in the nursery and the field. In order to facilitate litter 15N tracing, we conducted an experiment injecting large trees with 15N-NH4NO3 to create 15N-labelled litter, tracing the applied isotope into a full harvest of the canopy. Such labelled litter substitute was used to replace the litter layer in a Sitka Spruce plantation (Picea sitchensis L. (Bong.)), where the fate of this 15N from litter decomposition in the soil system was compared against the fate of 15N in deposition. Similarly, in potted Sitka Spruce saplings, we used combination treatments of 15N-labelled litter, soil-targeted 15N-deposition, and canopy targeted 15N-deposition, investigating 15N return in different age classes of above and below ground biomass. We found that i) 15N recovery in canopies (needles and branches) in our injected trees was almost all of the injected 15N five months after injection, ii) canopy application of NDEP led to 60 % 15N return in above-ground parts of saplings compared to 21 % in soil applications and iii) a litter-derived 15N source was retained 55 % more in topsoil, and 36 % more in roots, than a similar deposition 15N source applied as mineral fertilizer. We discuss the implications of such findings in the context of 15N return in different plant organs and ecosystem pools, seasonal variation in N content, and overall inferences of a forest ∆C/∆N effect. Our results suggest that the total ∆C/∆N effect driven by a high N sequestration from canopy uptake in wood is ~ 114:1, more than double that of 15N tracer experiments but not as high as upper estimates from correlative studies, and that litter-derived organic N is better retained in trees and soils in excess of similar amounts of mineral 15N from deposition. Existing forest 15N-fertilization experiments could under-estimate the overall ∆C/∆N effect of atmospheric N deposition.

Os reservatórios, embora importantes para diversas atividades humanas, podem trazer modificações negativas ao ambiente. Tais sistemas aquáticos propiciam condições favoráveis ao maior desenvolvimento da comunidade fitoplanctônica, associado ao aporte de nutrientes e ao fenômeno da eutrofização artificial. O estudo do processo de assimilação de macronutrientes por esta comunidade pode orientar planos para mitigação dos efeitos da eutrofização e contribuir para o entendimento dos fatores limitantes ao fitoplâncton. A presente pesquisa teve como objetivo principal quantificar a assimilação de três formas de nitrogênio pelo fitoplâncton com uso do traçador 15N, de modo a contribuir para o entendimento da influência da temperatura e da disponibilidade de luz sobre esse processo em amostras ambientais e em culturas laboratoriais. Os objetivos específicos visaram: 1) estimar a variação temporal da assimilação de nitrato, amônio e ureia em amostras ambientais do reservatório do Lobo (SP); 2) quantificar e comparar a assimilação das mesmas três formas nitrogenadas por duas espécies fitoplanctônicas isoladas em laboratório (a cianobactéria Microcystis aeruginosa e a clorofícea Scenedesmus sp.), para verificar a influência da luz e da temperatura sobre o processo de assimilação. Para o primeiro objetivo específico, amostras foram coletadas em quatro períodos do ano no reservatório do Lobo (out /14; jan, abr e jul /15), próximo a São Carlos (SP). Foram reproduzidas, em incubadoras, as condições de Radiação Solar Fotossinteticamente Ativa (RSFA) e temperatura observadas em campo e relativas às profundidades em que estavam disponíveis 50% e 10% da RSFA. Em laboratório, os traçadores nitrato (15NO3-), amônio (15NH4+) e ureia [(15NH2)2CO] foram adicionados em frascos de 250 mL, incubados por uma hora. Após etapa de filtração, os filtros seguiram para análise elementar de quantificação do nitrogênio particulado e espectrometria de massa para a determinação da composição isotópica (15N/14N). Para o segundo objetivo específico, após a obtenção das culturas isoladas de uma cianobactéria e uma clorofícea, foi realizada a quantificação da assimilação das formas de nitrogênio. Esta análise foi realizada por experimento fatorial, com dois fatores e três níveis (temperatura: 20, 24 e 28°C e luz: 50, 250 e 450 µE.m-2.s-1). Os resultados apresentados para as amostras ambientais sugeriram maior preferência da comunidade fitoplanctônica em assimilar amônio em janeiro (23,7 µgN.L-1.h-1) e julho (54,4 µgN.L-1.h-1) de 2015. Em abril de 2015, a assimilação foi maior para nitrato (38,5 µgN.L-1.h-1) e ureia (43,1 µgN.L-1.h-1). Foi observado, ao longo dos meses de coleta, aumento na assimilação total do nitrogênio, da mesma forma como a densidade total de fitoplâncton, o biovolume e a clorofila-a, sugerindo que a assimilação deve acompanhar o aumento do grau de trofia do reservatório. Os resultados para as culturas laboratoriais indicaram uma assimilação quase que total do amônio pelas duas espécies estudadas. Scenedesmus sp. mostrou-se mais adaptada a assimilar amônio (517,1 µgN.L-1.h-1) em condições de alta intensidade luminosa (450 µE.m-2.s-1), enquanto Microcystis aeruginosa assimilou melhor o amônio (160,0 µgN.L-1.h-1) em condições de 50 µE.m-2.s-1. Para as duas espécies, a temperatura de 20°C resultou em maiores taxas de assimilação. Com base nos resultados obtidos, busca-se oferecer subsídios para o controle do estado trófico e manejo de reservatórios subtropicais por meio da identificação da forma de nitrogênio preferencialmente assimilada pelo fitoplâncton e os respectivos fatores intervenientes. / The reservoirs, despite their importance for different human activities, can bring negative changes to the environment. Such aquatic systems hold favorable conditions to the development of the phytoplankton community, which is associated with nutrient inputs and to the artificial eutrophication. The study of macronutrients uptake by this community can aid in plans for mitigating the effects of eutrophication and contribute to the understanding of the limiting factors to phytoplankton. This research aimed at quantifying the uptake of three forms of nitrogen by the phytoplankton with of use the 15N isotope, in order to contribute to the understanding of the influence of temperature and light availability on this process in environmental samples and in laboratory cultures. The specific objectives were: 1) to estimate the temporal variation of the uptake of nitrate, ammonium and urea in environmental samples from the Lobo Reservoir (SP, Brazil); 2) to quantify and compare the assimilation of the same three nitrogen forms for phytoplankton species isolated in laboratory (the Cyanobacteria Microcystis aeruginosa and the Chlorophyceae Scenedesmus sp.), to verify the influence of light and temperature in the uptake process. For the first specific objective, samples were collected in four periods of the year in the Lobo Reservoir (Oct/14; Jan, Apr and Jul/15), close to São Carlos (SP). Under controlled lab conditions, Photosynthetic Active Radiation (PAR) and temperature conditions observed in the field (at depths corresponding to 50% and 10% of the PAR) were reproduced. The tracers nitrate (15NO3-), ammonium (15NH4+) and urea [(15NH2)2CO] were added in bottles of 250 mL and incubated for one hour. After filtration, the filters were sent to elemental analysis for quantification of particulate nitrogen and mass spectrophotometry for the determination of isotopic composition (15N/14N). For the second of specific objective, after obtaining isolated cultures of the Cyanobacteria and the Chlorophyceae, the quantification of nitrogen uptake was performed. This analysis was carried out by factorial experiment with two factors and three levels (temperature: 20, 24 and 28°C and light: 50, 250 and 450 µE.m-2.s-1). The results obtained for the environmental samples suggested greater preference of the phytoplankton community for ammonium uptake in January (23.7 µgN.L-1.h-1) and July (54.4 µgN.L-1.h-1) from 2015. In April 2015, the uptake rates were greater for nitrate (38.5 µgN.L-1.h-1) and urea (43.1 µgN.L-1.h-1). It was observed temporal increase in total nitrogen uptake, following the patterns for total phytoplankton density, biovolume and chlorophyll-a, suggesting that uptake can be related with the increasing trophic state of the reservoir. The results from the laboratory cultures showed almost total ammonium uptake by both studied species. Scenedesmus sp. seemed to be more adapted to uptake ammonium (517.1 µgN.L-1.h-1) in conditions of high light intensity (450 µE.m-2.s-1), while the respective condition for Microcystis aeruginosa uptaking ammonium (160.0 µgN.L-1.h-1) was 50 µE.m-2.s-1. For the two species, the temperature of 20ºC resulted in higher uptake rates. We expect our results will aid in trophic state control and management of subtropical reservoirs through the identification of the nitrogen forms preferentially assimilated by phytoplankton and the factors involved.

In durum wheat (<i>Triticum turgidum</i> L. var <i>durum</i>), grain protein concentration (GPC) and gluten quality are among the important factors influencing pasta-making quality. Semolina with high protein content produces pasta with increased tolerance to overcooking and greater cooked firmness. However, genetic improvement of GPC is difficult largely because of its negative correlation with grain yield, and a strong genotype x environment interaction. Therefore, identification of quantitative trait loci (QTL) for high GPC and the associated markers is a priority to enhance selection efficiency in breeding durum wheat for elevated GPC. At a physiological level, GPC is influenced by several factors including nitrogen remobilization from vegetative organs and direct post-anthesis nitrogen uptake (NUP) from the soil. Understanding the relationship between elevated GPC and nitrogen remobilization, and post-anthesis NUP will enable durum wheat breeders to develop varieties that not only produce high yield and high GPC, but also exhibit better nitrogen use efficiency. The objectives of this study were: (1) to identify and validate QTL for elevated GPC in two durum wheat populations; and (2) to determine if elevated GPC is due to more efficient nitrogen remobilization and/or greater post-anthesis NUP. A genetic map was constructed with SSR and DArT® markers in a doubled haploid population from the cross Strongfield x DT695, and GPC data were collected in replicated trials in six Canadian environments from 2002 to 2005. Two stable QTL for high GPC, QGpc.usw-B3 on chromosome 2B and QGpc.usw-A3 on 7A, were identified. Strongfield, the high GPC parent, contributed the alleles for elevated GPC at both QTL. These two QTL were not associated with variation in grain weight (seed size) or grain yield. QGpc.usw-A3 was validated in a second Strongfield-derived population as that QTL was significant in all six testing environments. Averaged over five locations, selection for QGpc.usw-A3 resulted in a +0.4% to +1.0% increase in GPC, with only small effects on yield in most environments. A physiological study of grain protein accumulation revealed that regardless of the growing condition, nitrogen remobilization was the major contributor for grain nitrogen in durum genotypes evaluated, accounting for an average of 84.3% of total GPC. This study confirmed that introgression of Gpc-B1 into Langdon resulted in increased GPC, and this GPC increase was due to higher N remobilization. Strongfield expressed greater N remobilization than DT695 and the semi-dwarf cultivar Commander, but N remobilization was not the determining factor for Strongfields elevated GPC. Strongfield expressed greater post-anthesis NUP than DT695. Similarly, a selection of six high-GPC doubled haploid (DH) lines from the cross DT695 x Strongfield expressed significantly greater post-anthesis NUP than six low-GPC DH selections, supporting the hypothesis that elevated GPC in Strongfield is derived from greater post-anthesis NUP. All six high-GPC DH selections carried the Strongfield allele at QGpc.usw-A3, suggesting this QTL maybe associated with post-anthesis NUP.

In durum wheat (<i>Triticum turgidum</i> L. var <i>durum</i>), grain protein concentration (GPC) and gluten quality are among the important factors influencing pasta-making quality. Semolina with high protein content produces pasta with increased tolerance to overcooking and greater cooked firmness. However, genetic improvement of GPC is difficult largely because of its negative correlation with grain yield, and a strong genotype x environment interaction. Therefore, identification of quantitative trait loci (QTL) for high GPC and the associated markers is a priority to enhance selection efficiency in breeding durum wheat for elevated GPC. At a physiological level, GPC is influenced by several factors including nitrogen remobilization from vegetative organs and direct post-anthesis nitrogen uptake (NUP) from the soil. Understanding the relationship between elevated GPC and nitrogen remobilization, and post-anthesis NUP will enable durum wheat breeders to develop varieties that not only produce high yield and high GPC, but also exhibit better nitrogen use efficiency. The objectives of this study were: (1) to identify and validate QTL for elevated GPC in two durum wheat populations; and (2) to determine if elevated GPC is due to more efficient nitrogen remobilization and/or greater post-anthesis NUP. A genetic map was constructed with SSR and DArT® markers in a doubled haploid population from the cross Strongfield x DT695, and GPC data were collected in replicated trials in six Canadian environments from 2002 to 2005. Two stable QTL for high GPC, QGpc.usw-B3 on chromosome 2B and QGpc.usw-A3 on 7A, were identified. Strongfield, the high GPC parent, contributed the alleles for elevated GPC at both QTL. These two QTL were not associated with variation in grain weight (seed size) or grain yield. QGpc.usw-A3 was validated in a second Strongfield-derived population as that QTL was significant in all six testing environments. Averaged over five locations, selection for QGpc.usw-A3 resulted in a +0.4% to +1.0% increase in GPC, with only small effects on yield in most environments. A physiological study of grain protein accumulation revealed that regardless of the growing condition, nitrogen remobilization was the major contributor for grain nitrogen in durum genotypes evaluated, accounting for an average of 84.3% of total GPC. This study confirmed that introgression of Gpc-B1 into Langdon resulted in increased GPC, and this GPC increase was due to higher N remobilization. Strongfield expressed greater N remobilization than DT695 and the semi-dwarf cultivar Commander, but N remobilization was not the determining factor for Strongfields elevated GPC. Strongfield expressed greater post-anthesis NUP than DT695. Similarly, a selection of six high-GPC doubled haploid (DH) lines from the cross DT695 x Strongfield expressed significantly greater post-anthesis NUP than six low-GPC DH selections, supporting the hypothesis that elevated GPC in Strongfield is derived from greater post-anthesis NUP. All six high-GPC DH selections carried the Strongfield allele at QGpc.usw-A3, suggesting this QTL maybe associated with post-anthesis NUP.

Os reservatórios, embora importantes para diversas atividades humanas, podem trazer modificações negativas ao ambiente. Tais sistemas aquáticos propiciam condições favoráveis ao maior desenvolvimento da comunidade fitoplanctônica, associado ao aporte de nutrientes e ao fenômeno da eutrofização artificial. O estudo do processo de assimilação de macronutrientes por esta comunidade pode orientar planos para mitigação dos efeitos da eutrofização e contribuir para o entendimento dos fatores limitantes ao fitoplâncton. A presente pesquisa teve como objetivo principal quantificar a assimilação de três formas de nitrogênio pelo fitoplâncton com uso do traçador 15N, de modo a contribuir para o entendimento da influência da temperatura e da disponibilidade de luz sobre esse processo em amostras ambientais e em culturas laboratoriais. Os objetivos específicos visaram: 1&#41; estimar a variação temporal da assimilação de nitrato, amônio e ureia em amostras ambientais do reservatório do Lobo &#40;SP&#41;; 2&#41; quantificar e comparar a assimilação das mesmas três formas nitrogenadas por duas espécies fitoplanctônicas isoladas em laboratório &#40;a cianobactéria Microcystis aeruginosa e a clorofícea Scenedesmus sp.&#41;, para verificar a influência da luz e da temperatura sobre o processo de assimilação. Para o primeiro objetivo específico, amostras foram coletadas em quatro períodos do ano no reservatório do Lobo &#40;out &#47;14; jan, abr e jul &#47;15&#41;, próximo a São Carlos &#40;SP&#41;. Foram reproduzidas, em incubadoras, as condições de Radiação Solar Fotossinteticamente Ativa &#40;RSFA&#41; e temperatura observadas em campo e relativas às profundidades em que estavam disponíveis 50&#37; e 10&#37; da RSFA. Em laboratório, os traçadores nitrato &#40;15NO3-&#41;, amônio &#40;15NH4+&#41; e ureia &#91;&#40;15NH2&#41;2CO&#93; foram adicionados em frascos de 250 mL, incubados por uma hora. Após etapa de filtração, os filtros seguiram para análise elementar de quantificação do nitrogênio particulado e espectrometria de massa para a determinação da composição isotópica &#40;15N&#47;14N&#41;. Para o segundo objetivo específico, após a obtenção das culturas isoladas de uma cianobactéria e uma clorofícea, foi realizada a quantificação da assimilação das formas de nitrogênio. Esta análise foi realizada por experimento fatorial, com dois fatores e três níveis &#40;temperatura: 20, 24 e 28&#176;C e luz: 50, 250 e 450 &#181;E.m-2.s-1&#41;. Os resultados apresentados para as amostras ambientais sugeriram maior preferência da comunidade fitoplanctônica em assimilar amônio em janeiro &#40;23,7 &#181;gN.L-1.h-1&#41; e julho &#40;54,4 &#181;gN.L-1.h-1&#41; de 2015. Em abril de 2015, a assimilação foi maior para nitrato &#40;38,5 &#181;gN.L-1.h-1&#41; e ureia &#40;43,1 &#181;gN.L-1.h-1&#41;. Foi observado, ao longo dos meses de coleta, aumento na assimilação total do nitrogênio, da mesma forma como a densidade total de fitoplâncton, o biovolume e a clorofila-a, sugerindo que a assimilação deve acompanhar o aumento do grau de trofia do reservatório. Os resultados para as culturas laboratoriais indicaram uma assimilação quase que total do amônio pelas duas espécies estudadas. Scenedesmus sp. mostrou-se mais adaptada a assimilar amônio &#40;517,1 &#181;gN.L-1.h-1&#41; em condições de alta intensidade luminosa &#40;450 &#181;E.m-2.s-1&#41;, enquanto Microcystis aeruginosa assimilou melhor o amônio &#40;160,0 &#181;gN.L-1.h-1&#41; em condições de 50 &#181;E.m-2.s-1. Para as duas espécies, a temperatura de 20&#176;C resultou em maiores taxas de assimilação. Com base nos resultados obtidos, busca-se oferecer subsídios para o controle do estado trófico e manejo de reservatórios subtropicais por meio da identificação da forma de nitrogênio preferencialmente assimilada pelo fitoplâncton e os respectivos fatores intervenientes. / The reservoirs, despite their importance for different human activities, can bring negative changes to the environment. Such aquatic systems hold favorable conditions to the development of the phytoplankton community, which is associated with nutrient inputs and to the artificial eutrophication. The study of macronutrients uptake by this community can aid in plans for mitigating the effects of eutrophication and contribute to the understanding of the limiting factors to phytoplankton. This research aimed at quantifying the uptake of three forms of nitrogen by the phytoplankton with of use the 15N isotope, in order to contribute to the understanding of the influence of temperature and light availability on this process in environmental samples and in laboratory cultures. The specific objectives were: 1&#41; to estimate the temporal variation of the uptake of nitrate, ammonium and urea in environmental samples from the Lobo Reservoir &#40;SP, Brazil&#41;; 2&#41; to quantify and compare the assimilation of the same three nitrogen forms for phytoplankton species isolated in laboratory &#40;the Cyanobacteria Microcystis aeruginosa and the Chlorophyceae Scenedesmus sp.&#41;, to verify the influence of light and temperature in the uptake process. For the first specific objective, samples were collected in four periods of the year in the Lobo Reservoir &#40;Oct&#47;14; Jan, Apr and Jul&#47;15&#41;, close to São Carlos &#40;SP&#41;. Under controlled lab conditions, Photosynthetic Active Radiation &#40;PAR&#41; and temperature conditions observed in the field &#40;at depths corresponding to 50&#37; and 10&#37; of the PAR&#41; were reproduced. The tracers nitrate &#40;15NO3-&#41;, ammonium &#40;15NH4+&#41; and urea &#91;&#40;15NH2&#41;2CO&#93; were added in bottles of 250 mL and incubated for one hour. After filtration, the filters were sent to elemental analysis for quantification of particulate nitrogen and mass spectrophotometry for the determination of isotopic composition &#40;15N/14N&#41;. For the second of specific objective, after obtaining isolated cultures of the Cyanobacteria and the Chlorophyceae, the quantification of nitrogen uptake was performed. This analysis was carried out by factorial experiment with two factors and three levels &#40;temperature: 20, 24 and 28&#176;C and light: 50, 250 and 450 &#181;E.m-2.s-1&#41;. The results obtained for the environmental samples suggested greater preference of the phytoplankton community for ammonium uptake in January &#40;23.7 &#181;gN.L-1.h-1&#41; and July &#40;54.4 &#181;gN.L-1.h-1&#41; from 2015. In April 2015, the uptake rates were greater for nitrate &#40;38.5 &#181;gN.L-1.h-1&#41; and urea &#40;43.1 &#181;gN.L-1.h-1&#41;. It was observed temporal increase in total nitrogen uptake, following the patterns for total phytoplankton density, biovolume and chlorophyll-a, suggesting that uptake can be related with the increasing trophic state of the reservoir. The results from the laboratory cultures showed almost total ammonium uptake by both studied species. Scenedesmus sp. seemed to be more adapted to uptake ammonium &#40;517.1 &#181;gN.L-1.h-1&#41; in conditions of high light intensity &#40;450 &#181;E.m-2.s-1&#41;, while the respective condition for Microcystis aeruginosa uptaking ammonium &#40;160.0 &#181;gN.L-1.h-1&#41; was 50 &#181;E.m-2.s-1. For the two species, the temperature of 20&#186;C resulted in higher uptake rates. We expect our results will aid in trophic state control and management of subtropical reservoirs through the identification of the nitrogen forms preferentially assimilated by phytoplankton and the factors involved.

Master of Science / Department of Agronomy / Ignacio Ciampitti / Soybean [Glycine max (L.) Merr.] yields have continuously increased over time. Seed yields are determined by the genotype, environment, and management practices (G × E × M) interaction. Closing yield gaps require a continuous improvement in the use of the available resources, which must be attained via implementation of better management decisions. Linear relationships between seed yield and nitrogen (N) demand are reported in the scientific literature. Main sources of N to the plant are the biological N fixation (BNF) and the soil mineralization processes. On overall, only 50-60% of soybean N demand is met by the BNF process. An unanswered scientific knowledge is still related to the ability of the BNF to satisfy soybean N demand at varying yield levels. Seed N demand not met by N fixation plus soil mineral N, is then fulfilled by the remobilization of N from vegetative organs during the seed filling period. An early remobilization process reduces the photosynthetic activity (leaves) and can limit seed yield. The objectives of this project were to: i) study yield improvements and contribution of N via utilization of contrasting N conditions under historical and modern soybean genotypes, and ii) quantify main seed N sources during the seed filling period. For objective one, four field experiments were conducted during the 2016 and 2017 growing seasons in Kansas, United States (US) and Santa Fe Province, Argentina (ARG). Those experiments investigated twenty-one historical and modern soybean genotypes with release decades from 1980s to 2010s. As for objective two, three field experiments were conducted during the 2015 and 2016 growing seasons in Kansas, US, studying three soybean genotypes: non-roundup ready (RR), released in 1997; RR-1, released in 2009; and RR-2, released in 2014. Across all studies, seeds were inoculated and tested under three N management strategies: i) control without N application (Zero-N); ii) 56 kg N ha-1 applied at reproductive growth stages (Late-N); and iii) 670 kg ha-1 equally split at three timings (Full-N). As for yield improvements and N limitation, soybean yield improvements from the 1980s to 2010s were documented, representing 29% increases in the US and 21% in ARG. Regarding N management, the Full-N fertilization produced a 12% increase in seed yields in the US and 4% in ARG. As for main seed N sources in objective two, remobilization accounted for 59% of seed N demand, and was negatively related to new N uptake occurring during the seed filling period. Seed N demand for greater yields was dependent on both, N remobilization and new N uptake, while for lower yields, seed N demand was mainly supported by the N remobilization process. These results suggest that: a) high seed yields are somehow limited by the availability of N to express their potential, although the question about N application still remains to be fully investigated, as related to the timing and the environment by plant interactions that could promote a N limitation in soybeans; b) remobilization accounts for majority (59%) of N sourced to the seed, and c) high yielding soybean (modern genotypes) rely on diverse N sources: the N remobilization process plus new uptake of N.

A process oriented modeling of an irrigated rice field in a semi arid area of Mali has been done with the help of computational tool CoupModel. The model has been used to simulate two levels of irrigation rates, in an attempt to test and see adequacy of a recommended irrigation rate and its environmental impact over the current management. A simpler simulation to represent less water demanding crops like sorghum or millet has also been done to indicate extent of the excess water and as alternative crop cultivation. Important processes and parameters to represent a rice cropping system have been identified and simulation was run for a 12 year period. Results show an irrigation amount of 916 mm delivers an overall 6 % increased yield. Results from the reduced irrigation also show a better output in surface runoff, nitrogen leaching and uptake, photosynthetic water use efficiency and fertilizer efficiency. Soil nitrogen and carbon storage shows nearly the same trend. Only nitrous oxide (N2O) emission rate increased by 13 % in the case of reduced irrigation. Simulation done for the other crops also shows a reasonable yield of sorghum or millet can be obtained with 46 % of water used for current rice irrigation.