Blends of controlled release and stabilized nitrogen (N) fertilizer represent an alternative to provide N at all corn growth stages, and is an option to reduce costs compared to the use of solely controlled release N. In this context, field experiments were conducted in Southeast Brazil with the use of a blend of polymer-sulfur coated urea (PSCU) and NBPT-treated urea (NBPTU) at a 70:30 ratio, applied at corn planting and incorporated into the soil. The objectives of the study were: i) to quantify and measure each fertilizer-derived N fate in the plants, and determine the nitrogen recovery efficiency of each N source in the blend; ii) to evaluate corn grain yield response to N rates (blend) in contrasting cropping systems, and to assess the posibility of reducing N rate when applying a blend of two enhanced efficiency N fertilizers compared to the application of regular urea; iii) understand and monitor changes in plant biomass and N uptake during the growing season. Fertilizer N contributed with less than 50% of the total plant N uptake at all evaluated corn growth stages (V4, V12, R2, and R6). At V4 growth stage, most of the N in the plant derived from fertilizer (NPDF) was provided by NBPTU, while later in the season, most of the NPDF was provided by PSCU. At harvest, most of the plant N was allocated in the grains (59%). Of the total plant N, 64% was supplied by the native soil N pool, 26% was provided by PSCU, and 10% by NBPTU. Therefore, NBPTU provided N to corn early in the season, while PSCU played a crucial role supplying N later in the season, as plants demand for N increased. Soil N was the main N source at all GS and this fraction decreased as N rate increased. At harvest, 64% of the total plant N was derived from the soil native N pool, 26% derived from PSCU, and 10% from urea. The measured fertilizer NRE of urea was in average 36%, and the estimated NUE from PSCU was 51%. In the second study, corn grain yield varied between sites, probably due to soil and climate characteristics of each site. Corn grain yield, N uptake, and biomass production were greatly impacted by fertilizer N. Grain yield and N uptake showed a quadratic response to N rates (blend). The blend of PSCU and NBPTU, applied at corn planting and incorporated into the soil proved to be a great strategy to attain yields at N rates below those needed when using regular urea. The third chapter focused on corn biomass and N uptake and partitioning throughout the growing season, and it was demonstrated that the amount of N uptake after flowering can reach up to 50% of the total plant N, thus, N availability must be guaranteed in late vegetative corn growth stages, and especially in the reproductive stages, which can be achieved by adopting enhanced efficiency N fertilizers such as the blend of PSCU and NBPTU used in this study. / A mistura de fertilizantes nitrogenados de liberação controlada e estabilizados representa uma alternativa para fornecer nitrogênio (N) em todos os estádios da cultura do milho, além de ser uma opção válida para reduzir custos em comparação ao uso exclusivo de produtos de liberação controlada. Neste sentido, conduziram-se experimentos de campo na região Sudeste do Brasil com a aplicação de um blend de ureia revestida com enxofre e polímeros (UREP) e ureia tratada com NBPT (U-NBPT), na proporção 70:30, aplicada na semeadura do milho, de forma incorporada. Os objetivos do trabalho foram: i) avaliar o destino do N dentro das plantas de milho proveniente dos fertilizantes misturados e determinar a eficiência de recuperação de cada um; ii) avaliar a resposta do rendimento de grãos de milho a doses de N (blend) em sistemas de produção contrastantes e avaliar a possibilidade de reduzir as doses de N quando aplicado o blend em comparação à ureia convencional; iii) entender e monitorar as mudanças da biomassa e o nitrogênio dentro das plantas de milho ao longo do ciclo da cultura. No primeiro estudo, o N na planta proveniente da UREP, da U-NBPT, e do solo (N-Solo) variaram ao longo do ciclo do milho. Contudo os fertilizantes nitrogenados contribuiram com menos de 50% do N total da planta em todos os estadios avaliados (V4, V12, R2 e R6). No estádio V4, a maior parte do N na planta proveniente de fertilizante (NPPF) foi fornecido pela U-NBPT, enquanto que nos estadios seguintes, a maior parte do NPPF foi fornecido pela UREP. O N-Solo foi o maior fornecedor de N para a planta, mas a contribuição diminuiu com o aumento das doses de N. Na colheita, 59% do total do N da planta foi alocado nos grãos. Do total de N da planta, 64% foi proveniente do N-Solo, 26% foi fornecido pela UREP, e 10% pela U-NBPT. A eficiência de recuperação da UREP e U-NBPT foram, respectivamente, 51 e 36%. No segundo estudo, o rendimento de grãos de milho variou entre locais, provavelmente devido às condições edafo-climáticas de cada área experimental. A aplicação do fertilizante nitrogenado influenciou o rendimento de grãos de milho, a produção de biomassa e acúmulo de N em todos os locais. O rendimento de grãos e acúmulo de N mostraram uma resposta quadrática às doses de N (blend). A incorporação do blend de UREP e U-NBPT na semeadura do milho mostrou-se como uma ótima estratégia para evitar perdas massivas de N e mostrou que pode atingir produtividade similar a ureia convencional com doses de N menores. O terceiro capítulo, com foco no acúmulo e particionamento da biomassa e N nas plantas de milho ao longo do ciclo, desmonstrou que a quantidade de N absorvido após o florescimento pode chegar a 50% do total de N acumulado nas plantas, pelo que adequada disponibilidade de N deve ser garantida nos estádios vegetativos finais e nos estádios reprodutivos da cultura do milho, o que pode ser conseguido com o uso de misturas de UREP e U-NBPT.

Blends of controlled release and stabilized nitrogen (N) fertilizer represent an alternative to provide N at all corn growth stages, and is an option to reduce costs compared to the use of solely controlled release N. In this context, field experiments were conducted in Southeast Brazil with the use of a blend of polymer-sulfur coated urea (PSCU) and NBPT-treated urea (NBPTU) at a 70:30 ratio, applied at corn planting and incorporated into the soil. The objectives of the study were: i) to quantify and measure each fertilizer-derived N fate in the plants, and determine the nitrogen recovery efficiency of each N source in the blend; ii) to evaluate corn grain yield response to N rates (blend) in contrasting cropping systems, and to assess the posibility of reducing N rate when applying a blend of two enhanced efficiency N fertilizers compared to the application of regular urea; iii) understand and monitor changes in plant biomass and N uptake during the growing season. Fertilizer N contributed with less than 50% of the total plant N uptake at all evaluated corn growth stages (V4, V12, R2, and R6). At V4 growth stage, most of the N in the plant derived from fertilizer (NPDF) was provided by NBPTU, while later in the season, most of the NPDF was provided by PSCU. At harvest, most of the plant N was allocated in the grains (59%). Of the total plant N, 64% was supplied by the native soil N pool, 26% was provided by PSCU, and 10% by NBPTU. Therefore, NBPTU provided N to corn early in the season, while PSCU played a crucial role supplying N later in the season, as plants demand for N increased. Soil N was the main N source at all GS and this fraction decreased as N rate increased. At harvest, 64% of the total plant N was derived from the soil native N pool, 26% derived from PSCU, and 10% from urea. The measured fertilizer NRE of urea was in average 36%, and the estimated NUE from PSCU was 51%. In the second study, corn grain yield varied between sites, probably due to soil and climate characteristics of each site. Corn grain yield, N uptake, and biomass production were greatly impacted by fertilizer N. Grain yield and N uptake showed a quadratic response to N rates (blend). The blend of PSCU and NBPTU, applied at corn planting and incorporated into the soil proved to be a great strategy to attain yields at N rates below those needed when using regular urea. The third chapter focused on corn biomass and N uptake and partitioning throughout the growing season, and it was demonstrated that the amount of N uptake after flowering can reach up to 50% of the total plant N, thus, N availability must be guaranteed in late vegetative corn growth stages, and especially in the reproductive stages, which can be achieved by adopting enhanced efficiency N fertilizers such as the blend of PSCU and NBPTU used in this study. / A mistura de fertilizantes nitrogenados de liberação controlada e estabilizados representa uma alternativa para fornecer nitrogênio (N) em todos os estádios da cultura do milho, além de ser uma opção válida para reduzir custos em comparação ao uso exclusivo de produtos de liberação controlada. Neste sentido, conduziram-se experimentos de campo na região Sudeste do Brasil com a aplicação de um blend de ureia revestida com enxofre e polímeros (UREP) e ureia tratada com NBPT (U-NBPT), na proporção 70:30, aplicada na semeadura do milho, de forma incorporada. Os objetivos do trabalho foram: i) avaliar o destino do N dentro das plantas de milho proveniente dos fertilizantes misturados e determinar a eficiência de recuperação de cada um; ii) avaliar a resposta do rendimento de grãos de milho a doses de N (blend) em sistemas de produção contrastantes e avaliar a possibilidade de reduzir as doses de N quando aplicado o blend em comparação à ureia convencional; iii) entender e monitorar as mudanças da biomassa e o nitrogênio dentro das plantas de milho ao longo do ciclo da cultura. No primeiro estudo, o N na planta proveniente da UREP, da U-NBPT, e do solo (N-Solo) variaram ao longo do ciclo do milho. Contudo os fertilizantes nitrogenados contribuiram com menos de 50% do N total da planta em todos os estadios avaliados (V4, V12, R2 e R6). No estádio V4, a maior parte do N na planta proveniente de fertilizante (NPPF) foi fornecido pela U-NBPT, enquanto que nos estadios seguintes, a maior parte do NPPF foi fornecido pela UREP. O N-Solo foi o maior fornecedor de N para a planta, mas a contribuição diminuiu com o aumento das doses de N. Na colheita, 59% do total do N da planta foi alocado nos grãos. Do total de N da planta, 64% foi proveniente do N-Solo, 26% foi fornecido pela UREP, e 10% pela U-NBPT. A eficiência de recuperação da UREP e U-NBPT foram, respectivamente, 51 e 36%. No segundo estudo, o rendimento de grãos de milho variou entre locais, provavelmente devido às condições edafo-climáticas de cada área experimental. A aplicação do fertilizante nitrogenado influenciou o rendimento de grãos de milho, a produção de biomassa e acúmulo de N em todos os locais. O rendimento de grãos e acúmulo de N mostraram uma resposta quadrática às doses de N (blend). A incorporação do blend de UREP e U-NBPT na semeadura do milho mostrou-se como uma ótima estratégia para evitar perdas massivas de N e mostrou que pode atingir produtividade similar a ureia convencional com doses de N menores. O terceiro capítulo, com foco no acúmulo e particionamento da biomassa e N nas plantas de milho ao longo do ciclo, desmonstrou que a quantidade de N absorvido após o florescimento pode chegar a 50% do total de N acumulado nas plantas, pelo que adequada disponibilidade de N deve ser garantida nos estádios vegetativos finais e nos estádios reprodutivos da cultura do milho, o que pode ser conseguido com o uso de misturas de UREP e U-NBPT.

Os fertilizantes nitrogenados desempenham função importante na nutrição e aumento da produtividade da cana-de-açúcar, porém alteram os processos de transformação de C e N do solo. Este trabalho teve por objetivo principal avaliar o efeito da aplicação consecutiva de fertilizantes minerais e orgânicos nas transformações microbianas, na mineralização do N, nos estoques de C, N total e N mineral, e na fertilidade do solo a médio prazo. Foram escolhidas duas áreas cultivadas com cana-de-açúcar no Estado de São Paulo nas quais experimentos com manejo da adubação nitrogenada foram conduzidos desde 2010 e 2011, com reaplicação anual dos tratamentos até 2014. Nestas áreas foi adotado o delineamento estatístico casualizado em blocos com quatro repetições. Os tratamentos foram: controle (sem adubação nitrogenada), adubação com as fontes nitrato de amônio, ureia, composto orgânico e Ajifer® na dose 100 kg ha-1 ano-1 N, e adubação com a fonte YaraBella NitromagTM nas doses de 50, 100, 150 e 200 kg ha- 1 ano-1 N. Para avaliar o efeito dos tratamentos na variação da fertilidade do solo e no teor de N orgânico hidrolisável (N-ISNT), foram realizadas amostragens de solo anuais nestes experimentos nas camadas de 0-20 e 20-40 cm. Para avaliar o efeito dos tratamentos nos processos de transformação do C e N do solo, em 2014 foram coletadas amostras de solo até 100 cm de profundidade nos tratamentos controle, composto orgânico, YaraBella NitromagTM nas doses 100 e 200 kg ha-1 ano-1 N e mata nativa, nas quais foram determinados a densidade do solo e teores de C orgânico, N total e N mineral (N-NH4+ e N-NO3- + N-NO2-). Adicionalmente, amostras da camada de 0-20 cm foram incubadas em laboratório para avaliar a disponibilidade temporal de N e os teores de C e N da biomassa microbiana e sua atividade. Os efeitos da adubação nitrogenada consecutiva foram pouco expressivos nas transformações microbianas, na disponibilidade temporal de N mineral e nos estoques de C e N do solo; entretanto, foi observado acúmulo de N mineral em camadas mais profundas (> 40 cm) para a dose de 200 kg ha-1 ano-1 N. Além disso, a área de mata apresentou melhores atributos microbiológicos e maior estoque de C e N comparado às áreas cultivadas com cana-de-açúcar. Em relação à variação da fertilidade do solo, observou-se que o aumento das doses de N potencializa a acidificação do solo em anos consecutivos, assim como promove diminuição do teor de bases trocáveis. A manutenção da fertilidade do solo, dos atributos microbiológicos, e dos estoques de C, N total e N mineral para produção sustentável de biocombustíveis depende do manejo adequado da fertilização nitrogenada no sistema de colheita dos canaviais sem prévia queima. De forma complementar e paralela a estes trabalhos, foi objeto de estudo a adaptação e validação de métodos colorimétricos na determinação de N inorgânico em extratos de solos tropicais. Os resultados revelaram que é necessário adicionar carvão ativado durante o processo de extração para que os métodos colorimétricos apresentem melhor exatidão e precisão. Recomenda-se utilizar solução extratora de KCl 2,0 mol L-1 e permitir tempo de reação de 30 min. A faixa linear de trabalho mais adequada varia de 0 a 7,5 e 0 a 10,0 mg L-1 de N para leituras de N-NH4+ e N-NO3- respectivamente. / Nitrogen fertilizers play an important role in nutrition and increased productivity of sugarcane, however, affect cycling of C and N in the soil. This study mainly aimed to evaluate the effect of the consecutive application of organic and inorganic sources of N in microbial soil transformations, in N mineralization, in C, total N and mineral N stocks, and soil fertility in the medium term. They were chosen two areas cultivated with sugarcane in the State of São Paulo in which experiments with management of nitrogen fertilization have been conducted since 2010 and 2011, with annual reapplication of treatments through 2014. In these areas was adopted the randomized blocks experimental design, with four replications. The treatments were: control (no nitrogen fertilization), fertilization with sources of ammonium nitrate, urea, organic fertilizer and Ajifer® at rate of 100 kg ha-1 yr-1 N, and fertilization with YaraBella NitromagTM source at rates of 50, 100, 150 and 200 kg ha-1 yr-1 N. To evaluate the effect of treatments on the variation of soil fertility and hydrolysable organic-N fraction (N-ISNT), annual soil samples were collected in these experiments at 0-0.2 and 0.2-0.40 m. To evaluate the effect of treatments on soil C and N transformations, soil samples were collected up to 1.0 m depth in the control, organic fertilizers, YaraBella NitromagTM at rates of 100 and 200 kg ha-1 yr-1 N treatments and bushland, in which were determined bulk density and organic C, total N and mineral N (NH4+-N and NO2--N + NO3--N). In addition, soil samples of the 0-0.2 m were incubated in the laboratory to assess the availability of N and the content of C and N microbial biomass and microbial activity. The effects of consecutive nitrogen fertilizer were inexpressive in microbial transformations, in the temporal availability of mineral N and C and N soil stocks; however, it was observed mineral N accumulation in deeper layers (> 0.4 m) at rate of 200 kg ha-1 yr-1 N. Moreover, the bushland showed better microbiological attributes and largest C and N soil stocks compared with areas cultivated with sugarcane. In relation to the variation of soil fertility, it observed that the increase in N rates enhances the soil acidification in consecutive years, and promotes decreased levels of exchangeable bases. The maintenance of soil fertility, the microbiological attributes, and the C, total N and mineral N soil stocks for sustainable biofuel production depends on proper management of nitrogen fertilization on crop system of without burning sugarcane fields. Complementarily and in parallel with these studies, it was the object of study the adaptation and validation of colorimetric methods for the determination of inorganic N in tropical soil extracts. The results revealed that it is necessary to add activated charcoal during the extraction process so that colorimetric methods exhibit better accuracy and precision. It is recommended to use extraction solution of KCl 2,0 mol L-1 and allow 30 min reaction time between addition of reagent and the spectrophotometer reading. The linear working range more suitable varies from 0 to 7.5 and 0 to 10,0 mg L-1 of N for measurement of NH4+-N and NO3--N respectively.

Os fertilizantes nitrogenados desempenham função importante na nutrição e aumento da produtividade da cana-de-açúcar, porém alteram os processos de transformação de C e N do solo. Este trabalho teve por objetivo principal avaliar o efeito da aplicação consecutiva de fertilizantes minerais e orgânicos nas transformações microbianas, na mineralização do N, nos estoques de C, N total e N mineral, e na fertilidade do solo a médio prazo. Foram escolhidas duas áreas cultivadas com cana-de-açúcar no Estado de São Paulo nas quais experimentos com manejo da adubação nitrogenada foram conduzidos desde 2010 e 2011, com reaplicação anual dos tratamentos até 2014. Nestas áreas foi adotado o delineamento estatístico casualizado em blocos com quatro repetições. Os tratamentos foram: controle (sem adubação nitrogenada), adubação com as fontes nitrato de amônio, ureia, composto orgânico e Ajifer® na dose 100 kg ha-1 ano-1 N, e adubação com a fonte YaraBella NitromagTM nas doses de 50, 100, 150 e 200 kg ha- 1 ano-1 N. Para avaliar o efeito dos tratamentos na variação da fertilidade do solo e no teor de N orgânico hidrolisável (N-ISNT), foram realizadas amostragens de solo anuais nestes experimentos nas camadas de 0-20 e 20-40 cm. Para avaliar o efeito dos tratamentos nos processos de transformação do C e N do solo, em 2014 foram coletadas amostras de solo até 100 cm de profundidade nos tratamentos controle, composto orgânico, YaraBella NitromagTM nas doses 100 e 200 kg ha-1 ano-1 N e mata nativa, nas quais foram determinados a densidade do solo e teores de C orgânico, N total e N mineral (N-NH4+ e N-NO3- + N-NO2-). Adicionalmente, amostras da camada de 0-20 cm foram incubadas em laboratório para avaliar a disponibilidade temporal de N e os teores de C e N da biomassa microbiana e sua atividade. Os efeitos da adubação nitrogenada consecutiva foram pouco expressivos nas transformações microbianas, na disponibilidade temporal de N mineral e nos estoques de C e N do solo; entretanto, foi observado acúmulo de N mineral em camadas mais profundas (> 40 cm) para a dose de 200 kg ha-1 ano-1 N. Além disso, a área de mata apresentou melhores atributos microbiológicos e maior estoque de C e N comparado às áreas cultivadas com cana-de-açúcar. Em relação à variação da fertilidade do solo, observou-se que o aumento das doses de N potencializa a acidificação do solo em anos consecutivos, assim como promove diminuição do teor de bases trocáveis. A manutenção da fertilidade do solo, dos atributos microbiológicos, e dos estoques de C, N total e N mineral para produção sustentável de biocombustíveis depende do manejo adequado da fertilização nitrogenada no sistema de colheita dos canaviais sem prévia queima. De forma complementar e paralela a estes trabalhos, foi objeto de estudo a adaptação e validação de métodos colorimétricos na determinação de N inorgânico em extratos de solos tropicais. Os resultados revelaram que é necessário adicionar carvão ativado durante o processo de extração para que os métodos colorimétricos apresentem melhor exatidão e precisão. Recomenda-se utilizar solução extratora de KCl 2,0 mol L-1 e permitir tempo de reação de 30 min. A faixa linear de trabalho mais adequada varia de 0 a 7,5 e 0 a 10,0 mg L-1 de N para leituras de N-NH4+ e N-NO3- respectivamente. / Nitrogen fertilizers play an important role in nutrition and increased productivity of sugarcane, however, affect cycling of C and N in the soil. This study mainly aimed to evaluate the effect of the consecutive application of organic and inorganic sources of N in microbial soil transformations, in N mineralization, in C, total N and mineral N stocks, and soil fertility in the medium term. They were chosen two areas cultivated with sugarcane in the State of São Paulo in which experiments with management of nitrogen fertilization have been conducted since 2010 and 2011, with annual reapplication of treatments through 2014. In these areas was adopted the randomized blocks experimental design, with four replications. The treatments were: control (no nitrogen fertilization), fertilization with sources of ammonium nitrate, urea, organic fertilizer and Ajifer® at rate of 100 kg ha-1 yr-1 N, and fertilization with YaraBella NitromagTM source at rates of 50, 100, 150 and 200 kg ha-1 yr-1 N. To evaluate the effect of treatments on the variation of soil fertility and hydrolysable organic-N fraction (N-ISNT), annual soil samples were collected in these experiments at 0-0.2 and 0.2-0.40 m. To evaluate the effect of treatments on soil C and N transformations, soil samples were collected up to 1.0 m depth in the control, organic fertilizers, YaraBella NitromagTM at rates of 100 and 200 kg ha-1 yr-1 N treatments and bushland, in which were determined bulk density and organic C, total N and mineral N (NH4+-N and NO2--N + NO3--N). In addition, soil samples of the 0-0.2 m were incubated in the laboratory to assess the availability of N and the content of C and N microbial biomass and microbial activity. The effects of consecutive nitrogen fertilizer were inexpressive in microbial transformations, in the temporal availability of mineral N and C and N soil stocks; however, it was observed mineral N accumulation in deeper layers (> 0.4 m) at rate of 200 kg ha-1 yr-1 N. Moreover, the bushland showed better microbiological attributes and largest C and N soil stocks compared with areas cultivated with sugarcane. In relation to the variation of soil fertility, it observed that the increase in N rates enhances the soil acidification in consecutive years, and promotes decreased levels of exchangeable bases. The maintenance of soil fertility, the microbiological attributes, and the C, total N and mineral N soil stocks for sustainable biofuel production depends on proper management of nitrogen fertilization on crop system of without burning sugarcane fields. Complementarily and in parallel with these studies, it was the object of study the adaptation and validation of colorimetric methods for the determination of inorganic N in tropical soil extracts. The results revealed that it is necessary to add activated charcoal during the extraction process so that colorimetric methods exhibit better accuracy and precision. It is recommended to use extraction solution of KCl 2,0 mol L-1 and allow 30 min reaction time between addition of reagent and the spectrophotometer reading. The linear working range more suitable varies from 0 to 7.5 and 0 to 10,0 mg L-1 of N for measurement of NH4+-N and NO3--N respectively.

In seasons of above normal precipitation, populations of annual weedy species increase in great abundance in semi-arid desert plant communities. These increases in biomass tie up a considerable portion of the available nitrogen of such ecosystems and may depress subsequent annual grass germination. A big sagebrush-annual bromegrass plant community was irrigated to simulate a spring growth period of abundant precipitation amenable to annual bromegras s productivity. Productivity and nitrogen content parameters were monitored throughout the spring and summer to evaluate the short and potential long term effects of this seasonal increase in "precipitation". Irrigation increased annual bromegrass productivity almost 50 percent. This increase was a result of the combined factors of increased soil moisture content and increased nitrogen availability due, apparently, to increased soil microfloral nitrification activity under low water water stress conditions. The increased annual growth resulted in a greater nitrogen uptake despite an observed decrease in irrigated bromegrass percent nitrogen values. Both the tie up and irrigation effect were maintained throughout the summer and into the subsequent annual germination period. However, total soil nitrogen levels and bromegrass nitrogen mineralization rates suggest that soil nutrient conditions for subsequently germinating bromegrass seedlings do not differ because of previous spring irrigation treatment.

The regeneration in oak-dominated stands following five silvicultural treatments was examined on four sites in the mountains of Virginia and West Virginia. Treatments included: silvicultural clearcut, leave-tree, commercial clearcut, shelterwood, and group selection. The effects of harvesting were compared among sites and among treatments. Oak regeneration dominance, measured by the relative density of dominant and codominant oak regeneration, was the most important variable calculated from the data. Oak regeneration dominance varied by site, but did not vary by silvicultural treatment; all treatments resulted in relatively low numbers. Therefore, the silvicultural treatments used here were not enough to overcome the site specific limitations to successful oak regeneration. Oak species also seemed to be less important in the regenerating stands than in their parent stands. The biggest losses in oak importance occurred on the intermediate and high quality sites; competitive oak regeneration was relatively scarce on two of the three sites with an oak site index (base age 50) of over 70 ft. Multiple linear regression analysis was used to identify the factors controlling oak regeneration at a smaller scale. The most important variables were those that described the oak stump sprouting potential, the understory and overstory oak component in the pre-harvest stand, post-harvest light and soil nitrogen levels. / Master of Science

The anthropogenic introduction of species into new ecosystems is a global phenomenon, and identifying the mechanisms by which some introduced species become dominant in their introduced ranges (i.e., invasive) is crucial to predicting, preventing, and mitigating the impacts of biological invasions. Introduced perennial C₄ grasses are invading semi-arid grassland and savanna ecosystems throughout the south-central U.S. We hypothesized that in these semi-arid ecosystems, where variable precipitation patterns strongly influence vegetation dynamics, the success of an invasive plant species may be due in part to ecophysiological traits that enable high performance in response to unpredictable water availability. We also hypothesized that increased primary productivity and decreased plant input quality associated with these grass invasions have the potential to alter ecosystem carbon and nitrogen cycling and storage by altering the ratio of inputs (productivity) to outputs (decomposition/respiration). We tested the first hypothesis by quantifying ecophysiological performance differences between an invasive C₄ grass, Bothriochloa ischaemum, and co-occurring C₃ and C₄ native grasses under wet and dry conditions in the field and under two levels of simulated precipitation frequencies in a greenhouse experiment. We tested the second hypothesis by examining whether increased primary productivity and decreased C₃:C₄ grass ratios in savanna grass-matrices associated with B. ischaemum invasion altered (1) plant input quality and thus nutrient cycling and/or (2) net ecosystem carbon uptake in invaded areas. B. ischaemum's success as an invader was not directly related to its ability to cope with precipitation variability and availability, but its ability to rapidly produce large amounts of biomass may allow it to directly out-compete native species. B. ischaemum invasion decreased plant input quality and soil nitrogen availability. B. ischaemum invasion shifted ecosystem C-uptake from being nearly year-round to occurring predominantly in the summer. Greater C-uptake during the summer and under drier conditions compensated for a shorter growing seasons in B. ischaemum-invaded areas and cumulative annual NEE was similar between invaded and native-dominated areas. We conclude that B. ischaemum's impacts on soil nitrogen availability and plant-canopy microhabitat may allow it to exclude native species from invaded areas, but that its impacts on ecosystem C sequestration may be small. / text

Innerhalb der letzten zwei Jahrzehnte ist die Entwaldungsrate auf Sumatra, Indonesien stark gestiegen, dies geht einher mit eine Umwandlung von Tieflandwäldern in Ölpalm- (Elaeis guineensis) und Kautschukmonokulturplantagen (Hevea brasiliensis). Es wurde festgestellt, dass Landnutzungsänderungen in landwirtschaftlichen Systemen die Bodennährstoffbestände sowie die Umsatzrate von Bodennährstoffen senkt, dies kann zu einer Abhängigkeit vom Einsetzen von Düngemitteln führen, die nur eine zeitweise Verfügbarkeit von Nährstoffen gewährleistet. Des Weiteren bedroht die Umwandlung von Wald in Monokulturen die hohe Biodiversität, welche in tropischen Wäldern vorherrscht, was wiederum die Funktionsweise des Ökosystems beeinflusst. Der Schwerpunkt dieser Arbeit lag darin, die Auswirkungen der Landnutzungsänderung auf Bodennährstoffhaushalt und Ökosystemdynamiken festzustellen, sowie die Mechanismen die für die Veränderungen verantwortlich sind zu verstehen. Alle Drei Studien waren Teil eines großen interdisziplinären Projekts welches die ökologischen und sozialen Effekte von tropischen Landnutzungsveränderungen untersucht. Die Probenentnahme für jede Studie erfolgte in der Region von Jambi auf Sumatra, Indonesien – ein Gebiet das früher dicht bewaldet war, aber eine starke Entwaldung erfahren hat. Es wurden zwei Landschaften ausgesucht, die sich über ihre vorherrschende Bodentextur und ihren Bodentyp definieren und die Region natürlich repräsentieren: zum einen waren dies lehmige Acrisole und zum anderem tonige Acrisole. In den beiden Bodenlandschaften wurden vier Systeme untersucht: Tieflandregenwald und regenerierter Wald durchsetz mit Kautschukbäumen (hier benannt als „Jungle-rubber“) sowie Monokulturen von Kautschuk (Sein bis 17 Jahre alt) und Ölpalmen (Neun bis 16 Jahre alt). Das Ziel der ersten Studie war zu bewerten, wie sich die Umsatzrate von Stickstoff (N) im Boden in Bezug auf die Umwandlung von Wald in Kautschuk- und Ölpalmplantagen verändert. Die Bruttoumsatzrate von Stickstoff im Boden wurde mit der 15N-Verdünnungsmethode mit in situ Inkubation der Bodenbohrkerne bestimmt. In den Lehm-Acrisolen, in denen die Bodenfruchtbarkeit gering war, waren auch die mikrobielle Biomasse, die Bruttostickstoffmineralisation und die Immobilisierung von Ammonium (NH4+) gering und es wurden keine signifikanten Veränderungen durch die Landnutzung aufgezeigt. Die Ton-Acrisole welche eine höhere Ausgangsfruchtbarkeit, bezogen auf die Referenzflächen, aufwiesen, waren auch einen höheren Anteil an mikrobielle Biomassen sowie durch höhere NH4+-Umwandlungsraten im Vergleich zu den Lehm-Acrisolen gekennzeichnet. In den Ton-Acrisolen hat die Umwandlung von Wald und Jungle-rubber in Kautschuk- und Ölpalmplantagen zu einer Verringerung der Bodenfruchtbarkeit geführt, was wiederum zu einer Reduzierung der mikrobiellen Biomasse und der NH4+-Umwandlungsraten beigetragen hat. Unsere Ergebnisse lassen annehmen, das je höher die Ausgangsbodenfruchtbarkeit und Stickstoffverfügbarkeit im Boden ist, desto höher ist die Reduktionen durch die Landnutzungsänderungen. Das Ziel der zweiten Studie war es, Veränderungen biochemischer Charakteristika des Bodens sowie des Nährstoffbestandes bis 2 m Bodentiefe in den verschiedenen Landnutzungssystemen zu erfassen und die Proportionen der Gesamtvarianz der biochemischer Bodencharakteristika zu bestimmen, die durch die räumlichen Komponenten in unserem experimentellem Design hervorgerufen werden. Der Tongehalb beeinflusst die Bodenfruchtbarkeit und die größeren Nährstoffbestände wurden in den Referenzflächen der Ton-Acrisolen gefunden. Bewirtschaftungspraktiken in den veränderten Landnutzungssystemen übten den größten Einfluss auf Boden-pH, Basensättigung, extrahierbaren Phosphor und austauschbares Natrium aus. Die Mehrheit der biochemischen Bodencharakteristika und der Nährstoffbestände wurden nicht signifikant durch Landnutzungsänderungen verändert. Basierend auf der Varianzkomponentenanalyse der verschachtelten räumlichen Struktur des experimentellen Designs, wurde die Gesamtvarianz von vielen biochemischen Bodencharakteristika durch die Abweichungen zwischen replizierten Plots und nicht durch die unterschiedliche Landnutzung erklärt. Dieses Ergebnis deutet darauf hin, dass wenn man signifikante Effekte von Landnutzungsänderungen auf biochemische Bodencharakteristika feststellen will, die Stichprobenzahl replizierter Plots pro Landnutzungssystem erhöht werden muss. Das Ziel der dritten Studie war es, zwischen direkten Landnutzungseffekten und indirekten „Bottom-up“-Effekten auf ober- und unterirdisch lebende Taxa zu differenzieren. Es wurden allgemeine „Multilevel path“- Modelle (eine Form von Strukturgleichungsmodellen), die eine Berechnung direkter und interaktiver Effekte von Landnutzung mit abiotischen Variablen und „Bottom-up“-Effekten zwischen biotischen Variablen zulassen, auf der Basis von Daten von Pflanzen, Mikroorganismen, Invertebraten der Streuschicht, baumbewohnende Ameisen, Vögeln und Umweltparametern (Boden- und Mikroklimaeigenschaften) entworfen. Die Ergebnisse der „Multilevel path“- Modelle zeigen, dass die Landnutzungsänderungen direkte Effekte auf Pflanzen, unterirdisch lebende Taxa einer niedrigen trophischen Ebene (z.B. Saprobionten und Herbivoren) und baumbewohnende Ameisen haben, fast alle Landnutzungsauswirkungen auf höhere trophische Ebenen von Invertebraten und Vögel waren jedoch „Bottom-up“-kontrolliert. Diese Studie lässt erkennen, dass Landnutzungsveränderungen, direkt und indirekt, ökologische Verschiebungen im großen Rahmen lenken. Die gefundenen Effekte auf höhere trophische Ebenen sind jedoch meistens von den Organismen der darunterliegenden trophischen Ebenen abhängig. Die Stickstoffumsatzraten im Boden und der Umfang der Stickstoffpools, welche in der ersten Studie gemessen wurden, wurden parallel mit Studien zur Stickstoffoxidemission und Stickstoffauswaschung des Bodens durchgeführt, um ein ganzheitliches Bild des Stickstoffhaushaltes in den veränderten Landschaft zu erhalten. Analysen zur Probenoptimierung wurden für die biochemischen Bodencharakteristika der oberen Bodenschicht bis 0,5 m aus der zweiten Studie durchgeführt, um festzustellen was die minimale Anzahl an Replikaten pro Landnutzungstyp ist, um signifikante Unterschiede zwischen den Landnutzungssystemen in unserem experimentellen Design festzustellen. Die Bodenkomponenten die in die „Multilevel path“- Modelle integriert waren, wurden erfasst und direkte Zusammenhänge zwischen diesen Bodeneigenschaften und der Biodiversität des Ökosystems und den Biomassen wurden untersucht, um ein besseres Verständnis davon zu bekommen, welche Rolle Bodennährstoffbeständen für die transformierten Systeme spielen. Insgesamt zeigen die Ergebnisse der drei Studien, dass die Bodennährstoffbestände eine wichtige Komponente des Ökosystems darstellt und Veränderungen der Bodennährstoffbestände durch Landnutzungsänderungen Auswirkungen auf die Biodiversität und die Funktionsweise des Ökosystems haben können.

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