O Níquel (Ni) é um micronutriente para as plantas, por ser componente estrutural das enzimas urease e hidrogenase, que desempenham função no metabolismo do nitrogênio (N) nas plantas leguminosas. A aplicação de Ni via tratamento de semente em soja pode potencializar o processo de fixação biológica de nitrogênio (FBN) com a finalidade de proporcionar maior produção de biomassa da parte aérea e de grãos. Objetivou-se com este trabalho avaliar o efeito da aplicação de doses de Ni, via tratamento de semente, no processo de FBN em plantas de soja, por meio da atividade de nitrogenase (ANase) e da abundância natural de 15N (? 15N?). Foi realizado um experimento em casa de vegetação, em condições controladas. O genótipo de soja BMX POTÊNCIA RR foi cultivado em solo arenoso e submetido à aplicação de 0; 45; 90; 135; 180; 360; e 540 mg kg-1 de Ni aplicado via tratamento de semente. As plantas de soja foram conduzidas até o estádio fenológico R7 (maturação dos grãos). Foram avaliadas a concentração de macro e micronutrientes nas folhas utilizadas para diagnose, determinou-se o teor de clorofila, a atividade enzimática da urease, bem como se quantificou a massa seca de nódulos, no estádio fenológico R1 (Início da floração: até 50% das plantas com flor). Foram ainda determinados: as produções de massa seca de parte aérea e de grãos, o índice de colheita, os acúmulos de Ni e de N na semente e a contribuição da fixação biológica de nitrogênio, por meio da técnica de abundância natural de 15N e indiretamente através da técnica da redução do acetileno (atividade da nitrogenase). Observou-se que as doses de Ni exerceram efeitos em todos os parâmetros avaliados. A aplicação de Ni na dose de 45 mg kg-1 aumentou a fixação biológica de N na ordem de 12% em relação ao tratamento controle (sem adição de Ni) e incrementou o acúmulo de Ni e de N no grão, com efeito na atividade da enzima urease. A aplicação de 45 e 90 mg kg-1 de níquel via semente resultou, respectivamente, no aumento de 67% e 77% da atividade da NAase em relação ao controle. Pela análise da abundância natural de 15N no grão (?15N?) observou-se que houve diferença na FBN entre os tratamentos e que a aplicação da dose de 45 mg kg-1, de Ni via semente, revelou que 99% do N acumulado no grão foi proveniente da FBN, enquanto que no tratamento sem adição de Ni via semente, a contribuição da FBN foi de apenas 77%. Nas condições do experimento, a aplicação de Ni na semente refletiu em maior produção de massa seca de parte aérea e na produção de grãos de soja / Nickel (Ni) is an essential micronutrient for plants due to its role on structural component of the enzymes urease and hydrogenase, which perform nitrogen (N) metabolism in legumes plants. Seed treatment with Ni in soybean might improve the symbiotic or biological nitrogen fixation (BNF) process, by increasing biomass production and grain yield. The objective of this study was to evaluate the effect of soybean seed treatment with Ni rates on the biological nitrogen fixation (BNF) process by evaluating the nitrogenase activity (NAase), the natural 15N abundance (? 15N ?) and urease activity in soybean plants, as well. The experiment was carried out in greenhouse with controlled conditions. The soybean cultivar used was BMX POTÊNCIA RR, which was grown in sandy soil, and submitted to application of 0, 45, 90,135, 180, 360 and 540 mg kg-1 of Ni on the seed treatment. Soybean plants were grown up to the R7 developmental stage (grain maturity). Macro and micronutrients concentration in the leaves used for diagnosis, as well as the chlorophyll content and urease activity have been determined on these tissues, and dry weight of nodules were evaluated in developmental stage R1 (flowering stage). The following measurements also were made: dry matter yield of plant top, dry matter yield of grains, the harvest index, uptake of Ni and N in the seed and the contribution of biological nitrogen fixation by 15N natural abundance and to indirectly way by the reduction of acetylene technique (nitrogenase activity), as well. It was observed that Ni rates influenced all parameters. Application of Ni rate of 45 mg kg-1 increased biological nitrogen fixation in the order of 12% as compared to the control. Ni rates of 45 and 90 mg kg-1 via seed treatment increased the NAase activity, respectively, in 67% and 77% compared to the control. Other benefits of Ni-soybean seeds treatment were the increased accumulation of Ni and N on the grain and its positive influence on the urease activity. By analyzing the natural abundance of 15N on the grain (?15N ?) it was observed that there were differences between treatments in the symbiotic nitrogen fixation. Around 99% of total N taken up in the seeds came from biological nitrogen fixation by supplying Ni-seed application rate of 45 mg kg-1, whereas in the control, the contribution of BNF was only 77%. In these experimental conditions, seed treatment with Ni reflected in higher dry matter production of plant top and grain yield

O Níquel (Ni) é um micronutriente para as plantas, por ser componente estrutural das enzimas urease e hidrogenase, que desempenham função no metabolismo do nitrogênio (N) nas plantas leguminosas. A aplicação de Ni via tratamento de semente em soja pode potencializar o processo de fixação biológica de nitrogênio (FBN) com a finalidade de proporcionar maior produção de biomassa da parte aérea e de grãos. Objetivou-se com este trabalho avaliar o efeito da aplicação de doses de Ni, via tratamento de semente, no processo de FBN em plantas de soja, por meio da atividade de nitrogenase (ANase) e da abundância natural de 15N (? 15N?). Foi realizado um experimento em casa de vegetação, em condições controladas. O genótipo de soja BMX POTÊNCIA RR foi cultivado em solo arenoso e submetido à aplicação de 0; 45; 90; 135; 180; 360; e 540 mg kg-1 de Ni aplicado via tratamento de semente. As plantas de soja foram conduzidas até o estádio fenológico R7 (maturação dos grãos). Foram avaliadas a concentração de macro e micronutrientes nas folhas utilizadas para diagnose, determinou-se o teor de clorofila, a atividade enzimática da urease, bem como se quantificou a massa seca de nódulos, no estádio fenológico R1 (Início da floração: até 50% das plantas com flor). Foram ainda determinados: as produções de massa seca de parte aérea e de grãos, o índice de colheita, os acúmulos de Ni e de N na semente e a contribuição da fixação biológica de nitrogênio, por meio da técnica de abundância natural de 15N e indiretamente através da técnica da redução do acetileno (atividade da nitrogenase). Observou-se que as doses de Ni exerceram efeitos em todos os parâmetros avaliados. A aplicação de Ni na dose de 45 mg kg-1 aumentou a fixação biológica de N na ordem de 12% em relação ao tratamento controle (sem adição de Ni) e incrementou o acúmulo de Ni e de N no grão, com efeito na atividade da enzima urease. A aplicação de 45 e 90 mg kg-1 de níquel via semente resultou, respectivamente, no aumento de 67% e 77% da atividade da NAase em relação ao controle. Pela análise da abundância natural de 15N no grão (?15N?) observou-se que houve diferença na FBN entre os tratamentos e que a aplicação da dose de 45 mg kg-1, de Ni via semente, revelou que 99% do N acumulado no grão foi proveniente da FBN, enquanto que no tratamento sem adição de Ni via semente, a contribuição da FBN foi de apenas 77%. Nas condições do experimento, a aplicação de Ni na semente refletiu em maior produção de massa seca de parte aérea e na produção de grãos de soja / Nickel (Ni) is an essential micronutrient for plants due to its role on structural component of the enzymes urease and hydrogenase, which perform nitrogen (N) metabolism in legumes plants. Seed treatment with Ni in soybean might improve the symbiotic or biological nitrogen fixation (BNF) process, by increasing biomass production and grain yield. The objective of this study was to evaluate the effect of soybean seed treatment with Ni rates on the biological nitrogen fixation (BNF) process by evaluating the nitrogenase activity (NAase), the natural 15N abundance (? 15N ?) and urease activity in soybean plants, as well. The experiment was carried out in greenhouse with controlled conditions. The soybean cultivar used was BMX POTÊNCIA RR, which was grown in sandy soil, and submitted to application of 0, 45, 90,135, 180, 360 and 540 mg kg-1 of Ni on the seed treatment. Soybean plants were grown up to the R7 developmental stage (grain maturity). Macro and micronutrients concentration in the leaves used for diagnosis, as well as the chlorophyll content and urease activity have been determined on these tissues, and dry weight of nodules were evaluated in developmental stage R1 (flowering stage). The following measurements also were made: dry matter yield of plant top, dry matter yield of grains, the harvest index, uptake of Ni and N in the seed and the contribution of biological nitrogen fixation by 15N natural abundance and to indirectly way by the reduction of acetylene technique (nitrogenase activity), as well. It was observed that Ni rates influenced all parameters. Application of Ni rate of 45 mg kg-1 increased biological nitrogen fixation in the order of 12% as compared to the control. Ni rates of 45 and 90 mg kg-1 via seed treatment increased the NAase activity, respectively, in 67% and 77% compared to the control. Other benefits of Ni-soybean seeds treatment were the increased accumulation of Ni and N on the grain and its positive influence on the urease activity. By analyzing the natural abundance of 15N on the grain (?15N ?) it was observed that there were differences between treatments in the symbiotic nitrogen fixation. Around 99% of total N taken up in the seeds came from biological nitrogen fixation by supplying Ni-seed application rate of 45 mg kg-1, whereas in the control, the contribution of BNF was only 77%. In these experimental conditions, seed treatment with Ni reflected in higher dry matter production of plant top and grain yield

Natural abundance of 15N varies greatly and unpredictably within and between environments. The unpredictable nature of 15N limits the use of N isotope natural abundance (d15N) in tracing the flow and fate of N in environments. Recent investigations have, however, revealed consistent and repeatable patterns of 15N in some ecosystem components. These patterns suggest that d15N may yet provide a tool to investigate and illuminate ecosystem N cycling processes. Identifying and quantifying the sources of isotopic variation must precede any significant advance in the application of this technique, and to this end an assessment of isotopic variation associated with major ecosystem components has been carried out in this thesis. d15N patterns have been established, hypotheses proposed and tested, and conclusions about the application of the technique are presented. 15N patterns in surface and groundwater were measured in a variety of different land-use catchments in an attempt to identify distinct isotopic 'fingerprints'. High levels of 15N variation were measured in both stream and groundwaters, resulting in strongly overlapping land-use 'fingerprints'. Environmental 15N variation in streams and groundwaters was found to be too great to differentiate between land-uses based on d15N alone. In contrast, the artificially 15N enriched signature of effluent N was used to trace its flow and fate, following irrigation, in a forested catchment. The effluent d15N signature allowed it to be traced into the major ecosystem components, permitting a first order N budget to be determined for effluent N storage and loss. N sources with significantly different 15N signatures to that of 'background ecosystem N' can therefore be used to trace the flow and fate of N in ecosystems. During the course of this work a number of higher and lower order plants were observed to have highly depleted (lt; -8 ) d15N signatures. Epiphytes and lithophytes, strongly reliant on atmospheric N sources, were consistently depleted in 15N, with signatures as low as -24 , measured in a range of environments. A similar level of depletion was measured in a wide range of plants growing in early primary succession sites (as low as -22.3 ), which could not be accounted for by any abiotic or biotic factor or significantly depleted N source. The absence of any measurable driver of depletion suggested a universal fractionating mechanism which acts in a wide range of environments and vegetation types. Diffusive uptake of atmospheric NH3(g) and the proportional uptake of a supplied N source were two proposed mechanisms that could theoretically account for the level and universal nature of depletion. Diffusive uptake of atmospheric NH3(g) was tested as a primary fractionating mechanism in plants. Strongly N deficient plants were capable of utilising NH3(g) as a nutritional source, but the level of 15N depletion measured in these plants closely approximated that of the inherent NH3(g) d15N signature. No significant additional fractionation is associated with NH3(g) diffusive uptake. Diffusive uptake of atmospheric NH3(g) by plants cannot alone account for the level of depletion measured in early primary succession plant communities. Proportional uptake of a N source as a primary fractionating mechanism was tested by growing plants in various concentrations and rates of applied N. Fractionation attributed to the proportional uptake of a supplied N source, as a consequence of P limitation or rapid flow over roots, resulted in a significant level of 15N depletion in plants. The level of depletion attributed to this mechanism was, however, not sufficient to account for the level measured in early primary succession plant communities. Individual 15N fractionating mechanisms cannot alone explain the level of depletion observed in early primary succession plants, however a combination of fractionating mechanisms can. Fractionation attributed to the proportional uptake of an already depleted N source, i.e., wet deposited N, largely accounts for the level of depletion measured in early succession plant communities. This two-step fractionation model can act on both higher and lower plants, independent of ecosystem biotic and abiotic factors. Additional, and less dramatic fractionations attributed to atmospheric NH3(g) uptake, mycorrhizal associations, internal remobilisation, and taxon-specific N acquisition strategies, will contribute to the level of d15N depletion. This thesis presents the first extensive survey of highly depleted d15N signatures in terrestrial vegetation. Furthermore, thorough testing of theoretically plausible mechanisms has resulted in a full account of the highly depleted d15N signatures measured in a wide range of vegetation types and environments.

<p>Soils of Northern Hemisphere forests contain a large part of the global terrestrial carbon (C) pool. Even small changes in this pool can have large impact on atmospheric [CO2] and the global climate. Soil respiration is the largest terrestrial C flux to the atmosphere and can be divided into autotrophic (from roots, mycorrhizal hyphae and associated microbes) and heterotrophic (from decomposers of organic material) respiration. It is therefore crucial to establish how the two components will respond to changing environmental factors. In this thesis I studied the effect of elevated atmospheric [CO2] (+340 ppm, ¹³C-depleted) and elevated air temperature (2.8-3.5 oC) on soil respiration in a whole-tree chamber (WTC) experiment conducted in a boreal Norway spruce forest. In another spruce forest I used multivariate modelling to establish the link between day-to-day variations in soil respiration rates and its δ¹³C, and above and below ground abiotic conditions. In both forests, variation in δ¹³C was used as a marker for autotrophic respiration. A trenching experiment was conducted in the latter forest in order to separate the two components of soil respiration. The potential problems associated with the trenching, increased root decomposition and changed soil moisture conditions were handled by empirical modelling. The WTC experiment showed that elevated [CO2] but not temperature resulted in 48 to 62% increased soil respiration rates. The CO2-induced increase was in absolute numbers relatively insensitive to seasonal changes in soil temperature and data on δ¹³C suggest it mostly resulted from increased autotrophic respiration. From the multivariate modelling we observed a strong link between weather (air temperature and vapour pressure deficit) and the day-to-day variation of soil respiration rate and its δ¹³C. However, the tightness of the link was dependent on good weather for up to a week before the respiration sampling. Changes in soil respiration rates showed a lag to weather conditions of 2-4 days, which was 1-3 days shorter than for the δ¹³C signal. We hypothesised to be due to pressure concentration waves moving in the phloem at higher rates than the solute itself (i.e., the δ¹³C–label). Results from the empirical modelling in the trenching experiment show that autotrophic respiration contributed to about 50% of total soil respiration, had a great day-to-day variation and was correlated to total soil respiration while not to soil temperature or soil moisture. Over the first five months after the trenching, an estimated 45% of respiration from the trenched plots was an artefact of the treatment. Of this, 29% was a water difference effect and 16% resulted from root decomposition. In conclusion, elevated [CO2] caused an increased C flux to the roots but this C was rapidly respired and has probably not caused changes in the C stored in root biomass or in soil organic matter in this N-limited forest. Autotrophic respiration seems to be strongly influenced by the availability of newly produced substrates and rather insensitive to changes in soil temperature. Root trenching artefacts can be compensated for by empirical modelling, an alternative to the sequential root harvesting technique.</p>

Intercropping, defined as two or more crops grown on the same land area at the same time, is a sustainable alternative to sole crops. Intercropping has been associated with multiple benefits, such as increased nutrient and soil organic carbon (SOC) cycling, decreased soil erosion and increased carbon (C) sequestration. A common intercropping practice is to integrate cereal and legume crops such as maize (Zea mays L.), and soybean (Glycine max (L.) Merr.). Most studies on intercropping have focused on yield, weed control, and land use efficiency in the tropics. Few studies have researched C and nitrogen (N) dynamics in temperate intercrops, with respect to soybean and maize residue stabilization. Soil from Balcarce, Argentina, was incubated for 140 days with soybean, maize, or no residue. Throughout the incubation, results illustrated the effect of residue application upon the soil, specifically through significantly higher amounts of light fraction (LF) C and LFN concentrations, soil microbial biomass (SMB) C and SMBN concentrations, higher microbial diversity, lower N2O production rates, in addition to distinct isotopic values in soil fractions and CO2 (p<0.05). Furthermore, it was observed from δ15N-TN and δ15N-LF that treatments with soybean residues included had higher N cycling (p<0.05), emphasizing the importance of including N-fixing legumes in complex agroecosystems. Significant changes over time in SMB and SMCS characteristics, and isotope values (p<0.05) indicated the preferential utilization of relatively young and easily accessible litter. Furthermore, the loss of labile material over the incubation resulted in more recalcitrant forms (such as older C and lignin) to be utilized. Slightly higher SOC, TN, LFC and LFN concentrations, as well as lower CO2 production rates suggested 2:3 (rows of maize:rows of soybean) as a more desirable intercrop design for C sequestration. The 1:2 intercrop design was observed to be more beneficial for microbial community structure, furthering the idea that intercropping is a beneficial alternative to sole cropping. This study improves knowledge in residue stabilization and C sequestration in complex agroecosystems, providing encouragement for the implementation of more sustainable management practices.

Intercropping, defined as two or more crops grown on the same land area at the same time, is a sustainable alternative to sole crops. Intercropping has been associated with multiple benefits, such as increased nutrient and soil organic carbon (SOC) cycling, decreased soil erosion and increased carbon (C) sequestration. A common intercropping practice is to integrate cereal and legume crops such as maize (Zea mays L.), and soybean (Glycine max (L.) Merr.). Most studies on intercropping have focused on yield, weed control, and land use efficiency in the tropics. Few studies have researched C and nitrogen (N) dynamics in temperate intercrops, with respect to soybean and maize residue stabilization. Soil from Balcarce, Argentina, was incubated for 140 days with soybean, maize, or no residue. Throughout the incubation, results illustrated the effect of residue application upon the soil, specifically through significantly higher amounts of light fraction (LF) C and LFN concentrations, soil microbial biomass (SMB) C and SMBN concentrations, higher microbial diversity, lower N2O production rates, in addition to distinct isotopic values in soil fractions and CO2 (p<0.05). Furthermore, it was observed from δ15N-TN and δ15N-LF that treatments with soybean residues included had higher N cycling (p<0.05), emphasizing the importance of including N-fixing legumes in complex agroecosystems. Significant changes over time in SMB and SMCS characteristics, and isotope values (p<0.05) indicated the preferential utilization of relatively young and easily accessible litter. Furthermore, the loss of labile material over the incubation resulted in more recalcitrant forms (such as older C and lignin) to be utilized. Slightly higher SOC, TN, LFC and LFN concentrations, as well as lower CO2 production rates suggested 2:3 (rows of maize:rows of soybean) as a more desirable intercrop design for C sequestration. The 1:2 intercrop design was observed to be more beneficial for microbial community structure, furthering the idea that intercropping is a beneficial alternative to sole cropping. This study improves knowledge in residue stabilization and C sequestration in complex agroecosystems, providing encouragement for the implementation of more sustainable management practices.

Soils of Northern Hemisphere forests contain a large part of the global terrestrial carbon (C) pool. Even small changes in this pool can have large impact on atmospheric [CO2] and the global climate. Soil respiration is the largest terrestrial C flux to the atmosphere and can be divided into autotrophic (from roots, mycorrhizal hyphae and associated microbes) and heterotrophic (from decomposers of organic material) respiration. It is therefore crucial to establish how the two components will respond to changing environmental factors. In this thesis I studied the effect of elevated atmospheric [CO2] (+340 ppm, 13C-depleted) and elevated air temperature (2.8-3.5 oC) on soil respiration in a whole-tree chamber (WTC) experiment conducted in a boreal Norway spruce forest. In another spruce forest I used multivariate modelling to establish the link between day-to-day variations in soil respiration rates and its δ13C, and above and below ground abiotic conditions. In both forests, variation in δ13C was used as a marker for autotrophic respiration. A trenching experiment was conducted in the latter forest in order to separate the two components of soil respiration. The potential problems associated with the trenching, increased root decomposition and changed soil moisture conditions were handled by empirical modelling. The WTC experiment showed that elevated [CO2] but not temperature resulted in 48 to 62% increased soil respiration rates. The CO2-induced increase was in absolute numbers relatively insensitive to seasonal changes in soil temperature and data on δ13C suggest it mostly resulted from increased autotrophic respiration. From the multivariate modelling we observed a strong link between weather (air temperature and vapour pressure deficit) and the day-to-day variation of soil respiration rate and its δ13C. However, the tightness of the link was dependent on good weather for up to a week before the respiration sampling. Changes in soil respiration rates showed a lag to weather conditions of 2-4 days, which was 1-3 days shorter than for the δ13C signal. We hypothesised to be due to pressure concentration waves moving in the phloem at higher rates than the solute itself (i.e., the δ13C–label). Results from the empirical modelling in the trenching experiment show that autotrophic respiration contributed to about 50% of total soil respiration, had a great day-to-day variation and was correlated to total soil respiration while not to soil temperature or soil moisture. Over the first five months after the trenching, an estimated 45% of respiration from the trenched plots was an artefact of the treatment. Of this, 29% was a water difference effect and 16% resulted from root decomposition. In conclusion, elevated [CO2] caused an increased C flux to the roots but this C was rapidly respired and has probably not caused changes in the C stored in root biomass or in soil organic matter in this N-limited forest. Autotrophic respiration seems to be strongly influenced by the availability of newly produced substrates and rather insensitive to changes in soil temperature. Root trenching artefacts can be compensated for by empirical modelling, an alternative to the sequential root harvesting technique.

A recuperação de funções e processos ecossistêmicos, entre outros atributos, é um dos indicadores mais importantes no processo de retorno de um ecossistema à sua trajetória histórica. A ciclagem de nutrientes é um atributo fundamental do ecossistema, e relaciona-se diretamente à regulação do funcionamento e do desenvolvimento dos ecossistemas e inclui, em um modelo geral, as entradas de nutrientes, as transferências internas entre plantas e solo e as saídas do sistema. A compreensão das mudanças nos processos biogeoquímicos durante a sucessão secundária em áreas em restauração ecológica ainda é incipiente, principalmente em áreas de florestas tropicais. Esse estudo tem por objetivo elucidar a dinâmica do nitrogênio ao longo do processo de restauração ecológica em áreas reflorestadas com espécies nativas da Mata Atlântica. A questão norteadora deste estudo é a seguinte: A restauração florestal com alta diversidade de espécies e predominância de espécies arbóreas nativas regionais restaura a dinâmica original do nitrogênio? Esse estudo visa investigar o funcionamento da ciclagem de nutrientes, com foco no nitrogênio, que é um elemento limitante à sucessão secundária, especialmente em florestas tropicais. Para tal, alguns indicadores da ciclagem do nitrogênio foram mensurados em uma cronosequência florestal formada por uma floresta natural preservada e florestas restauradas de diferentes idades (21 e 52 anos) reflorestadas com alta diversidade de espécies e predominância de nativas regionais. Os indicadores utilizados foram: 15N e teor de N da vegetação, serrapilheira e solo; razão N:P da vegetação e da serrapilheira; taxas líquidas de mineralização e nitrificação; teor de amônio, nitrato, N mineral e razão nitrato:amônio. As florestas foram amostradas entre agosto de 2008 e abril de 2009, nas seguintes estações: seca, transição entre seca e chuvosa, chuvosa, e transição entre chuvosa e seca. Foram encontrados padrões claros de mudanças na ciclagem do N ao longo da cronosequência estudada, incluindo diferenças nos valores de 15N foliar, teor de N, razão N:P, N mineral e taxas líquidas de mineralização e nitrificação, caracterizadas por um aumento de valores médios dessas variáveis ao longo da cronosequência. Os resultados encontrados sugerem que as florestas em processo de restauração, mesmo a de 52 anos, ainda não possuem uma ciclagem de N característica de uma floresta madura e, portanto, a recuperação da ciclagem de N ainda não foi completamente atingida. Entretanto, é possível afirmar que as florestas em processo de restauração estudadas estão seguindo uma trajetória de desenvolvimento caracterizada por uma ciclagem de N cada vez mais parecida com a de uma floresta natural madura, como a da floresta natural madura utilizada como referência. Através dos modelos de restauração utilizados para as florestas da cronosequência estudada, os processos da ciclagem do N são recuperados à medida que a floresta desenvolve-se, com uma clara tendência de mudança na economia de N para economia de P típica de florestas tropicais maduras. / The recuperation of ecosystem processes and functions, among other attributes, is one of the most important indicators in the process of return of an ecosystem to its historic trajectory. Nutrient cycling is a fundamental ecosystem attribute, and relates directly to regulation of functioning and development of ecosystems and includes, in a general model, nutrients entering, being transferred internally between plants and soil, end leaving the system. The comprehension of changes in biogeochemical processes during secondary succession in areas in ecological restoration is still incipient, mainly in tropical forests. This study aims to elucidate nitrogen dynamics along the process of ecological restoration in areas reforested with Atlantic Forest native species. The question driving this study is: Does ecological restoration with high diversity of species and predominance of regional native tree species restore nitrogen original dynamics? This research investigated nutrients cycling functioning, focusing on nitrogen, which is a limiting nutrient in secondary succession, particularly in tropical forests. In order to do so, some indicators of nutrient cycling were assessed in a forest chronosequence formed by a preserved natural forest and restored forests of different ages (21 and 52 years) reforested using high species diversity and predominance of regional native species. The indicators used were: 15N and N content in green foliage, litter and soil; N:P ratio of green foliage and litter; net mineralization and net nitrification rates; content of ammonium, nitrate, inorganic N, and nitrate:ammonium ratio. The forests were sampled between August 2008 and April 2009, in the following seasons: dry, dry-rainy transition, rainy, rainy-dry transition. Clear patterns of change in the N cycling along the studied chronosequence were found, including differences in green foliage 15N values, N content, N:P ratio, inorganic N and net mineralization and nitrification rates, characterized by an increase in the mean values of these variables along the chronosequence. The results found suggest that the forests in restoration process, even the 52 years old one, still do not present a N cycling characteristic of a mature forest and, therefore, the recuperation of the N cycling was not completely reached yet. However, it is possible to state that the forests in restoration process studied here are following a development trajectory characterized by a N cycling progressively more similar to what is common to a mature native forest, as the one used as the reference ecosystem in this study. Through the restoration models used for the forests studied, the N cycle processes are recovered as the forests develop, as they present a clear tendency of changing from N economy to P economy, typical to mature tropical forests.

O presente estudo teve como objetivo avaliar o efeito da inoculação com rizóbio no estabelecimento, crescimento inicial e em alguns aspectos da dinâmica de N em folhas de leguminosas (Fabaceae) arbóreas nativas plantadas por semeadura direta no campo, em área de mata ciliar no Estado de São Paulo. Foi avaliada a emergência de plântulas (% em relação ao número de sementes plantadas) até os 3 meses após a semeadura (MAS) e o crescimento em altura aos 13 MAS. O fracionamento isotópico do N foliar também foi avaliado, pelo método da abundância natural do 15N, além da concentração de N e a razão C/N foliar. Acacia polyphylla foi usada como planta-referência não-fixadora de N2. Em relação ao desenvolvimento inicial, houve grande variação entre as espécies estudadas. Acacia polyphylla e Enterolobium contortisiliquum apresentaram taxa de germinação mediana e rápido crescimento. Mimosa bimucronata e Parapiptadenia rigida tiveram crescimento relativamente rápido, porém baixo potencial germinativo. Erythrina speciosa e Poecilanthe parviflora, com crescimento lento, apresentaram germinação elevada no campo. As espécies fixadoras de N2 mostraram &#948;15 N médio de 2,7&#8240;, enquanto que a referência Acacia polyphylla se mostrou cerca de 4,0&#8240; mais enriquecida em 15N . Erythrina speciosa e Ormosia arborea apresentaram os menores valores de &#948;15N, enquanto que Parapiptadenia rigida, enriquecida em 15N, mostrou estar obtendo N exclusivamente do solo. Poecilanthe parviflora, Mimosa bimucronata e Enterolobium contortisiliquum apresentaram valores intermediários de &#948;15N, com grande variação entre os indivíduos amostrados. A concentração de N nos tecidos foliares variou entre 1,5% e 3,6%. De forma geral, esta variável não mostrou relação direta com o &#948;15N das plantas. A menor razão C/N foi observada em Enterolobium contortisiliquum e Erythrina speciosa, seguida por M. bimucronata. Nas condições experimentais locais, as plantas não responderam à inoculação com rizóbio em nenhum dos parâmetros avaliados (estabelecimento, crescimento inicial, &#948;15N, concentração de N e razão C/N). As espécies de crescimento mais lento e com alta incidência de herbivoria observados no campo (E. speciosa e O. arborea) foram também as que apresentaram &#948;15N na faixa considerada como de ocorrência da fixação biológica de nitrogênio. Os resultados indicam, para as outras espécies, maior aquisição de N proveniente do solo. / The aim of this work was to assess the effect of rhizobia inoculation on the establishment, early growth and some aspects of N dynamics in direct-seeded native legume (Fabaceae) trees in a riparian zone of São Paulo State, Southeastern Brazil. Seedling emergency (% of total planted seeds) was evaluated until three months after sowing (MAS), and the height of the trees were measured at 13 MAS. Isotopic N fractionation was also assessed by the 15N natural abundance method, as well as foliar N content and C/N ratio. Acacia polyphylla was used as a non-N2-fixing reference plant. Acacia polyphylla and Enterolobium contortisiliquum grew most rapidly, with moderate germination rate. Mimosa bimucronata and Parapiptadenia rigida were fast-growing but with low establishment rate. Erythrina speciosa and Poecilanthe parviflora showed low growing rate, but high germination potential. The N2-fixers showed an average foliar &#948;15N of 2,7&#8240;, while the reference-plant was 4,0&#8240; more <sup15>N enriched. Erythrina speciosa and Ormosia arborea showed the lowest &#948;15N values. The high &#948;15N of Parapiptadenia rigida probably indicated that it is obtaining N exclusively from the soil. Poecilanthe parviflora, Mimosa bimucronata and Enterolobium contortisiliquum showed intermediate &#948;15N values, with strong variations among the sampled individuals. Foliar N concentrations varied from 1,5% to 3,6%. In general, N concentrations were not correlated with foliar &#948;15N . The lowest C/N ratio was observed on E. contortisiliquum and E. speciosa , followed by M. bimucronata. In this field trial conditions, rhizobia inoculation did not show differences in all surveyed parameters (establishment, early growth, foliar &#948;15N, total N content and C/N ratio of trees). The slow-growing species, E. speciosa and O. arborea , that also had a high herbivore attack observed in the field, showed foliar &#948;15N in the biological N2 fixation range. The results suggested, for the other species, a larger soil N acquisition.

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