An understanding of ecosystem-level carbon (C) sequestration, or net ecosystem production (NEP), requires the separation of heterotrophic, microbial respiration (RH) from autotrophic, root-derived respiration (RA) as the components of RS (i.e., NEP = NPP - RH). However, separating these two sources in situ has been problematic since they are closely coupled. This study utilizes two similarly aged Pinus taeda L. stands, 8 and 9 years-old, aimed at quantifying these two respiration components through in-situ root severing. In order to use root-severing treatments to separate RS into RH and RA components, confirmation of carbohydrate depletion coupled to RA decline is crucial. This study evaluated the changes in CO2 flux rates and carbohydrate supply upon root severing in Pinus taeda L. using a controlled laboratory validating a two-part field study. The first field study used root-severing cores to test in-situ if respiration components can be attained based on the depletion of carbohydrate supply. The second field study was aimed at how future changes in climate might affect the ability of forests to store C and how modern forestry practices might affect changes and was conducted over the course of two installations, spring and summer 2012. In this study we examined the effects of fertilization (0 and 100.9 kg N ha-1 ) and throughfall reduction (0 and -30%) on total soil respiration (RS) as well as the heterotrophic contribution to RS, in a fully replicated (n=4), 2x2 factorial design. In the controlled lab experiment RS and RA declined by 86% and 95% respectively by the end of an 86 day trial and NSC carbohydrates declined by 60% for soluble, 29% for insoluble, and 43% for total (soluble + insoluble). The decline of RA was highly correlated to with the decline of NSC’s at 0.90, 0.69 and 0.93 for soluble, insoluble and total, respectively. The companion field study revealed a mean decrease 21±0.5% of over the final three dates when severed root respiration stabilized. In the second study, testing throughfall reduction and fertilization levels there were no fertilization by throughfall reduction interactions on the contribution of RH to RS in either the spring or summer; however, the main effect of throughfall reduction was significant in the spring. During the spring, the mean contribution of RH to RS for ambient throughfall plots was 96±6.4%, while the mean contribution under throughfall reduction was 68±1.9%. During the summer, there were no differences among treatments and the overall contribution of RH to RS was 78±1.6%. Collectively, both of these studies revealed that the severing of roots from their primary energy source and the subsequent depletion of stored NSC that the use of in-situ methods allows for the quantification of soil respiration components RA and RH. Using these estimates to model NEP in the short-term can be variable by season, however, long-term monitoring may simplify future NEP modeling scenarios / Master of Science

Due to concern over the increasing concentration of carbon dioxide in the atmosphere, forest researchers and managers are currently studying the effects of varying silvicultural and harvesting practices on the carbon dynamics of intensely managed forest ecosystems. Soil carbon dioxide efflux resulting from soil microbial activity and root respiration is one of the major components of the total carbon flux in forested ecosystems. In an effort to examine the response of soil carbon dioxide efflux to changes in soil factors, nutrient availability, temperature, and moisture, soil respiration rates were measured monthly over an entire year in a two-year-old loblolly pine (Pinus taeda L.) plantation subjected to fertilization and mulching treatments. A dynamic, closed-chamber infrared gas analysis system was used to measure efflux rates from plots treated with one of four treatment combinations including: nitrogen (115 kg/ha) and phosphorus (11.5 kg/ha) fertilization with black landscape cloth (mulch), fertilization without mulch, mulch without fertilization, and no treatment (control). For each treatment combination, plots were established at the seedling base and 1.22 m away from the seedling base to examine the effect of seedling roots on soil carbon dioxide efflux rates. Soil temperature and moisture were measured at each chamber position monthly and soil coarse fragments, soil nutrient levels, percent carbon, root biomass and coarse woody debris were measured beneath 64 chambers at the end of the study. Fertilization had no significant effect on efflux rates during any of our monthly sampling sessions despite the fact that fertilized seedlings experienced significant increases in both above and belowground biomass. Conversely, regression analysis of growing season soil carbon dioxide efflux rates revealed a slightly negative correlation with both total seedling nutrient uptake and biomass. Rates in plots with mulching were significantly higher than rates from non-mulched plots during five monthly measurement sessions, and higher rates in mulched plots during winter months was attributable to warmer soil temperatures. Rates at the seedling base were always significantly higher than rates in plots away from the seedling. Although rates were always higher at the seedling base, the variability observed was only weakly correlated with the amount of pine roots present beneath respiration chambers. Utilizing soil temperature and moisture, soil carbon, and cuvette fine root biomass in a regression model explained 54% of the variance observed in efflux rates across the yearlong study period. Soil temperature alone explained 42.2% of the variance, followed by soil carbon and soil moisture at 5.2% and 2.7% respectively. The amount of pine fine roots under measurement chambers accounted for only 2.4% of the variance. An additional 1.5% was explained by other factors such as soil phosphorus, coarse woody debris, non-pine root biomass, and soil calcium. An examination of the factors affecting the spatial patterns of soil carbon dioxide efflux revealed that total soil carbon and the amount of fine pine root biomass beneath cuvette base rings explain 38% and 11% respectively, of the observed variability in mean annual soil carbon dioxide efflux from differing plots. The most influential factor affecting soil carbon dioxide efflux during the yearlong study period was soil temperature and modeling of seasonal soil carbon dioxide efflux rates from managed forests using both soil temperature and moisture should be achievable with the establishment of data sets and statistical models covering a range of sites differing in productivity, stand age, and management intensity. The establishment of data sets and statistical models across a variety of forest sites should account for the changing influence of soil carbon levels, aboveground biomass, microbial activity, organic matter inputs, and root biomass on soil carbon dioxide efflux. / Master of Science

A considerable part of the global pool of terrestrial carbon is stored in high latitude soils. In these soils, repeated cycles of freezing and thawing creates soil motion (cryoturbation) that in combination with other cryogenic disturbance processes may play a profound role in controlling the carbon balance of the arctic soil. Conditions for cryogenic soil processes are predicted to dramatically change in response to the ongoing climate warming, but little is known how these changes may affect the ability of arctic soils to accumulate carbon. In this thesis, I utilize a patterned ground system, referred to as non-sorted circles, as experimental units and quantify how cryogenic soil processes affect plant communities and carbon fluxes in arctic soils. I show that the cryoturbation has been an important mechanism for transporting carbon downwards in the studied soil over the last millennia. Interestingly, burial of organic material by cryoturbation appears to have mainly occurred during bioclimatic events occurring around A.D. 900-1250 and A.D. 1650-1950 as indicated by inferred 14C ages. Using a novel photogrammetric approach, I estimate that about 0.2-0.8 % of the carbon pool is annually subjected to a net downward transport induced by the physical motion of soil. Even though this flux seems small, it suggests that cryoturbation is an important transporter of carbon over centennial and millennial timescales and contributes to translocate organic matter to deeper soil layers where respiration proceeds at slow rates. Cryogenic processes not only affect the trajectories of the soil carbon, but also generate plant community changes in both species composition and abundance, as indicated by a conducted plant survey on non-sorted circles subjected to variable differential frost heave during the winter. Here, disturbance-tolerant plant species, such as Carex capillaris and Tofieldia pusilla, seem to be favoured by disturbance generated by the differential heave. Comparison with findings from a previous plant survey on the site conducted in the 1980s suggest that the warmer temperatures during the last decades have resulted in decreased differential heave in the studied non-sorted circles. I argue that this change in cryogenic activity has increased abundance of plants present in the 1980s. The fact that the activity and function of the non-sorted circles in Abisko are undergoing changes is further supported by their contemporary carbon dioxide (CO2) fluxes. Here, my measurements of CO2 fluxes suggest that all studied non-sorted circles act as net CO2 sources and thus that the carbon balance of the soils are in a transition state. My results highlight the complex but important relationship between cryogenic soil processes and the carbon balance of arctic soils.

Submitted by Jadson Francisco de Jesus SILVA (jadson@uefs.br) on 2018-03-02T22:16:44Z No. of bitstreams: 1 clovesvilasboas_disserta??o_mestrado_2017.pdf: 2913418 bytes, checksum: bb279ced1f535288e5f541d897d80a7d (MD5) / Made available in DSpace on 2018-03-02T22:16:44Z (GMT). No. of bitstreams: 1 clovesvilasboas_disserta??o_mestrado_2017.pdf: 2913418 bytes, checksum: bb279ced1f535288e5f541d897d80a7d (MD5) Previous issue date: 2017-02-17 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior - CAPES / There is a great need for the development of more efficient systems for monitoring the dynamics of atmospheric carbon so that there is a better understanding of the interactions between the biosphere and the atmosphere. However, it is critical that these technologies have high coverage and low cost. In the vegetation, the process of biomass generation through photosynthesis is a determining factor in the way a vegetated area will appear radiometrically in the satellite images, therefore, the remote sensing becomes an alternative to the monitoring of this dynamics, having a High coverage and have a low cost. The objective of this work is to analyze the dynamics of CO2 fluxes in the Caatinga Biome by means of multispectral remote sensing, verifying the potential of multispectral images in the detection of CO2 fluxes in areas of preserved Caatinga and in a regenerated state. The study was carried out in areas of Caatinga in the municipality of Petrolina-PE and Araripina-PE, areas that are monitored by micrometeorological stations. The methodology adopted was based on the modeling of the Carbon Forest Sequestration Index (CO2flux) that measures the efficiency of the carbon sequestration process by vegetation, and proposes the integration of the Normalized Difference Vegetation Index (NDVI) with the Index of Photochemical Reflectance (PRI). For the database, 22 OLI (Landsat-8) multispectral scenes were used together with field-measured meteorological data to verify the relationship between the variables analyzed. The results showed that the vegetation of the Caatinga has seasonal variations in CO2 flows in regions with different vegetation types. The CO2flux index can be applied to determine the CO2 fluxes, presenting better adjustments when the CO2 data are compared using the 1 pixel reading in the image, showing to be more efficient in relation to the analysis with the footprint in. However, based on spectral models of vegetation, it was possible to determine the dynamics of CO2 flows in areas of preserved Caatinga and in regeneration state using data extracted from multispectral sensors. / H? uma grande necessidade no desenvolvimento de sistemas mais eficazes para o monitoramento da din?mica do carbono atmosf?rico, para que haja uma melhor compreens?o das intera??es entre a biosfera e a atmosfera. No entanto, ? fundamental que essas tecnologias possuam alta cobertura e um baixo custo. Na vegeta??o, o processo de gera??o de biomassa por meio da fotoss?ntese ? um fator determinante na forma como uma ?rea vegetada ir? aparecer radiometricamente nas imagens de sat?lite, portanto, o sensoriamento remoto vem a ser uma alternativa para o monitoramento dessa din?mica, por ter uma alta cobertura e possuir um baixo custo. O objetivo deste trabalho ? analisar, por meio de sensoriamento remoto multiespectral, a din?mica dos fluxos de CO2 no Bioma Caatinga, verificando o potencial das imagens multiespectrais na detec??o dos fluxos de CO2 em ?reas de Caatinga preservada e em estado de regenera??o. O estudo foi desenvolvido em ?reas de Caatinga no munic?pio de Petrolina-PE e Araripina-PE, ?reas monitoradas por esta??es micrometeorol?gicas. A metodologia adotada foi a partir da modelagem do ?ndice de Sequestro Florestal de Carbono (CO2flux) que mede a efici?ncia do processo de sequestro de carbono pela vegeta??o, e que prop?e a integra??o do ?ndice de Vegeta??o por Diferen?a Normalizada (NDVI) com o ?ndice de Reflect?ncia Fotoqu?mica (PRI). Foram utilizadas, para a base de dados, 22 cenas multiespectrais do sensor OLI (Landsat-8) juntamente com dados meteorol?gicos medidos em campo, a fim de verificar a rela??o entre as vari?veis analisadas. Os resultados mostraram que a vegeta??o da Caatinga tem varia??es sazonais nos fluxos de CO2 nas regi?es com diferentes tipos de vegeta??o. O ?ndice CO2flux pode ser aplicado para a determina??o os fluxos de CO2, apresentando melhores ajustes quando os dados de CO2 s?o comparados utilizando a leitura de 1 pixel na imagem, mostrando ser mais eficiente em rela??o a analise com as ?reas de influ?ncia (footprint) em rela??o aos pontos amostrais, no entanto, os dados de footprint apresentaram tamb?m correla??es significativas. Portanto, baseado nos modelos espectrais de vegeta??o foi poss?vel determinar a din?mica dos fluxos de CO2 em ?reas de Caatinga preservada e em estado de regenera??o utilizando dados extra?dos de sensores multiespectrais.

Submitted by Cássia Santos (cassia.bcufg@gmail.com) on 2015-11-26T11:30:41Z No. of bitstreams: 2 Dissertação - Arielle Elias Arantes -2015.pdf: 3887062 bytes, checksum: 7a0abb81f47daa3a2cac3f3c03fe9694 (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) / Approved for entry into archive by Luciana Ferreira (lucgeral@gmail.com) on 2015-11-27T07:10:12Z (GMT) No. of bitstreams: 2 Dissertação - Arielle Elias Arantes -2015.pdf: 3887062 bytes, checksum: 7a0abb81f47daa3a2cac3f3c03fe9694 (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) / Made available in DSpace on 2015-11-27T07:10:12Z (GMT). No. of bitstreams: 2 Dissertação - Arielle Elias Arantes -2015.pdf: 3887062 bytes, checksum: 7a0abb81f47daa3a2cac3f3c03fe9694 (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) Previous issue date: 2015-02-02 / Conselho Nacional de Pesquisa e Desenvolvimento Científico e Tecnológico - CNPq / The Brazilian savanna (known as Cerrado) is an upland biome made up of various physiognomies, from herbaceous to arboreal. In this work, vegetation greenness (EVI), precipitation (PPT), and evapotranspiration (ET) data for the 2000 to 2012 period were analyzed in order to understand the phenology of the major Cerrado natural and anthropic landscapes, as well as its relation to precipitation, net primary productivity (NPP), biomass, and evapotranspiration fluxes, according to different land use scenarios. Along the 13 years under analysis, for all vegetation types the mean wet season duration varied from eight to nine months, while the growing season lasted seven to eight months. The mean start of the wet and growing seasons were very close for all land cover types during these 13 years, with the start of the wet season in August or September (with the exception of 2002 and 2007), and the start of the growing season in September or October. The mean end of the wet and growing seasons occurred in April or May and in May or June, respectively. The start, the end, and the duration of the wet and the growing seasons varied according to rainfall volume and distribution, which are affected by the El Niño and La Niña phenomena. For example, during El Niño years, the start of the wet season occurred earlier in August, and the duration of the wet season was longer than eight months. If followed by a La Niña, the start of the growing season of the vegetation occurred later in October and the duration of the season was shorter than eight months. In addition to the rainfall variability (El Niño and La Niña) and the type of vegetation (natural or anthropic grasslands), the start, the end, and the duration of the wet and growing seasons are also affected by the spatial variation (latitude and longitude). Considering the spatial variation of the start of the wet and growing seasons, the beginning was earlier in the southern portion of the Cerrado biome, in August-September and September-October, and later in the north, in November-December and October-December, respectively. The end of the season showed the same behavior, i.e. an earlier end (March-April) in the southern portion for the wet and growing seasons, while much later in the northern portion (June-July). Regarding the seasonal and phenological behavior of the different vegetation types, the green-up for all Cerrado physiognomies started in mid-September to the end of October, at the onset of the rainy season, reaching peak values from December through January, and a gradual senescence, starting as early as March or April. The total Cerrado growing season biomass for 2002 was 28 gigatons of carbon and the evapotranspiration was 1336 gigatons of water. A sample-based response associated with the area occupied by each vegetation type showed that pasture and cropland had 52% and 22% less NPP and ET than natural landscapes. The mean growing season evapotranspiration and biomass for 2002 was 576 Gt of water and 12 Gt of carbon for pasture and croplands compared to 760 Gt of water and 16 Gt of carbon for the Cerrado natural vegetation. Considering a modeled future scenario (year 2050), the ET flux from natural Cerrado vegetation was 394 Gt less than in 2002 and 991 Gt less than in a potential scenario, characterized with only natural vegetation, while the carbon was eight Gt less than in 2002 and 21 Gt less than in a pre-conversion Cerrado (potential scenario). In 2050, the sum of the pasture and cropland ET fluxes increased by 405 Gt, relative to 2002, and the carbon by 11 Gt. At last, the different land use scenarios showed that the deforestation impact until now in the fluxes of evapotranspiration and in the biomass were greater than the future scenario. / A savana Brasileira (conhecida como o Cerrado) é um bioma de terras altas, formado por várias fitofisionomias de vegetação herbácea à arbórea. Neste trabalho, dados de vigor vegetativos (EVI), precipitação (PPT) e evapotranspiração (ET), para o período de 2000 a 2012, foram analisados com o intuito de compreender a fenologia das principais paisagens naturais e antrópicas do Cerrado, assim como a sua relação com a precipitação e a produtividade primária líquida (NPP), a biomassa e os fluxos de evapotranspiração, de acordo com diferentes cenários de cobertura e uso da terra. Nos 13 anos de análises, para todos os tipos de vegetação, a média da duração da estação chuvosa variou de oito a nove meses, enquanto a estação de crescimento durou de sete a oito meses. As médias do início da estação chuvosa e da estação de crescimento da vegetação foram próximas para os principais tipos de vegetação do cerrado, com o início da estação chuvosa em Agosto ou Setembro (com exceção dos anos de 2002 e 2007) e da estação de crescimento em Setembro ou Outubro. As médias do fim da estação chuvosa e da estação de crescimento ocorreram em Abril ou Maio e em Maio ou Junho, respectivamente. O início, o fim e a duração da estação chuvosa e da estação de crescimento da vegetação variaram conforme a quantidade e a distribuição da precipitação, a qual é afetada pelo fenômeno do El Niño e da La Niña. Por exemplo, durante anos de El Niño, o início da estação chuvosa ocorreu precocemente (em Agosto) e a sua duração foi superior a oito meses. Quando seguido por um La Niña, o início da estação de crescimento ocorreu mais tarde e a duração da estação foi inferior a oito meses. Além da variabilidade das chuvas (El Niño e La Niña) e do tipo de vegetação (gramíneas naturais ou antrópicas), o início, o fim e a duração da estação chuvosa e de crescimento da vegetação também são afetados pela variação espacial (latitude e longitude). Considerando a variação espacial do início da estação chuvosa e da estação de crescimento da vegetação, o começo foi mais cedo na porção sul do bioma Cerrado, em Agosto-Setembro e Setembro-Outubro, e mais tarde no norte, em Novembro-Dezembro e Outubro-Dezembro, respectivamente. O fim da estação teve o mesmo comportamento, com um fim mais cedo na porção sul, em Março-Abril, e mais tarde na porção norte, em Junho-Julho. Com relação ao comportamento sazonal e fenológico dos diferentes tipos de vegetação, a rebrota de todas as fitofisionomias de Cerrado começou em meados de Setembro até o fim em Outubro, com o início da estação chuvosa, atingindo os valores máximos em Dezembro a Janeiro; a senescência foi gradual, começando cedo (Março ou Abril). A biomassa total do Cerrado durante a estação de crescimento para 2002 foi 28 gigatoneladas (Gt) de carbono e a evapotranspiração foi 1336 Gt de água. A resposta baseada nas amostras associada com a área ocupada por cada tipo de vegetação mostra que a pastagem e a agricultura tiveram 52% e 22% menos NPP e ET do que as paisagens naturais. A média da evapotranspiração e da biomassa da estação de crescimento para 2002 foi de 576 Gt de água e de 12 Gt de carbono para a pastagem e agricultura, e de 760 Gt de água e a 16 Gt de carbono para a vegetação natural de Cerrado. Considerando o cenário futuro modelado para o ano de 2050, o fluxo de ET da vegetação natural foi 394 Gt menor do que em 2002 e 991 Gt menor do que no cenário potencial, caracterizado com apenas vegetação natural, enquanto o carbono foi 8 Gt menor do que 2002 e 21 Gt menor do que no cenário de pré-conversão do Cerrado (cenário potencial). Em 2050, a soma dos fluxos de ET da pastagem e agricultura aumentou em 405 Gt, relativamente a 2002, e o carbono em 11 Gt. Em fim, os diferentes cenários de uso da terra mostraram que o impacto do desmatamento até os dias de hoje nos fluxos de evapotranspiração e na biomassa foram maiores do que o cenário futuro.

Doctor of Philosophy / Department of Agronomy / Clenton E. Owensby / Most rangelands are harvested at some point during the year and removal of plant leaf area and biomass alters a host of ecosystem processes including gas exchange. An experiment was conducted in 2005 and 2006 to study the effects of clipping tallgrass prairie at different dates on water vapor and CO2 fluxes. A portable, non-steady-state chamber was designed to measure CO2 and water vapor fluxes from small plots in less than 40 s. A combination of sunlit and shaded readings allowed measurements of net carbon exchange (NCE) and ecosystem respiration (RE); by summing NCE and RE, gross canopy photosynthesis (GCP) was calculated. Throughout the two-year study, the chamber had a minimal effect on microclimate, i.e., average chamber temperature increased 2.9° C, while chamber pressure increased only 0.3 Pa during measurements, and photosynthetically active radiation attenuation was 10%. The immediate effect of all clipping treatments was a loss of leaf area that led to reductions in GCP, NCE, and RE and in most cases decreased water vapor flux. Further patterns of carbon flux were governed by the amount of water stress during canopy development, while water vapor flux rates varied with water availability. Canopies that developed during periods of low water stress quickly increased carbon flux rates following precipitation after a mid-season drought. However, flux rates of canopies, which developed during the mid-season drought, responded considerably slower to subsequent water availability. A separate experiment was conducted from June-October of 2006 to estimate GCP, leaf area index (LAI), and total aboveground biomass with a hyperspectral radiometer. Indices such as the Normalized Difference Vegetation Index and the Simple Ratio were used to estimate LAI and biomass had poor correlations with measured values. However, GCP was significantly correlated to all six indices derived in this study. While GCP measured from June-October was significantly correlated with all indices, removal of the senesced canopy scans recorded during October greatly increased the relationship.

La zone côtière représente seulement une petite partie de la surface océanique, mais elle joue un rôle important dans le cycle du carbone. Pour contribuer à préciser ce rôle, l’objectif de cette thèse était d’étudier les flux de carbone, en immersion et en émersion, des communautés benthiques intertidales vivant sur les estrans rocheux. La respiration et la calcification des principales espèces macrozoobenthiques ont été mesurées en laboratoire pour d’estimer les différentes adaptations métaboliques liées à une vie en milieu intertidal. En complément, les flux globaux de carbone des communautés ont été quantifiés aux interfaces air-sédiment et eau-sédiment grâce à des mesures in situ. D’une manière générale, la respiration de la communauté prévaut sur la production primaire, en conséquence les estrans rocheux semi-battus peuvent être considérés comme hétérotrophes. Grâce aux mesures de respiration en laboratoire et aux comptages d’espèces effectués sur les côtes rocheuses bretonnes, nous avons pu estimer la contribution du macrozoobenthos aux flux de carbone à une échelle régionale. La comparaison entre les résultats in situ et les études en laboratoire à permis de valider nos méthodes. / Coastal zone represents only a small part of ocean surface, but play a major role in carbon cycling. Ro help clarify this role; this thesis aimed to study carbon fluxes of intertidal benthic rocky shore communities during immersion and emersion. Respiration and calcification measurements of principal macrozoobenthic species were performed in the laboratory in order to evaluate their different metabolic adaptation to intertidal conditions. In addition, community carbon fluxes were quantified in situ at the air-sediment and water-sediment interfaces. Community respiration generally prevails on primary production indicating that semi-exposed intertidal rocky shore communities could be considered as heterotrophic. The contribution of macrozoobenthic organisms to carbon fluxes was estimated on a regional scale through respiration and calcification rates measured in the laboratory and species abundances recorded in Brittany (France) exposed rocky shores. The comparison between in situ and laboratory studies allowed our methods validation.

Soil organic carbon (SOC) concentrations and greenhouse gas (GHG) emissions are not uniform across the landscape, but assemble in "hotspots" in specific areas. These differences are mainly driven by human-induced activities such as agricultural management. 40-50% of the Earth's land surface is under agricultural land-use, for instance cropland, managed grassland and permanent crops including agro-forestry and bio-energy crops. Furthermore, 62% of the global soil C stock is SOC and the soil stores more than 3 times more C than the atmosphere. Thus, C sequestration in agricultural soil has a potentially important role in increasing SOC storage and GHG mitigation, and there is considerable interest in understanding the effects of agricultural management on SOC and GHG fluxes in both grasslands and croplands, in order to better assess the uncertainty and vulnerability of terrestrial SOC reservoirs. For the sake of discovering the agricultural management practices relating to the effective and sustainable C sequestration in agricultural lands in Europe, simulating future terrestrial C stocks and GHG budgets under varied agricultural management systems in major European ecosystems is essential. Using models is a useful method with the purpose of this and abundant studies have carried out. However, many model results have not been validated with reliable observed long-term data, while other studies have reported a strong impact of model initialisation on model result. Nevertheless, predictions of annual to decadal variability in the European terrestrial C and GHG ressources largely rely on model results. Consequently, finding the most appropriate and comprehensive model initialisation method for obtaining reliable model simulations became important, especially for process-based ecosystem models. In recent years, Zimmermann et al. (2007) have succeed in initialising the Rothamsted Carbon model (RothC) using a physical and chemical soil fractionation method. For that reason, we hypothesised that measured detailed SOC data would be useful to initialise ecosystem models, and this hypothesis should be tested for different process-based models and agricultural land-use and management. (...)

La concentration de Carbone organique de sol (COS) et les émissions de gaz à effet de serre (GES) ne sont pas uniformes à travers l’espace, mais se regroupent en “hotspots” dans des endroits spécifiques. Ces différences s’expliquent principalement par les activités anthropiques telles que la gestion agricole. 40-50% de la surface de la Terre est utilisé par l’agriculture, par exemple les terres cultivées, les prairies gérées et cultures permanentes, y compris l’agro-foresterie et de bio-cultures énergétiques. En outre, 62% du carbone globale est COS, et le sol conserve plus que 3 fois plus de C que l’atmosphère. Ainsi, la séquestration du carbone dans les sols agricoles joue un rôle potentiellement important dans l’augmentation de stockage de COS et l’atténuation des GES, et il y a un intérêt considérable pour comprendre les effets de la gestion agricole sur le COS et les flux de GES aux prairies et terres cultivées, afin de mieux évaluer l’incertitude et la vulnérabilité des réservoirs de COS. Afin de découvrir les pratiques de gestion agricole qui contribuent à la séquestration efficace et durable du carbone aux terres agricoles en Europe, il est essentiel de simuler les stocks futurs de carbone terrestriel et les budgets de GES par rapport aux systèmes de gestion agricole variés sur les grands écosystèmes européens. Dans ce contexte, la modélisation est une méthode utile, et la modélisation a déjà été utilisée dans beaucoup d’études. Cependant beaucoup de résultats de la modélisation n’ont pas encore été validés avec les données mesurées sur l’horizon long-terme, et d’ailleurs d’autres études ont constaté un fort impact de l’initialisation du modèle sur le résultat du modèle. Néanmoins, la variabilité des prévisions annuelles et décennales concernant le C et le GES en Europe dépendent des résultats du modèle. Par conséquence, il est important de trouver la meilleure méthode d’initialisation des modèles pour obtenir des résultats des modèles fiables, notamment pour les modèles d’écosystèmes dits “process-based”. Au cours des dernières années, Zimmermann et al. (2007) a réussit à initialiser le modèle de Rothamsted carbone (RothC) en utilisant une méthode (physique et chimique) de fractionation des sols. Pour cette raison, j’ai fait l’hypothèse que les données COS détaillées seraient utiles pour initialiser des modèles d’écosystème, et que cette hypothèse doit être testée avec les modèles différents par rapport aux gestions agricoles différentes. Les buts de cette thèse sont les suivants: i) évaluation des influences des gestions agricoles sur le stockage de COS, en utilisant des approches expérimentales et des approches de modélisation; et ii) déterminer la meilleur méthode d’initialisation des modèles. (...) / Soil organic carbon (SOC) concentrations and greenhouse gas (GHG) emissions are not uniform across the landscape, but assemble in “hotspots” in specific areas. These differences are mainly driven by human-induced activities such as agricultural management. 40-50% of the Earth’s land surface is under agricultural land-use, for instance cropland, managed grassland and permanent crops including agro-forestry and bio-energy crops. Furthermore, 62% of the global soil C stock is SOC and the soil stores more than 3 times more C than the atmosphere. Thus, C sequestration in agricultural soil has a potentially important role in increasing SOC storage and GHG mitigation, and there is considerable interest in understanding the effects of agricultural management on SOC and GHG fluxes in both grasslands and croplands, in order to better assess the uncertainty and vulnerability of terrestrial SOC reservoirs. For the sake of discovering the agricultural management practices relating to the effective and sustainable C sequestration in agricultural lands in Europe, simulating future terrestrial C stocks and GHG budgets under varied agricultural management systems in major European ecosystems is essential. Using models is a useful method with the purpose of this and abundant studies have carried out. However, many model results have not been validated with reliable observed long-term data, while other studies have reported a strong impact of model initialisation on model result. Nevertheless, predictions of annual to decadal variability in the European terrestrial C and GHG ressources largely rely on model results. Consequently, finding the most appropriate and comprehensive model initialisation method for obtaining reliable model simulations became important, especially for process-based ecosystem models. In recent years, Zimmermann et al. (2007) have succeed in initialising the Rothamsted Carbon model (RothC) using a physical and chemical soil fractionation method. For that reason, we hypothesised that measured detailed SOC data would be useful to initialise ecosystem models, and this hypothesis should be tested for different process-based models and agricultural land-use and management. (...)

Previous research has shown a close relationship between the terrestrial and aquatic carbon (C) cycles, namely that part of the C fixed via terrestrial net primary production (NPP) is exported to inland waters. In turn, it has been demonstrated that once in the freshwater system C can not only be transported laterally as dissolved organic carbon (DOC), particulate organic carbon (POC) and dissolved inorganic carbon (DIC) but is also mineralized and evaded back to the atmosphere as CO2, or buried in sediments. A number of hotspot areas of aquatic CO2 evasion have been identified but there are considerable gaps in our knowledge, particularly associated with understanding and accounting for the temporal and spatial variation of aquatic C fluxes at regional to global scales, which we know from local scale studies, to be substantial. In this thesis, three important regional hotspots of LOAC activity were identified, where significant gaps in our understanding remain.For the boreal region, an empirical model is developed to produce the first high resolution maps of boreal lake pCO2 and CO2 evasion, providing a new estimate for total evasion from boreal lakes of 189 (74–347) Tg C yr-1, which is more than double the previous best estimate. The model is also used along with future projections of terrestrial NPP and precipitation, to predict future lake CO2 evasion under future climate change and land-use scenarios, and it is found that even under the most conservative scenario CO2 evasion from boreal lakes may increase 38% by 2100. For the Amazon Basin, the ORCHILEAK land surface model driven by a newly developed wetland forcing file, is used to show that the export of C to and CO2 evasion from inland waters is highly interannually variable; greatest during wet years and lowest during droughts. However, at the same time overall net ecosystem productivity (NEP) and C sequestration is highest during wet years, partly due to reduced decomposition rates in water-logged floodplain soils. Furthermore, it is shown that aquatic C fluxes display greater variation than terrestrial C fluxes, and that this variation significantly dampens the interannual variability in NEP of the Amazon basin by moderating terrestrial variation. Finally, ORCHILEAK is applied to the Congo Basin to investigate the evolution of the integrated aquatic and terrestrial C fluxes from 1861 to the present day, and in turn to 2099 under a future climate and land-use scenario. It is shown that terrestrial and aquatic fluxes increase substantially over time, both over the historical period and into the future, and that these increases are largely driven by atmospheric CO2. The proportion of terrestrial NPP lost to the LOAC also rises from 3% in 1861 to 5% in 2099 and this trend is driven not only by atmospheric CO2 but also by climate change. This is in contrast to the boreal region where the proportion of NPP exported to inland waters is predicted to remain relatively constant, and to the Amazon, where a decrease has been predicted, due to differences in projected climate change. / L’état de l’art dans le domaine a montré qu’il y avait un lien étroit entre les cycles du carbone terrestre et aquatique :en effet, une partie du carbone fixé par photosynthèse (productivité primaire brute) est transférée vers les milieux aquatiques continentaux pour être ensuite transporté latéralement sous forme de carbone organique dissous (COD), de carbone organique particulaire (COP), de carbone inorganique dissous (CID). Durant ce transfert latéral, le carbone peut être minéralisé puis réémis vers l’atmosphère sous forme de CO2 ou enfoui dans les sédiments. Cependant, nous sommes encore loin de bien comprendre et surtout de quantifier les variations temporelles et spatiales des flux de carbones à l’échelle régionale et globale, même si les études faites à l’échelle locale nous montrent qu’elles sont importantes. Au cours de cette thèse, nous nous sommes focalisés sur 3 grandes régions pour lesquelles la connaissance des flux de carbone le long du continuum aquatique reliant les écosystèmes terrestres aux océans étaient encore très parcellaire.Pour la région boréale, un modèle empirique a été développé afin de produire les premières cartes à haute résolution de pCO2 et d’émission de CO2 pour les lacs boréaux. Les résultats du modèle nous ont permis de contraindre les émissions totales de CO2 pour les lacs boréaux à 189 (74-347) Tg C an-1, soit plus du double des estimations précédentes. Ce modèle a ensuite été couplé aux projections de production primaire brute terrestre et de précipitations afin de prédire les émissions de CO2 pour ces lacs pour différents scénarios de changement climatique et d’occupation des sols. Les résultats montrent que même en prenant le scénario le plus conservatif, les émissions de CO2 des lacs boréaux augmenteraient de 38% d’ici 2100.Pour le bassin de l’Amazone, le modèle d’écosystème terrestre ORCHILEAK, paramétré par de nouvelles donnés de forçage des zones humides, a été utilisé pour démontrer que l’export de carbone terrestre vers les réseaux fluviaux ainsi que les émissions de CO2 ont une très grande variabilité interannuelle :émissions élevées lors des années à forte précipitation et basses lors des années sèches. Cependant, la productivité nette de l’écosystème (PNE) Amazone et la fixation nette de carbone à l’échelle du bassin sont plus élevées lors des années humides, en partie dû au taux de décomposition de carbone organique réduit lorsque les sols sont saturés en eau. De plus, les résultats montrent que les flux de carbone des systèmes aquatiques ont une plus grande variabilité que les flux terrestres, ce qui atténue considérablement la variabilité interannuelle de la PNE du bassin de l'Amazone.Pour finir, nous avons appliqué ORCHILEAK au bassin du Congo afin d’étudier l’évolution intégrée des flux de carbone terrestres et aquatiques de 1861 à nos jours, ainsi que de projeter leur devenir au cours du 21eme siècle selon les scénarios de changement climatiques et de changement d’occupation des sols. Nous avons montré que les flux terrestres et aquatiques augmentent de façon significative durant la période historique et dans le futur, cette augmentation étant largement induite par l’augmentation du CO2 atmosphérique et, dans une moindre mesure, par le changement climatique. En particulier, la proportion de la productivité primaire brute terrestre exportée vers le continuum aquatique passe de 3% en 1861 à 5% en 2099. Ce résultat contraste avec ceux obtenu pour la région boréale où cette proportion reste relativement constante et pour l’Amazone où c’est une baisse qui est en fait prédite. Ces différences s’expliquent par des trajectoires de changement climatique distinctes pour ces 3 régions. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished