The solar radiation in general and UV radiation in particular has been recognized to stimulate plant litter decomposition through photochemical mineralization of photosensitive organic molecules, such as lignin, facilitating microbial decomposition, with great relevance role in dryland ecosystems where microbial activity is low, however little is known about how photodegradation could influence other ecosystems without moisture limitations and under what conditions may be favored, therefore the mechanisms has not yet been established. Decomposition in tropical ecosystem is a complex process and can be induced by a number of environmental factors with certain differences when compared to arid and semi-arid ecosystems. To assess the mechanisms underlying UV-B photodegradation, we designed a 300 days field experiment at a tropical ecosystem with high levels of annual precipitation and exposure litter to three levels of radiation combined with a biocide treatment. Results show that the removal of UV-B radiation decelerated plant litter decomposition during the later stage compared to litter exposure to full sun, however shaded litter had similar mass loss compared to litter exposed to full sun. Furthermore, differences in the decay constant among radiation treatments due to UV-B effect is independent of lignin loss. Overall, our study suggest that UV-B contributes to the plant litter decomposition through carbon losses, however, had no effect on nitrogen, neither lignin nor cellulose loss. However, more studies are needed in order to investigate the positive and negative effects of UV exposure on microbial activity in tropical ecosystems. / A radiação solar em geral e a radiação ultravioleta (UV) em particular têm sido reconhecida por estimular a decomposição da serapilheira através da mineralização fotoquímica de moléculas fotossensíveis, como a lignina, facilitando a decomposição microbiana, com um papel de grande relevância em ecossistemas áridos onde a atividade microbiana é baixa, no entanto pouco se sabe como a fotodegradação pode influenciar outros ecossistemas como por exemplo os mais úmidos e sob quais condições a fotodegradação é favorecida, portanto os mecanismos ainda não foram estabelecidos. Decomposição em ecossistemas tropicais é um processo complexo e pode ser influenciado por vários fatores ambientais e com certas diferenças quando comparada com ecossistemas áridos e semiáridos. Para avaliar os mecanismos subjacentes à fotodegradação via radiação UV-B, um experimento de campo de 300 dias foi estabelecido em um ecossistema tropical com alto índice de precipitação anual onde a serapilheira foi exposta a três níveis de radiações diferentes, combinada com um tratamento com biocida. Resultados mostram que a remoção da radiação UV-B desacelerou a decomposição da serapilheira durante o último estágio do experimento comparado com a serapilheira exposta a radiação ambiente, no entanto a serapilheira quando sombreada teve perda de massa similar à exposta a radiação ambiente. Além disso, diferenças na taxa de decaimento entre os tratamentos de radiação devido ao efeito da radiação UV-B foram independentes da perda de lignina. No geral, nosso estudo sugere que a radiação UV-B contribui com a decomposição da serapilheira através da perda de carbono, no entanto não teve efeito na perda de massa de nitrogênio, lignina e celulose. Portanto, mais estudos são necessários para investigar o efeito positivo e negativo da exposição à radiação UV-B na atividade microbiana e na decomposição da serapilheira em ecossistemas tropicais.

A diversidade da Mata Atlântica está constantemente ameaçada devido à perda de habitats provocada pela destruição e alteração dos ambientes naturais. Este fato é muito preocupante, pois os remanescentes florestais da Mata Atlântica encontram-se, em sua maior parte, em pequenos fragmentos altamente perturbados, acarretando em perda de biodiversidade da fauna existente. Muitos estudos demonstraram que mamíferos e aves especialistas (e. g. insetívoros) são muito sensíveis à fragmentação ambiental, tendendo a desaparecer em áreas abertas. A perda destes animais pode alterar a densidade de artrópodes e as taxas de herbivoria, ocasionando um efeito cascata (top- down), que causará influência até na decomposição e ciclagem de nutrientes. Entretanto o estudo de como esse efeito top-down ocorre em diferentes coberturas de solo ainda não foi testado, desta forma, objetivou-se investigar como as taxas de decomposição são modificadas pela exclusão experimental de vertebrados em áreas com diferentes coberturas vegetais na mata Atlântica. Parcelas de exclusão de vertebrados e parcelas controle foram alocadas em áreas com coberturas vegetais de pastagem e floresta, para verificar como ocorre o efeito top-down na decomposição. Utilizaram-se três tipos de serapilheira diferentes, uma gramínea (Brachiaria decumbens), uma espécie pioneira (Tibouchina sellowiana) e uma mistura de folhas de diferentes espécies da floresta primária adjacente. A técnica dos litter bags foi utilizada para avaliar as diferentes taxas de decomposição e foram coletados em intervalos de 16, 36, 71, 181 e 247 dias. O resíduo vegetal foi limpo, seco e pesado para obtenção das massas remanescentes (%) e taxa de decomposição, após isso o material foi triturado e pesado em subamostras para analises de nutrientes e compostos orgânicos (nitrogênio, carbono, fósforo, lignina, celulose e polifenóis). A taxa de decomposição não diferiu entre os tratamentos controle e exclusão de vertebrados, desta forma, não foi possível observar o efeito top-down da exclusão de vertebrados neste estudo, provavelmente devido à elevada biodiversidade da fauna do solo na área onde o experimento foi realizado e sugere-se repeti-lo em uma área menor e mais desconectada, com a finalidade de simular como o efeito cascata ocorre em pequenos fragmentos da mata Atlântica. Além disso, a decomposição foi mais rápida nos litter bags localizados na floresta do que na pastagem e as folhas de gramínea tiveram maior perda de massa do que as folhas de floresta primária e T. sellowiana, possivelmente devido às interações entre nitrogênio, lignina e a relação C:N dos resíduos vegetais. / The diversity of Atlantic forest is constantly threatened due to habitat loss caused by the destruction and alteration of natural environments, and most of the biome is now in small and fragmented areas. This fact is of a great concern, because the remaining areas in Atlantic forest are in small highly disturbed fragments, resulting in loss of biodiversity of the existing fauna, like mammals and birds. Many studies have shown that specialist mammals and birds (e.g. insectivores) are very sensitive to environment fragmentation and tend to disappear in open areas. The loss of these animals is likely to cause an increase in the density of arthropods and rates of herbivory, causing a top-down effect that may even influence the cycling of nutrients. We aimed to investigate how decomposition rates are modified by the experimental exclusion of vertebrates in an area with different land coverage in the Atlantic forest. Vertebrate exclusion plots and control plots were allocated in areas with different land coverage (pasture and forest) to evaluate the top-down effects in decomposition. We used three different kinds of leaves, being one grass (Brachiaria decumbens), one very common primary specie of the area (Tibouchina sellowiana) and a mix of primary forest leaves. Litterbags were used to evaluate the decomposition rate and it was collected during intervals of 16, 36, 71, 181 and 247 days. The vegetal residue was cleaned, dried and weighted to obtain the remaining mass (%) and decomposition rate, after that, the material was milled and weighted in subsamples for analyses of nutritional quality (N, C, P, lignin, cellulose and polyphenols). The decomposition rate was not different for the control and vertebrate exclusion plots, as a result, it was not possible to show the top-down effect in decomposition, possibly because of the high soil biodiversity in the area where the experiment was done. It would be very important to remake this experiment in a smaller and more disconnected area, in order to show how this cascade effect occur in Atlantic forest´s smaller fragments. Furthermore, the decomposition was faster at the forest plots than at the pastureland plots and the grass leaf litter presented the higher decomposition rate than the primary forest leaves and T. sellowiana and it was possibly caused by the interaction between nitrogen, lignin and C:N ratio.

A matéria orgânica particulada (MOP) é a principal fonte de energia em ecossistemas lóticos, podendo ser gerada dentro ou fora do ecossistema. Os processos de produção e decomposição são particularmente importantes para a ciclagem dos nutrientes no ecossistema, afetando as relações entre biodiversidade e suas funções e propriedades. No Brasil existem poucas informações sobre a dinâmica deste material orgânico nos sistemas aquáticos, apesar das grandes dimensões das bacias hidrográficas brasileiras, e da associação destas com a vegetação ripária. Sendo assim, o presente trabalho tem como objetivo a avaliação dos efeitos dos fatores reguladores da decomposição de matéria orgânica alóctone em sistemas aquáticos, bem como os padrões de retenção, acúmulo de detritos e ciclagem de nutrientes em três rios localizados em áreas distintas, sendo dois em área florestada conservada e outro localizado em área de pastagem. Os resultados obtidos contradizem as duas hipóteses iniciais na qual a espécie de gramínea C4 (B. brizantha), devido a alta relação C:N, alto teor de lignina e baixa concentração de nitrogênio, apresentaria menores taxas de decomposição quando comparada a espécie C3 (M. schottiania) e que a decomposição de ambas as espécies seria maior no rio situado na áreas de pastagem, devido a maior temperatura causada pela maior incidência de sol neste área, que estimularia a atividade microbiana e, portanto a decomposição do resíduo. A temperatura, frequentemente relatada na literatura como principal parâmetro ambiental regulador da decomposição, não se mostrou determinante no processo de decomposição, uma vez que as maiores de perda dos detritos ocorrem no rio situado na floresta, onde os menores valores de temperatura da água foram encontrados. Sendo assim, outros fatores se mostraram mais atuantes neste processo, como as características dos substratos e a concentração de nitrato dissolvido na água. Com relação à decomposição das espécies, a B. brizantha, apesar de nutricionalmente pobre, com baixa concentração inicial de nitrogênio e alta razão C:N, foi a espécie que teve maior perda de massa nos três rios analisados, devido sua baixa concentração de lignina, composto mais recalcitrante encontrado no tecido vegetal. A lignina também se mostrou determinante na dinâmica de perdas dos demais nutrientes das duas espécies, visto que este composto pode interagir com demais constituintes celulares, formando complexos que \"mascaram\" a decomposição dos compostos mais lábeis. Sendo assim, a interação entre quantidade de nutrientes e a forma na qual este estão disponíveis será de extrema importância para a definição da velocidade de decomposição dos detritos foliares no meio ambiente / The particulate organic matter (POM) is the main energy source in stream ecosystems, which may be generated inside or outside the ecosystem. The production and decomposition processes are particularly important for nutrient cycling at the ecosystem, affecting the relationships between biodiversity and its functions and properties. In Brazil, there is little information on the dynamics of organic matter in aquatic systems, besides the great dimensions of Brazilian watersheds, and its association of those with riparian vegetation. Thus, the present study aims to evaluate the effects of factors that regulate decomposition of allochthonous organic matter in aquatic systems, as well as the patterns of retention, accumulation of debris and nutrient cycling in three rivers located in different areas, two in a preserved forest area and one in a pasture area. Our results contradict the two initial hypothesis that decomposition would be slower for C4 plants (grass), due its higher C:N, higher lignin content and low nitrogen ratios, in any environment, and that decomposition rates would be faster in the pasture streams due to the higher water temperature caused by the high incidence of sun in this area, which would stimulate microbial activity, and thus the decomposition of the residue. Temperature, commonly reported in the literature as the main environmental regulator of decomposition, was not decisive in the decomposition process, since the larger losses of mass leaf occurred at the river located at the forest, where the lowest water temperature values were found. Therefore, other factors were more effective to determine the decomposition process, such as the characteristics of the substrate and the concentration of dissolved nitrate in the water. Regarding the differences at the decomposition rates of the species, B. brizantha, although nutritionally poor, with low initial nitrogen concentration and high C: N ratio, was the species with greater weight loss at the three rivers studied, due to its low lignin concentration, which is the most recalcitrant compound found in the plant tissue. Lignin also influenced the dynamic of nutrient losses of the two species, since this compound can interact with other cellular constituents, preventing the decomposition of the most labile compounds. Thus, the balance between the amount of nutrients and the form in which they are available will be extremely important to define the decomposition rate of leaf litter at the environment

Macroinvertébrés benthiques et hyphomycètes aquatiques représentent une part importante de la biocénose des cours d'eau de tête de bassin. D'un point de vue écologique, ils participent au recyclage du carbone organique à travers leur implication dans certaines fonctions écosystémiques telles que la décomposition de la litière. Ce processus est vital pour les cours d'eau forestiers ou bordés de ripisylve. Si celui-ci est aujourd'hui bien étudié en milieu tempéré, la contribution des deux types de décomposeurs (champignons et invertébrés) reste mal comprise en milieu tropical du fait de la variabilité éco-géographique prononcée de ce biome. Cette thèse qui concernait deux régions de Guinée jusque-là inexplorées, la Guinée Forestière et la Haute-Guinée, avait un double objectif. D'une part, il s'agissait de documenter la diversité taxonomique et trophique des communautés de macroinvertébrés benthiques et la composition des assemblages d'hyphomycètes aquatiques. D'autre part, la thèse visait à caractériser le processus de décomposition de la litière et les communautés de décomposeurs associés, notamment dans une perspective de quantification de la contribution de chacun des deux groupes. Cette dernière étude répondait ainsi à l'hypothèse d'une plus grande importance des décomposeurs fongiques aux faibles latitudes en comparaison des latitudes élevées où l'activité des décomposeurs invertébrés est prépondérante. Une étude exploratoire menée sur 12 rivières a révélé une diversité de 45 taxons correspondant à la faune macrobenthique de l'Afrique de l'Ouest, et incluant un nouveau taxon de crustacé pour la Guinée. Les deux régions d'étude se différenciaient dans la composition des groupes trophiques : les déchiqueteurs, essentiellement représentés par les crevettes, étaient dominants dans les cours d'eau de Guinée Forestière, et les racleurs abondaient dans ceux de savane en Haute-Guinée, caractérisés par une faible densité de la végétation riveraine. Par ailleurs, les prélèvements d'eau et d'écume ont décelé la présence de 29 espèces d'hyphomycètes aquatiques. A ces espèces viennent s'ajouter 9 autres identifiées sur les litières lors des expériences de décomposition portant ainsi à 38 le nombre d'espèces identifiées pour la Guinée au cours de cette thèse, dont 12 sont nouvelles pour le continent africain. Les expériences menées dans les deux régions montrent une décomposition rapide, probablement en partie liée à la forte teneur en nutriments des litières, et une faible diversité des décomposeurs associés. Dans les deux cours d'eau temporaires de savane étudiés, les invertébrés associés aux litières étaient absents. Comme une compensation, l'activité des champignons était accrue, lesquels accumulaient une biomasse mycélienne très élevée. La présence de déchiqueteurs potentiels a été notée dans les deux cours d'eau étudiés de Guinée Forestière mais leurs densités étaient faibles. Dans les deux régions, la différence entre les taux de décomposition totale et microbienne était faible, soulignant une importance minime des invertébrés et un rôle majeur des champignons et confirmant notre hypothèse. Il a également été montré au cours de cette thèse que les asséchements des cours d'eau avaient peu d'influence sur le taux de décomposition de la litière. Par ailleurs, une faible diversité des organismes décomposeurs, invertébrés et hyphomycètes aquatiques, ne semble pas altérer le processus de décomposition. Ainsi les lois générales basées sur les facteurs déterminants de la décomposition des litières tels que la qualité du matériel végétal ou un rôle équilibré entre les deux types de décomposeur, ne sont pas nécessairement transposables aux milieux tropicaux. Enfin, les résultats de cette thèse suggèrent que les conséquences du changement climatique sur le recyclage du carbone organique dans les eaux d'Afrique de l'Ouest, région prédite comme très affectée, pourraient être tempérées par la prédominance d'organismes peu vulnérables. / Benthic macroinvertebrates and aquatic hyphomycetes are major components of the biocenosis of headwater ecosystems. Both of them contribute to essential ecosystem functions like leaf litter decomposition as part of the organic carbon cycling. Such a process is vital for woodland streams or riparian tree-lined rivers. While this process is very well documented for temperate regions, the contribution of both types of decomposers, i.e. fungi and invertebrates, remains poorly understood in the tropics partly due to the large ecogeographic variability prevailing in this biome. The present thesis dealt with two unexplored regions of Guinea (Forested Guinea and Upper Guinea) and was motivated by a double objective. First, it aimed at documenting the taxonomic and trophic diversity of benthic macroinvertebrate communities together with the composition of aquatic hyphomycete assemblages. The second objective was to characterize leaf litter decomposition and leaf-associated decomposers, particularly in the perspective of quantifying the relative contribution of both decomposer types. The latter study specifically addressed the hypothesis of a greater importance of fungal decomposers at low latitudes compared to higher latitudes where the activity of invertebrate decomposers prevails. An exploratory survey conducted in 12 streams revealed the occurrence of 45 taxa belonging to the macrobenthic fauna of West Africa and including a new genus, Asellus, which completes the list of known crustacean taxa of Guinea. Both regions differed in the importance of the Functional Feeding Groups: shredders, mainly consisting in shrimps, dominated in Forested Guinea whereas scrapers were abundant in the savannah streams of Upper Guinea, which were characterized by scarce riparian vegetation. In stream water and foam, a total of 29 species of aquatic hyphomycetes were identified. Moreover, 9 additional species were found as sporulating on leaves in litter decomposition experiments, which led to a total of 38 species for Guinea with 12 being new for Africa. This thesis thus substantially expanded the list of known species for Africa. The experiments carried out in both regions showed a fast leaf litter decomposition, likely partly due to the high nutrient contents in litter, and a low diversity of leaf-associated decomposers. In the two studied temporary savannah streams, no leaf-associated invertebrates occurred. The latter was apparently compensated by a strong fungal activity as illustrated by very high mycelial biomass accrual. In the two studied streams of Forested Guinea, the occurrence of Caridina africana (Atyidae crustacean) as potential shredder could explain the leaf mass loss due to invertebrates, even though their density remained low. In both regions, the discrepancies between total and microbial decomposition rates were weak, highlighting a minute contribution of invertebrates and a major role of fungi thus supporting our hypothesis. The present results suggest that droughts resulted in low effects on the rate of leaf decomposition. Furthermore, leaf decomposition did not seem to be affected by the poor diversity of decomposers, i.e. invertebrates and aquatic hyphomycetes. Overall, the principles of leaf decomposition control by factors like the quality of leaf litter and the balanced involvement of both decomposer types do not appear to be fully applicable to tropical environments. Finally, the findings of this thesis suggest that the consequences of climatic change on the cycling of organic carbon in the aquatic ecosystems of West Africa, i.e. a region predicted to be particularly affected, could be mitigated by the dominance of organisms exhibiting a low vulnerability.

No description available.

The solar radiation in general and UV radiation in particular has been recognized to stimulate plant litter decomposition through photochemical mineralization of photosensitive organic molecules, such as lignin, facilitating microbial decomposition, with great relevance role in dryland ecosystems where microbial activity is low, however little is known about how photodegradation could influence other ecosystems without moisture limitations and under what conditions may be favored, therefore the mechanisms has not yet been established. Decomposition in tropical ecosystem is a complex process and can be induced by a number of environmental factors with certain differences when compared to arid and semi-arid ecosystems. To assess the mechanisms underlying UV-B photodegradation, we designed a 300 days field experiment at a tropical ecosystem with high levels of annual precipitation and exposure litter to three levels of radiation combined with a biocide treatment. Results show that the removal of UV-B radiation decelerated plant litter decomposition during the later stage compared to litter exposure to full sun, however shaded litter had similar mass loss compared to litter exposed to full sun. Furthermore, differences in the decay constant among radiation treatments due to UV-B effect is independent of lignin loss. Overall, our study suggest that UV-B contributes to the plant litter decomposition through carbon losses, however, had no effect on nitrogen, neither lignin nor cellulose loss. However, more studies are needed in order to investigate the positive and negative effects of UV exposure on microbial activity in tropical ecosystems. / A radiação solar em geral e a radiação ultravioleta (UV) em particular têm sido reconhecida por estimular a decomposição da serapilheira através da mineralização fotoquímica de moléculas fotossensíveis, como a lignina, facilitando a decomposição microbiana, com um papel de grande relevância em ecossistemas áridos onde a atividade microbiana é baixa, no entanto pouco se sabe como a fotodegradação pode influenciar outros ecossistemas como por exemplo os mais úmidos e sob quais condições a fotodegradação é favorecida, portanto os mecanismos ainda não foram estabelecidos. Decomposição em ecossistemas tropicais é um processo complexo e pode ser influenciado por vários fatores ambientais e com certas diferenças quando comparada com ecossistemas áridos e semiáridos. Para avaliar os mecanismos subjacentes à fotodegradação via radiação UV-B, um experimento de campo de 300 dias foi estabelecido em um ecossistema tropical com alto índice de precipitação anual onde a serapilheira foi exposta a três níveis de radiações diferentes, combinada com um tratamento com biocida. Resultados mostram que a remoção da radiação UV-B desacelerou a decomposição da serapilheira durante o último estágio do experimento comparado com a serapilheira exposta a radiação ambiente, no entanto a serapilheira quando sombreada teve perda de massa similar à exposta a radiação ambiente. Além disso, diferenças na taxa de decaimento entre os tratamentos de radiação devido ao efeito da radiação UV-B foram independentes da perda de lignina. No geral, nosso estudo sugere que a radiação UV-B contribui com a decomposição da serapilheira através da perda de carbono, no entanto não teve efeito na perda de massa de nitrogênio, lignina e celulose. Portanto, mais estudos são necessários para investigar o efeito positivo e negativo da exposição à radiação UV-B na atividade microbiana e na decomposição da serapilheira em ecossistemas tropicais.

A matéria orgânica particulada (MOP) é a principal fonte de energia em ecossistemas lóticos, podendo ser gerada dentro ou fora do ecossistema. Os processos de produção e decomposição são particularmente importantes para a ciclagem dos nutrientes no ecossistema, afetando as relações entre biodiversidade e suas funções e propriedades. No Brasil existem poucas informações sobre a dinâmica deste material orgânico nos sistemas aquáticos, apesar das grandes dimensões das bacias hidrográficas brasileiras, e da associação destas com a vegetação ripária. Sendo assim, o presente trabalho tem como objetivo a avaliação dos efeitos dos fatores reguladores da decomposição de matéria orgânica alóctone em sistemas aquáticos, bem como os padrões de retenção, acúmulo de detritos e ciclagem de nutrientes em três rios localizados em áreas distintas, sendo dois em área florestada conservada e outro localizado em área de pastagem. Os resultados obtidos contradizem as duas hipóteses iniciais na qual a espécie de gramínea C4 (B. brizantha), devido a alta relação C:N, alto teor de lignina e baixa concentração de nitrogênio, apresentaria menores taxas de decomposição quando comparada a espécie C3 (M. schottiania) e que a decomposição de ambas as espécies seria maior no rio situado na áreas de pastagem, devido a maior temperatura causada pela maior incidência de sol neste área, que estimularia a atividade microbiana e, portanto a decomposição do resíduo. A temperatura, frequentemente relatada na literatura como principal parâmetro ambiental regulador da decomposição, não se mostrou determinante no processo de decomposição, uma vez que as maiores de perda dos detritos ocorrem no rio situado na floresta, onde os menores valores de temperatura da água foram encontrados. Sendo assim, outros fatores se mostraram mais atuantes neste processo, como as características dos substratos e a concentração de nitrato dissolvido na água. Com relação à decomposição das espécies, a B. brizantha, apesar de nutricionalmente pobre, com baixa concentração inicial de nitrogênio e alta razão C:N, foi a espécie que teve maior perda de massa nos três rios analisados, devido sua baixa concentração de lignina, composto mais recalcitrante encontrado no tecido vegetal. A lignina também se mostrou determinante na dinâmica de perdas dos demais nutrientes das duas espécies, visto que este composto pode interagir com demais constituintes celulares, formando complexos que \"mascaram\" a decomposição dos compostos mais lábeis. Sendo assim, a interação entre quantidade de nutrientes e a forma na qual este estão disponíveis será de extrema importância para a definição da velocidade de decomposição dos detritos foliares no meio ambiente / The particulate organic matter (POM) is the main energy source in stream ecosystems, which may be generated inside or outside the ecosystem. The production and decomposition processes are particularly important for nutrient cycling at the ecosystem, affecting the relationships between biodiversity and its functions and properties. In Brazil, there is little information on the dynamics of organic matter in aquatic systems, besides the great dimensions of Brazilian watersheds, and its association of those with riparian vegetation. Thus, the present study aims to evaluate the effects of factors that regulate decomposition of allochthonous organic matter in aquatic systems, as well as the patterns of retention, accumulation of debris and nutrient cycling in three rivers located in different areas, two in a preserved forest area and one in a pasture area. Our results contradict the two initial hypothesis that decomposition would be slower for C4 plants (grass), due its higher C:N, higher lignin content and low nitrogen ratios, in any environment, and that decomposition rates would be faster in the pasture streams due to the higher water temperature caused by the high incidence of sun in this area, which would stimulate microbial activity, and thus the decomposition of the residue. Temperature, commonly reported in the literature as the main environmental regulator of decomposition, was not decisive in the decomposition process, since the larger losses of mass leaf occurred at the river located at the forest, where the lowest water temperature values were found. Therefore, other factors were more effective to determine the decomposition process, such as the characteristics of the substrate and the concentration of dissolved nitrate in the water. Regarding the differences at the decomposition rates of the species, B. brizantha, although nutritionally poor, with low initial nitrogen concentration and high C: N ratio, was the species with greater weight loss at the three rivers studied, due to its low lignin concentration, which is the most recalcitrant compound found in the plant tissue. Lignin also influenced the dynamic of nutrient losses of the two species, since this compound can interact with other cellular constituents, preventing the decomposition of the most labile compounds. Thus, the balance between the amount of nutrients and the form in which they are available will be extremely important to define the decomposition rate of leaf litter at the environment

La décomposition des résidus de culture est un processus essentiel en sols de grandes cultures car elle représente la principale source carbone (C) exogène de ces systèmes. Bien que la dégradation des végétaux de diverses quantités et qualités soit bien connue, ses conséquences sur les communautés du sol et sur les fonctions qui en découlent sont moins maîtrisées. Pour mieux comprendre ces effets, nous avons suivi à des étapes clés de la décomposition l’évolution de composantes majeures de la diversité du sol (micro-organismes, nématodes, microarthropodes et macrofaune) en lien avec les principaux flux de C, d’azote (N) et les activités enzymatiques associées pour différentes quantités et qualités de litières. Les expériences ont été menées à différentes échelles spatiales et temporelles (champ, mésocosme et microcosme).La quantité et qualité de litières ont des effets distincts sur les communautés du sol. La quantité de litière impacte les détritivores, et la qualité de litière affecte plus généralement les principaux groupes fonctionnels. La litière labile affecte plus la voie bactérienne et les macro-détritivores en début de décomposition, et la litière récalcitrante la voie fongique et les méso-détritivores en fin de décomposition. Ces évolutions sont reflétées par les dynamiques enzymatiques. Plus spécifiquement, la régulation top-down de la faune (manipulations de L.terrestris et de nématodes) a montré de forts impacts sur les enzymes. De manière similaire, la composition initiale du micro-réseau trophique du sol a fortement conditionné les dynamiques enzymatiques. Au contraire, la minéralisation du C de la litière demeure principalement dictée par sa qualité initiale. Cette thèse démontre l'importance de la gestion des litières sur les fonctions biologiques des sols agricoles. Faire varier la qualité et quantité des litières apparaît comme un levier pour moduler la santé des sols et certains services écosystémiques à long terme. / Crop residues decomposition is an essential process in cultivated lands since it constitutes the main source of organic matter in these systems. Although the underlying mechanisms of residues degradation of varying qualities and quantities are well-known, their consequences on soil biota and the related functions are less understood. To better understand this process, we chose to follow at key steps of decomposition the evolution of major components of soil diversity (microorganisms, nematodes, micro-arthropods and macrofauna), the evolution of litter quality, the main carbon (C) and nitrogen (N) fluxes and enzyme activities associated, for different quantities and qualities of litter added. The experiments were set up at different spatial and temporal scales (field, mesocosm and microcosm experiments) in function of our objectives.Litter quantity and quality have distinct effects on soil biota. Litter quantity influences the detritivores, and litter quality influences more generally the main functional groups. Labile litter impacts more the bacterial energy channel and the macrodetritivores at the first stages of decomposition, and recalcitrant litter impacts more the fungal energy channel and the mesodetritivores at the latter stage of decomposition. These evolutions are mirrored by the enzymatic dynamics. More specifically, fauna top-down regulations (L.terrestris and nematodes manipulations) exhibit high effects on soil enzymes. Similarly, soil micro-food web initial composition drives enzymatic dynamics. Contrastingly, litter C mineralization depends mainly on its initial quality. This PhD shows the importance of litter management on the biological functions of cultivated soils. Varying litter quality and quantity appear as a mean to influence soil health and some ecosystem services on the long term.

I investigated the effects ofhabitat loss and subsequent restoration on invertebrate community structure and ecosystem functioning in a mined peat bog in the North Island, New Zealand. In an experimental trial, the impact of peat bog habitat loss and isolation on the invertebrate community associated with Sporadanthus ferrugineus (Restionaceae) was investigated. Potted S. ferrugineus plants were exposed to invertebrates at various distances up to 800 m from an intact habitat (the presumed source population) over 18 weeks. Invertebrates rapidly colonised the experimental plants, with all major Orders and trophic groups present on Sc ferrugineus within 6 weeks. However. with increasing distance away from the undisturbed habitat, there was a significant decrease in total richness and abundance of invertebrates associated with the potted plants. Additional tests showed that even a moderate degree of isolation (i.e. greater than 400 m) from the intact habitat caused an almost complete failure of 'Batrachedra' sp. to colonise its host plant, at least in the short-term, The density of eggs and larvae, and the average larval size of 'Batrachedra' sp. (Lepidoptera: Coleophoridae) colonising Si ferrugineus plants, as well as the proportion of Si ferrugineus stems damaged by 'Batrachedra' sp. herbivory, all decreased logarithmically with increasing distance from the intact habitat. Surprisingly, though, the rate of recovery of the insect-plant interaction following experimental habitat restoration was remarkably rapid (i.e. between 3Y2 and 6 years). After just 6 years there was no significant difference in insect-plant interactions between the intact peat bog sites and any of the experimentally restored sites up to 800 m away. These results suggest that the degree of isolation from undisturbed habitat has a major impact on the rate and patterns of restoration recovery in the invertebrate community and that some insect-plant interactions can recover rapidly from habitat loss with restoration management. Restoration of mined peat bogs in northern New Zealand is initiated by establishing a native vegetation cover to minimize further peat degradation. The effects of various restoration techniques on litter decomposition, microbial community activity and beetle community composition were investigated within an experimental trial, These treatments included translocation ofpeat bog habitat (direct transfer of islands), milled peat islands with no seed and milled peat islands with seed, and were compared with an unrestored mined site and an undisturbed peat bog. In all the response variables measured, the undisturbed peat bog sites had significantly higher decomposition rates and microbial respiration rates, and significantly higher abundance and species richness of beetles than any of the restoration treatments. Inaddition, the technique used to restore mined peatlands had a significant effect on the beetle community composition and litter decomposition processes. Despite a rapid initial change in the beetle community following habitat translocation, the direct transfer islands were still the most similar in beetle species composition to the undisturbed peat bog. Microbial activity and decomposition rates were higher in the direct transfer and mined peat surface after 6 months. However, even after 12 months, decomposition rates in the restored habitats were still far from reaching the levels recorded in the undisturbed peat bog. The results suggest that beetle community structure and ecosystem processes such as decomposition and microbial activity rates may be able to recover faster with certain restoration techniques, such as direct transfer of intact habitat islands. Subsequently, I examined long-term beetle community reassembly on islands that had been restored by creating raised areas ofprocessed peat with the addition of Leptospermum scoparium seed. Monitoring of different-aged restored islands representing the full range of restoration ages (up to 6 years) available at the peat mine, indicated that as the peat islands became older and the vegetation structure became more complex, the abundance, species richness and composition of the beetle community became increasingly similar to the community in the undisturbed peat bog. Despite this, distinct differences between the intact peat bog and older restored peat islands still persisted, even after 6 years, particularly at an individual species level. However, it is predicted that within 12 years the restored peat islands will share 100% ofbeetle species in common with the undisturbed peat bog. Taken together, these results indicate that restoration is effective in initiating the recovery of beetle assemblages and ecosystem processes (such as litter decomposition and microbial community activity) in cut-over peat bogs. However, it is estimated to take at least 12 years before pre-mining communities and functions are attained, and ongoing monitoring to develop an understanding of the longer-term dynamics of such ecosystems and processes is clearly required.

Malgré l'importance reconnue des forêts tropicales humides dans la régulation du climat et du cycle global du carbone, la biogéochimie des forêts tropicales reste moins bien appréhendée que celle d'autres biomes. En particulier, il existe encore de larges incertitudes quant aux limitations nutritives des processus ou encore sur le rôle de la diversité pour les fonctions écosystémiques. La diversité spécifique élevée des arbres de forêt amazonienne se traduit localement par une forte hétérogénéité de la qualité des apports de litière foliaire, ces apports constituant une ressource primordiale d'énergie et de nutriments pour les organismes saprophages. Cependant les conséquences d'une telle hétérogénéité des litières pour le fonctionnement souterrain sont encore peu connues dans ce milieu. L'objectif de cette thèse est d'établir une compréhension mécaniste des effets de la qualité des apports de litière ainsi que du rôle de leur diversité sur la décomposition. J'ai combiné des fertilisations factorielles de carbone (C), d'azote (N) et de phosphore (P) à l'utilisation d'une large gamme de stœchiométrie C : N : P des litières (issues de différentes espèces d'arbres) en forêt tropicale de Guyane française et en laboratoire pour évaluer la nature et l'étendue des contraintes énergétiques et nutritives imposées par la qualité des litières sur les activités des décomposeurs. Bien que la perte en masse des litières dans ce système d'étude apparait largement expliquée par la qualité des différentes formes de C des litières, suggérant un fort contrôle de la disponibilité en énergie sur la décomposition, les ajouts externes de C n'ont pas permis de mettre en évidence cette apparente contrainte énergétique. Cependant, je montre que la décomposition des litières tropicales est limitée conjointement par N et P, et que l'amplitude de cette co-limitation est fortement reliée à la disponibilité en P des litières ainsi qu'à leur stœchiométrie N : P. Ainsi, même si le P apparaît plus profondément limitant dans ce système (en accord avec l'hypothèse généralement admise d'un fort déficit en P en forêt tropicale humide), l'accès à l'N foliaire semble également contraindre la décomposition. Je mets également en évidence des effets de diversité des mélanges de litières, pour la plupart synergiques (les taux de décomposition observés des mélanges étaient généralement plus élevés que ceux prédits à partir des taux de décomposition des espèces seules) et largement amplifiés par la présence de la faune détritivore. De plus, en présence de faune, il apparait qu'une forte dissimilarité stœchiométrique dans les mélanges de litières favorise des effets synergiques. Autrement dit, l'association de litières ayant des stœchiométries dissimilaires semble favoriser un meilleur équilibre nutritionnel pour la faune saprophage, stimulant ainsi la décomposition. Finalement, je montre que ces effets synergiques dans les mélanges sont renforcés à travers les effets à long terme des apports de litières issus des différentes espèces d'arbres contribuant à ces mélanges. Ce résultat suggère que la complémentarité de l'utilisation des ressources émerge à travers des interactions à long terme entre les arbres et les organismes décomposeurs. / The high tree species diversity in Amazonian rainforest translates into a high variation of leaf litter quality input to the soil. These inputs constitute a major resource of nutrients and energy for saprophageous organisms, particularly in tropical rainforests growing on old and highly weathered soils and consequently impoverished in rock-derived mineral nutrients. However the consequences of such leaf litter heterogeneity for belowground functioning are still poorly understood. In this thesis, I aim to develop a better mechanistic understanding of leaf litter quality effects and litter diversity on decomposition. Using a factorial fertilization of carbon (C), nitrogen (N), phosphorus (P) and a wide range of leaf litter C : N : P stoichiometries (derived from different tree species) in French Guiana rainforest and in laboratory, I aim to assess the nature and the extent of energetic and nutritional constraints imposed by litter quality on decomposer activities. Although the litter mass loss in this study system is greatly explained by the litter C quality, suggesting a strong control of energy availability on decomposition, external C additions do not alleviate this apparent energetic constraint. However, I find that litter decomposition is conjointly limited by N and P and that the extent of this NP co-limitation is widely related to leaf litter P content and to litter N : P stoichiometry. Thus, even if P appears more profoundly limiting in this system (in accordance to the well-recognized P deficiency hypothesis in tropical rainforest), the litter N access seems also constrain the decomposition. Moreover, I find significant litter diversity effects in litter mixtures and most of them are synergistics (observed mass loss in mixture was more often higher than predicted mass loss on the basis of single species decomposition). These litter diversity effects are largely higher in presence of soil fauna and increase with stoichiometric dissimilarity in mixtures. In other words, the association of stoichiometrically dissimilar litter appear promote a better nutritional balance for the saprophageaous fauna, and thus, stimulate the decomposition. Finally, I show that synergistic effects in mixtures are reinforced toward long-term presence of tree species contributing to the litter mixtures. This result suggest that complementarity effects on mixed litter decomposition may emerge through long-term interactions between aboveground and belowground biota.