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International audience; The sustainability of fast-growing tropical Eucalyptus plantations is of concern in a context of rising fertilizer costs, since large amounts of nutrients are removed with biomass every 6–7 years from highly weathered soils. A better understanding of the dynamics of tree requirements is required to match fertilization regimes to the availability of each nutrient in the soil. The nutrition of Eucalyptus plantations has been intensively investigated and many studies have focused on specific fluxes in the biogeochemical cycles of nutrients. However, studies dealing with complete cycles are scarce for the Tropics. The objective of this paper was to compare these cycles for Eucalyptus plantations in Congo and Brazil, with contrasting climates, soil properties, and management practices. The main features were similar in the two situations. Most nutrient fluxes were driven by crown establishment the two first years after planting and total biomass production thereafter. These forests were characterized by huge nutrient requirements: 155, 10, 52, 55 and 23 kg ha_1 of N, P, K, Ca and Mg the first year after planting at the Brazilian study site, respectively. High growth rates the first months after planting were essential to take advantage of the large amounts of nutrients released into the soil solutions by organic matter mineralization after harvesting. This study highlighted the predominant role of biological and biochemical cycles over the geochemical cycle of nutrients in tropical Eucalyptus plantations and indicated the prime importance of carefully managing organic matter in these soils. Limited nutrient losses through deep drainage after clear-cutting in the sandy soils of the two study sites showed the remarkable efficiency of Eucalyptus trees in keeping limited nutrient pools within the ecosystem, even after major disturbances. Nutrient input–output budgets suggested that Eucalyptus plantations take advantage of soil fertility inherited from previous land uses and that long-term sustainability will require an increase in the inputs of certain nutrients.

Sustainable production of eucalypt plantations forests is closely dependent on conservative management of soil organic matter stocks and N availability, mainly in short rotation stands set up on low fertility sites. The aims of this study conducted in Brazil and Congo were: i) to examine the effect of mixtures of Eucalyptus sp. and Acacia mangium on stand growth, biomass allocation, net primary productivity and biological N2 fixation, and ii) to assess the behaviour of the two species in contrasted pedo-climatic conditions.

The aim of this study was to study the development of the root system of the 1-41 clone of Eucalyptus in Congo. Root architecture was studied in an age serie stands. A root typology was proposed with five classes of roots and three main functions: soil exploration with both lateral and vertical roots, soil exploitation with a network of fine roots and re exploitation of the soil zone by late emitted roots. All of these root types appeared at all the studied stages because of reiterations that allow the root system to explore a large soil volume. The results of root respiration study were used to parameterize a root respiration model with root respiration partitioned between a growth component, a maintenance component and a nutrient absorption component. The model was then applied using field data of fine root biomass and nitrogen content of whole tree biomass to estimate fine root respiration of a Eucalyptus stand. The simulated value fits reasonably well with independent estimates of root respiration in this site. Estimations of fine root production and turnover were highly variable depending of the methods used. The more realistic one predicts a turnover of 4 year-1, i.e. a high recycling of carbon and nutrient by belowground litter. Results of this study offer promising perspective for developing an architectural model of root development and its use to establish carbon, water and nutrient budget in Congolese Eucalypt plantation., L'objectif de cette étude a été d'étudier le développement du système racinaire du clone d'Eucalyptus 1-41 au Congo. L'rchitecture du système racinaire a été abordée dans une chronoséquence. Elle a permis de définir une typologie composée de cinq catégories de racines assurant trois fonctions essentielles : l'exploration grâce aux racines latérales et pivotantes, l'exploitation du milieu assurée par un réseau de racines fines et la réexploitation du milieu, qui se fait par la mise en place tardive d'un système racinaire traçant. Tous ces types racinaires se mettent en place au cours du temps, grâce à une stratégie réitérative, qui permet au système racinaire de se déployer dans un volume important de sol. L'étude de la respiration racinaire a permis de paramétrer un modèle décomposant la respiration totale en des termes dépendants de la croissance, de l'entretien et de l'absorption minérale. Les paramètres de ce modèle ont ensuite été appliqués sur des données de biomasse et minéralomasse au champ afin d'estimer la respiration des racines fines à l'échelle du peuplement. Les résultats obtenus se rangent bien dans la gamme des mesures in situ au champ. L'étude de la production des racines fines a permis de mettre en évidence une forte variation des estimations en fonction des méthodes de prélèvement utilisées. Cependant l'estimation la plus réaliste donne un turnover racinaire de l'ordre de 4 an-1, impliquant un important retour d'éléments minéraux et de carbone dans le sol. Les résultats de cette étude offrent de bonne perspective pour développer un modèle architectural utile pour l'établissement des bilans de carbone, d'eau et de nutriments des plantations d'Eucalyptus au Congo.

International audience; Soil CO2 efflux was measured in clear-cut and intact plots in order to quantify the impact of harvest on soil respiration in an intensively managed Eucalyptus plantation, and to evaluate the increase in heterotrophic component of soil respiration because of the decomposition of harvest residues. Soil CO2 effluxes showed a pronounced seasonal trend, which was well related to the pattern of precipitation and soil water content and were always significantly lower in the clear-cut plots than in the intact plots. On an annual basis, soil respiration represented 1.57 and 0.91 kgCm_2 yr_1 in intact and clearcut plots, respectively. During the first year following harvest, residues have lost 0.79 kgCm_2 yr_1. Our estimate of heterotrophic respiration was calculated assuming that it was similar to soil respiration in the clear-cut area except that the decomposition of residues did not occur, and it was further corrected for differences in soil water content between intact and clear-cut plots and for the cessation of leaf and fine root turnover in clear cut. Heterotrophic respiration in clear-cut plots was estimated at 1.18 kgCm_2 yr_1 whereas it was only 0.65 kgCm_2 yr_1 in intact plots (41% of soil respiration). Assumptions and uncertainties with these calculations are discussed.

More than 40,000 ha of clonal eucalypt plantations have been established in the Pointe-Noire region on very deep sandy soils. As the precipitation are low (1200 mm/yr"') and the water requirements of Eucalyptus are high, it is very important to study the water uptake strategy of such plantations. This study focuses on the spatial variability of the root distribution, which is an important data to establish hydrological models. A previous study, using the method of root impacts counting, was conducted on a six-year-old planted crop where a TDR monitoring was carried out, up to 5m depth. It showed 1) A stréng vertical anisotropy of the distribution of thin mots, the density of roots decreasing sharply with depth 2) A horizontal anisotropy decreasing when the distance of the profile from the stump increases 3) A grouping of thin roots due to the possible influence of preferential ways of drainage, link to a severe hydrophobicity of the upper soil. This work aims to complete these results by studying, at différent ages of the rotation, the spatial localization of the root system of stands planted with the saine clone as previously.

Coordonnée par le Cirad en partenariat avec INRAE et l'IRD, l'étude ≪ 4 pour 1000 ≫ Outre-mer dresse un bilan inédit des stocks de carbone du sol des territoires ultramarins. Les auteurs formulent des recommandations opérationnelles et de recherche pour preserver ces stocks élevés et répondre aux grands enjeux actuels de l'agriculture face au changement climatique au niveau national et territorial. Si le potentiel des sols métropolitains a fait l'objet d'une evaluation en 2019, celui des territoires ultramarins était jusqu'ici méconnu. Dans le cadre de la présente étude, les auteurs dressent pour la premiere fois un état des lieux des connaissances pour la Guadeloupe, la Martinique, la Guyane, La Reunion, Mayotte, Saint-Martin et Saint-Barthélemy.