Chapitre 5. Stocks de carbone dans les sols des zones agro-écologiques du Bénin

Introduction Situé en Afrique de l’Ouest, le Bénin s’allonge perpendiculairement au littoral du Golfe du Bénin entre les méridiens 1° et 4° et les parallèles 6° et 12° Nord. Les climats y sont chauds et humides. Les conditions pédologiques et la végétation du Bénin sont caractéristiques de la zone intertropicale. La surface totale cultivable représente 62,5 % de la superficie du pays et 20 % seulement des surfaces cultivables sont effectivement exploitées, soit 12,2 % du territoire national (..

Carbone des sols en Afrique

Les sols sont une ressource essentielle à préserver pour la production d’aliments, de fibres, de biomasse, pour la filtration de l’eau, la préservation de la biodiversité et le stockage du carbone. En tant que réservoirs de carbone, les sols sont par ailleurs appelés à jouer un rôle primordial dans la lutte contre l’augmentation de la concentration de gaz à effet de serre. Ils sont ainsi au centre des objectifs de développement durable (ODD) des Nations unies, notamment les ODD 2 « Faim zéro », 13 « Lutte contre le changement climatique », 15 « Vie terrestre », 12 « Consommation et production responsables » ou encore 1 « Pas de pauvreté ». Cet ouvrage présente un état des lieux des sols africains dans toute leur diversité, mais au-delà, il documente les capacités de stockage de carbone selon les types de sols et leurs usages en Afrique. Il propose également des recommandations autour de l’acquisition et de l’interprétation des données, ainsi que des options pour préserver, voire augmenter les stocks de carbone dans les sols. Tous les chercheurs et acteurs du développement impliqués dans les recherches sur le rôle du carbone des sols sont concernés par cette synthèse collective. Fruit d’une collaboration entre chercheurs africains et européens, ce livre insiste sur la nécessité de prendre en compte la grande variété des contextes agricoles et forestiers africains pour améliorer nos connaissances sur les capacités de stockage de carbone des sols et lutter contre le changement climatique

Importance des manifestations des symptômes de déficience minérale du palmier à huile (Elaeis guineensis Jacq.) dans les plantations paysannes au Sud-Bénin en Afrique de l’Ouest

Objectif : Des symptômes de déficience minérale ont été observés dans les plantations paysannes des grandes zones élaeicoles au Bénin. Cette observation a suscité la présente étude de dénombrement des différents types de symptômes de déficience minérale. L’’objectif de l’étude était de connaître les manifestations des symptômes les plus observés et leur degré d’importance A cet effet, des symptômes de déficience minérale ont été étudiés sur la base de la description de manifestation des symptômes faite par Fairhust et Caliman puis Jacquemard.Méthodologie et résultats : Au total, 214 plantations dont 108 et 106 respectivement dans les départements de l’Ouémé et du Plateau ont été étudiées. Ces plantations sont installées sur des sols ferrallitiques, ferrugineux, hydromorphes et sur des vertisols. Quatre types de symptômes de déficience minérale ont été dénombrés dans les deux zones avec leur fréquence de manifestation pour l’azote (5,63±0,47 et 6,48±0,48 %), le potassium (18,78±0,58 et 26,69±0,59 %), le magnésium (1,68±0,18 et 2,72±0,19 %) et le bore (4,89 ±0,39 et 3,53±0,40 %). La manifestation des symptômes de déficience en azote et en magnésium était observée sur les palmiers de tous les types de sol sauf sur sols ferrugineux. Les palmiers à huile sur sols ferrallitiques ont présenté plus fréquemment des symptômes de déficience en potassium comparativement aux sols hydromorphes lesquels par contre ont présenté plus de symptômes de déficience en bore. Le taux moyen de manifestation de l’ensemble des symptômes de déficience était de 45,92% avec une faible manifestation sur les sols régulièrement fertilisés. Conclusion et application des résultats : Des analyses de sol et le diagnostic foliaire sont nécessaires pour confirmer et mieux expliquer l’effectivité de ces carences afin d’élaborer la cartographie de la déficience minérale des plantations de la zone d’étude. L’effet de ces déficiences minérales sur la croissance et le fonctionnement physiologique devra également être déterminé dans la perspective de l’amélioration de la nutrition minérale des palmiers à huile au sud-Bénin. Mots clés : Fertilité des sols ; diagnostic foliaire ; nutrition des plantes ; plantation de palmier à huile. English Title: Importance of mineral deficiency symptoms manifestation in the farmers’ oil palm plantation in southern Benin, West AfricaObjective: The manifestation of nutrient deficiency symptoms was observed throughout farmers’ oil palm plantation of the major zones of oil palm production in south Benin. To better understand this issue, the present study was carried out and aimed to assess the level of manifestation of the different types of nutrient deficiency symptoms and their severity. Methodology and Results: For that purpose, the mineral deficiency symptoms were studied on the basis of nutrient deficiency symptoms description made by Fairhust et Caliman puis Jacquemard. , The survey has been conducted in 214 farmers’ plantations (108 and 106 in the departments of Ouémé and Plateau respectively). These plantations were grown on acrisols, ferric and luvisols, hydromorphic soil and vertisol. Four types of nutrient deficiency symptoms were identified in the plantation nitrogen (5.63 ± 0.47 and 6.48 ± 0.48 %), potassium (18.78 ± 0.58 and 26.69 ± 0.59 %), magnesium (1.68 ± 0.18 and 2.72 ± 0.19 %) and boron (4.89 ± 0.39 and 3.53 ± 0.40 %). Nitrogen and magnesium deficiency symptoms were observed on oil palm of all soil types except ferric and luvisols. Oil palms grown on acrisol presented more potassium deficiency symptoms than the hydromorphic soils, which showed boron deficiency symptoms. The average rate of all nutrient deficiency symptoms was 45.92 % with a weak manifestation on the fertilized soils. Conclusion and application of results: Soil analysis and foliar diagnosis are necessary to confirm these nutrient deficiencies in order to map the mineral deficiency of oil palm plantations in the study area. The effect of these nutrient deficiencies on plant growth and physiological parameters will be also determined in order to improve oil palm mineral nutrition in the south of Benin.Key words: Soil fertility; foliar diagnosis; plant nutrition; oil palm plantation

Improving the methodology for root biomass estimation in monocotyledonous tree plantations: case of oil palm (Elaeis guineensis. Jacq) in West Africa

International audienceBackground and aims Root biomass is one of the most widely used parameters to characterise root growth and belowground carbon stock. Our aim was to define a standard method to estimate the root biomass of young and adult oil palm trees in commercial plantations. Methods Three methods based on the sampling excavation volume were compared using the same sampled tree. Work time and the number of workers required for each operation were recorded. We compared two large excavation volumes based on Voronoi tessellation and the standard root auger coring method in one 2-year-old and one 16-year-old commercial oil-palm plantation in Benin, West Africa. Results Oil palm total root biomass was estimated at 0.84 and 22.23 Mg ha(-1) in the 2-year-old and 16-year-old plantation, respectively. Compared to the reference method, the simplified Voronoi trench method estimated slightly higher (+5%) and lower (-17%) total root biomass with no significant differences but required 2 and 3 times more labour time, respectively, while the auger method estimated significantly lower (-23% and - 53%) total root biomass in the 2-year-old and 16-year-old plantation, respectively. Coarse and fine root biomass were significantly higher under the windrow than under the footpath zones. Conclusion The simplified Voronoi trench method required twice as much labour time as the auger method but was most efficient way to estimate oil palm total root biomass, irrespective of the age of the plantation

Effect of land use change on carbon stocks in an agricultural region of southern Benin

International audienceLand use change is one of the most important determinants of carbon storage and dynamics in ecosystems. Areas in the proximity of metropoles undergo land use changes but are poorly studied for their soil and biomass carbon budget, especially in Africa. Close to Benin's economic capital, the Lama Territory located on the Allada Plateau and is subject to high demographic and urbanization pressures. Carbon (C) stocks in the region are likely to be rapidly changing. This research assessed the land use changes and quantified the spatial distribution and variation of C stocks between 2000 and 2018 on the Ferralsols of the study area. Random forest models using spectral bands of LANDSAT images and some spectral indices as predictors were calibrated to classify the land use. C stocks of four C pools (aboveground, belowground biomass, litter, and soil) were mapped with InVEST (integrated valuation of ecosystem services and tradeoffs) model. Land use change occurred in 61% of the Ferralsol area between 2000 and 2018. The surface of forests and crop-plantation associations decreased in benefit of tree plantations, adult palm groves, and built-up areas. With the loss of forest surface and the increasing urbanization, C stocks of the region decreased by -218 Gg C with 4% uncertainty (-175 Gg C in soil pools, and - 125 Gg C in aboveground biomass with both 6% uncertainty) between 2000 and 2018. Results highlight: (i) the need for soil C stock accounting to avoid underestimation of C stocks evolution in a rural region; and (ii) the relevance of tree plantations and deforestation control to maintain the C stocks in tropical areas

Effect of land use change on carbon stocks in an agricultural region of southern Benin

International audienceLand use change is one of the most important determinants of carbon storage and dynamics in ecosystems. Areas in the proximity of metropoles undergo land use changes but are poorly studied for their soil and biomass carbon budget, especially in Africa. Close to Benin's economic capital, the Lama Territory located on the Allada Plateau and is subject to high demographic and urbanization pressures. Carbon (C) stocks in the region are likely to be rapidly changing. This research assessed the land use changes and quantified the spatial distribution and variation of C stocks between 2000 and 2018 on the Ferralsols of the study area. Random forest models using spectral bands of LANDSAT images and some spectral indices as predictors were calibrated to classify the land use. C stocks of four C pools (aboveground, belowground biomass, litter, and soil) were mapped with InVEST (integrated valuation of ecosystem services and tradeoffs) model. Land use change occurred in 61% of the Ferralsol area between 2000 and 2018. The surface of forests and crop-plantation associations decreased in benefit of tree plantations, adult palm groves, and built-up areas. With the loss of forest surface and the increasing urbanization, C stocks of the region decreased by -218 Gg C with 4% uncertainty (-175 Gg C in soil pools, and - 125 Gg C in aboveground biomass with both 6% uncertainty) between 2000 and 2018. Results highlight: (i) the need for soil C stock accounting to avoid underestimation of C stocks evolution in a rural region; and (ii) the relevance of tree plantations and deforestation control to maintain the C stocks in tropical areas

Allometric equations for estimating oil palm stem biomass in the ecological context of Benin, West Africa

correction à cet article : 10.1007/s00468-018-1791-z et WOS : 000457471000025International audienceKey messageThe models we developed used palm stem diameter at 1.5m, stem density measured by core sampling at 1.5m, and the height of the palm to estimate palm stem biomass.AbstractAllometric equations based on non-destructive methods were developed to estimate palm stem biomass. Twenty mature palms of different ages were subject to destructive and non-destructive measurements: stem height from the collar to the bottom of the frond at the 33rd position in the crown (frond 33), dry weight, the diameter and thickness of cylindrical slices sectioned at different heights along the stem, and the dry weight of samples of stem tissue taken from the core of each slice. The densities of the stem slices obtained using the destructive method (D-true) and the non-destructive method (D-core) were linearly correlated: D-true=1.062xD(core) (R-2=0.99) independently of the height, age and genetic origin of the palm. Stem density varied with height, reaching its maximum at around 1.5m, after which it remained more or less constant to the top of the stem. Stem linear density (SLD) estimated from the product of the density and the section of the stem fell to a threshold value that remained more or less constant above 1.5m. SLD between observed slices was estimated by interpolation based on a hyperbolic model to compute the true biomass of each palm. An equation derived from the integral of the hyperbolic model was fitted to estimate palm stem biomass as a function of wood density observed by non-destructive sampling at 1.5m on the stem and of the diameter at the same height. With this equation, palm stem biomass can be estimated with an error of 5%

Diversity and socio-economic aspects of oil palm agroforestry systems on the Allada plateau, southern Benin

International audienceIn southern Benin, oil palm is mainly cultivated by smallholders who associate it with successive crops over the three first years when the trees are immature. There is little information available on the diversity of the practices used in these young plantations and their socio-economic background, although this information is essential for proposing opportunities to improve the performance and sustainability of these systems. This study used focus groups and a survey of 54 farms to test how the selection of practices was linked with the farmer’s objectives. A conditional inference tree was used to compare the selection of the practices against socio-economic characteristics of the farmer. We identified the crops that were the most frequently associated with young oil palm trees across the region and found that the higher the fertilizer application on the associated crop the lower the application on the trees. The practices varied markedly in terms of economic indicators, reasons for selection, socio-economic characteristics of the farmers and the location of the farms. We showed that financial resources, especially for hiring external labor, household size and inclusion in social networks were important factors to take into account when evaluating alternative practices for oil palm plantations in this area

Biomass and soil carbon stocks of the main land use of the Allada Plateau (Southern Benin)

International audienceThe inventory of the carbon (C) pools in Africa's ecosystems is not well documented, although it is crucial to support climate mitigation policies. We quantified the C stocks in plant biomass, woody necromass, litter and soil (0-30 and 30-100 cm) for the five main land usesforest, tree plantation, young and adult palm groves, croplandsof Ferralsols on the Allada plateau in southeast Benin. Forests have the highest total C stocks (389 ± 54 Mg C ha À1) compared with other land uses (222 ± 33, 154 ± 6, 105 ± 2, 77 ± 3 Mg C ha À1 in tree plantations, adult palm groves, young palm groves and croplands, respectively). The C stocks are higher in the biomass than in the soil (0-100 cm), e.g. in the forest, stocks were 279 ± 54 Mg C ha À1 in the biomass versus 83 ± 2 Mg C ha À1 in the soil. Differences of soil C stocks between land uses are low (% 28 Mg C ha À1) and concentrated in topsoils. The structure and species diversity of the forest partly explained the variability and the high C biomass compared to tree plantations. Type of forest and plantations is important to consider in conserving C stocks in landscapes

Comparative analysis of nutritional status and growth of immature oil palm in various intercropping systems in southern Benin

Beninese smallholders associate food crops and cash crops with immature oil palms to reduce field maintenance costs and gain income before the palms reach productive phase. Little is known about the effects of these crops on the nutritional status and growth of the palms in their immature phase even though the yield of adult palms can be affected by the management practices during this phase. The objective of this study was to evaluate the most common oil palm-based intercropping systems found in southern Benin in terms of nutritional status and growth of the palm. Within 15 oil palm farms, we compared 15 immature oil palm fields where the crop succession associated with the oil palms was dominated by maize, cassava, tomato, and pineapple. The nutrient concentrations in the soil and the palm leaves, and growth indicators were measured at the end of the immature phase. We found that the palm growth indicators were the lowest in the successions with pineapple. N and P nutrition of the immature palms was satisfactory but K was deficient in all systems, especially in those with pineapple. The K levels in the soils and palm leaves were correlated. Rough field budgets comparing the amounts of N and K applied to the crop successions with their N and K exports from non-returning products indicated that soil indigenous K supply would be particularly depleted in the systems with pineapple. We concluded that the young oil palms were affected by the competition for K exerted by the crop successions with pineapple even though they were the most fertilized in the region. The high profitable crop is therefore associated with the lowest growth rates of the immature palms. The mineral fertilizer management in these oil palm temporary intercropping systems should be improved