Effet des champignons mycorhiziens Arbusculaires sur le phosphore des sols tropicaux et implication dans la biosynthèse du caroténoïde du manioc

Open Access Journal; Published online: 31 March 2019Objectifs : Evaluer du point de vue agronomique les avantages attribués aux champignons mycorhiziens arbusculaires (CMA) par rapport aux sols des zones des forêts humides et vérifier le rôle que le phosphore mobilisé par ces champignons jouerait dans la biosynthèse du caroténoïde. Méthodologie et résultats : Deux variétés de manioc étaient soumises aux inoculations apportées seules ou combinées aux autres fertilisants. Les analyses de sol effectuées en amont et en aval du manioc ont permis de constater des changements significatifs en ce qui concerne l'acidité mesurée à 4,6 de pH avant le manioc et 6,3 après inoculation des CMA. Des changements sont également observés sur la structure du sol où on a observé des modifications partant de la structure particulaire au départ à une structure grumeleuse après application de fumier combiné aux inoculations des CMA. Ces inoculations ont fait augmenter la teneur du phosphore dans le sol à 7,5 %, l’azote à 4 % et le carbone à 13%. Le rendement du manioc a donné des moyennes de 55 t ha-1 de racines sous inoculation contre 21 t ha-1 de racines lorsque le sol n’était pas traité. On a noté des modifications significatives du caroténoïde total dans la racine de manioc lorsque le sol était inoculé et était plus pourvu en phosphore. Conclusions et champs d’application des résultats : les champignons mycorhiziens sont présents dans les sols tropicaux des forêts humides et peuvent être multipliés sous le sorgho. Lorsqu’ils sont inoculés en champ de manioc, ils permettent à la fois des accroissements de rendement du manioc, la disponibilisation du phosphore autrefois complexé par les cations acides du sol et l’accélération de la biosynthèse du caroténoïde total du manioc jaune. La possibilité de réaliser des multiplications en cascade de ces champignons et leur conditionnement sur des substrats stériles permettra de fabriquer des inocula locaux qui pourront être utilisés comme fertilisant biologique en lieu et place des fertilisants minéraux conventionnels. Objectives: To evaluate agronomically the advantages attributed to Arbuscular Mycorrhizal Fungi (AMF) compared to soils in humid forest zones and verify the role that phosphorus mobilized by these fungi would play in the biosynthesis of carotenoid of yellow cassava. Methodology and results: Two varieties of cassava were inoculated alone or in combination with other fertilizers. Soil tests carried out before cassava cultivation and after harvest showed significant changes in acidity measured at 4.6 pH before cassava and 6.3 after inoculation with AMF. Changes were also observed in soil grain size with 71% sand initially and 65.5% after application of manure combined with inoculations (LSD.05 = 2.7%). These inoculations increased the soil phosphorus content to 7.5%, nitrogen to 4% and carbon to 13%. Cassava yield averaged 55 t ha-1 under inoculation versus 21 t ha-1 when the soil was untreated. Significant changes in total carotenoid in the cassava root were noted when the soil was inoculated and had a higher phosphorus content. Conclusions and application findings: Mycorrhizal fungi are present in tropical soils of moist forests and can be propagated under sorghum. When inoculated in the cassava field, they allow both increases in cassava yield, the availability of phosphorus once complexed by acidic soil cations and the acceleration of the total carotenoid biosynthesis of yellow cassava. The possibility of performing cascade multiplications of these fungi and their conditioning on sterile substrates will make local inocula that can be used as biological fertilizer instead of conventional mineral fertilizers

Enhancing yield and profitability of cassava in the savannah and forest zones of Democratic Republic of Congo through intercropping with groundnut

Objective: This study was conducted to simulate intercropping of cassava and groundnut as practiced by farmers in the savannah and forest zones of Democratic Republic of Congo (DRC). It aims to (i) evaluate the intercropping of two most cultivated food crops (Cassava and Groundnut) under conditions of marginal soils of the tropics and (ii) propose alternatives to farmers that can enhance productivity.Methods and results: At each location, a split plot design was used with Cassava branching habit (branching type –Erect, Branched and medium habit) as the main plot and System that is association with groundnut (intercrop) or without (monocrop) as the subplot. A combined model with sites was fitted using the Mixed Model with SAS v9.4. The results suggest that in the equatorial and savannah zones the performance of both crops was influenced by the type of branching of cassava stem used in the intercrop with erect types yielding more. Non-significant differences were observed in groundnut yield for all effects. The calculation of Land Equivalent Ratio (LER) and economic analyzes drawn from the marginal rate of return (MRR) clearly showed an advantage of intercropping over the sole cropping system.Conclusion and application: The practice of intercropping groundnut and cassava is advantageous. However, the choice of the variety of cassava based on the branching habit influences the growth and productivity of groundnuts. Optimal production of the two crops and profit was attained where the cassava variety has erect habit. Local varieties with medium habit were generally unhealthy, performed poorly and are therefore not recommended in intercropping systems.Keywords: Productivity; Land Equivalent Ratio; cropping systems; marginal rate of retur

Effect of Tillage and the Position of Cuttings on Cassava Root and Leaf Yields in the Savanna and Forest Fallow Zones of the Democratic Republic of Congo

In Democratic Republic of Congo (DRC), cassava covers more than half of the area under cultivation and is permanently consumed by more than 70% of the population for its roots and about 80% for its leaves, which are one of the main vegetables in the country. National cassava production suffered a setback in the early 1990s and an annual arithmetic reduction of the cassava production, from this period to the start of a program on the rehabilitation of the crop. The main objective of the program was to develop varieties with high dry yield and resistant to cassava diseases and pests mainly the cassava mosaic disease (CMD) because the latter was identified as the major cause for the low production recorded. As and when the program gained momentum, many other factors appeared to have important role for the effective recovery of cassava production. Soil fertility management and cultural practices are amongst them. This study was conducted in this context in two different agro-ecological zones (Litoy in Kisangani hinterlands in forest zone and Plateau de Bateke at the East of Kinshasa in savannah zone). The soil of Litoy is heavier than the one of Plateau des Bateke whose sand content is higher than 90%. The method of land preparation influenced the production of cassava roots and leaves in the grassy fallows of the forested areas of the Kisangani region. A 45% increase of the root yield was obtained in this area when the soil was plowed. An stable yield of 14 t/ha was obtained in the Bateke plateau savannah regardless of the land preparation method. The position/orientation of cuttings at planting did not indicate significant statistical differences. However, higher gross profit margins were obtained on plowed land with US$2500/ha compared to US$1500/ha on no tilled land

Growth, Productivity, Biomass and Carbon Stock in Eucalyptus saligna and Grevillea robusta Plantations in North Kivu, Democratic Republic of the Congo

Initiated by the World Wildlife Fund (WWF) more than a decade ago in North Kivu, single-species plantations of Eucalyptus saligna and Grevillea robusta constitute, with other village plantations, the current legal source of wood-energy for the communities bordering the Virunga National Park (PNVi). This study assesses the growth and productivity of these plantations in two sites with different soil and climatic conditions to predict their production over time. The study also assesses the carbon stock and long-term CO2 fixation in the biomass of the studied plantations to deduce their contribution to climate change mitigation. Non-destructive inventories were carried out during three consecutive years in 20 E. saligna and 12 G. robusta plantations in Sake and Kirumba. Analysis of the data revealed that both species have similar diametric growth while height growth and productivity were significantly higher in the E. saligna plantations. The productivity of E. saligna was also higher in Kirumba than in Sake, while that of G. robusta was higher in Sake than in Kirumba. The differences observed were mainly related to species, silviculture, altitude and concentration of bioavailable elements in the soils. The analysis of productivity evolution over time allowed us to determine optimal rotations at 8 and 12 years, respectively, for E. saligna and G. robusta plantations. The relationships between biomass or carbon stock and tree diameter were not different between the studied species but were significantly different at the stand level. If silviculture was standardized and plantations carefully monitored, carbon stock and long-term CO2 fixation would be higher in G. robusta plantations than in E. saligna plantations. These results indicate that while for productivity reasons E. saligna is the favoured species in wood-energy plantations to quickly meet the demand of the growing and disadvantaged population living in the vicinity of PNVi, carefully monitored G. robusta plantations could be more interesting in terms of carbon credits. To simultaneously optimise wood-energy production and carbon storage in the plantations initiated in North Kivu, E. saligna and G. robusta should be planted in mixture. In addition, species and site characteristics adapted silvicultural management practices must be applied to these plantations, which are very important for the region, its population and its park. Finally, the economic profitability as well as the sustainability of the plantations should be assessed in the longer term in North Kivu