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2011) and moderate P availability (Laclau et al. They are acidic with low Al and Fe oxide amounts (Mareschal et al. 2011), nutrient-poor with low soil organic matter (SOM) content, i.e., low C and N concentration and CEC (Nzila et al. These soils are mainly coarse textured Ferralsols (Mareschal et al. This includes savannas of the Congolese coastal plains on inherently nutrient-poor soils with low potential for agriculture. Large areas of the native savannas span on over 6 million ha in Central Africa (Schwartz and Namri 2002). For this, the forest floor plays a key role in tightening the P cycle, minimizing the P losses. This shows that interactions between plants, microorganisms and soil can sustain the demand of this ecosystem. However, the mixed-species forest stands immobilized greater P in organic forms, preventing the P losses by leaching and contributing to sustain the P demand in the medium term. In relation with the fast mineralization in this environment, highly soluble orthophosphate was the dominant P form in both forest floor and soils. SOM were related to the OM recalcitrance of the forest floor, indicating this higher microbial activity of the forest floor in this stand might be favouring the incorporation of C into the mineral soil. Thermal analysis and 13C NMR analysis revealed a more advanced stage of humification in forest floor of acacia-eucalypt stands, suggesting a greater microbial activity in its litter. Thermal analysis (differential scanning calorimetry (DSC) and thermogravimetry (TGA)) and nuclear magnetic resonance (solid state 13C CPMASS and NMR and 31P-NMR) spectroscopy have been applied to partially decomposed forest floor and soils of pure acacia and eucalypt, and mixed-species acacia-eucalypt stands. In the present study, OM quality and P forms in forest floor and soil were investigated to get more insight on the C and P dynamics useful to sustainable management of forest plantations. In spite of this, the relationship between SOM quality, C and other nutrient dynamics are not well understood. Therefore, SOM stability might play a crucial role in regulating soil-plant processes. In tropical environments, most of the nutrient processes are determined by the high rates of organic matter (OM) mineralization.
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However, some indicators suggest a perturbation in SOM and P dynamics might affect the sustainability of the system in the medium and long term. Eucalypt trees were expected to benefit from N 2 fixed by acacia. Acacia was introduced in eucalypt plantations established in the Congolese coastal plains to improve soil fertility and tree growth. Land-use change and forest management may alter soil organic matter (SOM) and nutrient dynamics, due in part to alterations in litter input and quality.