Abstract
The area of rubber plantations, the primary source of natural rubber, has increased significantly in recent decades. Monoculture management of rubber plantations can lead to a gradual decline in soil fertility as replanting cycles progress. The export of logging residues from the plots between two cycles would contribute to this impoverishment. In this context, fertilisation is a common practice to improve tree growth at young ages. The objective of this thesis was to study the influence of logging residue management on biogeochemical cycles and soil functioning in immature rubber plantations. The main nutrient fluxes were assessed during the first 3 years after planting in 2 industrial plantations in Côte d'Ivoire with contrasting soil conditions. On both sites, an experimental set-up consisting of a logging residue gradient (from total export to double restitution) coupled with fertiliser application or not was set up on 5 ha. The results show that the restitution of logging residues had a positive effect on the overall biogeochemical functioning of the plantation and that it did not lead to significant losses of elements through erosion or deep drainage. Tree growth, nutrient accumulation in the stand, soil health and soil nutrients stocks were improved in a scenario of logging residue restitution compared to a scenario of export. The cover crop planted in the interline is an essential compartment in the mineral and biological functioning of the soil at the beginning of the planting cycle. At this stage, a strong decoupling between the supply and demand of nutrient is observed: the nutritional needs of the rubber tree stand are low compared to the quantity of nutrient released by the decomposition of logging residues. Thus, the cover crop allows the conservation of a part of the nutrient resulting from this decomposition, by reducing losses through erosion and deep drainage, in a very intense biological cycle. The input-output balances of mineral elements at the soil level are positive in the first two years in the case of logging residue restitution, but they become negative for most elements from the third year of plantation, mainly due to the increase in stand requirements. This work shows that the restitution of logging residues, combined with the installation of a cover crop, is an appropriate practice to increase the productivity and sustainability of a rubber plantation. The combination of these two practices could (1) help avoid the decline in soil fertility associated with successive replanting cycles observed on some tropical sites and (2) support the high nutritional requirements of the stand at the beginning of the plantation cycle. Our results could be the basis for the development of a nutrient budget model to help manage fertilisation.