Decomposition of logging residues in immature rubber plantations and effect on soil fertility

The decomposition of crop residues plays a crucial role in nutrient biogeochemical cycles, contributing to the maintenance of soil fertility over successive replanting cycles in tropical perennial crops. In rubber plantations, the main source of natural rubber worldwide, the role of crop residues (hereafter referred to as logging residues) in nutrient cycling has been little studied. This study aimed to gain insight into the dynamics of nutrient release during the decomposition of logging residues in intensively managed rubber plantations, until their complete decay. The impact of contrasting practices of logging residue management on soil nutrient stocks was determined over 3.5 years after clearcutting. The experiment was replicated at 2 sites with contrasting soil conditions in Côte d'Ivoire. Four treatments (replicated in four blocks at each site) were set up with increasing amounts of logging residues at planting. Initial stocks of nutrients, and then decomposition rates and nutrient releases were monitored over 3.5 years after clearcutting, for leaves, twigs, branches and trunks. Soil nutrient stocks were determined in the 0-30 cm layer from clearcutting (i.e., 6 months before planting) up to three years after planting. About 94 % of the total nutrients in logging residues were released over the first three years after replanting rubber trees. All nutrient stocks in the topsoil increased drastically in the treatments with large amounts of residues. Recovery rates were calculated to evaluate soil efficiency in retaining nutrients released from logging residues: for all nutrients (except N), recovery rates were below 100 % at both sites, indicating possible nutrient losses. This indicates that further studies are necessary for a thorough understanding of the fate of nutrients originating from logging residues, particularly regarding deep drainage losses and nutrient uptake by trees. This study highlights the importance of improving practices to maximize nutrient storage and uptake such as i) uniformly spreading logging residues across the plot, ii) sowing legume cover crops and iii) planting trees as early as possible after clearcutting. Leaving logging residues on-site, either partially or fully, could reduce dependence on mineral fertilizers and promote more sustainable natural rubber production.