The effects of slash management on nutrient cycling and tree growth in Eucalyptus plantations in the Congo

Since 1978, 42,000 ha of clonal Eucalyptus plantations have been established around Pointe-Noire. These plantations are growing on sandy and acidic soils with small reserves of available nutrients; therefore, risks of nutrient deficiencies exist in the medium or long-term. The effects of slash and litter management practices on soil properties, tree nutrition and growth were quantified to evaluate harvesting methods with respect to sustainable management of the replanted sites. The experimental design was a randomized complete block with four replications and six treatments: R, all aboveground organic residues removed; WTH, whole-trees harvested and removed; SBH, stemwood and bark removed; SH, stemwood-only removed; DS, double slash retained; B, stemwood-only removed and all organic residues burned. After 12 months removal of all slash material (R) had a marked negative effect on total biomass (5.9 t ha-1 vs a mean of 7.2 t ha-1 for the other treatments). During the first 18 months of the study no significant differences among treatments were detected (P < 0.05), but at 24 months, tree growth was significantly reduced where all organic residues were removed (R), compared to DS (12.9 m3 ha-1 per year vs 22.2 m3 ha-1 per year). The best growth was measured in plots with the highest organic residues (treatments DS and SH). Nutrient content in the aboveground biomass of the I-year-old stand was strongly dependent on the type of slash and litter management. Differences in tree nutrient concentrations in various treatments demonstrated the ability of trees to store available nutrients in aboveground biomass. The rate of decomposition of organic residues during the first 8 months after harvesting ranged from 36% (WTH) to 56% (SH). Between 7 and 24 months, the annual production of inorganic nitrogen in the top soil (0-13 cm layer) was 71.6, 66.7, and 85.7 kg ha-1 per year for R, SH, and DS, respectively. The rate of soil nitrification was high, and therefore nitrogen leaching may have occurred in the early development stage when trees are not able to take up all the available nitrogen.