Balanço nutricional e sustentabilidade da produtividade de povoamentos de Eucalyptus dunnii Maiden em função do método de colheita da madeira

Abstract

In forest stands, the maintenance of the productive capacity of the soil is directly related to the techniques used throughout the production process. Within this perspective, the forest harvest is one of the most impactful, due to its large export of nutrients through the removal of biomass. Therefore, the selection of the most appropriate harvesting method is essential to ensure the productivity of future rotations and the sustainability of the soils. In this context, the objective of the present study was to evaluate the impact of different harvesting systems on the stock and export of nutrients in Eucalyptus dunnii stands, with 11 years of age established in Uruguay. For this purpose, quantification of the biomass above ground of eucalyptus trees was carried out. Initially, three plots were delimited in the study area, in which the diameters at breast height (DBH) of all trees were measured. With this information, the diameters were divided into five classes, with two trees being divided per class, at ground level. The biomass was fractionated in the components: leaves, branches, stem wood, stem bark, pointer wood and pointer bark. These components were weighed and a representative sample was obtained from each of them, to check the moisture content and analyze the concentrations of nutrients. For the quantification of the litter accumulated on the soil, 24 samples were collected, with the aid of a 25 cm x 25 cm frame, which were weighed and sent to determine the concentrations of nutrients. Based on the amounts of nutrients in each of these biomass and litter compartments, a simulation of the export of nutrients in different harvesting systems was carried out - full tree (removal of all biomass from the tree above the ground), RMa + Ca (removal wood with bark) and cut-to-length (removing only the biomass from the stem wood) - the Biological Utilization Coefficient and the Potential Number of Rotations for each system were determined and, based on these results, were inferences were made regarding the sustainability of production. Above-ground biomass was 256.2 Mg ha-1, of which 79.6% was represented by the stem wood component, 11.3% by the stem bark, 6.0% by the branches, 2.4% by the branches. leaves, 0.6% for the pointer's wood and 0.1% for the pointer's bark. The total stock of macronutrients in the biomass was 1202.1 for Ca; 587.5 for N; 302.7 for Mg; 301.3 for K and 71.2 kg ha-1 for P, the micronutrient stock was 55692.7 for Mn; 21387.0 for Fe; 1563.8 for B; 1517.2 for Zn and 907.6 g ha-1 for Cu. In the stem wood there was a greater stock of nutrients, except for the elements K, Ca and Mn, which were allocated in greater proportions in the bark. The amount of litter was 18.2 Mg ha-1. In this biomass, there was an accumulation of 142.4; 123.5; 72.7; 24.5 and 7.0 kg ha-1 for the macronutrients Ca, N, Mg, K and P, respectively, and 22197.3; 4562.0; 392.4; 361.2 and 242.1 g ha-1 for the micronutrients Mn, Fe, Zn, B and Cu, respectively. Considering the cut-to-length system, compared to the full tree, there was a reduction in the export of nutrients of 78.2% to Mn; 71.4% for K; 64.2% for Ca; 56.5% for B; 55.1% for Mg; 49.6% for Zn; 45.1% for Cu; 44.1% for N; 31.2% for P and 30.2% for Fe. As for Biological Utilization Coefficient, it was found that, in general, micronutrients had the highest values and, considering the components of the tree, the wood was the element that obtained the highest rate of conversion of nutrients into biomass, followed by the components bark, branches and leaves. Analyzing the results of the NPR, it was observed that the harvesting only of the stem wood provided an increase in this parameter, in comparison to the harvesting of the whole tree, especially for Ca, which went from 5.4 to 17.0, and Mg, which was 11.8 to 27.4, in full tree and cut-to-length systems, respectively. Cut-to-length was the system that presented the least export of nutrients, due to the maintenance of residues, which is the most suitable for maintaining the nutritional sustainability of the soil.