Biomassa e nutrientes no corte raso de um povoamento de Pinus taeda L. de 17 anos de idade no município de Cambará do Sul - RS

Abstract

This study was conducted in a 17 years old Pinus taeda stand in Cambará do Sul county (RS), in a typical Haplumbrept soil, and had as objectives: to evaluate the biomass and nutrients in the different trees components and in the litter accumulated upon the soil; to determine the nutrients variation along the stem, in the bark and in the wood, and to determine the relative height to take samples to evaluate the nutrients content in these components; to quantify the available nutrients stock in the soil down to 60 cm of depth; to evaluate the nutritional impact from different biomasses harvest intensities; to establish a nutritional balance and to estimate the number of possible rotations based on the nutrients stock available in the system and the nutrients input by rainfall. Biomass and nutrients were estimated using regression equations adjustments, with the harvest (cut) of 18 trees distributed in 6 diametric classes and the litter based on sampling units (with known area). Nutrients stock in soil was determined based on soil density and the amounts of nutrients contained on each 10 cm depth layers. The sampling point of bark and stem wood was determined using confidence interval, with the nutrients amount evaluation at 10%, 30%, 50%, 70% and 90% of the trees height, using as reference the average amount according to the biomass. The nutritional impact considered the nutrients removal due to the above ground biomass intensity utilization. Nutritional balance and the number of rotations, as a function of three biomass harvesting methods, considered the removal of nutrients at the 9 and 12 years thinning and the clear cutting at 17 years; the available nutrients stock in the soil and in the harvesting residues and the inputs by the rainfall. The above ground biomass was estimated as 253.56 Mg ha-¹, being 62.4% in the stem wood, 13.6% in the roots, 8.9% in the live branches, 7.4% in the bark, 4.8% in the needles and 2.9% in the dried branches. Considering the total biomass above ground, the nutrients stock was (kg ha-¹): N = 567.90; P = 42.37; K = 206.66; Ca = 261.19; Mg = 96.03; S = 93.87; B = 1.73; Cu = 0.84; Fe = 13.36; Mn = 11.09 and Zn = 1.43; the stem, the wood and the bark contributed with 44.0%, 39.0%, 53.8%, 50.5%, 52.9%, 61.6%, 50.4%, 37.6%, 41.2%, 54.8% and 52.9%, respectively, for the total N, P, K, Ca, Mg, S, B, Cu, Fe, Mn and Zn amounts. Litter showed a biomass accumulation of 14.93 Mg ha-1, containing (kg ha-¹): N = 152.73; P = 7.79; K = 10.05; Ca = 47.78; Mg = 12.87; S = 8.94; B = 0.20; Cu = 0.17; Fe = 85.05; Mn = 4.49 and Zn = 0.41. Soil is upper layer nutrients stock available was (kg ha-1): N = 1205.8; P = 15.5; K = 209.5; Ca = 1838.0; Mg = 449.7 and S = 163.3. The most adequate point for collecting bark and wood stem samples is at 30% of the total height. In the simulations studied, an increase in biomass utilization decreased the site capacity to maintain successive culture cycles, starting from a estimative of 17 years; these were: 17 for N, 27 for Mg, 41 for Ca, and undefined cycles (sustainable) for P, K and S, if only the stem wood is removed; and 3 for P, 4 for N, 7 for K, 7 for Mg, 9 for Ca e 10 for S, if the total above ground biomass is harvested.