Avaliação da sustentabilidade nutricional de plantios de Pinus taeda L. usando um balanço de entrada-saída de nutrientes

Abstract

The knowledgement of the nutritional sustainability of the practices used in plantations is necessary for its adequate management, mainly when used in low fertility sites, with slash-burning, and with no fertilizer application, like P.taeda plantations in south of Brazil. Given the increasing importance of the species, a study was done to investigate the nutritional sustainability of cenarios, based on eight representatives sites of Parana and Sao Paulo States. These sites, within 19 to 29 years-old, were measured and the nutrient stocks was determined by sampling 4 tress per site, plus the forest floor. Soil nutrient stocks were also established based on soil profiles description and analysis. Yield and nutrient stocks were adjusted for a 25-years cycle, together with the nutrient rainfall inputs. The studied cenarios were: i) harvesting (removing just debarked logs or the hole trees), ii) salsh-management (minimum cultivation or burning), iii) erosion (none or 2 cm per cycle). The number of nutrient cycles (N, P, K, Ca and Mg) to zero the soil- forest system stocks were used as the sustainability indicator. Simulations were carried out with a VBA implementd program. Mean annual increments ranged from 11 to 27 m3 ha-1 yr-1, with site index from 18 to 29 m (base age of 25 years). MAI was positively correlated with minerable N (r2 = 0,722) and available P (r2 = 0,702). These two nutrients also differ from the others due to their higher concentarion on the aboveground part of the soil-forest (50 and 66%, respectively). The cycle number analysis showed that N is the only nutrient that never has a positive balance. For all nutrients the most impactant practice is the burning, followed by the hole tree harvesting and then by erosion. The average nutritional limitation risk was: N > P > Mg > K,Ca, based on positive balances and number of cycles. Taking N as the best indicator for sustainability, the most productive sites tend to be the less sustainables, considering their growth rate. The long term sustainability of P.taeda palantations depends on the use of conservacionist practices together with fertilization practices in some cases.