Ciclagem de nutrientes e decomposição de serapilheira em plantio florestal, na região de Itaparica, Pernambuco

Abstract

In areas where species of economic interest are cultivated, soil nutrient exports are high. Concomitantly, the semi-arid soils are often of high natural fertility and the use of biochar may protect this fertility for a longer period, especially in pure plantations of forest species. The establishment of pure plantations of angico and exotic species in brazilian semi-arid, such as eucalyptus, can compromise the contribution of litter and nutrient cycling, thus, it is important to know the litter supply capacity and nutrients of these crops in a semi-arid environment, where mineral fertilization is not recommended. Therefore, the objective of this study was to evaluate changes in soil chemical attributes and nutrition of pure plantations of eucaliptos (Eucaliptus urophylla x Eucaliptus tereticornis) and angico-vermelho (Anadenanthera colubrina var. Cebil), at 41 months under fertilization of lake sediments, fishery tank residues and biochar, produced by means of the burning of algaroba remaining in the area, as well as to evaluate the contribution of litter and nutrients of these in addition, estimating the rate of decomposition and nutrient release of eucalypt leaf material in the Pernambuco’s semi-arid. After 41 months of application of the natural fertilizers and biochar (July 2017), samples of eucalyptus and angico leaves were collected, as well as the soil of the experimental area was sampled. For leaf analysis of N, P, K, Ca and Mg, twenty - five leaves of the middle third of the crowns of the trees were collected. Soil samples were also collected up to a depth of 0.30 m, 15 cm apart from the base of the trees. The litter deposition was quantified and the contentes of N, P, K, Ca and Mg were determined at 45, 90, 135, 180, 225, 270, 315 and 360 days, allowing to calculate the nutrient supply over time. The litter was fractionated into twig, leaf, reproductive material, miscellany and bark. To evaluate the decomposition of the leaf fraction of eucalyptus were used nylon bags and the remaining weight of these bags, as well as the contents of N, P, K, Ca and Mg were determined in the same time intervals used to evaluate litter deposition. The litter decomposition constant and the nutrient release constant in the eucalypt leaf material were estimated, as well as the half-life and time for 95% of the leaf material to be decomposed and to release the nutrients. The soil K+ and P contents were influenced by the natural fertilizers and the biochar increased the C content of the soil. Angico cultivation reduced soil K + and P levels and eucalyptus cultivation reduced Mg2+ levels. The fish tank residue was responsible for the elevation of P levels in both eucalyptus and angico, and was recommended for naturally poor soils in P. The angico had the highest levels of N, P, K and Ca, suggesting a high demand of this species by these nutrients. The eucalyptus was more demanding in Mg than the angico, suggesting that its cultivation in semi-arid environment should be recommended in soils rich in Mg2+. The eucalyptus litter contribution was higher than that of the angico, but both were influenced by rainfall, in which the highest depositions occurred in the dry periods of the year. The application of biochar negatively influenced the deposition of eucalyptus litter and had no effect on angico. The material contributed by angico was richer nutritionally than eucalyptus, but the contribution of nutrients of leaf fraction of eucalyptus litter was higher than that of angico. The decomposition rate of eucalyptus leaf litter was 1.5 mg day-1, suggesting that the eucalyptus cultivation in an ecologically based production system, managed with natural fertilizers, the microbial activity is intensified, accelerating the nutrient cycling. The nutrient release time of the leaf fraction of the eucalyptus litter in this ecologically based production system was a maximum of 33 days, suggesting that the foliar deposition being renewed within this time interval, so that the continuous nutrient cycle be preserved.