Contribuição de espécies arbóreas para a ciclagem de nutrientes em florestas tropicais seca e úmida

Abstract

Nutrient cycling is a natural process, responsible for interactions between forest vegetation and soil. The production, accumulation and decomposition of the litter are the steps that enable this cycling. In tropical forests this cycle changes with the forest environment. The largest amount of litter deposited is of foliar origin, varying nutritionally according to the species. Studies evaluating nutritional contents in forest ecosystems find different results regarding the order of nutrients. In tropical environments this differentiation is observed in both dry and wet forests and may be related to vegetation. Therefore, the composition of the species in each environment has a different role in the nutrient dynamics. Thus, the following hypotheses were tested: i: nutrient content, content and efficiency should vary between species and between environments, ii: in more fertile environments efficiency is more individualized among species, iii: nutritional contents in the dry forest are greater than in the rainforest, however, the contribution of litter is lower; iv: decomposition rates should vary between species and between environments, in the dry forest moisture limitation promotes slower decomposition. This study aims to evaluate the dynamics of nutrient cycling in dry and humid tropical forests and the contribution of each species in this cycle, determining the stocks in leaf biomass, litter and soil. The work was carried out in two areas with different typologies, both in Pernambuco, an area of Atlantic Forest and Caatinga. The chemical composition, content, nutritional efficiency, litter production and decomposition were studied in the species with the highest absolute density by phytosociological study. The biological efficiency of nutrients N, P, K, Ca and Mg showed variation among the environments, presenting higher N and Mg efficiency in the rainforest and for P and K in the dry forest. In the dry forest, the species Pilocarpus spicatus and Guapira opposita were important for nutrition; in the rainforest, Dialium guianese stood out. The litter production of the species in the dry forest environment was higher than the amounts collected in the rainforest environment. The species Metrodorea mollis, Guapira opposita and Brosimum Guianense stood out. The contents of the litter Dialum guiamnense and Pilocarpus spicatus were higher. The contribution of nutrients was influenced by the biomass emphasizing Metrodorea mollis, Guapira opposita and Brosimum Guianense. In both environments it was possible to identify species that presented different patterns in the decomposition process, such as Guapira opposita and Brosimum guianense. Among environments it was observed that decomposition in tropical rainforest occurs at a faster rate than the dry forest. Absorption and utilization rates were higher in rainforest species, while the cycling index was higher in dry forest species, as a result of the relationships between nutrient stocks in leaf biomass and litter of the species, besides the stocks in the soil of these environments.