Ciclagem de nutrientes em função da sazonalidade climática e do gradiente de altitude em floresta tropical seca

Abstract

The sustainable development of the Araripe region, in Pernambuco, Brazil, should be based on the conservation of its natural resources. The native domain of this area is the dry tropical forest (Caatinga) which has as main climatic characteristic long periods of drought in addition to present areas of high altitudes. This research aimed to evaluate forest nutrition and nutrient cycling as a function of climatic seasonality and altitude gradient in fragments of dry tropical forest. The fragments are located in the municipality of Araripina/PE and were classified in three environments, according to altitude: Lowland (<600 m), Slope (600 to 750 m) and Plateau (>750 m). The evaluations occurred during the wet, wet-dry, dry and dry-wet periods between 2016 and 2017, classified according to the municipality historical monthly rainfall in the last 20 years. In each environment, the phytosociological survey was carried out to define the five species with the highest absolute density. In the dry and humid periods, the pH and the TOC contents, N, P, K+, Ca2+ and Mg2+ were determined in the soil. In leaves, the N, P, K, Ca and Mg contents were calculated and their contents and the biological utilization efficiency of these nutrients were calculated. Also, 20 plots per environment were installed, where the production and the accumulation of litter were quantified. In their most representative fractions, the content and the contribution of N, P, K, Ca and Mg were determined and the rate of decomposition of the accumulated litter and its renewal times was estimated. Observing N, P, K, Ca and Mg stocks in leaf biomass, leaf litter and soil, the biological cycle of these nutrients was evaluated by means of absorption, utilization and cycling rates. The efficiency of biological utilization of nutrients was concentrated in few species in more fertile soil and, in less fertile soils, shared among the species. The TOC and nutrient contents were concentrated during the dry season and in the topsoil. The ability to absorb nutrients was shared between species in the dry period and concentrated in a single species in the wet period. In relation to the litter, its production was influenced by the altitude gradient, of seasonal character, with additions in the dry period. Its accumulation was greater in the Lowland. The concentration and the contribution of the most representative fractions of the litter produced and accumulated presented the descending order: Ca>N>K>Mg>P. In addition, the dry period compromised the decomposition of accumulated litter which, in the Lowland, slowly decomposes. In the wet period, the decomposition times were higher for the Plateau. In the dry period were higher for the Lowland. The fraction of branches and barks is the one that stays the longest on the forest floor. As for the biological cycle of nutrients, Slope showed the highest rates of absorption and use. The cycling of the cations K, Ca and Mg was higher in the Lowland, whereas for the N and P, in the Plateau. The Lowland was the environment with the highest rate of cycling.