Abundância natural de N em solos e folhas de áreas de campina e campinarana da reserva biológica da campina – INPA, região de Manaus, Amazonas

Abstract

Vegetations growing on white-sand soils, which are known as “campinas” are very much different of those on terra-firme dense forests, in terms of structure and dynamics. In addition, soils of “campina” are very poor in nutrients, presenting a more closed cycling. The aim of this work was to investigate the dynamics of N- cycling using the 15 N natural abundance of leaf and soil. The studied site was a gradient “campina” to “campinarana” (transition between “campina” and dense forest) vegetation at Reserva Biológica da Campina of INPA. It was determined the 15 N natural abundance of the soils and eight tree species that co-occur in the site. The main questions were: does the vegetation physiognomy influence the isotopic composition of the studied tree species? If there is a variation, is it influenced by the use of many strategies to acquire N-sources? And how different is “campina” 15 N natural abundance in relation to dense forest? Vascular epiphytes (Araceae, Bromeliaceae and Orchidaceae) that were present in the studied trees had their isotopic composition determined as well in order to compare their own signatures with their host ones and to determine their possible N souces. The δ 15 N values for the studied tree species were very similar to those found in other N-limitated ecosystems, ranging from -9,6 to +1,6 ‰. Aldina heterophylla Spruce ex Benth (Caesalpiniaceae) had more enriched values than the non-leguminous species, which could have been divided into two groups. This could be an evidence of the use of N sources on different depths or even from micorrhizal fungi association. It was observed a progressive enrichment in the isotopic signatures toward “campinarana” site. The observed values showed that rainfall and temperature are not the unique factor influencing the isotopic composition of these vegetations. Epiphytes δ 15 N values were higher than their host trees ones, also being divided into groups accordingly to the main N source acquired. The use of different N sources seems to be a common strategy for both epiphytic plants and tree species, since they occur in nutritionally stressed environments.