Avaliação dos teores de nitrogênio, fósforo, potássio, cálcio e magnésio, e variação natural do δ¹³C em florestas de inundação na Amazônia Central

Abstract

In this study, the results of N, P. K, Ca and Mg contents in canopy and understory leaves, stem wood and litter in floodplain (várzea and igapó)² in the central part of Amazonia are shown. Signifícant differences were noticed in the nutrient contents of the two kinds of vegetation. Wetland forests were the ones that showed the highest contents of those nutrients. That could be the effect of the sediments brought down from the Andes, which are being deposited on the várzea every year. Different patterns of distribution of foliar nutrients were noticed between the two kinds of vegetation, varying from nutrient to nutrient. In the várzea forests the nutrient contents varied in the following order: N > Ca > K > Mg > P, while in the igapó forests the sequence was: N > K > Ca > Mg > P. Among the families and species that were studied it was not possible to notice a distribution pattern for the elements analyzed, once the variation found within the same species were as large as those noticed among the different families. The variation can be due to environmental or even to genetic factors of the sampled plants. The patterns for the use of phosphorus and calcium in várzea forests were less efficient when compared to igapó forests, which themselves were less efficient than terra firme forests. For the nitrogen it was not possible to notice any pattern clearly. The possible explanation for such a fact it that there could be a second source in the biological nitrogen fíxation from the atmosphere. In the forests studied the leguminosae trees (2.91 %) had a higher nitrogen content than non-leguminosae ones (2.02%). The average values of δ¹³C show that there is a rising gradient between the under canopy (understory leaves) (-32.9: -33.5%0) and canopy (-32.2;-32.9%0) leaves for várzea and igapó, respectively. This fractionating ¹³C / ¹²C could be due to a higher contribution of CO2 from biogenic origin. The values of δ¹³C found in the different compartments in both sites, várzea and igapó, do not differ in a significant way. However, significant differences were noticed among the averages of δ¹³C in the different families that were studied.