Dinâmica de nutrientes na solução do solo em um sistema agroflorestal em implantação.

Abstract

The agroforestry systems constitute an alternatíve to produce foods, wood and other non-wooden products, also playing an excellent role in the conservation and maintenance ofthe functions oftropical ecosystems. That being so, a agroforestry system with maize, cowpea, jack beans and cassava in rotation were studied in the area of CEPLAC, following the procedures ofthe Program" SHIFT - Studies on Human Impact on Forests and Floodplains in the Tropics ", in Benevides, Pará, near Belém. During the studied period between 1995/1996, the following parameters were registered: precipitation and saturation ofthe soil water (tensiometers in the depths 10, 45 and 145 cm), the chemical composition of the solution of rain (rain gauges) and the ground (lysimeters in depths 25, 40, 60 and 110 cm), the exported harvest ofthe field and its content of nutrients, and the residue from the remaining biomass in the field and its content ofnutrients. The emphasis was in the study ofthe fluctuations in the chemical composition ofthe soil solution and in its dependence on the human activity in the field, aiming to get adaptable information to the reality ofsmall and medium producers. The most important observation verified was that the N was the element with the biggest relative importance in this work, for being present in bigger amounts in the soil solution in the band of25 cm, followed by the Ca, Mg, K, CI and Na. The effective participation ofthe CI and the Na in the cycling processes was a surprise, probably generated by the high amount ofthese in the rainwater. The S-SO4 and the AI had little importance for presenting small amounts. The other elements, Fe, Mn, H, P-PO4 appear in very low XI amounts. A system of rotational handling, and in association, suggests a faster and more intensive dynamics, thus the concentration of the elements studied in the soil solution was always higher in the agroforestry system than that in the forest. However, the observations during the studied period showed that the Na, CI and AI are less sensitive to the handling of the vegetation, whereas the Ça, Mg, K, N-NH4, N-N03 and S-S04 are more affected by the developed activities inside the agroforestry system. Specifically regarding the Na and CI, the concentration of these elements in the forest suffered influences :from the action of rain, :from impact of the dry deposition (bum) and :from the sea water, even so the participation of these ones in the agroforesty system is corroborated. The handling activities developed inside the agroforestry system (plantation, weeding, thinning, harvesting, pruning) generated additions and losses of nutrients promoting a fluctuation in the course of time. These fluctuations were observed mainly for the Ca, K, N-N03, CI and Na. The K was put into motion faster than the Ca in the soil solution. The S-S04 presented a small fluctuation, contrary to the N-N03 and the Cl. For the N-N03, Ca, Mg and K, the cause of the fluctuations was the sarne, that is, release and absorption.