Nutrientes e carbono no solo em áreas com diferentes Sistemas de uso da terra na região do Alto Solimões (Benjamin constant- AM)

Abstract

The present work was conducted in two traditional rural communities in the upper Solimões region, in the state of Amazonas. Its purpose was to evaluate the soil chemical characteristics (organic C, total N, P, K available, Ca, Mg and Al exchangeable) in areas cleared for cultivation established following the clearing slashing and burning of different-aged primary forests and secondary growth plots (3, 5, 10 and 20 years of fallow period). It also studied how the time of the fallow period influenced the fertility in the soil of forest and secondary growth plots. An entirely random design with subdivided plots with three replicates, was used. The systems studied in the plots were related with the burning and of the vegetation, with and without the burning and, in the subplots, the fallow time (3, 5, 10 and 20 years) and the forest, as control. Four secondary growths, one forest and five areas cleared for planting, which summed up to ten systems that when replicated in both communities totalled 20 systems, were evaluated. In the local edaphoclimatic conditions, the primary and secondary vegetation slash and burn for the implantation of planting areas, promoted an increase on the contents of phosphorus and potassium, and exchangeable calcium and magnesium. Nonetheless, there was no significant effect either of the burn or the time of the fallow period on the soil s organic carbon and total nitrogen contents. The secondary growth fallow period time contributed to a significant increase on the pH index and exchangeable calcium and magnesium contents in the soil, as well as to the reduction of exchangeable aluminium. Onthe other hand, there was no increase on the available phosphorus and potassium contents. The change on exchangeable calcium and magnesium contents suggests that short fallow periods may not be sufficient for the reposition of the contents of those nutrients in the soil, being liable to compromise the yield on the cleared for planting and secondary growth areas. In these Cambissolos, the findings point out five years of fallow to be the minimum time needed for attaining exchangeable calcium, magnesium and aluminium values equivalent to those found in a twenty tear old fallow period.