José Frutuoso do Vale Júnior

Gênese, química e mineralogia de solos derivados de sedimentos pliopleistocênicos e de rochas vulcânicas básicas em Roraima, Norte Amazônico

The influence of parent material, varying from Plio-Pleistocene sediments and basic volcanics, and topographic position, to soil genesis, was studied in a topolitossequence typical of savanna at the Cauame River catchment, Roraima. We aimed to describe the soil variability under savanna by means of chemical, physical and mineralogical attributes. Fourteen soil profiles were described and samples, allowing the identification of the following classes: Yellow Latosol, Red Latosol, Yellow and Red-Yellow Argisols, Plinthosols, Gleisols, Fluvic Neosols and Vertisols. The mineralogy was determined by DRX. The pedogenetical processes and soil characteristics were closely related with topographical-hydrological variation and underlying lithology. Soils derived from pre-weathered sediments of the Boa Vista Formation are generally of low fertility, with high acidity and kaolinite mineralogy and low Fe-oxides contents. Soils developed from Basalt or under the influence of alluvia with mafic contribution all showed high base saturation (eutric) and clay mineralogy with the presence of 2:1 clays, with varying amounts of kaolinite. The occurrence of Vertisols, degraded carbonatic nodules and eutric soils under savanna indicate Quaternary drier paleoclimates in this part of north Amazonia, during which carbonates were formed in the lower horizons of Vertisols, in closed depressions.

Atributos físico-hídricos de um Argissolo amarelo sob floresta e savana naturais convertidas para pastagem em Roraima

The broad soil and ecosystem diversity of the state of Roraima is gradually being replaced by pasture and, depending on the management system adopted, this can be associated with soil physical degradation. Within this context, the aim of the present study was to evaluate and compare the soil physical attributes of an Argissolo Amarelo (Ultisol) under savanna and forest natural ecosystems, and their conversion into a cattle pasture system. The primary treatments were natural savanna (NS), savanna converted to pasture (CS), natural forest (NF) and forest converted to pasture (CF). The secondary treatments were the soil sampling depths of 0-10, 10-20 and 20-40 cm. The following physical properties were evaluated: texture, bulk density (SD) and particle density (PD), resistance to penetration (PR), total porosity (TP), gravimetric moisture (GM), available water capacity (AWC), and soil organic matter (SOM). The SD was greater in both savanna areas, without differences between CS and NS at any depth, associated with the cohesive character of savanna soils. The average PD value was typical of kaolinite soils. PR varied as a function of the areas and the depths; the effect of conversion on this property was observed. TP was greater in NF and less in CS, and TP decreased along with depth. AWC was greater in NF only at the 0-10 cm depth. SOM was greater in the NF and CF areas. The study concluded that conversion of the natural environment into pasture systems caused significant changes in PR, SD, AWC, TP and SOM, with major expressiveness in CF, due to the removal of natural vegetation, leading to the loss of SOM, which is an important conditioner of soil physical and water properties. In CS areas, suppression of the natural vegetation did not lead to the same expressiveness, mainly due to the significantly lower SOM contents.

RELAÇÕES SOLO–VEGETAÇÃO EM “ILHAS” FLORESTAIS E SAVANAS ADJACENTES, NO NORDESTE DE RORAIMA

Studies on soils of forest islands within the savanna domain are key for understanding processes of landscape formation and evolution. We characterized the morphological, physical and chemical properties of soils at four different forest fragments that occur in the savanna-forest mosaic of northeastern Roraima, north Amazonia. The methodology was based on transects crossing the entire island, from east-west and north-south direction, digging up five soil profiles for sampling and classification. In addition, the neighboring savannas were also sampled following the same strategy, at 100 m long transects departing from the border, allowing comparisons to be made. Latosols were the dominant soil class in all four islands, followed by Ultisols and Plinthosols. All soils were dystric , with low CEC and acid. Better chemical and physical conditions were observed in forested soils compared with surrounding savannas, in a given soil class. Thus, in spite of no variation on soil class at different phytophysionomies at a given gradient, specific chemical and physical attributes were significantly varied, exerting a positive effect for the establishment of forest vegetation. Despite their occurrence side-by-side in the savanna- forest mosaic, the present -day climate agrees with the existence of savanna in the flat landforms, whereas forest islands are conditioned by subtle, yet significant, soil physico-chemical variations, with no need to invoke any paleoclimate for explaining this relationship. Further in depth studies may contribute for testing the hypothesis that Quaternary shifts of the expansion and contraction of forest may represent paleoclimate relicts isolated within the savanna domain.