Boris Volkoff

Consequências do cultivo e do pousio sobre a matéria orgânica do solo sob floresta natural na Amazônia Oriental

In an area of yellow podzolic latossol, five differents conditions of soil use, were compared: natural forest, recently, one and five years of annual crops, and three years of fallow following two years of annual crops. The organic matter was characterized by grain-size fractionation method. In the soils after burring and one year of cropping, plant residues in the litter and 0-15 em layer werw much less abundant than under the natural vegetation. Such evolution continued up to five years of cultivation, but was cultivation, but was reserved when the soil was abondoned and fallow developed. Carbon of the very fine fractiones (less than 50 μm), which concentrates the clay-humic complex material, remained almost unchanged. Chemical fractionation of this material shewed that under annual crops the humus composition changed, and the amount of fulvic acids, principally those soluble in sodium hydroxide, increased, contrary to the humic acid. This results suggest that hamus incorporated non-polymeric material from the crops, and that a de-polymerisation process of the pre-existing clay-humic complex material ocuured. This process stopped under fallow and the humus then returned to its initial equilibrium.

Landscape and soil regionalization in southern Brazilian Amazon and contiguous areas: methodology and relevance for ecological studies

Soils of a large tropical area with differentiated landscapes cannot be treated uniformly for ecological applications. We intend to develop a framework based on physiography that can be used in regional applications. The study region occupies more than 1.1 million km2 and is located at the junction of the savanna region of Central Brazil and the Amazon forest. It includes a portion of the high sedimentary Central Brazil plateau and large areas of mostly peneplained crystalline shield on the border of the wide inner-Amazon low sedimentary plain. A first broad subdivision was made into landscape regions followed by a more detailed subdivision into soil regions. Mapping information was extracted from soil survey maps at scales of 1:250000-1:500000. Soil units were integrated within a homogenized legend using a set of selected attributes such as taxonomic term, the texture of the B horizon and the associated vegetation. For each region, a detailed inventory of the soil units with their area distribution was elaborated. Ten landscape regions and twenty-four soil regions were recognized and delineated. Soil cover of a region is normally characterized by a cluster composed of many soil units. Soil diversity is comparable in the landscape and the soil regions. Composition of the soil cover is quantitatively expressed in terms of area extension of the soil units. Such geographic divisions characterized by grouping soil units and their spatial estimates must be used for regional ecological applications.