Influência da forma e posição da encosta nas características do solo e na regeneração natural de espécies florestais em áreas de pastagens abandonadas.

Abstract

The aim of this work was to evaluate the influence of the position along a toposequence and hillslope form on chemical and physical-hydric soil properties, and their relationship with the natural regeneration of vegetation in areas of abandoned pastures. A total of eight areas in the municipality of Itaboraí - Rio de Janeiro State were evaluated. In the shoulder, backslope and footslope positions of the relief toposequence concave and convex slope shapes were identified. Two experimental plots (10 x 20 m) were installed in each section of the toposequence, one for each slope shape, and evaluations related to soil and vegetation type were proceeded. Chemistry and physical-hydric soil characterization was evaluated at 0-10cm depth. Chemical analysis (pH, Al, Ca, Mg, P, K, C e N) and physical hydric attributes of soils based on granulometry, porosity (macro, micro and total porosity), soil bulk density, hydraulic conductivity, and soil mechanical resistance to penetration (RMSP) were carried out. The analysis of vegetation consisted of dry matter evaluation, aboveground regrowth rate of pasture and quantification, and species richness of trees and herbaceous shrubs in natural regeneration. The areas were grouped into 2 based on textural class: Group 1, the clayey areas, and Group 2 medium texture areas. Soil fertility in all the areas showed low levels of sum of bases and base saturation, and high levels of aluminum saturation, which were not influenced by topographic conditions, although the concave slope shape seem to promote the removal of bases. The soil organic carbon and total nitrogen contents were highest on the clayey texture areas. Chemistry and physical-hydric characterization of soil differed among different topographic conditions in medium texture areas, where the shoulder and backslope showed higher porosity (macro and total porosity). The RMSP was not influenced by topographic conditions. More than 90% of the analyzed points showed declivities ranging from 20 and 75 %, reducing values of pH, SB and V (%), and increasing values of m (%). The grass biomass values exceed 10 Mg/ha. The regrowth rate was high showing an average ranging from 984 to 1.356 kg/ha/month, and the clayey areas had lowest values. Low species richness was observed, with greatest diversity of arboreal growth on concave slope; while the herbaceous shrub regeneration tended to be highest in backslope and footslope. The regeneration in terms of species richness was lowest in B. humidicola areas, for both strata and in herbaceous shrub cover. A complementary analysis showed that the geographical orientation of the slopes had a close relationship with the values of natural regeneration, while south-facing slopes showed highest species richness. The backslope in concave slopes showed lowest regrowth in the pasture, and these areas were identified as those with best natural regeneration when compared to other areas studied in this research.