DIVERSIDADE E ATIVIDADE MICROBIANA DO SOLO AMAZONICO SOB ESTRESS HIDRICO 

Selma Batista (UFRJ); Paulo Hargreaves (UFRJ); Lêda Mendonça-hagler (UFRJ)

Amazon Rainforest is characterized by its high biodiversity. In extreme drought conditions the forest is susceptible to fires. Climatic changes resulting in alterations on water availability can affect ecosystem functioning. The Dry Forest Project was conducted at National Forest of Tapajós, Brazil, aiming to evaluate the effect of prolonged drought on Amazon Rain Forest. Two one-hectare plots were selected based on similar vegetation structure and richness. The dry forest (DF) plot was covered to restrict rain fall reaching the soil and the primary forest (PF), a control plot which was not covered. Our objective was to study the effect of hydric stress on soil microbial communities and their enzymatic activity within the Dry Forest experiment, with the aid of conventional and molecular biology techniques. Genotypic characterization of bacterial isolates was done by BOX-PCR; bacterial community profiles were assessed using 16S rDNA-DGGE profiles (Eubacteria, Proteobacteria, Actinomicetales). Soil analysis revealed that both soils were acid (pH 4.0). Counts of heterotrophic bacteria, fungi, cellulose degraders, proteolytic bacteria and phosphate-solubilizers were determined (12samples/plot). A decrease on microbial counts was detected at the dry (DF) soil, reaching two orders of magnitude for cellulose degraders. Enzymatic activity tests indicated that in DF soil, cellulase, protease and phosphatase activities were reduced by 40, 60 and 30% respectively, in comparison to PF control soil. Bacteria isolates were mainly Gram-positive bacilli (62% DF soil and 47% PF soil). The prevalent genera isolated from these soils were, Bacillus, Corynebacterium and Micrococcus. Gram-positive rods were further investigated by BOX_PCR and revealed high genetic diversity in both soils. 16S-rDNA DGGE profiles were on average 80% similar between DF and PF soils. Differences were better resolved when working with specific bacterial groups. This study did not reveal significant changes to the Amazon rainforest soil microbiota as a result of drought. It appears that the soil maintains a diverse population of microbes that are resistant to environmental changes. Enzymatic activity decreases in the absence of rainfall resulting in increasing leaf litter. The danger to the forest under dry conditions may not be from a loss in microbial activity or function but rather an increased risk of forest fire.