The Amazon Basin covers almost 25% of South America. With about 7.5 million km2, it extends into the territory of nine countries and accounts for 70% of tropical forests around the globe. Only in Brazil the total area is 5.1 million km2 (Fearnside, 2005). Despite its great beauty and exuberance, the Amazon rainforest is found in soils of low fertility, while its maintenance depends on the cycling of nutrients from vegetation covering (Cenciani et al., 2009).

The quality and soil fertility are defined from the point of view of some essential attributes that maintain the agricultural productivity, namely as: soil ability to promote plant growth, water supply and nutrient processing, efficient gases exchange in the atmosphere-soil interface and the activity of micro and macro organisms (Dilly & Nannipieri, 2001). In this context it is highlighted the role of soil microbial biomass (SMB), defined as the living portion of soil organic matter, excluding roots and larger organisms than, approximately, 5000 pm3. The microbial biomass comprises the dormant and the metabolically active organisms in the soil; performing a primary role for maintenance and the products of microbial recycling are then absorbed by plant roots (Cenciani et al., 2009).

Soil quality or even "soil health" can be analyzed by the activity of microbial biomass, one of few active fractions of organic matter, sensitive to tillage and that can be quantified. Overall SMB comprises about 2-3% of total organic carbon in the soil, thus indicating it to be a sensible parameter to evaluate the quality of soils submitted to different management strategies, or to pollution impacts. The development of indirect methods for measurement of SMB such as the incubation-fumigation (IF) (Jenkinson & Powlson, 1976), the substrate induced respiration (SIR) (Anderson & Domsch, 1978), the content of ATP in microbial cells (Jenkinson & Ladd, 1981) and the extraction-fumigation (EF) method (Vance et al., 1987) facilitated the assessment of the SMB compartment.

Some studies previously carried out in chronosequences forest to pasture in Amazonia have shown that SMB is reduced after 3 years of establishing pastures, but their levels are raised in older pastures, and reach similar contents in the native forest. Several studies quantified the main elements (C, N, P, S) immobilized into microbial cells at different soil depths (Feigl et al., 1995 a, b; Fernandes et al., 2002; Cenciani et al., 2009).

Overall SMB reflects the contents of total organic matter, representing an efficient and sensitive parameter in assessing the quality of soils under different management or impacts of pollution. In Brazil, some studies realized in chronosequences forest to pastures in Amazonia have shown that microbial biomass is reduced in the early years (about three to five years), but increases in older pastures reaching levels similar to those of the native forest (Feigl et al., 1995 a, b; Fernandes, 1999). The ability of SMB to increase again in older pastures, reaching values closer to the native forest suggests that the microorganisms of such soils have high resilience, or the capacity for growth and physiological activity, even after the impact of slash-and-burn of the native forest.

The stability of a system determines its ability to continue working under stress conditions, for both natural and those induced by human action (Orwin & Wardle, 2004). Since the microorganisms are the key players of the conversion of soil organic matter and the availability of nutrients, its resilience directly affects plant productivity and the stability of forest and agricultural ecosystems (Orwin & Wardle, 2005). For this reason it is essential to understand how microorganisms respond to environmental disturbances, as well as the factors involved in this response.