Karina Cenciani1, Andre Mancebo Mazzetto2, Daniel Renato Lammel1, Felipe Jose Fracetto2, Giselle Gomes Monteiro Fracetto2, Leidivan Frazao2, Carlos Cerri1 and Brigitte Feigl1

1Centro de Energia Nuclear na Agricultura/Laboratorio de Biogeoquimica Ambiental

2Escola Superior de Agricultura "Luiz de Queiroz"

Brazil

1. Introduction

Amazonia is a natural region formed by the Amazon River Basin and covered by the largest equatorial forest in the world, covering an area of 6,915,000 km2, of which 4,787,000 km2 are in Brazil. Due to the large size and low population density, it is considered to be the best — well preserved Brazilian biome. Amazonian tropical forest soils are supposed to hold high microbial biodiversity, however the human impact has been extensive in the last decades, coupled with uncontrolled wood removal and the concomitant advancement of agricultural frontier (Fearnside, 2005).

Under the current scenario it is notorious the importance of Amazonia to the Brazilian ecosystem and even worldwide. Precisely because of this the images of slash-and-burn of the forest produce a strong impact on the public opinion. More than 60 million hectares were deforested. Of this total an estimated 35 million hectares were replaced by pastures for beef production, one million hectares were occupied with perennial crops, three million hectares with annual crops, and more than 20 million hectares support secondary vegetation called "capoeira" or fallow (Fig. 1).

What’s occurring in the pastures at the Amazonia, as well in other tropical regions is the loss of the productive capacity after 4 to 10 years of use due to overgrazing, invasion of unpalatable weed species, loss of soil fertility and cultivation of inadequate grass species (Fernandes et al., 2002). It is estimated that 30 to 50% of pastures in the Brazilian Amazon are in advanced stage of degradation, giving rise to the fallow sites. In general, the establishment of pastures is done with simple technology and no use of fertilizers. Its maintenance depends almost exclusively on the nutrients contained in the ashes produced during burning of the original vegetation. Fallows also play an essential role for recovery of native species, as it reassimilates part of carbon and nitrogen that were released when slash — and-burn of native vegetation was used (Fernandes et al., 2002; Schroth et al., 2002).

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 (Cenciani et al., 2009).

In recent years many technological advances and the development of new and independent cultivation techniques led microbiologists to explore more precisely the "black box" of soil microbial diversity. This new knowledge is contributing to our better understanding of the distribution and abundance of soil microorganisms, the effect of community structure on ecosystem functioning, the effects of land use changes on microbial communities and hence in the ecosystem.

Traditional methods were usually based on specific cultivation media in laboratory conditions, in which only 1-3% of the soil microbes present conditions for growth. For this reason much research have been developed using generic properties, such as the microorganisms basal respiration, enzymatic activity, mineralization of soil organic matter, among others, that under controlled laboratory conditions represent rough estimates of the metabolic functions of microbial biomass, reflecting its physiology as whole soil community (Ananyeva et al., 2008).

Considered one of the most important "hot spots" in the world, Amazonia has an important role in the discovery of new species of plants, animals and microorganisms, which may be important for the functionality of different ecosystems. However there are limited studies addressing the impacts of land use changes under the Amazonia microbial communities and their functions in the soil. Within this context bacterial and fungal communities, considered the most abundant groups of microorganisms in the soil, can act as important indicators of environmental stresses induced by the use of Amazonian soils.

Soil microbial diversity is usually assessed as species and genetic diversity rather than as structural and functional diversity. However, in terms of soil quality, these two last forms of diversity may be equally important due to the microorganism’s functional redundancy. The importance of functional and catabolic diversity lies in the fact that only based on changes in the genetic diversity; it is not possible to infer whether some functions of soils were lost or not (Mazzetto et al., 2008).

A soil with high redundancy of functions is probably able to maintain well-balanced its ecological processes, even under a disturbance. This approach, defined as resilience, refers to the buffering effects of external disturbances to the ecosystem. In a soil system the reduction of microbial diversity can be an important indicator of the loss of resilience and, consequently, the soil quality. The abundance of some species of microorganisms seems not to be as important as the maintenance of their genetic and functional diversities. This is because the abundance reflects more immediately the short-term microbial fluctuation, while the diversity reveals the balance between the number of microorganisms and the functional domains in the soil (Kennedy, 1999; Lavelle, 2000).

The main objective of our chapter is to describe the relationship between the genetic and functional diversity approaches to study the microbial ecology and the impact of different land uses under soil microorganisms in Amazonia.