Soil Microbial Biomass Under Native Cerrado and Its Changes After the Pasture and Annual Crops Introduction

Abstract

The Brazilian savanna (Cerrado) soils were incorporated into the agricultural production process in the 1970’s. Soils were initially occupied by pastures, and later used for cropping (Ferreira et al., 1997). An area of over 12 million hectares is cultivated with annual crops under different systems of soil management (Bayer et al., 2004). The introduction of pastures and/or annual crops utilizing different management systems promoted changes in the dynamics of soil organic matter (SOM). The SOM improves the soil structure and regulates its biological activity, as well as being directly linked with the ability to accumulate water and maintain the soil fertility. The introduction of agricultural systems in the Cerrado soils, with intensive land use, has brought direct consequences for the chemical, physical and biological properties of soil, with losses in its quality. Continued land use for the cultivation of grains, fibers and cultivated pastures can generate a rapid process of degradation, loss of organic matter, biological processes and imbalance in the flow of nutrients. The conversion of Cerrado into pastures and croplands made by the slash-and-burn process which causes major impacts on soil fertility. In order to improve the soil conservation, maintain and increase crop productivity, a number of practices were introduced, such as the elimination of crop residues caused by burning, the adoption of conservation tillage, and the management of crop residues (Mielniczuk et al., 1983). As a consequence, SOM increased not only due to the reduction of losses caused by biological decomposition and erosion but also due to the increase of plant residues on soil surface (Bayer et al., 2000). It is widely known that SOM improves soil structure (Feller & Beare, 1997) and regulates soil biological activity (Bayer & Mielniczuck, 1999), in addition to its role in water holding capacity and soil fertility maintenance (Dick, 1983). The dynamics of SOM is different in clayey and sandy soils and it is highly influenced by different management practices and climate conditions in each region. The stocks of SOM decrease when the soil is exposed to intensive tillage systems due to increasing losses caused by water erosion and microbial oxidation (Silva et al., 1994). However, little information is available about the effects of agricultural management practices on the dynamics of SOM in the Cerrado