Microbial Biomass in Native Amazonian Ecosystems and its Adaptation to Deforestation and Pasture Introduction and Management

Abstract

Changes in biogeochemical cycles after land conversion from native forest to pasture have been focused in several aspects, like soil organic matter (SOM) quantity and quality, emissions of greenhouse gases, changes in hydrological and limiting elements cycles (Feigl, Melillo and Cerri 1995, Matson et al. 1997, Neill et al. 1997a,b, 2001, Garcia-Montiel et al. 2000). The decomposition of SOM and the mineralization of nutrients are mediated by microorganisms, and the significant environmental changes associated with land conversion are quite likely to alter their three most critical functions, i.e. labile source and immediate sink of C, N, P and S, and substrate transformations (Dalal 1998). Microbial biomass has even been proposed as a sensitive indicator of soil quality and soil health (Doran and Zeiss 2000, Schloter et al. 2003). As with any reservoir, its size is important not only because it provides an indication of slower, less easily detectable SOM changes, but also because it represents an important labile pool of plant available nutrients. Temporal measures of these changes are currently scarce. Users and policymakers require measures of microbial biomass in order to feed models to develop sustainable cropping systems. Assessment in soil microbial biomass (SMB) was facilitated in the last 20 years by physiological, biochemical and chemical techniques, including chloroform fumigation incubation (CFI) (Jenkinson and Powlson 1976) and chloroform fumigation extraction (CFE) (Brookes et al. 1985, Vance et al. 1987) among many others. As pointed out by Dalal (1998), it is difficult to compare soil microbial biomass values obtained by different methods in different laboratories due to variants of kEC values (the fraction rendered extractable by fumigation, used to convert the measured data into biomass C), soil moisture contents, incubation temperatures, SOM content, and instrumentation and analytical techniques. Most of the data presented in this chapter were obtained at Nova Vida Ranch (10◦30‘S, 62◦30‘W) in Rondonia state, Brazil (Fig. 12.1). The local climate is humid tropical, with an annual average temperature of 25.5◦C and precipitation of 2,200 mm (Bastos and Diniz 1982) including a dry season from May to September. Mean daily temperature for the warmest and coolest months varies less than 5◦C.