Abstract
It is well-known that Amazon tropical forest soils contain high microbial biodiversity. However, anthropogenic actions of slash and burn, mainly for pasture establishment, induce profound changes in the well-balanced biogeochemical cycles. After a few years the grass yield usually declines, the pasture is abandoned and is transformed into a secondary vegetation called "capoeira" or fallow. The aim of this study was to examine how the clearing of Amazon rainforest for pasture affects: (1) the diversity of the Bacteria domain evaluated by Polymerase Chain Reaction and Denaturing Gradient Gel Electrophoresis (PCR-DGGE), (2) microbial biomass and some soil chemical properties (pH, moisture, P, K, Ca, Mg, Al, H + Al, and BS), and (3) the influence of environmental variables on the genetic structure of bacterial community. In the pasture soil, total carbon (C) was between 30 to 42 % higher than in the fallow, and almost 47 % higher than in the forest soil over a year. The same pattern was observed for N. Microbial biomass in the pasture was about 38 and 26 % higher than at fallow and forest sites, respectively, in the rainy season. DGGE profiling revealed a lower number of bands per area in the dry season, but differences in the structure of bacterial communities among sites were better defined than in the wet season. The bacterial DNA fingerprints in the forest were stronger related to Al content and the Cmic:Ctot and Nmic:Ntot ratios. For pasture and fallow sites, the structure of the Bacteria domain was more associated with pH, sum of bases, moisture, total C and N and the microbial biomass. In general microbial biomass in the soils was influenced by total C and N, which were associated with the Bacteria domain, since the bacterial community is a component and active fraction of the microbial biomass. Results show that the genetic composition of bacterial communities in Amazonian soils changed along the sequence forest-pasture-fallow.
Highlights
The huge Amazon River Basin of 4.6 x 106 km2 contains the largest equatorial forest of the world
The aim of this study was to assess the impact that land use changes in the Amazon region have had on the structure of the Bacteria domain
The conversion of forest to pasture for cattle grazing and the abandonment to fallow affected soil chemical properties, microbial biomass and the structure of bacterial communities
Summary
The huge Amazon River Basin of 4.6 x 106 km contains the largest equatorial forest of the world. By the implementation of development programs for the northern region of the country in the 1970s, more than 3.7 x 105 km of native vegetation were replaced by pasture for beef production (Cerri et al, 2003). The burning of forest vegetation and land use changes to pastures have increased the soil pH, C and N contents, exchangeable cations and decreases in exchangeable acidity (Moraes et al, 1995; Fernandes et al, 2002). In several areas of grassland in the Amazon (Veiga, 1995) as well as in other tropical regions The secondary vegetation that gradually develops in such areas is called “capoeira” and has the ability to restore soil nutrients and to assimilate the carbon released during cutting and burning of the original forest (Zarin et al, 2001; Schroth et al, 2002)
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