Abstract
The Southern Grasslands or Pampa biome covers 176,496km2 (2.07%) of the Brazilian territory, concentrated in the southernmost state with 63% covered by this biome. When adding Uruguay and Argentina, the biome area reaches 750,000km2. Rangelands are being substituted by crops and commercial eucalyptus, with potential impacts on ecological and hydrological response of watersheds and river basins. Studies of isotopic natural abundance may be used to understand vegetation dynamics in the past. We evaluated the natural abundance of 13C in soil profiles (ten soil layers down to 1.40m) at different positions of the landscape, in order to associate it with the vegetation dynamics in the studied environment. Two grassland areas and a forest area in the Pampa region of southern Brazil were sampled. Isotopic signature δ13C in the soil profile of the two grassland areas was dissimilar: one area is in the range of C4 plant formation, and the other followed the same trend up to the first 1.00m but from this depth on δ13C is in an intermediate range, which could have occurred due to the influence of C3 and C4 plants. Forest vegetation presents a clear change of predominant vegetation in a more-recent past period. Characteristic values of soil formed by C3 photosynthetic cycle plants are predominant in surface layers, whereas those of C4 plants predominate in deeper soil layers. Our hypothesis that soil classes, texture, and position in landscape are related to the presence/absence of riparian forest, as evaluated by the 13C isotopic abundance in soils from these sites, is accepted. Quantification of isotopic natural abundance allowed verifying changes in vegetation based on photosynthetic types predominant in the environments of grassland and native forests of the Pampa biome, and may provide information to manage this ecosystem.
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