The carbon isotope record in soils along a forest-cerrado ecosystem transect : implications for vegetation changes in the Rondonia state, Southwestern Brazilian Amazon region

This paper presents carbon isotope data on soil organic matter (SOM) collected along an ecosystem transect that includes a wooded savanah (cerrado), a tropical semideciduous forest (cerradao), a forest transition type and a tropical forest. The study area is located in the Rondonia state, southwestern Brazilian Amazon region. 14C data of total soil organic matter and charcoal indicate that the organic matter in these soils is a least Holocene in age. The forest and forest transition sites are characterized by delta 13C soil depth profiles generated typically by C3 plants, indicating no major changes in plant communities have occurred in this region during the time period represented by the isotope data. In contrast, the cerrado and cerradao have experienced significant vegetation changes during the Holocene. The delta 13C data (-3% to -2.7%) obtained in the deepest part of the profile at the cerradao site show the expansion of the C3 forest vegetation into this region during early Holocene. A vegetation change consisting of increased C4 plant influence is reflected in the 13C-enriched values (-1.9% to -1.6%) at both sites during the middle Holocene. The recent part of the 13C record shows a clear expansion of C3 vegetation, particularly at the cerradao site. The regression/expansion of the forest and savannah vegetation documented at the cerradao and cerrado sites is probably related to changes from a humid to a drier climate and a return to more humid conditions and is in agreement with palaeoclimatic information reported for Brazil and the Bolivian Altiplano. This study suggests that large areas in the Amazon basin have been affected by vegetation changes during the Holocene and that soil organic matter in the transition areas between savannah and forest ecotones contains a valuable palaeorecord of vegetation changes in the Amazon region. (Résumé d'auteur