Abstract
ABSTRACTWe investigated the variation of stable (813C) soil carbon isotopes in relation to depth in seven of the most important savanna areas to adjacent contiguous forests in the Amazon region. The 813C of bulk organic matter in all profiles from forested sites increased with soil depth. In forest profiles from Amapá, Alter do Chão, and Roraima, the enrichment was less than 3.5%o between deeper soil and surface layers, suggesting that 03 plants have remained the dominant vegetation cover. On the other hand, in forest soil profiles from Humaitá and Carolina sites, the 813C enrichment was greater than 3.5%o, indicating the influence of past 04 vegetation or a mixture of C3/C4 vegetation (woody savanna). The surface 813C values in the savanna profiles were 5–13%o greater than the comparable forest profiles, indicating the influence of C4 vegetation. Two kinds of isotopic distribution were observed in deeper layers. The savanna profiles at Alter do Chão, Chapada dos Parcels, and Redenção had relatively constant 813C values throughout the profile, suggesting minor past changes in the vegetation composition. In profiles at Amapá, Roraima, Humaitá, and Carolina, 813C values decreased with depth from the surface and converged with comparable forest values, suggesting more woody savanna in the past than exists currently.RESUMONós investigamos neste estudo a variação em profundidade dos isótopos estáveis de carbono (δ13C) da matéria organica do solo (SOM) em sete áreas de savanas e florestas da região Amazo̧nica. Os valores de δ13C da SOM aumentaram com a profundidade do solo. Nos perfis em floresta do Amapá, Alter do Chão e Roraima o enriquecimento isotópico com a profundidade foi menor que 3,5%0, sugerindo que plantas do tipo C3 foram sempre o tipo de vegetação dominante. Por outro lado, nos perfis em floresta de Humaitá e Carolina, o enriquecimento isotópico foi maior que 3,5%o, indicando a influ○ncia no passado de uma vegetação do tipo 04, ou uma mistura de vegetação C3/C4 (savana lenhosa). Os valores de δ13C na superflcie do solo em savanas fbram cerca de 5 a 13%o maiores que os perfis em floresta, evidenciando a influ○ncia da vegetação C4. Dois tipos de distribuição isotópica foram observados em camadas mais profundas. Nas savanas de Alter do Chão, Chapada dos Parecis e Redenção os valores δ13C foram constantes ao longo do perfil do solo, sugerindo que não houveram mudanças significativas na vegetação. Nos perfis do Amapá, Roraima, Humaitá e Carolina, os valores de δ13C diminuiram com a profundidade do solo, aproximando‐se aos valores encontrados na floresta, sugerindo a exist○ncia no passado de uma savana mais lenhosa que a actual.
Highlights
N6s investigamosneste estudo a variago em profundidade dos isbtopos estbveis de carbon0 (6I3C) da matkria orginica do solo (SOM) em sete hreas de savanas e florestas da regiSo Amazhica. 0 s valores de 6I3C da SOM aumentaram corn a profundidade do solo
Several lines of evidence, including paleolimnological, paleofaunal (Rancy 1993), and the carbon isotopic composition of soil organic matter (Desjardins et al 1996, Pessenda et al 1996, Gouveia et al 1997), suggest that the dynamics of expansion and contraction of savanna regions result from climatic fluctuations during the Quaternary period (Prance 1982, Sarmiento & Monasterio 1975)
The presence of charcoal dated from ca 3900 to 1800 years B.P. found in the northern Amazon (Desjardins et al 1996) suggests the occurrence of more recent climatic changes in that region as well as in other regions of Amazonia (Servant et al 1993)
Summary
Four of these sites (Amapi, Alter do Ch5o, Humaiti, and Roraima) are "isolated" savanna pockets that are completely surrounded by forest, and three (Chapada dos Parecis, Redengo, and Carolina) are classified as non-isolated savannas. We sampled five depth intervals (00.05, 0.1-0.2, 0.5-0.6, 1.0-1.2, and 1.4-1.6 m) in two soil cores in the savanna and two in the surrounding or dosest forest. The forest and savanna core sites were chosen on flat ground in undisturbed vegetation well away from the present ecotone (savanndforest boundary). As isolated savannas varied in size from several hectares to tens of square km, the distance from the ecotone boundary to the sampling location for savanna varied from 200 m to 40 km.Only the Chapada dos Parecis (Fig. 1) had no forest nearby. The conventional radiocarbon age, expressed as years before present (B.P.) was estimated assuming a 14Chalflife of 5568 years
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