Abstract
We investigated the variation of stable (δ13C) soil carbon isotopes in relation to depth in seven of the most important savanna areas to adjacent contiguous forests in the Amazon region. The δ13C 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‰ between deeper soil and surface layers, suggesting that C3 plants have remained the dominant vegetation cover. On the other hand, in forest soil profiles from Humaitá and Carolina sites, the δ13C enrichment was greater than 3.5‰, indicating the influence of past C4 vegetation or a mixture of C3/C4 vegetation (woody savanna). The surface δ13C values in the savanna profiles were 5–13‰ 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 Parecis, and Redenção had relatively constant δ13C values throughout the profile, suggesting minor past changes in the vegetation composition. In profiles at Amapá, Roraima, Humaitá, and Carolina, δ13C values decreased with depth from the surface and converged with comparable forest values, suggesting more woody savanna in the past than exists currently.
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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