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.
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Vol. 34 • No. 1