Past vegetation changes in Amazon savannas determined using carbon isotopes of soil organic matter

We investigated the variation of stable (delta(13)C) soil carbon isotopes in relation to depth in seven of the most important savanna are-as to adjacent contiguous forests in the Amazon region. The delta(13)C of bulk organic matter in all profiles from forested sites increased with sod depth. In forest profiles from Amapa, Alter do Chao, and Roraima, the enrichment was less than 3.5parts per thousand between deeper soil and surface layers, suggesting that C-3 plants have remained the dominant vegetation cover. On the other hand, in forest soil profiles from Humaita and Carolina sites, the delta(13)C enrichment was greater than 3.5parts per thousand indicating the influence of past C-4 vegetation or a mixture Of C-3/C-4 vegetation (woody savanna). The surface delta(13)C values in the savanna profiles were 5-13parts per thousand greater than the comparable Forest profiles, indicating the influence of C-4 vegetation. Two kinds of isotopic distribution were observed in deeper layers. The savanna profiles at Alter do Chao, Chapada dos Parecis, and Redencao had relatively constant delta(13)C values throughout the profile, suggesting minor past changes in the vegetation composition, In profiles at Amapa, Roraima, Humaita, and Carolina, delta(13)C values decreased with depth from the surface and converged with comparable forest values, suggesting more woody savanna in the past than exists currently.