Origin and dynamics of soil organic matter and vegetation changes during the Holocene in a forest-savanna transition zone, Brazilian Amazon region

Carbon isotope data an soil organic matter (SOM) were collected along an ec osystem transect 90 m in length that includes a tropical forest on the plat eau, a transitional forest-savanna and savanna in a depression. Total organ ic carbon data show a significant increase in carbon content from sites rep resenting forest ecosystem to sires representing savanna ecosystem. It was hypothesized that carbon accumulation in the depression is controlled by fl ooding conditions that slow down carbon decomposition and in part by carbon transport from the upper part of the transect (the savanna and the transit ion forest-savanna areas) into the depressions by water during the rainy se ason. The origin of the carbon was confirmed by using soil C-13 analysis. T he savanna sites located in the depression showed delta C-13 values between -19.5 parts per thousand and -22.5 parts per thousand indicating a mixture of C-3 and C-4 plants. The vegetation cover in the depression is predomina ntly C-3 grasses with delta C-13 values of about -27 parts per thousand and -26 parts per thousand. In the site under savanna located at an elevation slightly higher, the delta C-13 value was more enriched (-16 parts per thou sand) showings the predominance of C-4 plants (delta C-13 of -13.6 parts pe r thousand). At the forest-savanna transition and in the forest ecosystem t he delta C-13 values were characteristic of C-3 plants (-25 parts per thous and and -28.1 parts per thousand). C-14 and C-13 data indicate that the org anic matter of mixed origin has been deposited for at least the last 7000 y ears in the savanna depressions. The C-13 pattern observed in the soil orga nic matter profiles indicate a predominance of C-3 plants in the early part of the Holocene. About 7000 to 4000 years ago, the data show the influence of C-4 plants, indicating forest regression associated with a drier climat e than at present. The more recent C-13 records suggest forest expansion, a nd the return to a climate similar to the present.