Hydromorphic soil, topographic depression and vegetation development history by using δ13C and 14C in rondônia state (SW Brazilian Amazon)

Abstract

δ13C and 14C from Soil Organic Matter (SOM) were used to differentiate vegetation fluctuations in a contact forest-savannah. The study was carried out within a typical ecosystem of the SW Brazilian Amazon, characterized by lowered plateaus with waterlogged topographic depressions. The toposequence included five soil profiles located in forest (F), forest-savannah transition (FS), savannah (S1), savannah depression border (S2) and savannah in the depression (S3). The δ13C values have shown that at ~200cm depth, with ages ~ 12,000 to 10,000 B.P., δ13C values of - 27%o to -27.7%o indicate a homogeneous C3 forest vegetation. At 100cm, as ~ 6000 to - 5000 B.P., an uniform enrichment of - 20.2%o to -22.3%o indicate a mixture of C3 forest and C4 savannah reflecting forest regression and savannah expansion. Higher δ13C values (-15.9 to -18.796o) at 50-60cm whose ages were estimated as ~4700 to - 3800 B.P. suggest a maximum expansion of C4 savannah grass in response to drier climate conditions. More depleted 13C value in S3 profile (-20.996%) is attributable to a plant community consisting beside of C4 savannah grass predominantly of C3 savannah grass. Possibly due to an adaptive advantage of the C3 photosynthetic pathway in response to changing environmental conditions, C3 grass emerged after the assumed initiation of depression formation at the time.