4.4 Sombric-like horizons of the Nilgiris versus the African sombric horizons So far, we have shown that one of the hypotheses proposed to account for the presence of sombric horizons in central Africa is consistent with the observations reported in the present study. In the Nilgiris, the dark subsurface horizons are the remnants of humification processes that have taken place under a former grassland vegetation. In Africa, the sombric horizons have many different facies (Frankart, 1983) and, as Van Wambeke (1992) observes, they occur in environments where their origin may be related to many different types of events. As far as we know, no analytical data similar to those discussed in this paper are available. Thus, we are certainly not in a position to extend our hypothesis to cover the whole African area where sombric horizons were reported to occur. However, we may underline coincidences. Beside similarities in terms of elevation and nature of the soil parent materials, there are now reports (e.g. Bonnefille et al., 1990) indicating that in Burundi, especially, the end of the Pleistocene was also characterized by an important extension of the savanna at elevations that were more recently colonized by the montane forest. On the other hand, in an early paper by Ruhe (1956), a climatic and associated vegetation change was already suggested to be at the origin of the development of sombric horizons. Finally, there is another puzzling similarity. As is clearly the case in the Nilgiris and in Central Africa as well, the dark subsurface horizons frequently have chemical and physical properties similar to those that are presently recognized as soil andic properties (Neel, 1983; Buol and Eswaran, 1987; Mutwewingabo, 1989).
5 Conclusion The soils of the Nilgiris showing the clearest colour contrast between their dark subsurface horizons and their brown top A horizons have recorded the vegetation changes that they experienced since the end of the Pleistocene in their organic matter. SOM of the dark subsurface horizons derives from the grassland vegetation that existed more than 10,000 years ago, whereas top soil OM have the 13 C signature of organic constituents deriving from the C3-type vegetation that replaced the grassland more recently. On the other hand, the different horizon colours observed, as well as the different origins of their organic matter, are also reflected in the chromatic and chemical properties of their humic acids. In depth, the humic acids belong to the A-type category of Kumada (1987) whereas, in the top A horizons, similar highly aromatic and dark humic acids are absent.
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