Occurrence of sombric-like subsurface A horizons in some andic soils of the Nilgiri Hills (Southern India) and their palaeoecological significance

This paper deals with four pedons (three Fulvudands and one Andic Haplorthox) located at high elevations (2000–2400 m a.s.l.) in the Nilgiri Hills on an old laterized surface of South India, and characterized by thick (50–100 cm) epipedons whose colour tends to darken with depth. In two of them, the colour contrast between the top and the bottom A horizons was so well expressed that they were morphologically very similar to the African Oxisols reported to have sombric subsurface horizons. Soil organic matter (SOM) of the different selected A horizons was characterized by its mean residence time (MRT) and its δ 13C value, whereas their humic acids (HAs) were examined by techniques that made it possible to determine their chemical composition and their chromatic properties. It was found that the darker colour of the subsurface A horizons of these soils was related to three joined characteristics: a SOM δ 13C signature indicating a C4-type vegetation (i.e. a grassland-type) origin, the presence of humic acids belonging to the A-type (i.e. melanic) category of Kumada (Kumada, K., 1987. Chemistry of soil organic matter. Developments in Soil Science, Elsevier, Tokyo), and large mean residence time values. In contrast, the top A horizons had a more recent SOM, mainly inherited from a C3-type vegetation, and humic acids (HAs) that were less condensed than the A-type HAs. It was also found that where the presence of sombric-like subsurface A horizons was the most evident, the vegetation change recorded by the soils (i.e. the change from a grassland-type vegetation towards a C3-type vegetation) was expressed on the thickest part of their epipedon. These observations led us to propose that the occurrence of sombric-like subsurface A horizons resulted from the following sequence of events: (i) an exceptionally thick accumulation of organic matter deriving from grassland followed by (ii) its progressive replacement by less dark organic matter derived from a C3-type vegetation. This proposal was found to be in reasonable agreement with the history of the vegetation changes experienced by the Nilgiri Hills since the end of the Pleistocene. On the other hand, it is also very similar to one of the hypotheses proposed by Van Wambeke (Van Wambeke, A., 1992. Soils of the Tropics—Properties and Appraisal. McGraw-Hill, New York) to explain the frequent occurrence of sombric horizons in the Oxisols and Ultisols of the high-altitude areas of Central Africa.