grass versus forest) under which the soils have evolved (Shoji, 1988), it could then be hypothesized that the darkening of colour with depth observed in Nilgiri soils might be indicative of vegetation changes. The purpose of this paper is to present observations and analytical data to document this hypothesis. Firstly, we will show that both the organic matter and the humic acids of the Nilgiri Andisols or andic soils keep the signature of the different types of vegetation under which these soils evolved during the last millennia. Afterwards, we will demonstrate that whenever a darkening of colour is observed in their thick A horizons, this morphological modification is closely associated with a change in the origin of their organic matter. In other words, we will show that the occurrence of dark, sombric-like subsurface A horizons in the Nilgiri soils can clearly be related to the vegetation and, most likely, to the associated climatic changes that have affected this area since the end of the Pleistocene. Finally, we will examine the implications of the present study concerning the genesis of the African sombric horizons.
2. Materials and methods 2.1. Study area and its environment. The Nilgiri Hills (also called the Nilgiris, 2000 to 2500 m a.s.l.) correspond to the Precambrian charnockite massif rising at the junction of the Western and Eastern Gh â ts in Southern India (Fig. 1). They result from residual erosion (Gunnell and Louchet, 2000) and have been deeply laterized during the Tertiary (Subramanian and Murthy, 1976; Subramanian and Mani, 1981). Demangeot (1975) has given a good description of the geomorphic features of this strongly rolling surface formed by low areas (or basins) of convex landforms separated by hill ranges. The thick regoliths inherited from this previous weathering cycle constitute, in most places, the present day parent materials of the soils. In the Nilgiris, the climatic conditions are characterized by a mean annual temperature of about 15 ° C and by rainfall regimes showing very drastic changes over short distances along a West-East gradient. In consequence, precipitations range from 5000-2500 mm year -1 in the western part subjected to the south-west monsoon, from 2000-1500 mm in the eastern part receiving convective rains, and from 1200-900 mm in the central part exposed to a mixed regime. The native vegetation, largely disturbed by human activities, corresponds to a mosaic of stunted, high-elevation, evergreen forest (locally called ‘shola’) dominated by Lauracea, Myrtacea and Ericacea species, and grasslands of different floristic compositions including a
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