Interaction of geomorphic processes and long-term human impact in the soil evolution: A study case in the tropical area at Veracruz, Mexico

Lahar deposits in karstic tropical regions of eastern Mexico provide favorable conditions for developing deep weathered profiles. In addition, the long-term human impact strongly modifies the pedogenetic trends. To un-derstand the interplay of geomorphic processes, pedogenesis and human impact (ancient and recent), we studied soils at the eastern sector of the Transmexican Volcanic Belt, where limestone hills from the Sierra Madre Ori-ental converge. A geomorphon map was created to evaluate the relationship between soil profiles and their geomorphic position. Then, seven soil profiles from different geomorphological and land use contexts were studied: one "natural" profile at a limestone foothill, where the forest is still preserved; three "Dark" profiles, which are in parcels cultivated with sugarcane; and three "Red" profiles, two of them nowadays not cultivated, and one in a coffee plantation. Micromorphology, grain size distribution, magnetic susceptibility, pH, electric conductivity, and clay mineralogy were determined, in all genetic horizons. Micromorphological observations showed two different kinds of pedogenetic patterns. The first one, corresponded to the "Dark" profiles and partly to the natural profile, was characterized by a quite heterogeneous composition where coarse particles were immersed in a fine material. The second pattern was observed in the "Red" soils, documented by the over-whelming dominance of fine clayey material colored with reddish ferruginous pigment. Clay mineralogy was domain by a kaolinitic component in all the profiles except at Cueva, where kaolinite was accompanied by smectite. We associate such patterns with the effect of two generations of lahars which produced two types of sediments and land surfaces of different ages. Sometimes the deposits of different generations were overlapped in one profile, generating pedocomplexes. Morphology (macro and micro) of the "Dark"profiles showed strong similarities to the Amazonian Dark Earths, suggesting they were the result of the long-term human impact.