Characterization, functioning and classification of two volcanic soil profiles under different land uses in Central Mexico

Volcanic soils constitute an important resource for agriculture and forestry in Central Mexico, as well as in various world regions. They exhibit unique properties and high productive potential related to the amorphous materials they contain. The relationship between amorphous materials, soil characteristic and functioning, has not been well studied. The objectives of the present work were to assess the influence of land use (agricultural and forest), topography and other soil forming factors on physical, chemical and mineralogical characteristics and pedological processes responsible for soil genesis and soil classification of two volcanic soil profiles derived from andesitic parent material located 150 m away from each other within the same toposequences. The toposequence is located in the Trans-Mexican Volcanic Belt (TMVB), a highly populated region of Central Mexico that provides part of the water for Mexico City megapolis. A series of field and laboratory techniques including physical, chemical, micromorphological, X-ray diffraction (XRD), transmission electron microscopy (TEM), infra-red analysis, Mössbauer spectroscopy were used. The main factor affecting the present morphology of the soil profiles was the topography. The mineralogical features of the upper layers of the maize profile (Pachic Andosol), indicate redistribution of soil material from the upper part of the toposequence. The land use change favored this redistribution. Deeper horizons of this profile were developed from volcanic ashes deposited in situ. hematite and ferrihydrite, considered markers of evolution in redistributed soil material were observed in this profile associated with allophane. The presence of hematite has been reported for the first time in Mexican Andosols. The present characteristics of the forest profile (Dystric Cambisol) are mainly due to the pedological process of the volcanic ash layers remaining in situ after the redistribution and volcanic breccia. It was concluded that the forest profile evolved from an Andosol to an Inceptisol, which was evidenced by desaturation, loss of silica and organic carbon. In this profile the Fe minerals were associated with the presence of gibbsite and halloysite. The position in the toposequence and the physical and chemical characteristics of these profiles define their present functioning, such as losses by erosion and C dynamics.