Biological and mineralogical features of Andisols in the Mexican volcanic higlands

In the montane grasslands of Mexico, perennial grass and black Andisols are linked in a complex mechanism of soil formation in which both biogenic and mineral products interact. In this ecosystem, typical of the volcanic highlands, a sequence of profiles has been studied by macro- and micro-morphology, scanning and transmission electron microscopy (TEM), infra-red spectroscopy of soil samples and humic acid (HA) extracts, radiocarbon dating and pollen record, soil moisture and available water determinations and soil water analyses. The grass Muhlennberrgia macroura is important in the soil-forming process as it is the host of micro- and meso-fauna which produce abundant organic micro-aggregates. Soil formation began 7550 years ago and continued through several climatic and vegetation changes. The soil thickens as its age increases. Mineral neoformation is oriented towards allophane and halloysite, rather than Al-rich minerals such as imogolite and gibbsite, and high Si contents are observed in the soil water. The black Andisols have high water and carbon storage capacities and play a prominent role in regulating drainage. Presentday destruction of the grassland for potato cultivation will decrease the waterer reserve of these volcanic highlands.