ANDOSOL-FORMING PROCESS LINKED WITH SOIL FAUNA UNDER THE PERENNIAL GRASS MULHEMBERGIA-MACROURA

On the Cofre de Perote volcano, Mexico, at an elevation of 3000 m, veg etation is dominated by pine trees and Mulhembergia macroura grass. Th e grass undergoes a specific decomposition process whereby dead leaves and roots at different stages of decomposition accumulate below the p lant, making a mat and forming a hummock of soil. Soil thin sections w ere prepared from the plant necromass and underlying soil to examine b iological features by optical microscopy. Small organic fragments and soil aggregates were observed under a scanning electron microscope, co upled with a microprobe. C and N contents were measured, soil samples were analyzed by infrared spectroscopy and preliminary quantitative sa mpling of meso and macrofauna was executed in the plant residues and t he underlying soil strata. Observations and analyses showed that soil microaggregates were faecal pellets from fauna, mainly Enchytraeids an d Acari, living in the rooted soil below the grass. These biological a ggregates exhibited a concentric internal structure and a silicon-rich coating which presumably protects them against microbial decompositio n. Organic matter showed a predominance of aliphatic components upon a romatic components and appeared to be stable within the whole soil pro file. As a consequence of plant die-off and mesofaunal activity, organ ic products accumulate and soil thickness and carbon storage increase with time. (C) 1998 Elsevier Science B.V. All rights reserved.