Andosol-forming process linked with soil fauna under the perennial grass Mulhembergia macroura

Abstract

On the Cofre de Perote volcano, Mexico, at an elevation of 3000 m, vegetation is dominated by pine trees and Mulhembergia macroura grass. The grass undergoes a specific decomposition process whereby dead leaves and roots at different stages of decomposition accumulate below the plant, making a mat and forming a hummock of soil. Soil thin sections were prepared from the plant necromass and underlying soil to examine biological features by optical microscopy. Small organic fragments and soil aggregates were observed under a scanning electron microscope, coupled with a microprobe. C and N contents were measured, soil samples were analyzed by infrared spectroscopy and preliminary quantitative sampling of meso and macrofauna was executed in the plant residues and the underlying soil strata. Observations and analyses showed that soil microaggregates were faecal pellets from fauna, mainly Enchytraeids and Acari, living in the rooted soil below the grass. These biological aggregates exhibited a concentric internal structure and a silicon-rich coating which presumably protects them against microbial decomposition. Organic matter showed a predominance of aliphatic components upon aromatic components and appeared to be stable within the whole soil profile. As a consequence of plant die-off and mesofaunal activity, organic products accumulate and soil thickness and carbon storage increase with time.