The humus profile of an alpine ericaceous heath (Savoie, France): A high-resolution micromorphological analysis

The characteristics used to distinguish between Mor and Moder differ among the various classifications of humus forms. Mor is often associated with ericaceous heathland that develops at high altitude or latitude, or in lowlands when soils are nutrient-poor, sandy, and acidic. This study applies a new micromorphological method on three humus profiles from an alpine heathland in the French Alps (Savoie, altitude 1950 m) to better understand the underlying processes. Microscopic observations of horizontal sections of humus monoliths, gradually cut with dissecting instruments, allow us to characterize the proportions of humus components, the micromorphology of each component, and the microstructural organization of the humus. The analysis shows the importance of millipedes for the biological activity of the humus profile in the alpine ericaceous heath. These animals feed indiscriminately on litter (remains of ericaceous, moss, and herb vegetation), which therefore does not have time to accumulate. They also sometimes feed on humified organic matter when climatic conditions force these animals to live at depth. As a result, humus undergoes macrostructuration (aggregates and galleries). When the level of organic matter accumulation is high, this activity remains within the framework of a holorganic context, but when this level is low, millipedes consume a significant proportion of minerals and create a mineral–organic mixture. This work illustrates the distinctive characteristics of these ecosystems and emphasizes the importance of a thorough description of the humus forms by addressing various criteria, which facilitates the classification of transition humus forms.