Micromorphology of organic matter and humus in Mediterranean mountain soils

•Thin sections of topsoils can be successfully used for the study of humus.•Scanned images of thin sections of topsoils may be enough for humus classification.•Studied Amphi humus had laminated fabric, small excrements and fungal elements.•Studied Mull humus had larger excrements and faunal pores.

Humus classification is one of the most useful approaches when studying the dynamics of SOM in an ecosystem. Until now, soil micromorphology has seldom been applied to the determination of humus types, in spite of the close relationship between soil organic matter and soil structure. Micromorphological techniques were applied to the study of humus soils under forest and pasture in the Catalan Pre-Pyrenean region in order to characterize particulate organic matter, its degree of decomposition and its relationship with the physico-chemical and biotic properties of topsoil. Fourteen thin sections were studied from the humus profiles of 5 soils (2 Typic Ustifluvent, 2 Typic Calciustept and 1 Typic Ustorthent). Nine fabric units related to organic matter were identified and counted through a petrographic microscope and in scanned images of the thin sections. The results helped to classify two humus forms (Amphi and Mull) that had statistically different qualities and quantities of micromorphological features and assemblages. Amphi humus forms were characterized by a laminated fabric in the OL horizon and by a loose dropping fabric in the OF and OH horizons; with droppings of mesofauna (enchytraeids, springtails and mites), and fungal elements (sclerotia, hyphae). Mull humus forms had larger amounts of earthworm casts and feces of Isopods/Arthropods/Diptera together with faunal pores. The use of scanned thin sections proved to be useful for the general determination of the humus forms, although for a detailed organic matter study the use of the optical microscope is essential.