THE RELEVANCE OF HUMUS FORMS FOR LAND DEGRADATION IN MEDITERRANEAN MOUNTAINOUS AREAS

In the Gavarras (NE Spain), a large number of plots on respective schi sts, leucogranite and granodiorite was studied for their soils and veg etation. Results were used to check conclusions from earlier studies o f Mediterranean forest soils (mostly shallow Regosols and Cambisols) o n such acidic to intermediate rocks. They confirmed that the humus for m depends on catenary position and lithology, and that aggregate stabi lity and infiltration characteristics of the upper mineral soil horizo n relate to humus form type. Aggregate stability of the topsoil was fo und to be relatively high in mor and mull type humus forms, but differ ences with moder type humus forms were not statistically significant. Differences in aggregate stability are attributed to the presence of s table humus-clay-iron complexes in mulls and to high fungal activity a nd organic matter content of mors. Low infiltration rates were only en countered in topsoils with mor type humus form, in line with results f rom the earlier studies. In deeper soil horizons with low organic matt er content, aggregate stability will be largely related to soil reacti on and base saturation. On leucogranite and granodiorite, these were f ound to vary strongly, most probably largely due to local differences in fast acid neutralizing capacity (ANC,). These local differences are primarily attributed to differences in the mineralogical composition and texture of the soil material, connected with differences in lithol ogy and/or brought about by erosion, colluviation and soil formation. Consonant with earlier studies, it is concluded that the susceptibilit y of these forest soils to erosion largely depends on properties of th e upper mineral soil horizon, which are controlled by or related with humus form development. General trends in the latter are clear and can be used to predict this susceptibility. In the case of land degradati on, which implies a more severe erosion, deeper soil horizons are also involved. Spatial variability in properties of these horizons, releva nt for degradation, is considerable and not clearly related to humus f orm development. Accordingly, the above mentioned trends in humus form development cannot be used to predict the susceptibility to land degr adation. (C) 1998 Elsevier Science B.V. All rights reserved.