The relevance of humus forms for land degradation in Mediterranean mountainous areas

In the Gavarras (NE Spain), a large number of plots on respective schists, leucogranite and granodiorite was studied for their soils and vegetation. Results were used to check conclusions from earlier studies of Mediterranean forest soils (mostly shallow Regosols and Cambisols) on such acidic to intermediate rocks. They confirmed that the humus form depends on catenary position and lithology, and that aggregate stability and infiltration characteristics of the upper mineral soil horizon relate to humus form type. Aggregate stability of the topsoil was found to be relatively high in mor and mull type humus forms, but differences with moder type humus forms were not statistically significant. Differences in aggregate stability are attributed to the presence of stable humus–clay–iron complexes in mulls and to high fungal activity and organic matter content of mors. Low infiltration rates were only encountered in topsoils with mor type humus form, in line with results from the earlier studies. In deeper soil horizons with low organic matter content, aggregate stability will be largely related to soil reaction and base saturation. On leucogranite and granodiorite, these were found to vary strongly, most probably largely due to local differences in fast acid neutralizing capacity (ANC_f). These local differences are primarily attributed to differences in the mineralogical composition and texture of the soil material, connected with differences in lithology and/or brought about by erosion, colluviation and soil formation. Consonant with earlier studies, it is concluded that the susceptibility of these forest soils to erosion largely depends on properties of the 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 degradation, which implies a more severe erosion, deeper soil horizons are also involved. Spatial variability in properties of these horizons, relevant for degradation, is considerable and not clearly related to humus form development. Accordingly, the abovementioned trends in humus form development cannot be used to predict the susceptibility to land degradation.